Laminar Burning Velocity in Aviation Fuels: Conventional Kerosene, SAFs, and Key Hydrocarbon Components

Abstract

Sustainable aviation fuels (SAFs) are vitally important for aviation decarbonization. The laminar burning velocity (LBV), a key parameter reflecting the combustion behavior of fuel/oxidizer mixtures, serves as a fundamental metric for evaluating SAF performance. This paper systematically reviews and evaluates the LBV experiment method and the performance of traditional aviation fuel, SAFs produced via different pathways, and individual components (n-alkanes, iso-alkanes, cycloalkanes, and aromatic hydrocarbons, as well as the impacts of isomers and homologues) in aviation fuels. It is found that LBV values of different SAFs exhibit significant fluctuations, approaching or slightly deviating from those of conventional aviation fuels. Carbon number, branching degree, substituent types, and testing methods in the components all affect LBV performance. Specifically, increased branching in iso-alkanes reduces LBV, cyclohexane and benzene show higher LBV than their methylated counterparts (methylcyclohexane and toluene), and n-alkylcyclohexanes/benzenes with short (C1–C3) side chains demonstrate minimal LBV variation. Spherical flame methods yield more consistent (and generally lower) LBV values than stagnation flame techniques. These findings provide insights for optimizing SAF–conventional fuel blends and enhancing drop-in compatibility while ensuring operational safety and usability.

1. Introduction

Under extreme operational conditions, aircraft engines may encounter in-flight shutdown incidents. A critical factor influencing successful reignition at high altitudes and recovery from windmilling conditions involves the combustion and ignition properties of aviation fuels [1]. The autoignition delay represents a fundamental chemical reaction timescale, significantly influencing operational limits under critical combustion scenarios, including flame blowout and ignition thresholds. To accommodate modern aircraft requirements for increased cruising speeds and higher engine pressure ratios, the chemical autoignition reactions of aviation fuels must occur rapidly, achieving ignition nearly instantaneously [2]. Auto-ignition delay is therefore difficult to measure accurately, and the LBV is often used as a surrogate measure of aviation fuel reactivity [3]. This approach is justified since the turbulent flame combustion rate within gas turbines can be scaled relative to the laminar flame speed, accounting for factors such as fuel evaporation and mixing. Thus, the timescales associated with combustion in a laminar flame are comparable in magnitude to those observed within gas turbine combustion chambers. Turbulent flame speed can also be obtained from models that include parameters such as laminar flame speed, turbulence intensity, density, turbulent Reynolds number, etc. [4]. The LBV is critical for developing and validating chemical kinetic mechanisms and turbulent combustion models [5].

The LBV offer insights into the reactivity, heat release, and diffusivity of fuel/air mixtures [6]. As a fundamental parameter of premixed combustion, the LBV exhibits multiscale coupling relationships with flame temperature, residence time, and combustion emission characteristics [7,8,9]. From the perspective of reaction kinetics, the LBV shows a positive correlation with flame temperature due to the temperature sensitivity of Arrhenius-type reaction rates. Elevated temperatures not only accelerate chain reactions involving key radicals (H/O/OH) but also enhance the transport of reactive species through increased thermal diffusivity. This thermo-chemical coupling directly modulates the residence time characteristics in the flame zone: higher LBV reduces the residence time of reactants in high-temperature regions, which can effectively suppress the growth of polycyclic aromatic hydrocarbons (PAHs) and soot nucleation, while potentially leading to incomplete oxidation of intermediate products such as CO. Regarding emission characteristics, the increase in LBV influences pollutant formation through dual mechanisms: improving combustion efficiency reduces unburned hydrocarbon (UHC) emissions, while steeper temperature gradients at the flame front promote the formation of thermal NOx [10].

Traditional aviation kerosene consists of hundreds of chemical compounds, including n-alkanes, iso-alkanes, cycloalkanes, and aromatic hydrocarbons. Organic constituents possessing carbon numbers typically between C8 and C16 account for approximately 95% of aviation kerosene mass, following a normal distribution centered around C12 [11]. Such compositional complexity significantly hinders effective numerical modeling, and the most widely used solution is to select a limited number of mixtures of pure hydrocarbons as surrogate fuels to simplify the computational process [12]. Surrogate fuels, chosen to best match the physical and chemical properties of real fuels, typically comprise two to three high-mass fraction components found in aviation kerosene. These surrogates are extensively studied to elucidate fundamental combustion data and validate kinetic models [13]. Consequently, research on LBV for aviation kerosene primarily focuses on n-alkanes within the C10–C14 range, with considerably limited investigation into non-key components, particularly iso-alkanes and cyclic compounds [14].

Reducing aviation carbon emissions is essential for the industry’s long-term sustainability. The International Air Transport Association (IATA) has announced its commitment to achieving Fly Net Zero by 2050 [15], and the development of SAF technology stands as the most crucial means to achieving this goal [16]. Unlike conventional petroleum-based aviation kerosene, SAFs can be derived from biomass, waste oils, or solid waste. Consequently, SAFs differ in composition and distribution from traditional aviation kerosene. For instance, in Fischer–Tropsch hydroprocessed synthesized paraffinic kerosine (FT-SPK), synthesized paraffinic kerosine hydroprocessed esters and fatty acids (HEFA-SPK), synthesized iso-paraffins (SIPs), alcohol-to-jet synthetic paraffinic kerosene (ATJ-SPK), and hydroprocessed hydrocarbons, esters, and fatty acids (HC-HEFAs), the maximum mass fraction of aromatics cannot exceed 0.5%. Particularly in SIPs, the mass fraction of saturated hydrocarbons must exceed 98% [17]. Due to the presence of aromatic compounds, petroleum-derived fuels exhibit lower LBV compared to SAFs [18]. This leads to differences in combustion flame lengths and changes in thermal loads on combustion chamber walls or in the recirculation zone, thereby affecting engine performance [19]. To minimize the pronounced influence of hydrocarbon fuel composition and proportion on engine functionality, certified SAFs must be blended with conventional aviation kerosene, with a maximum blending ratio of 50% [20]. To meet airworthiness requirements, ensuring aircraft and aviation engine safety and performance remain at the same level as traditional fuels [21], in-depth research on the LBV of SAFs and its individual components is essential.

2. LBV Test Methods

The LBV is an intrinsic fuel parameter controlled by pressure, temperature, and equivalence ratio. For premixed fuel–air combustion, the prevalent LBV testing methods include the Bunsen burner method, spherical flame method, stagnation flame method, heat flux method, annular stepwise diverging tube, and externally heated diverging channel method [22]. The characteristics, advantages, and disadvantages of each LBV testing method are summarized in

Table 1. Six LBV testing methods and information.

Method	Earliest Year	Characteristic	Advantage	Disadvantage
Spherical flame method	1934	Plotting p-t curves or r-t curves	Controllable initial temperature and pressure conditions	Significantly affected by flame stretch
Stagnation flame method	1985	Fixes the effect of flame stretch on LBV	No downstream conductive heat losses	Extrapolation required; complex for liquid fuel measurement
Heat flux method	1994	Direct measurement of adiabatic flame speed	Direct measurement method; apt for liquid fuel measurement	Failure under both high pressure (>10 atm) and high temperature Effective when LBV < 80 cm/s
Bunsen burner method (tracer particles)	1959	Introduces tracer particles	Traditional method with the simplest structure	Inaccurate due to boundary layer near burner rim, difficulties in flame anchoring
Annular stepwise diverging tube	2011	Introduces tracer particles	Capable of the measurement of high-LBV fuels	Further investigation in both experiments and simulations needed
Externally heated diverging channel method	2011	Introduces tracer particles	Direct measurement of burning velocities under a large range of temperature	Investigation of the condition of higher pressures and high-LBV fuels needed

.

The spherical flame method entails confining a premixed fuel/air mixture within a sealed spherical chamber and igniting it at the center of the chamber under a known equivalence ratio and initial temperature and known pressure conditions [23]. The spherical flame propagates outward into the unburned mixture while the LBV is measured. Flame stretch, arising from phenomena such as unsteady flame propagation, non-uniform flow fields, flame curvature variations, and diffusion effects, significantly influences LBV measurements. The stagnation flame method involves supplying identical premixed gases into two nozzles placed at a certain distance apart to achieve uniform velocities at the nozzle exits [24]. Igniting the ejected mixture leads to the formation of a stable stagnation flame at a fixed position after collision, allowing for the measurement of the radial propagation velocity of the laminar flame. This approach partially mitigates the influence of flame stretch on the measured flame propagation speed. In the heat flux method, the flow of unburned mixture is directed orthogonal to a stationary flame front at the burner exit plane, enabling direct measurement of the unburned fuel/air mixture velocity and thus obtaining the LBV of the fuel [25]. The Bunsen burner method is the conventional approach for measuring the LBV of various fuel/air mixtures [26]. The LBV is defined as the normal component of the unburned gas velocity along the flame front and is calculated according to the following formula:

$$S_L=u_g\mathit{sin}\alpha$$

(1)

where S_L represents LBV, u_g denotes the velocity of the unburned gas, and α represents the half-cone angle of the conical flame, shown in Figure 1d. One approach to testing the LBV of a Bunsen burner involves the utilization of the conservation of mass principle:

$$S_L={\displaystyle \frac{\dot{Q}}{A_f}}$$

(2)

where $\dot{Q}$ and A_f are the volumetric flow rate and average flame area, respectively. The use of tracer particle methods enables a clearer determination of the flame edge position, facilitating a more precise calculation of the flame area. The early annular diverging tube (ADT) method first used a conical insert in a quartz tube to form a ring-shaped divergence. Later, the conical magnetic core was replaced by a stepped magnetic core to form the annular stepwise diverging tube (ASDT) [27]. In the ASDT, the flames exhibited minimal wrinkling or elongation, allowing the measurement of the critical LBV of a stationary flame. The development of the annular stepwise diverging tube aimed to overcome limitations in combustion spatial length and scale, reducing the influence of heat losses and flame elongation on actual flames. In the externally heated diverging channel method, a premixed fuel–air mixture enters through a rectangular intake at one end of the channel and ignites at the outlet, stabilizing at a specific position within the channel [28]. External heating of the channel walls aids flame stabilization and maintains nearly adiabatic conditions. By applying conservation of mass at the flame front, the laminar adiabatic burning velocity can be determined. In this method, the flame structure is significantly influenced by the flow field, and the flame front must remain planar without stretching or curvature to accurately calculate LBV values. A schematic diagram illustrating the principles of the six LBV testing methods is provided in Figure 1.

In the aforementioned methods for measuring LBV, flame stretch fundamentally affects the measurement, a phenomenon that significantly contributed to the inaccuracy and errors in numerous measurement data during the last century [29]. For instance, in the early stages, the spherical flame method struggled to achieve an ideal flame front, and the correction methods lacked uniformity, resulting in significant discrepancies in LBV measured by different researchers at different times. Even with the introduction of particle tracing techniques, the Bunsen burner method could only marginally mitigate its impact. By contrast, the other four methods showed much better agreement across tests. Subsequently, in the subsequent analysis of LBV for each component of aviation fuel, this issue will be summarized and discussed. Looking ahead, the primary focus in the upgrading and innovation of LBV measurement methods lies in eliminating the significant increase in laminar speed caused by flame stretch and minimizing thermal losses resulting from non-combustion.

3. LBV of Conventional Aviation Fuel

3.1. LBV of Jet A and Its Surrogate Fuel

Research on the LBV of aviation fuels typically involves surrogate fuels. Surrogate components are typically selected from representative components of alkanes, cycloalkanes, and aromatic compounds contained in traditional aviation fuels. The selection and proportion of components are crucial factors determining whether the performance of the surrogate fuel closely resembles that of traditional aviation fuels. Experimental conditions and methods used to measure LBV for Jet A and its surrogate fuels are summarized in

Table 2. Experimental conditions and measurement methods for the LBV of Jet A and its surrogate fuel.

Surrogate Fuel/Jet A	Experimental Conditions			LBV Measurement Methods	Ref.
	T (K)	P (atm)	Φ
n-Decane, methylcyclohexane, toluene (82.1/7.9/10.0 and 59.6/-/40.4 mol%)	400	1	0.7–1.4	Jet wall stagnation flame configuration	[30]
n-Dodecane, n-propylbenzene, 1,3,5-trimethylcyclohexane (66.2/15.8/18.0 mol%)	473	1	0.6–3.0	Laminar burner	[31]
n-Decane, n-propyl-benzene, propyl-cyclohexane (76.7/13.2/10.1 wt%)	400–473	1–10	0.7–1.3	High-pressure Bunsen burner	[6]
n-Decane, methylcyclohexane, benzene, butylcyclohexane (40/20/20/20 vol%)	400	1	0.7–1.4	Heat flux method	[32]
Jet A	400, 473	1	0.7–1.4	Counterflow burner	[33]
Jet A	400, 450, 470	1	0.7–1.4	Counterflow burner	[5]
Jet A	473, 550	1	0.7–1.1	Bunsen burner	[34]
Jet A	473	1	0.9–1.4	Bunsen burner	[35]
Jet A	470	1	0.75–1.5	Jet wall stagnation flame configuration	[36]
Jet A	473	1, 3, 6	0.6–2.0	Bunsen burner	[37]

.

Table 3. Information on the composition and properties of Jet A and its surrogate fuel.

Composition of Surrogate Fuel	H/C Ratio	Mol Weight (g·mol−1)	Density (kg/m3)	Boiling Point (K)	Ref.
Jet A	1.91	153.0	815.9	423–533	[6,30]
n-Decane, methylcyclohexane, toluene (82.1/7.9/10.0 mol%)	2.11	133.8	735.8	374–447	[30]
n-Decane, toluene (59.6/40.4 mol%)	1.77	122.0	762.4	384–447	[30]
n-Dodecane, n-propylbenzene, 1,3,5-trimethylcyclohexane (66.2/15.8/18.0 mol%)	2.03	148.0	764.8	412–489	[31]
n-Decane, n-propyl-benzene, propyl-cyclohexane (76.7/13.2/10.1 wt%)	2.06	136.9	747.0	432–447	[6]
n-Decane, methylcyclohexane, benzene, butylcyclohexane (40/20/20/20 vol%)	1.83	111.6	782.8	353–454	[32]

summarizes the composition and property data of Jet A and its surrogate fuel, with some values sourced from references, while others calculated based on surrogate component ratios where reference data were unavailable.

Denman et al. employed particle image velocimetry in a stagnation flame technique to investigate the LBV of commercial Jet A kerosene and surrogate fuels [30]. The LBV measurement results of Jet A kerosene were notably lower than previous test results, especially under rich fuel conditions, indicating considerable compositional differences among commercial batches. Blend 1 (82.1% n-decane, 7.9% methylcyclohexane, 10% toluene, molar fraction) and blend 2 (59.6% n-decane, 40.4% toluene, molar fraction) exhibited significant differences in LBV under lean conditions, stemming from their distinct chemical kinetics mechanisms and reactivity. Blend 1 closely matched the results of Jet A from Ref. [5] but exceeded the LBV values measured for the current Jet A sample under rich conditions. Blend 2 matched the LBV values of the target Jet A well across the entire equivalence ratio range. Liu et al. selected molar ratios of 66.2% n-dodecane, 15.8% n-propylbenzene, and 18.0% 1,3,5-trimethylcyclohexane as surrogate fuels for Jet A-1 and proposed a detailed chemical kinetics mechanism involving 401 species and 2838 reactions [31]. Their simulations for fuels and single components involved in experiments accurately predicted LBV data obtained from jet stirred reactor (JSR) measurements, with deviations of around 10% within the Φ = 0.6–1.5 range, albeit with slight underestimations outside this range of equivalence ratios. The uncertainty of LBV measurements primarily stemmed from cone angle determination, ranging from 2% to 13% and depending on Φ values. Additionally, inaccuracies in mass flow controllers, possible pressure fluctuations, and ideal-gas assumptions also contributed to measurement uncertainties. Wu et al. improved the high-pressure laminar flame combustion chamber and used OH* chemiluminescence and the kerosene–PLIF technique to capture flame profiles [6]. A composition of 76.7 wt% n-decane, 13.2 wt% n-propylbenzene, and 10.1 wt% propylcyclohexane adequately reproduced the LBV characteristics of Jet A-1, obtaining α values for different equivalence ratios based on experimental results, the power law equation, and extended formulation, as shown in the following formula [38].

$$S_L={S_{L0}\left({\displaystyle \frac{T}{T_0}}\right)}^{\alpha }$$

(3)

where S_L0 was defined as LBV at T = 400 K and P = 0.1 MPa. Under conditions of T = 423 K, P = 0.1–0.8 MPa, and Φ = 0.7–0.8, based on high-pressure measurements of Jet A-1/air mixtures, power exponent β at equivalence ratios of 0.7 and 0.8 was −0.235 and −0.198, respectively.

$$S_{L0}\left(\phi \right)=S_{L0,\phi =1}+S_{L\mathrm{0,1}}\left(\phi -1\right)+{S_{L,2}\left(\phi -1\right)}^2+{S_{L,3}\left(\phi -1\right)}^3$$

(4)

This indicates that under lean conditions, the pressure dependence exponent increases with the equivalence ratio, as shown in Formula (5). Idirsov et al. selected liquid volume fractions of 40% n-decane, 20% butylcyclohexane, 20% methylcyclohexane, and 20% benzene as alternative fuels for Jet A and measured fuel LBV using the heat flux method [32]. At 400 K, differences in LBV data between alternative fuels and kerosene were approximately 5–10 cm/s across the entire Φ range, suggesting incomplete evaporation of all fractions in kerosene and leading to experimental error in the results. Formulas (3) and (5) enable extrapolation of LBV data, permitting cross-comparison of LBV measurements obtained under different temperature or pressure conditions across studies. While extensive research exists on empirical temperature/pressure dependency correlations, extrapolation formulas for the equivalence ratio are virtually absent [38,39,40]. However, all LBV investigations invariably include measurements at the stoichiometric ratio (Φ = 1.0) and its neighboring equivalence ratios (0.9–1.2). Whether higher or lower equivalence ratios are experimentally examined varies among studies. For meaningful cross-study comparisons, LBV data should be compared at identical equivalence ratios.

$$S_L={S_{L0}\left({\displaystyle \frac{P}{P_0}}\right)}^{\beta }$$

(5)

At P₀ = 1 atm, the LBV of Jet A at various temperature conditions is depicted in Figure 2, and the corresponding references are listed in Table 2. The LBV trends of Jet A observed in different studies align: the LBV increases with the equivalence ratio from Φ = 0.6 to 1.0, reaching a maximum at equivalence ratios of 1.05 or 1.1 (highlighted in green), beyond which the LBV decreases with further increases in the equivalence ratio. The color variation across the four temperature points in the graph vividly illustrates that Jet A’s LBV decreases with increasing temperature, with maximum LBV values (LBV_{_max}) occurring around 120, 80, 70, and 60 cm/s at 550 K, 470 K, 450 K, and 400 K, respectively. Differences in LBV at the same temperature across studies likely come from Jet A source/batch differences, test procedures, equipment calibration, and experimental conditions.

3.2. LBV of RP-3 and Its Surrogate Fuel

RP-3 is the most widely used fuel for Chinese aircraft engines. It is a traditional petroleum-derived aviation fuel known for its mature and simple production process, low cost, and long-term storage capability [41]. Liu et al. blended 14% n-decane, 10% n-dodecane, 30% iso-hexadecane, 36% methylcyclohexane, and 10% toluene (molar%) as a surrogate for RP-3 [42]. In a constant volume combustion reactor, both RP-3 kerosene and alternative fuels exhibited an increase in LBV with decreasing P₀ or increasing T₀, reaching a maximum at Φ = 1.1 (conditions including T₀ = 400, 420, 450, 480 K; P₀ = 0.1/0.3 MPa; Φ = 0.7–1.4). The authors also established the reduction reaction mechanism for the alternative fuels, comprising 181 species and 872 reactions. In most cases, the LBV obtained from experimental data of alternative fuels and simulations based on the reduction reaction mechanism differed from the actual tested LBV of RP-3 by less than 5%. Furthermore, the authors validated that alternative fuels could represent RP-3’s low-temperature oxidation and ignition delay time, and the proposed simplified mechanism could predict these chemical properties of RP-3. Huang et al. selected 22% n-dodecane, 15% iso-hexadecane, 48% decalin, and 15% 1,2,4-trimethylbenzene as alternative fuels for RP-3 and tested it in a constant-volume combustion bomb [43]. The conclusions obtained validated the aforementioned results, indicating that the LBV of both RP-3 kerosene and alternative fuels increased with increasing Φ before decreasing, with a peak around Φ = 1.1; increasing the initial temperature or decreasing the initial pressure led to an increase in LBV. The LBV of RP-3 at different pressures but similar temperatures in the two aforementioned works is summarized in Figure 3.

3.3. LBV of JP-8

JP-8 is a Jet-A-derivative fuel formulated with additives for thermal stability, giving it a chemical composition very similar to that of Jet A-1. Due to its lower freezing point, JP-8 can serve both as both coolant and fuel within fuel systems, making it widely utilized by the US military in aircraft engines. Far et al., employing a combination of a spherical vessel and a shadowgraph system, observed that the LBV of JP-8 increases with temperature and decreases with pressure [44]. The LBV data were fitted to the following power law correlation:

$$S_L=S_{L0}\left[1+a_1\left(1-\phi \right)+a_2{\left(1-\phi \right)}^2\right]{\left({\displaystyle \frac{T}{T_0}}\right)}^{\alpha }{\left({\displaystyle \frac{P}{P_0}}\right)}^{\beta }$$

(6)

In the equation, S_L0 denotes the LBV at P₀ = 1 atm, T₀ = 500 K, and Φ = 1, where a₁, a₂, α, and β are constants. This correlation is specifically applicable to the LBV of JP-8/air flames. It is valid within the pressure range of 1 atm < P < P_cr, temperature range of 500 K < Tu < T_cr, and equivalence ratio range of 0.8 < Φ < 1.0. Here, T_cr and P_cr represent the critical temperature and pressure at which cell formation initiates. P_cr can be obtained from empirical tables in the text, while T_cr can be calculated from the following equation:

$$T_{cr}=T_i{\left({\displaystyle \frac{P_{cr}}{P_i}}\right)}^{{\displaystyle \frac{\gamma -1}{\gamma }}}$$

(7)

where T_i and P_i denote the initial temperature and pressure, respectively, while γ represents the ratio of specific heat at constant pressure to that at constant volume.

During the actual flight of an aircraft, the occurrence of in-flight shutdown may subject the combustion chamber of aviation engines to extremely low pressures, necessitating a comprehensive understanding of the flame propagation characteristics of aviation fuel under low-pressure conditions. Liu et al. innovatively incorporated a fan-stirred device into the constant-volume bomb apparatus to homogenize the gas-phase mixture within the chamber [1]. Additionally, with 6.5 kW band heaters, they were integrated into the cylindrical stainless steel combustion chamber to achieve a maximum temperature of 573 K and facilitate experiments at varying initial pressures. The LBV also increases with increasing initial temperature and decreasing initial pressure. Across all temperatures and pressures, the LBV_{_max} of aviation kerosene occurs at an equivalence ratio of 1.1. Based on experimental findings, Formula 6 is fitted and is applicable within the pressure range of 25–100 kPa, temperature range of 400–480 K, and equivalence ratio range of 0.7–1.5. The reference conditions for the formula are T₀ = 450 K and P₀ = 100 kPa.

$${S_L}^0=\left[199{\phi }^4-878{\phi }^3+1242{\phi }^2-613\phi +117\right]{\left({\displaystyle \frac{T}{T_0}}\right)}^{1.76}{\left({\displaystyle \frac{P}{P_0}}\right)}^{-0.15}$$

(8)

With the exception of occasional data points under rich fuel conditions, the formula demonstrates good consistency with experimental test values (within 5% error). In conditions where there is significant deviation between the two sets of results, due to the instability of flame propagation, the uncertainty in data testing outweighs the deviation in fitting results. The fitted temperature exponent (α = 1.76) is positive, attributed to faster reaction rates and mass diffusion at higher temperatures, which enhance flame propagation. The pressure exponent (β = −0.15) is negative, because lower pressure suppresses hydrodynamic and diffusive–thermal flame instabilities.

3.4. The Effect of Additives on Aviation Fuel LBV

As a species with significant radiative properties, water’s influence on combustion radiation directly impacts the operational state of aviation engines under harsh conditions. But currently, most research is focused on the effect of water on the LBV of simple fuel systems, such as syngas mixtures, propane–air mixtures, and oxygen-enriched methane [45,46,47]. Zheng et al. demonstrated that radiation reabsorption of water in polluted air significantly affects the accurate prediction of aviation fuels’ LBV [48]. In high-enthalpy vitiated air with mole fractions of 21% O₂, 12% H₂O, and 67% N₂, PR-3’s LBV increases initially with increasing equivalence ratio, peaking at Φ = 1.15, with S_Lmax,ADI = 67.30 cm/s and S_Lmax,SNB = 70.03 cm/s (420 K, ADI = adiabatic model, SNB = statistical narrow-band model). With increasing equivalence ratio, the relative difference between SNB and ADI simulation results decreases initially, then increases at atmospheric pressure. At lean conditions (Φ = 0.7), the maximum relative difference of (S_L,SNB-S_L,ADI)/S_L,ADI = 12.60%. According to simulation results and sensitivity analyses, the influence of radiation reabsorption on LBV stems primarily from chemical rather than physical factors, such as optical thickness and the absorption coefficient. The key elementary reaction enhancing LBV is CH₂OH + H = CH₃ + OH, with radiation reabsorption inhibiting this reaction’s effect on LBV simulation results initially increasing then decreasing with an increasing equivalence ratio. Considering pressure variations within the 1~10 atm range, the reaction H + O₂ = O + OH, dominated by OH radicals’ sensitivity to radiation reabsorption, becomes predominant, enhancing the impact of radiation reabsorption on LBV simulation result differences with increasing pressure. Conversely, slight increases in LBV simulation result differences between 10 and 15 atm are controlled by direct radiation effects. At 15 atm pressure, radiation reabsorption’s influence on LBV simulation result differences reaches 12.41%.

Liquefied natural gas (LNG) is considered a promising aviation alternative fuel due to its abundant resources, low cost, and low greenhouse gas (GHG) emissions. However, its direct application necessitates changes in the design of fuel tanks and fuel delivery systems due to LNG’s low-temperature liquid state. Moreover, the low energy density and slow combustion rate of methane pose new requirements for engine and combustion chamber design. Therefore, blending LNG with traditional aviation fuels appears to be a method to reduce GHG emissions in the aviation industry without altering existing aircraft and engine structures [49]. Liu et al. investigated the fundamental combustion characteristics of CH₄/RP-3/air mixtures in a constant-volume combustion bomb, finding that LBV_{_max} occurs at an equivalence ratio of 1.1, independent of initial temperature, pressure, and methane mixture ratio [50]. LBV increases with higher initial temperatures and lower initial pressures, following a similar trend as pure RP-3 and pure CH₄. Increasing the methane mixture ratio reduces the LBV of the blend fuel, with a significant decrease observed after the methane mixture ratio exceeds 0.8. Sensitivity analysis indicates that highly reactive components strongly correlated with LBV, such as H and OH, exhibit reduced peak molar fractions with increasing methane mixture ratio, leading to decreased LBV. Additionally, the addition of methane reduces the Arrhenius factor of the blend fuel, exp (T_a/2 T_ad), which significantly impacts LBV, particularly when the methane mixture ratio exceeds 0.8, explaining the observed LBV trend. Some differences exist between Liu et al.’s study and previous research [51]. Firstly, under conditions of an initial temperature of 420 K and initial pressure of 0.1 MPa, the equivalence ratio corresponding to the maximum LBV differs. Secondly, at Φ = 0.8, the LBV of fuel without added methane is minimal, while at Φ = 1.2/1.3, the blend fuel with a methane mixture ratio of 0.8 exhibits the lowest LBV. Thirdly, the influence of methane addition on LBV is significant when Φ < 0.9 or Φ > 1.1, while it is less pronounced when 0.9 < Φ < 1.0. The impact of methane addition on RP-3 LBV may arise from changes in density, flame equilibrium temperature, heat capacity, and heat release rate.

Similarly, renewable ethanol biofuels can be blended with aviation fuel to mitigate energy crises and facilitate the achievement of the aviation industry’s fly net-zero objectives. However, their lower energy density and higher water solubility limit their potential to fully replace aviation kerosene. Therefore, blending ethanol into aviation kerosene is a more practically effective approach [52]. Liu et al. obtained the LBV of ethanol/RP-3 premixed flames in a constant-volume chamber [16]. The results revealed that LBV_{_max} occurred at Φ = 1.1, gradually decreasing on both sides. Increasing the initial temperature, decreasing the initial pressure, and increasing the ethanol blending ratio all enhanced the LBV of the premixed flame, with ethanol exhibiting the most significant promoting effect at Φ = 1.4. The thermal and dilution effects of ethanol blending lead to a reduction in LBV, while chemical effects enhance LBV and dominate. According to sensitivity analysis and reaction pathway analysis, the addition of ethanol increases the abundance of active free radicals H, O, and OH, accelerating the generation rates of representative species CH₃, CH₂O, and HCO, thereby enhancing LBV. From a molecular reaction perspective, the rapid decomposition of C₂H₅OH into CH₂O, followed by oxidation to CO₂ without needing extensive breaking, helps increase the LBV of the blend.

4. LBV of SAFs

In recent years, as demands for fuel sustainability and a reduction in carbon emissions have escalated, an increasing number of studies have shifted from conventional aviation fuels towards SAFs. Compared to traditional fuels, SAFs offer advantages such as ample reserves, lower emissions, and anticipated lower commodity prices. Assessing the LBV of SAFs aids in understanding their combustion performance and disparities with conventional fuels, constituting a pivotal aspect of SAF performance certification. Depending on the feedstock for alternative fuel production, they are generally categorized as synthetic fuels and renewable or bio-jet fuels. The former refers to alternative aviation fuels produced via the Fischer–Tropsch synthetic method from natural gas or coal, while the latter encompasses aviation fuels derived from biomass using various methods [33]. Below, we present introductions to SAFs obtained through these two pathways.

4.1. LBV of Synthetic Fuels

Synthetic fuels are categorized as natural gas-to-liquid (GTL) or coal-to-liquid (CTL) fuels based on their feedstock. Although both use the Fischer–Tropsch process, variations in preparation equipment and procedures result in significant compositional variations among GTL fuels [53]. Consequently, LBV disparities are observed in GTL fuels obtained from different sources. GTL fuels primarily include Syntroleum S-8 fuel, Shell GTL, and other sources of GTL fuels.

Ji et al. determined the LBV of traditional fuels (JP-7 and JP-8) and alternative fuels (synthetic fuel S-8, Shell-GTL, and bio-jet fuel R-8, derived from animal/vegetable oil) using the stagnation flame method, finding no significant difference between measured GTL fuel and bio-jet fuel LBV [54]. In contrast, traditional fuels exhibited smaller LBV values due to higher concentrations of cycloalkanes and aromatics. Kumar et al. studied Syntroleum S-8 using the stagnation flame method and compared its LBV at temperatures of 400, 450, and 470 K with Jet A and pure alkanes (n-decane, n-dodecane) [5]. Their results indicated similar combustion characteristics between alternative and traditional fuels. This contrasts with the conclusion of Ji et al., possibly due to the different traditional aviation fuels used in the two studies [54]. Hui et al. measured various fundamental combustion characteristics, including LBV, of three Fischer–Tropsch produced synthetic fuels and three bio-jet fuels [33]. LBV measurements of alternative fuels were conducted using the stagnation flame method at 400 K and 470 K, with no significant differences from Jet A found.

Additionally, there is substantial work on LBV testing, mechanistic studies, and simulation calculations concerning synthetic fuels and their surrogate fuels. Kick et al. conducted LBV measurements using the Bunsen burner method for GTL fuel and a blend with 20% hexanol [19]. Within the measured range of 423–473 K, the experimental results are similar to the simulation results of their proposed surrogate fuels (composed of n-decane, iso octane, and 1-hexanol), which involve 3479 reactions and 490 species. Subsequently, Kick et al. extended similar studies to GTL with 20% 1-hexanol or 50% naphthenic [35], validating against models proposed by Dagaut et al. [55,56] and demonstrating the practical potential of surrogate fuels. Wang et al. measured the LBV of Syntroleum S-8 using the spherical flame method and a surrogate fuel mixture of 32% iso-octane, 25% n-decane, and 43% n-dodecane, extending the study from 0.5 atm to 4.3 atm pressure and a 490 K-to-620 K temperature range, showing high consistency with simulations using Ranzi’s chemical kinetics mechanisms in a one-dimensional steady premixed flame code from CANTERA. Z [57]. Wang et al. extended the study to diluted conditions of the same fuel, obtaining similar results using the same simulation method [58]. Ibrahim et al. investigated the LBV of liquefied petroleum gas with a composition of 60% butane, 20% isobutane, and 20% propane, finding that adding CH₄, H₂, or O₂ increases LBV, whereas CO₂/O₂ mixtures decrease it [59]. Yu et al. studied the LBV of S-8 fuel with a surrogate fuel composition of 32% iso-octane, 25% n-decane, and 43% n-dodecane, finding higher agreement with a one-dimensional steady premixed flame code from CANTERA using Ranzi’s chemical kinetics mechanisms compared to their proposed detailed kinetics model (DKM) [60]. Keesee et al. investigated two synthetic jet fuels (Syntroleum S-8 and Shell GTL) and resolved the difficulty in accurately measuring gas-phase fuel equivalence ratios using an in situ laser absorption technique [53]. Both fuels exhibited an LBV_{_max} of 60 cm/s under conditions of 403 K and 1 atm pressure.

The LBV test results of S-8 fuel near 400 K from four representative studies are depicted in Figure 4. Despite similar temperature and pressure conditions during measurement, significant variations in LBV are evident. Keesee et al. reported an LBV_{_max} of approximately 57 cm/s, notably lower than the 60–66 cm/s range observed by others [53]. Additionally, Keesee’s LBV_{_max} occurred at an equivalence ratio skewed towards rich conditions, reaching 1.3 compared to others, which were nearer to 1.1. Consequently, Keesee’s LBV values under lean conditions were significantly lower than the others but exhibited higher values under rich conditions. Hui et al. recorded the highest LBV of the studies, peaking at 66 cm/s [33], surpassing Ji et al. (63 cm/s) and Kumar et al. [5] (61 cm/s). Under lean conditions, LBV data from Hui, Ji, and Kumar were comparable. Apart from differences and errors in testing methods and processes, one of the primary sources of variation in S-8 fuel LBV curves is the differing compositions of fuels used by different researchers. Ji et al., Keesee et al., and Hui et al. utilized S-8 fuels with different average molecule compositions of C₁₀H_22.7, C_11.8H_25.6, and C_11.8H_25.5, respectively [54]. Compared to S-8 fuel, LBV test results for Jet A exhibited smaller discrepancies among scholars, with an LBV_{_max} of around 58–62 cm/s under the same experimental conditions. LBV trends with the equivalence ratio were highly consistent, and the LBV_{_max} at different temperatures corresponded to equivalence ratios of 1.05–1.1, as shown in Figure 2.

4.2. LBV of Bio-Jet Fuel

Due to the abundance of available biomass sources, the variety of bio-jet fuels far exceeds that of synthetic fuels. Plant oil sources (algae, camelina, jatropha, etc.), animal fats (tallow), and waste oil are the primary raw materials for bio-jet fuel [33,61]. The compositional standards of bio-jet fuel prepared according to different standard paradigms are not uniform, resulting in significant discrepancies in the lateral comparison of LBV performance in various studies. The LBV values of several typical bio-jet fuels are summarized in Figure 5. At approximately 1 atm and 420 K, the LBV_{_max} of two pure bio-jet fuels are around 62 and 55 cm/s, respectively, both lower than the LBV_{_max} for a blend of microalgae oil and RP-3 (80 cm/s). Furthermore, the LBV_{_max} of these two pure bio-jet fuels occurs at equivalence ratios of around 1.0 and 1.2, which contrasts with the trend observed for conventional aviation fuels. According to Figure 2, the range of LBV_{_max} for Jet A at 1 atm and for 470 K is 78–86 cm/s, the LBV_{_max} (84 cm/s) of bio-jet fuel from camelina also falls within this range, and bio-jet fuel prepared from corn stover lignin exhibits a lower LBV_{_max} (75 cm/s). At 1 atm and 480 K, the LBV_{_max} for a blend of microalgae oil and RP-3 is approximately 90 cm/s. According to Figure 3c, under the same conditions, the LBV_{_max} of RP-3 is only 81 cm/s. Therefore, based on the data from these studies, it is difficult to conclude whether traditional jet fuel or bio-jet fuel has a higher LBV. Apart from the use of different testing methods, it may also be because the production process of bio-jet fuel originates from the laboratory rather than following ASTM standards, or because the prepared bio-jet fuel has undergone component optimization to achieve performance close to that of conventional jet fuel.

When various bio-jet fuels are compared to conventional fuel, significant uncertainty is observed, with some bio-jet fuels showing comparable or slightly deviating LBV values. Richter et al. measured the LBV of farnesane (2,6,10-trimethyldodecane), which is produced from sugar using the Bunsen flame method at 473 K, and found LBV values close to those of Jet-A [37]. They proposed a reaction model based on these results, which showed good agreement with actual data in terms of ignition delay times and LBV. Abi Nurazaq et al. determined the LBV of a third-generation biofuel derived from renewable feedstocks (biomass, waste cooking oil, and agricultural wastes) by the spherical flame method, finding LBV values similar to those of Jet A [64]. Their study indicated that under lean and rich conditions, the LBV is respectively increased and suppressed by chain branching reactions (O₂ + H =OH + O) and chain propagation reactions (and O₂ + H = HO₂ (+M)). Xu et al. investigated the LBV of corn stover lignin via catalytic hydrodeoxygenation (HDO) at atmospheric pressures of 1, 2, and 4 bar and temperatures of 443 and 473 K, revealing lower LBV values for the bio-aviation fuel candidate compared to Jet A-1, RP-3, and n-decane under lean conditions, but higher values under rich conditions [63]. The bio-aviation fuel candidate they adopted contains approximately 67.29 mol% cycloparaffins, and it is believed that a high proportion of cyclic hydrocarbons is the reason for the low LBV. Hwang et al. measured the LBV of two high-performance hydrocarbon fuels using the Bunsen burner method at 550 K, finding LBV peak values biased towards rich conditions, resulting in a lower LBV for high-performance hydrocarbon fuels compared to Jet A under lean conditions [34]. Chong et al. observed an LBV_{_max} biased towards rich conditions when studying palm methyl esters (PMEs) using the stagnation method [36]. The PMEs’ LBV was found to be closer to that of diesel but lower than Jet A’s LBV. Xu et al. determined the LBV of a bio-jet fuel primarily composed of ethers, esters, alcohols, and ketones derived from rice husk using the spherical flame method at temperatures of 358, 388, and 418 K [65]. The LBV of this fuel lay between those of its two main components, ethanol and ethyl acetate. Additionally, research has investigated the influence of blending ratios of bio-jet fuels on aviation fuel LBV performance. Wu et al. studied the effect of lignocellulosic-biomass pyrolysis oil (mainly composed of anisole, 4-methylanisole, and ethylvalerate) on the LBV of commercial gasoline using the Bunsen flame burner method and found minimal impact on commercial gasoline LBV even with 10% alternative fuel blending under testing temperatures of 400–470 K and pressures of 0.1–0.8 MPa [66]. Liu et al. measured the LBV of a blend of algae-derived biocrude with RP-3 aviation kerosene over temperature ranges of 420–480 K and pressure ranges of 1–3 atm, indicating that the blending ratio has a greater influence on LBV than temperature and pressure [62].

To support the airworthiness certification of SAFs and assess their combustion performance and safety in practical applications, we tentatively propose recommending standard characterization protocols: (1) The T₀ of unburned fuel/air mixtures should be set at 400/425/450/475/500/550 K, with this wide range designed to simulate real scenarios such as cold starts, low-temperature ignition, and high-altitude reignition conditions; (2) the P₀ of unburned fuel/air mixtures should be set at 0.25/0.5/0.75/1/2/5 atm to cover extreme flight conditions, particularly for fighter aircraft; (3) the equivalence ratio should be tested across 0.6–1.5 at intervals of 0.5 to thoroughly simulate combustion performance under both low-emission and high-power modes while exploring engine safety boundaries; (4) the maximum blending ratios for SAFs from different production processes and feedstocks should follow ASTM D7566, for instance, FT-SPK and HEFA-SPK capped at 50% and SIP and HC-HEFAs limited to 10%; (5) within the equivalence ratio range of 0.9–1.2, the LBV performance gap between maximum-blend SAFs and conventional jet fuel should not exceed 5%, while remaining below 7.5% for Φ < 0.85 or Φ > 1.2; and (6) under identical conditions with equivalent additive content (e.g., water, methane, ammonia, LNG), the LBV performance deviation between maximum-blend SAFs and conventional jet fuel should remain around 7.5%.

5. LBV of Key Hydrocarbon Components in Aviation Fuel

Hydrocarbons, as the primary constituents of aviation fuel, play a crucial role in determining combustion characteristics. By employing the stagnation flame method, Davis and Law corrected significant measurement errors in alkanes caused by flame stretch, thus obtaining more accurate LBV data [67]. Their findings revealed that, excluding methane, n-alkanes have nearly identical LBVs. Since Law et al.’s work on the impact of flame stretch on LBV, accounting for flame stretch effects has become standard practice in LBV measurements. Konnov et al. compiled extensive LBV data for low-carbon alkanes, documenting whether numerical corrections for flame stretch were applied. However, aviation fuels predominantly comprise hydrocarbons with a carbon number distribution centered around C12, typically following a normal distribution from C8 to C16 [11]. Consequently, LBV data for low-carbon alkanes alone are insufficient for studying aviation fuel components. Comprehensive research and a summary of LBV characteristics for n-alkanes, iso-alkanes, cycloalkanes, and aromatic hydrocarbons in aviation fuels are necessary. Such research will enhance the understanding of how various components influence the LBV of aviation fuels and improve the combustion performance of both traditional aviation fuels and sustainable aviation fuels through targeted component blending.

5.1. LBV of N-Alkanes in Aviation Fuel

N-alkanes are a primary component of both traditional aviation fuel and sustainable aviation fuels. These compounds exhibit an increase in LBV with rising temperatures [24,68] and a decrease with increasing pressure [24,69]. Among the n-alkanes, n-decane has been the most extensively studied for its LBV. Moghaddas et al. measured the LBV of n-decane under lean conditions (0.7 ≤ Φ ≤ 1) using the spherical flame method [70], achieving results consistent with those of Zhao et al., Kumar et al., and other researchers [71]. However, the spherical flame method used by Moghaddas et al. encounters issues with flame front irregularities under varying temperatures and pressures, complicating data acquisition under rich conditions. This issue has been addressed by other measurement methods. Kumar et al. utilized the counterflow flame method to investigate n-decane combustion at different temperatures [68], achieving results consistent with the predictions of Kumar et al. [72] and the PoliMi mechanisms [73]. The ability to measure LBV over a wide temperature range (338–650 K) in Kumar et al.’s study is attributed to the novel counterflow flame method. Their experimental data confirmed Zhao et al.’s prediction that the temperature exponent decreases as the equivalence ratio increases, reaching a minimum near 1.1.

For higher carbon n-alkanes, Kumar et al. investigated the LBV variation of n-decane and n-dodecane at different temperatures using the stagnation flame method [72]. Their study found Zhao et al.’s simulation predictions accurate under lean conditions but underestimated LBV in fuel-rich mixtures [71]. Rajesh et al. employing the spherical flame method [24], also investigated n-dodecane’s LBV, and compared the results with PoliMi mechanisms and other models [73]. They attributed the discrepancies in rich conditions near an equivalence ratio of 1.4 to hydrodynamic instability effects. Ji et al. conducted extensive research on the LBV of n-alkanes ranging from C5 to C12, noting that non-linear extrapolation for correcting flame stretch effects provides closer results to those of simulations, deriving values 1~4 cm/s smaller than those of linear extrapolation [74]. Data on LBV for higher-carbon alkanes are relatively scarce [75]. Li et al. measured the LBV of n-tetradecane and n-hexadecane using the counterflow flame method combined with digital particle image velocimetry, indicating that the LBV of n-tetradecane and n-hexadecane are very similar [18]. Zeng et al. introduced a model incorporating gas-phase pyrolysis of n-tetradecane that closely matched previous experimental results [18].

At present, research on the LBV of n-decane and n-dodecane is gaining popularity. This is partly due to their frequent use as components in surrogate fuel formulations and the abundance of relevant research and data, which can be utilized to validate the testing accuracy of the constructed equipment. For instance, Wu et al. verified the accuracy of their Bunsen burner system by initially measuring the LBV of n-decane when studying the LBV of Jet A-1 and its surrogate fuels [6]. Similarly, Le Dortz et al. validated the precision of their experimental setup by first measuring the LBV of n-decane and n-dodecane before investigating the emission characteristics of Jet A-1 and its surrogate fuels using the spherical flame method [76]. Information on LBV tests for different n-alkanes is shown in

Table 4. Information on LBV tests for different n-alkanes.

Carbon Number	Experimental Conditions		LBV Measurement Methods	Ref.
	P (atm)	T (K)
1–8	-	-	-	[22,77]
10	1	338, 360, 400, 470, 500, 610, 650	Externally heated diverging channel method	[69]
10	1, 2	470	Spherical flame method	[69]
10	1	403	Stagnation flame method	[74]
10	1	400, 425	Spherical flame method	[76]
10	1	360, 400, 470	Stagnation flame method	[72]
10	1	400	Externally heated diverging channel	[78]
10	1	398	Stagnation flame method	[13]
10	1	398	Spherical flame method	[79]
10	1	398	Spherical flame method	[80]
10	1	398	Spherical flame method	[81]
12	1, 2, 3, 4	400, 425, 450	Spherical flame method	[24]
12	1	400	Spherical flame method	[82]
12	1	403	Stagnation flame method	[74]
12	1	400, 470	Stagnation flame method	[72]
12	1	400, 425	Spherical flame method	[76]
12	1	400	Spherical flame method	[82]
14	1	423, 443	Stagnation flame method	[18]
14	1	423, 443	-	[75]
16	1	443	Stagnation flame method	[18]

.

Taking n-decane as an example, the LBV_{_max} obtained from different years and testing methods is plotted in Figure 6. It can be observed that, firstly, the LBV results obtained after 2010 are significantly lower than those of earlier data, and the data tend to converge. The error of the results measured using the spherical flame method in 2013 was also significantly smaller than that in 2011. This is mainly attributed to scholars’ deepening understanding of LBV mechanisms in recent years. The testing platforms constructed can better avoid heat dissipation and generate stable flame fronts, and the correction principles for flame stretch effects are also more refined. For example, the 1992 study by Sher et al.—conducted without an appropriate flame stretch correction method—claimed that n-butane could reach an LBV of 82 cm/s at 400 K and 1 atm, a figure now regarded as a considerable overestimation [83]. Moreover, the spherical flame method yields relatively small values with good consistency across different studies. In contrast, the values obtained from the stagnation flame method are relatively high, with significant differences among the data. From the perspective of the uncertainty range, except for the data from 2010 and unmarked data, the standard deviations of LBV_{_max} measured by the spherical flame method and the stagnation flame method were similar, both around 1 cm/s. The standard deviation of LBV_{_max} measured by the externally heated diverging channel method was larger, around 3 cm/s. Additionally, both the Bunsen burner method and the heat flux method achieved standard deviations for LBV_{_max} of around 1 cm/s [6]. However, when testing blended fuel mixtures or when the equivalence ratio leaned more toward the fuel-lean or fuel-rich side, the uncertainty of the test results typically increased [34,35]. The externally heated diverging channel method, as a newer measurement technique, offers the advantage of a wide temperature measurement range. However, due to varying approaches in handling external heat control and flame deformation near the outlet wall, this method is currently in the experimental stages. It can be observed that the LBV results obtained through this method are similar to those from the spherical flame method, but further validation is required to enhance its accuracy.

Figure 7 compiles LBV measurements for n-alkanes. The overall trend from highest to lowest LBV is 443 K > roughly 423 K > roughly 400 K > 373 K > 353 K, indicating that temperature exerts a stronger influence on LBV than carbon chain length. Around 400 K, the LBV differences among n-pentane, n-heptane, n-nonane, n-decane, and n-dodecane are fairly small, consistent with Davis and Law [67]. However, as the carbon number increases, there are certain variations in factors such as chemical structure, bond energy, thermophysical parameters, transport properties, and adiabatic flame temperature (T_ad), which may lead to trends in LBV. The T_ad represents the highest temperature achievable in a combustion process under adiabatic (or perfectly insulated) conditions, and it influences the laminar flame speed by affecting the reaction kinetics and thermal diffusivity of the mixture [84]. The methods depicted in the figure include the heat flux method, spherical flame method, and counterflow flame method. Each method has differences in testing principles, result accuracy, and errors, leading to limited comparability between the data. In addition to the testing methods, the flow rates of the fuel/air premixed gas vary across different experimental conditions. The oxygen in the air sustains the combustion process by providing a continuous supply of oxidizer, while also controlling the stoichiometric ratio and supporting flame propagation [84]. Therefore, differences in the premixed gas flow rates are also one of the reasons for the limited comparability among the data. Furthermore, the LBV_{_max} of n-alkanes corresponds to an equivalence ratio of 1.05–1.1, independent of carbon number and initial temperature, and decreases with increasing lean and rich conditions. Previous studies on n-alkanes have not been comprehensive enough, with a lack of LBV data for undecane and pentadecane, for example.

5.2. LBV of Iso-Alkanes in Aviation Fuel

The experimental conditions, testing methods, and publication years of LBV tests for iso-alkanes are summarized in

Table 5. Summary of LBV test information for different iso-alkanes.

Iso-Alkanes	Experimental Conditions		LBV Measurement Methods	Year	Ref.
	P (atm)	T (K)
Isobutane	1, 2, 5, 10	298	Spherical flame method	2018	[91]
Isobutane	1	298–398	Flat flame burner, spherical flame method	2016	[90]
2,2,4-Trimethylpentane	1	298, 400, 470	Stagnation flame method	2010	[93]
2,2,4-Trimethylpentane	1	298, 358, 398	Heat flux	2013	[87]
2,2,4-Trimethylpentane	1	353	Stagnation flame method	2012	[94]
2-Methylheptane	1	353	Stagnation flame method	2012	[94]
3-Methylheptane	1	353	Stagnation flame method	2012	[94]
2,5-Dimethylhexane	1	353	Stagnation flame method	2012	[94]
2,2,4,6,6-Pentamethyl-heptan	1, 3	473	Bunsen burner	2023	[95]

. For C4 alkanes, Davis et al. highlighted in their study of n-butane and isobutane that with an increase in the number of branches in saturated alkanes of the same carbon number, the LBV gradually decreases [67]. Studies by Li et al. and Zhou et al. on butane also support this observation. Under conditions of 1 atm and 298 K, with an equivalence ratio near 1.1, the LBV_{_max} for isobutane is 33.7 cm/s, while that for n-butane reaches 37.1 cm/s [90,91]. Similarly, Marshall et al. obtained an LBV of 37.3 cm/s for n-butane under the same conditions [92], and Konnov et al. summarized the LBV for n-butane in the range of 35–40 cm/s [22]. Isobutane does not exist in aviation kerosene, but research on this iso-alkane with the lowest carbon number clearly shows that the LBV of iso-alkanes is lower than that of n-alkanes with the same carbon number.

For C8 alkanes, Ji et al. measured the LBV_{_max} values at approximately 51.8 cm/s, 47.7 cm/s, 47.6 cm/s, 45.4 cm/s, and 45.0 cm/s for n-octane, 2-methylheptane, 3-methylheptane, 2,5-dimethylhexane, and iso-octane (2,2,4-trimethylpentane), respectively, under conditions of 353 K and 1 atm, with an equivalence ratio near 1.04 [94]. Through the analysis of combustion mechanisms, they attributed the decrease in LBV to the generation of non-reactive resonance-stabilized intermediates such as propene, isopropyl, and isobutene. In a study by Dirrenberger et al. on the impact of ethanol addition to commercial fuels and iso-octane on LBV, it was found that ethanol content below 15 vol% had no significant effect on the measured fuel LBV [87]. Kumar et al. measured the LBV of iso-octane at multiple temperatures, and after extrapolating the values to 353 K [93], they were found to be consistent with the results from Ji et al. and Dirrenberger et al. For C12 alkanes, Saraee et al. extended the testing results of ATJ fuel to 473 K under 1 atm conditions, revealing an LBV_{_max} of 78.3 cm/s near an equivalence ratio of 1.1 [95]. The ATJ fuel, composed of 90%, is confirmed to be iso-dodecane (2,2,4,6,6-pentamethylheptane). N. Rajesh et al. measured the LBV_{_max} of n-dodecane at 79.7 cm/s under conditions of 473 K and 1 atm [24]. Thus, for hydrocarbon fuels with a carbon number of 12, the presence of branching still leads to a decrease in LBV. Figure 8 displays the LBV_{_max} values for C4, C8, and C12 alkanes, showing a direct observation of the reduction in LBV performance due to an increase in the number of branches in the molecular structure of isomers, despite variations in completion years and testing methods across the studies.

5.3. LBV of Cycloalkanes in Aviation Fuel

Representative cycloalkanes in aviation fuels predominantly include cyclopentane, cyclohexane, and their derivatives. Kumar et al. employed an externally heated diverging channel approach to measure the LBVs of methylcyclohexane over a wide temperature range (300–610 K) and compared their findings with prior data [96]. By investigating reactivity at 400 K and 610 K, they observed that enhanced formations of C₂H₄ and C₂H₃ radicals with increasing temperature led to higher LBVs for methylcyclohexane. Lubrano Lavadera et al. reported that mixing ammonia with methylcyclohexane and toluene reduced the LBV of both cycloalkanes due to combined thermal and kinetic effects [97]. Zhao et al. measured LBVs of cyclopentane and cyclohexane over broad pressure and temperature ranges, finding good agreement with the JetSurF2.0 and Tian models while showing that cyclohexane’s LBV exceeds cyclopentane’s [98], as indicated by the green triangles in Figure 9. This difference arises because cyclopentane generates larger amounts of methyl and allyl intermediates, which consume H radicals and promote chain termination, while cyclohexane produces more ethyl and 1,3-butadiene species that further generate vinyl radicals, triggering chain branching and faster reactions. Ji et al. measured and compared the LBVs of cyclohexane and multiple cyclohexane derivatives, attributing lower LBVs of mono-substituted cyclohexanes to increased propene and allyl radical formation [99]; Wu et al. likewise arrived at conclusions aligned with Zhao et al. and Ji et al., proposing that the symmetric ring of cyclohexane favors the formation of 1,3-butadiene and produces less propene [100]. Liu et al. investigated five six-carbon cycloalkanes (cyclohexane, benzene, cyclohexene, 1,3-cyclohexadiene, and 1,4-cyclohexadiene), noting that the JetSurf model predicts their LBVs satisfactorily, except for that of benzene [101]. Ranzi et al. summarized data for a variety of hydrocarbons and oxygenated fuels, suggesting that cycloalkanes often exhibit LBVs similar to those of the corresponding alkanes [102], consistent with Davis et al. [67].

Table 6. Information on LBV testing for various cycloalkanes.

Cycloalkane	Experimental Conditions		LBV MeasurementMethods	Year	Ref.
	T (K)	P (atm)
Cyclopentane	353, 403, 453	1, 2, 5	Spherical flame method	2018	[98]
Cyclohexane	353, 403, 453	1, 2, 5	Spherical flame method	2018	[98]
Cyclohexane	403	1, 5	Spherical flame method	2023	[101]
Cyclohexane	353	1	Stagnation flame method	2011	[99]
Methylcyclohexane	353	1	Stagnation flame method	2011	[99]
Ethylcyclohexane	353	1	Stagnation flame method	2011	[99]
n-Propylcyclohexane	353	1	Stagnation flame method	2011	[99]
n-Butylcyclohexane	353	1	Stagnation flame method	2011	[99]
n-Hexane	353	1, 2, 5, 10, 20	Spherical flame method	2012	[100]
Cyclohexane	353	1, 2, 5, 10, 20	Spherical flame method	2012	[100]
Methylcyclohexane	353	1, 2, 5, 10, 20	Spherical flame method	2012	[100]
Ethylcyclohexane	353	1, 2, 5, 10, 20	Spherical flame method	2012	[100]
Methylcyclohexane	300~610	1	Externally heated diverging channel method	2021	[96]
Methylcyclohexane	338	1	Heat flux method	2022	[97]

compiles the experimental conditions, methods, and publication years for LBV tests on various cycloalkanes.

The summarized LBV peaks of n-alkanes and cycloalkanes from multiple articles at 353 K and 1 atm are presented in Figure 9, with several insights offered: Firstly, there is no distinct trend in LBV performance between n-alkanes and cycloalkanes of the same carbon number. For C5 and C7 alkanes, n-pentane exhibits a higher LBV than cyclopentane, and n-heptane has a higher LBV than methylcyclopentane. However, for C6 alkanes, n-hexane’s LBV is lower than cyclohexane’s LBV. Secondly, comparing the LBV performance of cyclohexane and its derivatives in the works of Ji et al. and Wu et al. [100], (represented by red circles and blue squares, respectively), it is evident that the LBV peak of cyclohexane is significantly higher than that of its derivatives. Moreover, the LBV performance differences among methyl, ethyl, propyl, and butyl derivatives of cyclohexane are minimal, which may be attributed to bond energies. Thirdly, while Ji et al. utilized the stagnation flame method for testing, Wu et al. employed the spherical flame method. Although both studies exhibit a similar trend in LBV variations as “cyclohexane > n-hexane > methylcyclohexane ≈ ethylcyclohexane,” the LBV values for each alkane measured differ by 1~3 cm/s. This suggests a consistent pattern of LBV variation with compositional structure across the two studies, albeit with variations in absolute values due to different measurement methods. The observation that the LBV results obtained via the stagnation flame method are higher than those obtained via the spherical flame method is consistent with the conclusions drawn for n-alkanes.

5.4. LBV of Aromatic Hydrocarbons in Aviation Fuel

In aviation fuel, aromatic hydrocarbons play a positive role in swelling, lubricity, thermal stability, etc.

Table 7. Information on LBV testing of different aromatic hydrocarbons.

Aromatics	Experimental Conditions		LBV MeasurementMethods	Year	Ref.
	T (K)	P
Benzene	450	304 kPa	Spherical flame method	2005	[103]
Toluene	450	304 kPa	Spherical flame method	2005	[103]
Ethylbenzene	450	304 kPa	Spherical flame method	2005	[103]
m-Xylene	450	304 kPa	Spherical flame method	2005	[103]
n-Propylbenzene	450	304 kPa	Spherical flame method	2005	[103]
Toluene	298, 400, 470	1 atm	Stagnation flame method	2010	[93]
Benzene	353	1 atm	Stagnation flame method	2012	[104]
Toluene	353	1 atm	Stagnation flame method	2012	[104]
o-Xylene	353	1 atm	Stagnation flame method	2012	[104]
m-Xylene	353	1 atm	Stagnation flame method	2012	[104]
p-Xylene	353	1 atm	Stagnation flame method	2012	[104]
1,3,5-Trimethylbenzene	353	1 atm	Stagnation flame method	2012	[104]
1,2,4-Trimethylbenzene	353	1 atm	Stagnation flame method	2012	[104]
n-Propylbenzene	353	1 atm	Stagnation flame method	2012	[104]
Toluene	298, 358, 398	1 atm	Heat flux	2014	[87]
Benzene	358, 423	1, 2, 5, 10, 20 atm	Spherical flame method	2017	[105]
Toluene	358, 423	1, 2, 5, 10, 20 atm	Spherical flame method	2017	[105]
Ethylbenzene	358, 423	1, 2, 5, 10, 20 atm	Spherical flame method	2017	[105]
sec-Butylbenzene	423	1, 2, 5, 10 atm	Spherical flame method	2020	[107]
Pentylbenzene	473	1, 2, 5, 10 atm	Spherical flame method	2021	[108]
Isobutylbenzene	423	1, 2, 5, 10 atm	Spherical flame method	2021	[106]
n-Butylbenzene	423	1, 2, 5, 10 atm	Spherical flame method	2021	[106]
tert-Butylbenzene	423	1, 2, 5, 10 atm	Spherical flame method	2021	[106]
Toluene	338	1 atm	Heat flux method	2022	[97]
Benzene	403	1, 5 atm	Spherical flame method	2023	[101]

summarizes LBV measurement information for various aromatics. Johnston et al. provided an earlier comparison of LBVs for benzene and its derivatives, concluding that benzene > ethylbenzene > n-propylbenzene > toluene > m-xylene [103]. Their study noted that when using the spherical flame method to measure LBV under fuel-rich conditions, flame-front instabilities can arise—an issue that also prevented Moghaddas et al. from assessing n-decane LBV under fuel-rich conditions at 1 atm [70]. Ji et al. measured LBVs for numerous aromatics and observed that increased methylation generates more benzyl or benzyl-like groups, promoting radical chain termination and reducing LBV [104]. They also highlighted significant experiment–simulation discrepancies for aromatics. Wang et al. investigated the LBVs of benzene, toluene, and ethylbenzene over 1–20 atm, finding that the LBV values consistently ranked (from smallest to largest) as toluene, ethylbenzene, and benzene [105]. Zhang et al. measured the LBVs for four butylbenzene isomers—arranged in ascending order as isobutylbenzene, n-butylbenzene, tert-butylbenzene, and sec-butylbenzene—and attributed differences to variations in the formation of methyl, cyclopentadienyl, and benzyl radicals, which strongly affect LBV at higher pressures and under fuel-rich conditions [106,107]. Zhang et al. then measured the LBV of n-pentylbenzene and, together with other alkylbenzene data from the literature, analyzed the combustion mechanisms of alkylbenzene, concluding that methyl-, ethyl-, and propylbenzene decompose more readily at lower temperatures, while n-butyl- and n-pentylbenzene exhibit nearly identical decomposition temperature ranges [108].

Figure 10 compiles the maximum LBVs of benzene and its derivatives at 1 atm across various temperatures. As the temperature increases, the maximum LBV of benzene and its derivatives also increases. At approximately 353 K and 423 K, benzene and its derivatives follow similar LBV trends, with benzene and toluene exhibiting the largest and smallest LBVs, respectively. The LBVs of toluene, ethylbenzene, and propylbenzene display a “decrease–increase–decrease” pattern, matching the trends Johnston et al. reported at 304 kPa and 450 K [103].

6. Combustion Mechanism Model and Simulation Accuracies

Combustion mechanism models play an indispensable theoretical and predictive role in LBV research. By constructing detailed reaction networks comprising hundreds to thousands of elementary reactions, these models can elucidate the key kinetic factors influencing LBV at the molecular level. They not only enable quantitative assessment of the contributions of various radicals to chain reactions in flame propagation, but also accurately characterize the influence of fuel molecular structure on reaction pathway selectivity. Through sensitivity analysis and reaction path tracking, kinetic models can identify the decisive reaction steps governing LBV variations, providing a theoretical foundation for explaining the experimentally observed nonlinear relationships between LBV and equivalence ratio, temperature, and pressure. In the

Table 8. Information on combustion mechanism model and simulation accuracies.

Fuel	Combustion Mechanism Models	Simulation Conditions		Simulation Accuracies	Ref.
		T (K)	P (atm)
Jet A and its surrogate fuel	JetSurF 2.0	400	1	Accurately predicts the measured velocity profiles.	[30]
Jet A and its surrogate fuel	Chemical kinetics mechanism involving 401 species and 2838 reactions	473	1	Φ = 0.6–1.5, with deviations underpredicted by about 10%.	[31]
RP-3 and its surrogate fuel	Chemical kinetics mechanism involving 181 species and 872 reactions	400, 420, 450, 480	1, 3	Less than 5% deviation.	[42]
GTL fuel–20% hexanol–air mixtures	Chemical kinetics mechanism involving 3479 reactions and 490 species	423, 473	1	Φ < 1.2, experiments are underpredicted; Φ > 1.2, predictions are slightly higher.	[19]
Syntroleum S-8 and its surrogate fuel	Ranzi’s chemical kinetics mechanisms	550	1	Simulated results are about 5–10% higher than the measured value.	[57]
Syntroleum S-8 and its surrogate fuel	Ranzi’s chemical kinetics mechanisms	400, 473	1	The chemical mechanism showed better agreement with experimental data.	[60]
n-Decane	PoliMi mechanisms	up to 610	1	Measurements agree well with PoliMi model predictions across all mixtures.	[68]
n-Dodecane	JetSurF 2.0/PoliMi mechanisms	425	1, 4	At 1 bar, the LBV deviations of the JetSurF 2.0 and PoliMi mechanisms were within 5% and 8%, respectively. At 4 bar, the JetSurF 2.0 mechanism maintained deviations within 10%, whereas the PoliMi mechanism exhibited deviations exceeding 15% for Φ > 1.2.	[24]
n-Pentylbenzene	Chemical kinetics mechanism involving 313 species and 1975 reactions	473	1, 2, 5, 10	The model reliably predicts experimental data across all tested pressures and equivalence ratios.	[108]

, we summarize the mechanism models mentioned in the text along with their simulation accuracies.

7. Conclusions

SAFs represent a primary tool for achieving net-zero emissions in the aviation industry by 2050. Their combustion performance must closely match that of traditional aviation fuels in terms of availability and safety. Therefore, this study systematically evaluates the LBV performance of traditional aviation fuels, SAFs, and different components in aviation fuels, yielding the following conclusions:

1. The LBV of different traditional aviation fuels (Jet A/Jet A-1/RP-3/JP-8) increases with rising temperature and decreases with increasing pressure, supported by empirical formulas. The peak LBV typically occurs at equivalence ratios between 1.05 and 1.1. Discrepancies in LBV levels among studies examining the same fuel may stem from variations in fuel batches, principles of testing methods, instrument calibration, and measurement precision. Radiation reabsorption of water significantly influences the LBV performance of aviation fuels, primarily attributed to the CH₂OH + H = CH₃ + OH elementary reaction and OH radicals. The impact of CH₄ addition on LBV varies across different literature sources, but a consistent finding is that under fuel-rich conditions, CH₄ reduces the LBV of the fuel mixture. The addition of ethanol results in increased activity of H, O, and OH radicals, along with rapid oxidation to CO₂ without the need for pyrolysis, thereby enhancing the LBV of the blended fuel’s premixed flame.

2. Whether synthetic fuels or bio-jet fuels, the LBV of different SAFs exhibits significant fluctuations, approaching or slightly deviating from that of conventional aviation fuels. The maximum LBV of SAFs corresponds to an equivalence ratio of between 1.0 and 1.2, differing from the range of 1.05 to 1.1 observed in conventional aviation fuels. These differences alter flame lengths during combustion, shifting the optimum fuel-lean or fuel-rich conditions for flame propagation and impacting engine performance. The performance disparity between sustainable and conventional aviation fuels in terms of LBV stems from compositional differences.

3. By synthesizing and organizing data from multiple studies, it was observed that the LBV of n-alkanes does not show a significant trend with carbon number, but this may be due to the low comparability of results from different publication years and test methods. For iso-alkanes of the same carbon number, the LBV decreases with increased branching. Cyclohexane exhibits a higher LBV compared to its derivatives and cyclopentane. Furthermore, the LBV trends of toluene, ethylbenzene, and propylbenzene show a sequential “decrease-increase-decrease” pattern relative to benzene.

4. Priority of future work: Future research should prioritize comparative studies on the LBV of SAFs and conventional jet fuels, with experimental conditions and evaluation criteria potentially referencing recommended standard characterization protocols. Investigations into methods for optimizing the LBV performance of SAFs are essential to facilitate their drop-in compatibility and airworthiness certification. Regarding key aviation fuel components, future LBV testing and combustion mechanism studies should focus on understudied hydrocarbon species, such as odd-carbon-numbered long-chain n-alkanes (C11, C13, C15), n-alkylcyclohexanes/benzenes with side chains exceeding four carbons, and highly branched isomers. Systematic comparisons of LBV variations among n-alkanes, iso-alkanes, cycloalkanes, and aromatic hydrocarbons of identical carbon numbers are warranted, with mechanistic interpretations based on molecular structure, combustion reaction pathways, and physicochemical properties. All such investigations must employ consistent experimental conditions and standardized testing methodologies to ensure comprehensive and reliable findings. For instance, comparative analysis of LBV testing methods reveals that the spherical flame method yields smaller values with high consistency, while the stagnation flame method yields larger values with relatively larger discrepancies between data points.