Ignition and Combustion Characteristics of Aluminum Hydride-Based Kerosene Propellant

Abstract

Aluminum hydride (AlH₃) is a promising candidate for enhancing the combustion performance of liquid fuels due to its high energy density and exceptional hydrogen storage capacity. This study investigated the ignition and combustion characteristics of μ-AlH₃ particles in kerosene droplets using TG-DSC analysis, high-speed imaging, laser ignition, and combustion product characterization, with comparisons to micron- and nano-aluminum powders. Results showed that the exothermic combustion of hydrogen released from AlH₃ decomposition lowered the primary oxidation temperature of aluminum, leading to more intense combustion with smaller ejected particles. The particle size of kerosene droplets containing AlH₃ rapidly decreases due to the escape of hydrogen. The heat released by the combustion of hydrogen significantly accelerates the combustion of droplets, and the fastest combustion rate is observed at a concentration of 1% AlH₃. The combustion products of kerosene droplets containing AlH₃ are smaller than those of kerosene droplets containing aluminum, indicating that their combustion efficiency is higher. A combustion model for AlH₃-based kerosene droplets was developed, demonstrating less than 10% error in predicting ignition delay and burning rates. These findings provide valuable insights for the application of AlH₃ in liquid fuels.

1. Introduction

To enhance the performance of liquid rocket motors, it is essential to develop fuels with higher energy characteristics [1,2,3,4]. An effective approach is to incorporate energetic metal particles into traditional hydrocarbon fuels, creating a stable suspension that enhances combustion properties and energy density of the fuel. Micron- and nano-metal particles, in particular, are in the spotlight for their high energy density, short ignition delay, and efficient heat and mass transfer properties. Additionally, the incorporation of micron- or nano-sized metals can facilitate fuel evaporation or induce micro-explosions, thereby enhancing ignition probability and thermal conductivity. As a result, micron- and nano-metal particles are considered promising additives to improve the combustion performance of liquid fuels [5,6,7,8,9].

Among the metals commonly added to fluid fuels, aluminum is a representative energetic material [10]. It is typically added to fluid fuels in micron or nanometer particle sizes, with nanofluidic fuels being the dominant focus of research and nanoscale energy-containing materials show great advantages over micron-scale materials [11,12,13,14]. Previous studies have shown that adding aluminum nanoparticles to fuel significantly improves combustion performance, leading to improved fuel characteristics, such as increased energy density, reduced ignition delay, and other benefits [15,16,17,18,19]. For instance, the addition of aluminum nanoparticles to kerosene has been shown to reduce the overall ignition delay time [20,21]. Similar to the evaporation and combustion of pure kerosene droplets, the evaporation and combustion of kerosene droplets containing aluminum powder also adhere to the classic d² law [22,23]. Additionally, the presence of nano-aluminum particles reduces bubble formation in the kerosene droplets. As the concentration of nanoparticles increases, a more pronounced microburst phenomenon is observed in heptane fuel [24,25]. These improvements contribute to the enhanced combustion performance of fluid fuels.

However, the addition of aluminum powder to kerosene presents two main challenges. First, the agglomeration of metal nanoparticles in the base fluid can significantly affect the flow and combustion properties of nanofluidic fuels [22]. Second, the surface of aluminum nanoparticles is highly prone to oxidation, leading to the formation of a dense oxide layer. Due to their larger specific surface area, aluminum nanoparticles exhibit a greater percentage of this oxide layer, which negatively impacts their ignition properties [26,27]. In contrast, the addition of aluminum hydride (AlH₃) to kerosene leads to the generation of substantial amounts of hydrogen during combustion and decomposition. This hydrogen helps to break up particle aggregates into stable granular dispersions, preventing reaggregation and significantly enhancing the combustion of aluminum. Therefore, we wondered whether the incorporation of aluminum hydroxide into kerosene droplets could achieve better combustion performance than the incorporation of micrometer or nanometer aluminum into kerosene droplets [28,29]. The thermal decomposition and oxidation reaction of AlH₃ can be divided into one weight loss stage and two weight gain stages [30,31]. The reaction amounts and rates in each stage are different and are affected by the oxygen concentration. The poor stability of AlH₃ can be improved through coating [32].

Aluminum hydride exhibits excellent hydrogen storage capacity [33], with a high hydrogen storage density of 10.1% (0.148 g/cm³), which is twice that of liquid hydrogen [34,35]. The combustion process of AlH₃ in air occurs in several stages [36,37]. In the first phase, hydrogen flame off the surface of the sample. In the second phase, hydrogen is consumed, and the flame descends to contact the sample, igniting and initiating low-temperature combustion of aluminum, similar to the combustion of ultra-dispersed aluminum powder (UDAP). In the third phase, the combustion transitions to high-temperature combustion, reaching temperatures of 2000–2400 °C. In the fourth phase, the hydrogen flame is consumed, and the flame descends again to ignite and promote the low-temperature combustion of aluminum [38,39]. Finally, in the fifth phase, the combustion continues into the high-temperature phase, reaching temperatures of 2000–2400 °C. Shock tube studies have revealed that the combustion mechanism of AlH₃ at high temperature and pressure occurs in two steps. The first step involves dehydrogenation and oxidation. The combustion behavior of the residual aluminum after dehydrogenation is similar to that of micron-sized aluminum particles. In the early stages of combustion, a large amount of hydrogen is released near the particle surface, and its combustion accelerates the ignition and combustion of the aluminum particles. Additionally, it was found that the heating rate and reaction atmosphere during the ignition and combustion of AlH₃ significantly influence the onset temperature of hydrogen production from decomposition, ignition temperature, and activation energy [40].

Previous studies have largely overlooked three key aspects. First, while current research primarily focuses on the ignition temperature, combustion temperature, and combustion time of AlH₃, there is limited attention given to how its physical and chemical properties influence its flame characteristics. Moreover, studies on the dynamic changes in combustion intensity and morphology during the entire combustion process of AlH₃ are scarce. Second, the impact of AlH₃ on the ignition of individual kerosene droplets has not been thoroughly explored. This aspect is crucial for understanding the application prospects of aluminum hydride in liquid fuels. Studying the application prospects of aluminum hydride in the field of high-energy liquid fuels is of great importance. Third, research on the combustion mechanism of single droplet combustion of aluminum hydride-containing kerosene is lacking, and no model currently exists to explain the ignition delay and combustion rate of such droplets.

Hence, this study aims to address the following objectives: (1) the thermal reaction process and combustion characteristics of AlH₃ particles, (2) the effects of varying AlH₃ content on the combustion characteristics and products of kerosene droplets, and a comparison with micron- and nano-aluminum kerosene droplets, and (3) the development of a combustion kinetics model for droplets containing AlH₃. To achieve this, we utilized techniques such as TG-DSC analysis, combustion diagnostics, high-speed imaging, and combustion product analysis. The findings of this study are crucial for guiding the application of AlH₃ in fluid fuels.

2. Experimental Methods

2.1. Materials and Preparation

For our experiments, Novacentrix (Round Rock, TX, USA) provided micron-sized aluminum powder (99%, nominal diameter 29 μm), nano-aluminum powder (99%, nominal diameter 50 nm), and aluminum hydride powder (99%, nominal diameter 29 μm). Silicon carbide (SiC) fibers, used for suspending prepared kerosene droplets, were supplied by the School of Materials Science and Engineering at Northwestern Polytechnical University (Xi’an, China), while the kerosene droplets were purchased from Sigma-Aldrich (St. Louis, MO, USA). AlH₃, a white hexagonal crystal, is known for its dense, transparent structure, which features well-defined crystal planes and edges. As illustrated in Figure 1c, the AlH₃ particles in this experiment display surface characteristics such as pores and fine particles. These traits are likely the result of slow hydrogen release and partial oxidation during storage. The preparation methods for nanofluid and microfluid fuels are outlined in Refs. [41,42,43,44]. Micron-aluminum (μ-Al), nano-aluminum (n-Al), or micron-aluminum hydride (μ-AlH₃) particles were incorporated into kerosene-based fuel at mass fractions of 1%, 5%, 7.5%, or 10%. The mixture was then subjected to ultrasonication for 10 min to ensure uniform dispersion of the particles [45]. The composition of the nanofluid and microfluid fuels is provided in

Table 1. Composition of the fluid fuels.

Number	Droplet	Metal Particle	Metal Content
G-0	kerosene
G-1	kerosene	μ-Al	1%
G-2	kerosene	μ-Al	5%
G-3	kerosene	μ-Al	7.5%
G-4	kerosene	μ-Al	10%
G-5	kerosene	n-Al	1%
G-6	kerosene	n-Al	5%
G-7	kerosene	n-Al	7.5%
G-8	kerosene	n-Al	10%
G-9	kerosene	μ-AlH3	1%
G-10	kerosene	μ-AlH3	5%
G-11	kerosene	μ-AlH3	7.5%
G-12	kerosene	μ-AlH3	10%

.

2.2. Experimental Setup and Procedure

The experimental setup is shown in Figure 1d, with the physical diagram provided in Figure 1. The prepared kerosene fuel droplet is suspended on a SiC fiber, with an initial diameter set at approximately 1 mm. The electric heating wire is set to a temperature of 700 °C, and the thermal environment during the heating process is recorded. The design of the experimental apparatus and measurement methods for droplet combustion are based on Refs. [42,46]. A high-speed camera (Phantom M340, Vision Research Inc. (Wayne, NJ, USA)) was used to capture the combustion process of the kerosene droplets, and the emission spectrum was recorded using an Avantes AvaSpec-2048 (Avantes, Apeldoorn, The Netherlands). After the droplets were burned out, the combustion products were collected and examined under a scanning electron microscope (SEM: ZEISS, EVO MA, ZEISS (Oberkochen, Germany)). To study the thermal reactions of micron-aluminum, nano-aluminum, and micron-aluminum hydride, thermogravimetric differential scanning calorimetry (TG-DSC) (Netzsch: STA 499 F5 (Netzsch, Selb, Germany)) was used, heating the three materials from 20 °C to 800 °C at a rate of 5 °C/min in CO₂ atmosphere. Additionally, the ignition and combustion behavior of the three materials were investigated using a laser ignition device [41]. The laser igniter power was set to 200 W, and the self-sustained burning time for each material was recorded.

3. Results and Discussion

3.1. Thermal Oxidation Characteristic

The thermal characteristics of AlH₃ in the atmosphere were investigated using TG-DSC. Its thermal reaction process, illustrated in Figure 2a,b, can be divided into three main stages. Detailed data are provided in

Table 2. TG and DTG characteristics for μ-AlH₃ particles.

Samples	TG			DTG
	Ti	Te	MC	Tp	Lmax
AlH3	25.2	178.0	−5.6	185.3	24.1
	399.2	590.4	30.6	553.4	7.8
	590.5	810.1	7.2	654.3	−2.8

Note: T_i, initial reaction temperature (°C); T_e, final reaction temperature (°C); MC, mass change (%); T_p, peak temperature of mass change (°C); L_max, maximum mass change rate (% min⁻¹).

. In the first stage, known as the weight-loss stage, AlH₃ decomposes to produce hydrogen. As the temperature increases, the mass begins to decrease. The TG curve indicates a significant weight loss between approximately 25 °C and 178 °C, which is attributed to the decomposition of AlH₃ and the release of hydrogen, with a 5.6% weight loss during this temperature range. The mass remains relatively stable as the temperature rises to 399.2 °C, suggesting that no further chemical reactions occur during this period. The second stage is the weight-gain stage, which corresponds to the oxidation of aluminum. During this phase, the mass increases rapidly as aluminum reacts with oxygen to form alumina (Al₂O₃). The TG curve shows a significant weight gain starting at 399.2 °C, with a weight gain of 30.6% observed between 399.2 °C and 590.4 °C.

Comparison with previous results of other researchers on the thermogravimetry of nanoparticle powders reveals that the weight gain of nanoparticle powders of aluminum begins at 573.3 °C and ends at 652.0 °C [47], which shows that the exothermic oxidation of hydrogen produced by the decomposition of aluminum hydride results in a lowering of the temperature of the primary oxidation of aluminum. In the third stage, which is also a weight-gain stage, the aluminum undergoes high-temperature oxidation. The TG curve indicates a secondary weight gain starting at 590.5 °C, with an additional 7.2% increase in mass between 590.5 °C and 810.1 °C. This weight gain is due to the continued oxidation of aluminum at higher temperatures. However, the maximum heat release and rate of weight gain in the secondary weight-gain stage are lower than those in the primary oxidation stage. This is because the oxide layer formed during the oxidation process undergoes a transformation through several phases: from γ-Al₂O₃ to δ-Al₂O₃, then to θ-Al₂O₃, and finally to α-Al₂O₃. The final α-Al₂O₃ layer has a very high density, which impedes the further progression of the oxidation reaction [48,49].

The DSC curve of aluminum hydride powder during the thermal reaction process is shown in Figure 2b. As observed, the DSC curve displays two distinct exothermic peaks and one endothermic peak throughout the entire reaction process. The first exothermic peak occurs between 158 °C and 192 °C, attributed to the oxidation of hydrogen released from the thermal decomposition of AlH₃, which generates heat. The second exothermic peak, occurring between 400.1 °C and 605.5 °C, corresponds to the oxidation of aluminum, also resulting in heat release. The specific temperature peaks and associated heat releases are summarized in

Table 3. DSC curves for μ-AlH₃ particles.

Samples	Exotherm Peak				Endotherm Peak
	Tp	Ti	Te	Q	Tp	Ti	Te	Q
AlH3	177.1	158.0	192.2	24.5	657.9	645.8	667.8	−2.3
	546.4	400.1	605.5	7.7

Note: T_i, initial reaction temperature (°C); T_e, final reaction temperature (°C); T_p, peak temperature (°C); Q, heat release (J g⁻¹).

. Finally, the third endothermic peak, observed between 645.8 °C and 667.8 °C, is associated with the melting of aluminum before the secondary oxidation phase. The endothermic peak occurs at 657.9 °C, just before the weight gain from oxidation.

3.2. Laser Ignition Experiment

Laser ignition experiments were conducted on aluminum powders with particle sizes of 29 μm (micron) and 50 nm (nano), as well as AlH₃ powder with a particle size of 29 μm. The peak flame brightness during the combustion of each sample is shown in Figure 3c–e. Among the samples, the flame brightness of nano-aluminum was higher than that of micron-aluminum. The flame morphology of aluminum hydride, however, differed from the other two. This difference is attributed to the combustion of hydrogen released during the decomposition of AlH₃. The flame from aluminum hydride was more stable, positioned higher, and exhibited a more intense combustion compared to the aluminum samples. During the combustion of both micron and nano-aluminum, particulate ejection occurred, with aluminum particles in the molten state being expelled, forming spherical combustion products. In contrast, the aluminum particles ejected from aluminum hydride were smaller in size, confirming that aluminum hydride offers superior combustion performance over both micron and nano-aluminum.

The emission spectra for the three samples during combustion are shown in Figure 3d. A characteristic AlO peak at 486.3 nm was observed for all three samples. The self-sustained combustion time, shown in Figure 3e, was nearly halved for micron-sized aluminum hydride compared to micron-sized aluminum. This is due to the faster hydrogen release during the combustion of aluminum hydride, which, with its smaller particle size and larger specific surface area, exhibits a faster combustion rate compared to micron-sized aluminum.

3.3. Droplet Ignition

The time required for the droplet to ignite is referred to as the ignition delay time, which consists of physical and chemical delays. The physical delay is the time needed for the mixture of droplet vapor and oxygen to reach the energy for the combustion reaction [50,51]. In this experiment, droplet combustion is a premixed combustion process, where the droplet and oxygen are already mixed. Therefore, the physical concentration necessary for combustion, while the chemical delay is the time required for the mixture to reach the activation delay can be ignored, and only the chemical delay time is considered. In the context of aerospace engine fuels, reducing ignition delay can significantly enhance fuel utilization [52]. In this study, the ignition delay was determined by recording the time interval between the complete heating of the heating wire and the first appearance of the droplet flame, as shown in Figure 4a.

As shown in Figure 4b, the addition of μ-Al particles increases the ignition delay time of the fuel, making it longer than that of pure kerosene droplets. This is most evident at a micron-aluminum concentration of 7.5%, where the ignition delay reaches 0.037 s, compared to 0.024 s for pure kerosene. When μ-Al particles are added to kerosene, their suspension stability in the fuel is greatly reduced, causing them to settle quickly. When the droplet ignites, a significant amount of μ-Al particles has settled at the bottom of the droplet. These deposited particles reduce the probability of radiation being absorbed multiple times by the droplet, thus lowering its radiation absorption capacity. Consequently, the droplet ignites from the bottom. Interestingly, the ignition delay time decreases as the concentration of μ-Al particles increases. Although a higher particle concentration results in more deposition, the high thermal conductivity of aluminum particles causes localized cooling rather than heating. This cooling effect at the bottom of the droplet promotes fuel evaporation, facilitating the attainment of ignition conditions more quickly, which leads to a slight reduction in ignition delay. A similar trend is observed with n-Al particles added to kerosene.

As shown in Figure 4b, the ignition delay time of kerosene droplets containing aluminum hydride initially decreases and then increases as the aluminum hydride concentration increases. The minimum ignition delay time of 0.023 s is observed at an aluminum hydride concentration of 5%. This reduction is likely due to the hydrogen released during the decomposition of AlH₃, which accelerates droplet evaporation. However, as the AlH₃ content continues to increase, the ignition delay time starts to rise. We hypothesize that, at higher concentrations, the decomposition of AlH₃ produces a large amount of hydrogen gas, which forms a hydrogen-rich layer around the droplet. This hydrogen layer acts as an insulating barrier, hindering heat transfer to the droplet and thereby increasing the ignition delay.

3.4. Combustion Characteristics of Fuel Droplet

3.4.1. Kerosene Combustion

As shown in Figure 5a, in the high-temperature environment generated by the electric heating wire, the kerosene droplet absorbs heat and evaporates, forming a vapor cloud around the droplet. When the vapor cloud reaches the required concentration for ignition, the droplet ignites, and a distinct flame appears around it. The kerosene continues to evaporate and burn, and the droplet shrinks in size, following a linear relationship with time until it completely burns out, entering the stable combustion stage. During this process, the flame intensity is low internally but high externally. Throughout the combustion, the flame maintains a smooth transition, with little change in the flame intensity distribution.

3.4.2. Combustion of μ-Al&Kerosene and n-Al&Kerosene

For the μ-Al&kerosene and n-Al&kerosene droplets, the combustion process begins similarly to that of pure kerosene: the droplets first absorb heat from the environment and vaporize. After a period of preheating, the droplets ignite, and, once ignited, they continue to vaporize and burn steadily. The combustion characteristics of this process are similar to those of pure kerosene, involving preheating, ignition, and stable combustion. As the liquid component of the droplet burns off, the micronized aluminum particles in the droplet accumulate on the silicon carbide filaments, forming clumps. The heat generated by the combustion of kerosene is not sufficient to ignite the micron-sized aluminum agglomerates. For the n-Al&kerosene droplets, a similar sequence occurs: the droplets first absorb heat and vaporize, then ignite after a period of preheating, continuing to burn steadily. When the liquid fuel evaporates, the aluminum nano-powder collects on the silicon carbide filaments and forms agglomerates. The combustion of the kerosene generates sufficient heat to elevate the temperature within these aluminum agglomerates. Oxygen from the surrounding environment diffuses to the surface of the agglomerates, initiating oxidation. The aluminum agglomerates are ignited and emit an intense white light. The combustion products then remain on the filaments.

3.4.3. Combustion of μ-AlH₃&Kerosene

Figure 5d shows the combustion process of a single droplet of kerosene containing AlH₃ powder. This is because the kerosene is doped with aluminum hydride (AlH₃), which decomposes at high temperatures to produce hydrogen and aluminum. So this decomposition process affects the combustion behavior, the hydrogen generated during decomposition initially dissolves in the kerosene. As decomposition progresses, the rising hydrogen concentration may lead to bubble formation. When the pressure inside the bubble (hydrogen pressure) exceeds the sum of the surface tension of the droplet and the external pressure, the bubble escapes into the surroundings. Once the hydrogen stored in the droplet escapes, the droplet diameter decreases abruptly, and at this stage the burning rate increases significantly due to the heat generated by the combustion of the escaping hydrogen. At the same time, the aluminum particles produced by the decomposition of AlH₃ enhance the ability of the droplet to absorb radiant heat. As a result, the droplet absorbs more heat per unit time, leading to faster evaporation and combustion. Once the liquid phase of the droplet burns out, a micro-explosion occurs, a phenomenon not observed with other fuels. This highlights the superior performance of AlH₃ as a kerosene additive.

3.4.4. Combustion Rate Properties

In order to quantitatively analyze the effect of the three fuel additives on the droplet combustion rate at different concentrations, three experiments were conducted for each sample and the average droplet diameter was recorded. As can be seen in Figure 6a–c, the normalized diameters of nanoscale and microscale aluminum droplets decrease with time during the stable combustion phase, which follows the D² law, similar to the results of previous studies by other authors [22]. The D² law describes the linear relationship between the square of the droplet diameter and combustion time, where the slope represents the burning rate constant (K) [11,53]. In the combustion process of kerosene droplets containing aluminum particles, the aluminum powder first absorbs heat. As the temperature of the aluminum particles increases, they release energy, which, in turn, accelerates the combustion of the kerosene droplet. This process can be divided into two stages. In the first stage, the efficiency of heat absorption by the aluminum powder is primarily influenced by the particle size and concentration of the aluminum powder. Larger particle size and higher concentration require more energy. For micron-aluminum/kerosene droplets, the large particle size of micron aluminum means that the energy generated throughout the stable combustion process is not sufficient to activate all micron-aluminum particles. As a result, the entire stabilized combustion process stays in the first stage, resulting in a slower combustion rate. The lowest burning rate is at 7.5% micron-aluminum concentration, 0.897 mm²s⁻¹. In contrast, due to its larger specific surface area, nano-aluminum enables faster combustion of kerosene droplets compared to micron-aluminum. Kerosene droplets containing nano-aluminum require less energy during the first stage of aluminum particle activation.

Aluminum nanoparticles are uniformly dispersed at low concentrations (1%), while the high thermal conductivity of the nanoparticles promotes heat transfer within the droplets, accelerating preheating and volatilization. Aluminum nanoparticles are uniformly dispersed at medium concentration (5%) Elevated particle concentration leads to van der Waals forces dominance and formation of large-sized agglomerates, which reduces the effective specific surface area, slows down the rate of aluminum oxidation, and diminishes its combustion-promoting effect. This is when the lowest burning rate is reached, at 0.855 mm² s⁻¹. High solid content leads to the rise of droplet viscosity, inhibiting internal flow and external oxygen diffusion, hindering the volatile components and air mixing, the formation of localized oxygen-deficient zones, combustion efficiency decreases. High concentration of particles may absorb too much radiant heat, delaying the rise of droplet surface temperature and postponing the volatilization process. The combustion rate of aluminum nanoparticles picks up at high concentrations (>5%), and after the particle concentration exceeds the critical value, a continuous thermal conductivity path is formed, which significantly improves the overall thermal conductivity of the droplet, accelerates internal heat transfer, and shortens the preheating time.

However, for kerosene single droplets containing aluminum hydride, because aluminum hydride decomposes at high temperature to produce hydrogen and aluminum. So this decomposition process affects the combustion behavior. Hydrogen produced by decomposition is initially dissolved in the kerosene, and as the decomposition proceeds, the hydrogen concentration increases and bubbles may form. When the pressure inside the bubble (hydrogen pressure) exceeds the sum of the droplet surface tension and the external pressure, the bubble escapes Once the hydrogen stored in the droplet escapes, the droplet’s diameter decreases abruptly, the escaping hydrogen burns to produce heat, and the rate of combustion increases significantly. The combination of these factors results in the kerosene droplets containing aluminum hydride burning faster than the kerosene droplets containing aluminum nanoparticles. When the particles of aluminum hydride are dispersed uniformly at low concentration (1%), the high diffusion rate and fast combustion characteristics of hydrogen can accelerate the local flame propagation and enhance the gas-phase reaction rate. The fastest burn rate of 1.208 mm² s⁻¹ was observed at this stage. At the same time, the additional heat released by hydrogen combustion enhances droplet evaporation. When aluminum hydride is in medium concentration (5%), it may lead to agglomeration, which reduces the effective reaction area; at the same time, the viscosity of droplets increases, which inhibits internal convection and heat transfer, and slows down the evaporation. At high concentrations (>5%) the residue may form a porous structure (rather than a dense layer), allowing rapid diffusion of fuel vapors through the pores and easier penetration of oxygen into the droplet, while at the same time releasing greater amounts of hydrogen and generating more heat from the combustion of hydrogen, which promotes a more pronounced rate of combustion.

3.5. Combustion Residue Morphology

Figure 7 presents scanning electron micrographs of the combustion residues from the three samples. The micrographs reveal that the combustion behavior of the three fuel types differs significantly. For the micron-sized aluminum kerosene droplets, the combustion was incomplete, and the aluminum particles remained attached to the silicon carbide filaments in spherical forms. In contrast, both nano-aluminum and aluminum hydride droplets combusted completely, with the resulting alumina crystals adhering to the silicon carbide filaments in filamentous and flocculent structures. The combustion residue morphologies of nano-aluminum and aluminum hydride were similar, indicating a higher combustion efficiency compared to micron-sized aluminum. However, the combustion residue of aluminum hydride was more porous and fluffier, with a more prominent white luster of alumina. The residue from nano-aluminum, on the other hand, was relatively more prone to aggregation. Thus, the combustion efficiency of aluminum hydride kerosene droplets is superior to that of both micron-sized aluminum and nano-aluminum kerosene droplets, owing to its more complete combustion and the favorable morphology of its combustion residues.

3.6. Ignition and Combustion Model of Kerosene Droplets Containing Aluminum Hydride

The above text analyzed the combustion process of kerosene droplets containing aluminum hydride. Many researchers have conducted studies on the ignition and combustion mechanisms of aluminum-based kerosene droplets [54,55,56]. Building upon this body of work, we have refined the ignition and combustion model for aluminum-based kerosene droplets, tailored to the experimental conditions of our study. The heat and mass transfer mechanisms involved in this process are illustrated in Figure 8a. As the temperature of the droplet increases, the rate of evaporation accelerates. Simultaneously, the concentration of kerosene vapor decreases, while the concentration of oxygen exhibits a radial increase. Prior to ignition, oxygen is completely consumed, leading to complete combustion. During this process, the aluminum particles suspended within the droplet effectively absorb external radiant heat through multiple reflections, thereby enhancing heat transfer and increasing the droplet’s combustion rate. During both the evaporation and combustion stages, the mass flow rate of kerosene vapor on the droplet surface can be determined using the following equation:

$${\dot{\mathrm{m}}}_{f,e}=4\pi r_s{\displaystyle \frac{{\lambda }_g}{c_{pg}}}\ln \left[1+{\displaystyle \frac{c_{pg}(T_{\infty }-T_s)}{Q_v}}\right]$$

(1)

$${\dot{\mathrm{m}}}_{f,c}=4\pi r_s\left\{{\displaystyle \frac{{\lambda }_g}{c_{pg}}}\ln \left[1+{\displaystyle \frac{c_{pg}(T_{\infty }-T_s)}{Q_v}}\right]+{\displaystyle \frac{D_o{C}_{\infty }}{\beta }}\right\}$$

(2)

where $r_s$ is the droplet radius, ${\lambda }_g$ is the gas thermal conductivity, $c_{pg}$ is constant-pressure specific heat capacity for the gas, $T_{\infty }$ is the ambient temperature, $T_s$ is the droplet surface temperature, $Q_v$ is latent heat of vaporization for the fuel, $D_{\infty }$ is diffusion coefficient, $C_{\infty }$ is ambient oxygen concentration, $\beta$ is Oxygen-to-fuel equivalence ratio. The heat flux density at the surface during evaporation and combustion of a droplet is determined according to the following equation:

$$q_e=2\pi r_s{\lambda }_{g}Nu(T_{\infty }-T_s)\ln \left(1+B_y\right)/B_y+{\epsilon }_h\sigma A{T}_h^{4}k{\epsilon }_f$$

(3)

$$q_c=2\pi r_s{\lambda }_{g}Nu(T_{\infty }-T_s)\ln \left[\left(1+B_y\right)\left(1+B_o\right)\right]/B_y+{\epsilon }_h\sigma A{T}_h^{4}k{\epsilon }_f$$

(4)

where $Nu$ is Nusselt number, $B_y$ is the Spalding mass transfer number, ${\epsilon }_h$ is emissivity for the heating source, σ is the blackbody radiation constant, $A$ is area of heating source, $T_h$ is the temperature of heating source, $k$ is ratio of radiation transmitted to the droplet to the total radiation from the heating source, ${\epsilon }_f$ is the radiation absorption coefficient, $B_o$ is the oxidation reaction. ${\epsilon }_f$ of the droplet is determined by the combination of the radiation absorption coefficients of the kerosene and the Al particles. This combined absorptivity can be estimated using Equation (5)

$${\epsilon }_f=f_{Al}{\epsilon }_{Al}+(1-f_{Al}){\epsilon }_k$$

(5)

The flame front temperature of a fuel droplet satisfies Equation (6)

$${\displaystyle \frac{T_f-T_s}{T_{\infty }-T_s}}=\left[{\displaystyle \frac{B_o{Q}_f}{c_{pg}(T_{\infty }-T_s)}}+1\right]/\left[1+{\displaystyle \frac{B_o}{B_y}}\left(B_y+B_o\right)\right]$$

(6)

The equations for conservation of mass and energy during the ignition and combustion of the droplet are Equations (7) and (8), respectively,

$${\displaystyle \frac{d\left(\frac{4}{3}\pi r_s^3{\rho }_f\right)}{dt}}={\dot{\mathrm{m}}}_f$$

(7)

$$\left(m_{Al}{c}_{pl}+m_f{c}_{pf}\right){\displaystyle \frac{dT}{dt}}=q_c-{\dot{\mathrm{m}}}_f{Q}_v$$

(8)

where $c_p$ is the specific heat capacity, $Q_v$ is the latent heat of vaporization of kerosene.

Combined with the real situation of the experiment, we set the boundary conditions as “Still air”, p_∞ = 1 atm, T_∞ = 300 K, T_n = 650 K, T_h = 1000 K. The initial condition is r_d = 0.5 mm, T_s = T_i = 300 K. Droplet combustion follows the principles of conservation of mass and conservation of energy, and these equations described above describe the processes that control the behavior of droplets, and by solving these control equations, the time evolution of important parameters such as surface temperature and droplet size can be determined.

In the combustion stage of kerosene droplet containing aluminum hydride, in addition to the evaporative combustion of burning kerosene, the aluminum hydride in kerosene droplet is decomposed by heat to produce hydrogen, and the decomposition rate is determined by the Equation (9) [57,58]

$${\displaystyle \frac{d{m}_{Al{H}_3}}{dt}}=-A{e}^{\left(-{\displaystyle \frac{E_a}{RT}}\right)m_{Al{H}_3}}$$

(9)

where $E_a$ is activation energy of the decomposition reaction of aluminum hydride.

Research theory based on bubble dynamics [59,60,61]. The partial pressure of hydrogen gas in a kerosene droplet produced by the decomposition of aluminum hydride can be determined by Equation (10), and the critical partial pressure that allows the hydrogen gas to escape from the kerosene droplet is determined by Equation (11).

$$p_{H_2}={\displaystyle \frac{m_{H_2}RT}{\frac{4}{3}\pi r_s^3}}$$

(10)

$$p_{crit}=p_{amb}+{\displaystyle \frac{2\sigma }{r_s}}$$

(11)

When the partial pressure of dissolved hydrogen exceeds the critical pressure, hydrogen gas escapes ($p_{H_2}>p_{crit}$).

The mass change of the escaped hydrogen is determined by the following Equation (12), and the mass of hydrogen is conserved

$${\dot{m}}_{H_2,out}=k_{out}(p_{H_2}-p_{crit})$$

(12)

$${\displaystyle \frac{d{m}_{H_2}}{dt}}={\displaystyle \frac{3}{2}}{\displaystyle \frac{d{m}_{Al{H}_3}}{dt}}-{\dot{m}}_{H_2,out}$$

(13)

Mass and energy conservation during liquid phase evaporation and combustion of fuel droplets

$${\displaystyle \frac{d\left(d^2\right)}{dt}}=-{\displaystyle \frac{4{\lambda }_g}{{\rho }_f{c}_{pg}}}\ln \left(1+{\displaystyle \frac{c_{pg}(T_{\infty }-T_s)}{Q_v}}\right)-{\displaystyle \frac{d{m}_{Al{H}_3}}{2\pi {\rho }_{f}dt}}$$

(14)

$$mc{\displaystyle \frac{dT}{dt}}=q_c-{\dot{\mathrm{m}}}_f{Q}_v+{\dot{m}}_{H_2,out}{Q}_{H_2}$$

(15)

Figure 8c–h present a comparison between model predictions and experimental observations. The error between the model predictions and experimental values for ignition delay and combustion velocity is less than 10%, indicating that the model accurately captures the combustion behavior trends of kerosene droplets observed in the experiments. Notably, the error is slightly larger at higher concentrations, suggesting that the prediction deviation increases as the additive concentration rises. Overall, the model’s predictions exhibit a strong correlation with the experimental results, providing reliable validation of the model’s accuracy and further supporting the validity of the theoretical framework.

4. Conclusions and Future Perspectives

This study systematically investigated the combustion characteristics of aluminum hydride (AlH₃) in kerosene and compared its performance with micron- and nano-aluminum additives. Key findings include:

(1) AlH₃ undergoes a two-phase reaction—initial hydrogen release (5.6% mass loss) followed by aluminum oxidation (30.6% mass gain). The exothermic oxidation of hydrogen lowers the primary oxidation temperature of aluminum compared to pure nano-aluminum.

(2) Laser ignition tests revealed that AlH₃ exhibits faster self-sustained combustion (nearly half the time of micron-aluminum) and more intense flames due to hydrogen release. Ejected aluminum particles were smaller, indicating superior combustion efficiency.

(3) AlH₃-doped droplets showed a non-linear ignition delay trend, with the shortest delay at 5% concentration (0.023 s).

(4) The hydrogen gas released by the decomposition of AlH₃ escapes, causing a sudden reduction in the droplet size and micro-explosion. The heat released by the combustion of hydrogen gas significantly increases the combustion rate of the droplets. The concentration of AlH₃ with the fastest combustion rate is 1% (1.208 mm²/s).

(5) SEM analysis confirmed more complete combustion with AlH₃, yielding finer, porous alumina residues.

(6) A developed combustion model for AlH₃-kerosene droplets predicted values showed a strong correlation with experimental data, with errors in ignition delay and combustion rate predictions of less than 15% and 10%, respectively, confirming the model’s accuracy and reliability.

In summary, AlH₃ demonstrates superior combustion properties over conventional aluminum additives, offering potential for high-energy liquid fuel applications. Based on this research, the work we are going to attempt includes:

(1) Further studies will explore the trade-offs between AlH₃ loading, stability, and combustion efficiency to identify optimal formulations for specific industrial applications.

(2) Large-scale synthesis, long-term storage stability, and safety protocols for AlH₃-based fuels need systematic evaluation to facilitate practical adoption.

(3) Experimental validation in real-world combustors or thrust chambers will be conducted to assess performance under operational conditions.

At the same time, the potential contributions of our work to the industrial sector include:

(1) Provides a viable pathway to develop next-generation, high-energy-density liquid fuels.

(2) Offers insights into mitigating nanoparticle agglomeration and improving combustion uniformity via hydrogen-assisted mechanisms.

(3) The developed combustion model serves as a tool for optimizing fuel design and predicting combustion behavior in industrial systems.