Search Results (15)

Nitrous oxide (N₂O) has attracted increasing attention as an oxidizer for space propulsion systems due to its non-toxic nature and favorable handling characteristics. Its relatively high vapor pressure allows self-pressurization, while its wide storage temperature range makes it attractive for a range of space applications. In parallel with broader efforts to identify alternatives to conventional toxic propellants, numerous studies have investigated liquid propulsion systems based on N₂O combined with hydrocarbon fuels, spanning both premixed fuel blends and non-premixed bipropellant configurations. This review summarizes experimental and system-level studies on N₂O–hydrocarbon propellant combinations, including ethylene, ethane, ethanol, propane, acetylene, methane, dimethyl ether, and propylene. Results reported by different research groups reveal clear differences among propellant combinations in terms of vapor pressure, thermal stability, chemical reactivity, and ignition delay. These differences have direct implications for injector design, mixing strategies, ignition mechanism, and system safety. By bringing together recent results from the literature, this paper aims to clarify the practical trade-offs associated with fuel selection in N₂O-based premixed and bipropellant systems and to provide a useful reference for the design and development of future space propulsion concepts. Full article

A three-dimensional numerical investigation using ethylene–oxygen was conducted to examine the characteristics of detonation waves in a non-premixed rotating detonation engine (RDE) across three equivalence ratio conditions: fuel-lean, stoichiometric, and fuel-rich. The study aims to identify the distinct timescales associated with detonation wave propagation within the combustor and to analyze their impact on detonation wave behavior, emphasizing the influence of equivalence ratio and injector behavior on detonation wave characteristics. The results indicate that the wave behavior varies with mixture concentration, with the ethylene injector demonstrating greater stiffness compared to the oxygen injector. In lean mixtures, characterized by excess oxidizer, waves exhibit less intensity and slower progression toward equilibrium, resulting in prolonged reaction times. Rich mixtures, with excess fuel, also show a delayed approach to equilibrium and an extended chemical reaction timescale. In contrast, the near-stoichiometric mixture achieves efficient combustion with the highest thermicity, rapidly reaching equilibrium and exhibiting the shortest chemical reaction timescale. Overall, the induction timescale is generally 2–3 times longer than its respective chemical reaction timescale, while the equilibrium timescale spans a broad range, reflecting the complex, rapid dynamics inherent in these chemical processes. This study identifies the role of the characteristic chemical timescale in influencing the progression of pre-detonation deflagration in practical RDEs. Prolonged induction times in non-ideal conditions, such as those arising from equivalence ratio variations, promote incomplete reactions, thereby contributing to pre-detonation phenomena and advancing our understanding of the underlying flow physics. Full article

Large-scale industrial burners are essential components in various industries including power generation and chemical processing. Enhancing their energy efficiency and reducing emissions, particularly nitrogen oxides (NOx), requires a combination of experimental research and computational fluid dynamics (CFD) simulations. While there exist numerous emission control techniques, the main focus of the present review study was the passive control technique. The result of this review indicates that biodiesel fuel crude palm oil (CPO) was found to reduce emission components, particularly carbon components and particulate matter (PM). Moreover, it also mitigates cavitation within the injector’s orifice, reducing wear and tear. Although cavitation enhances spray atomization and creates finer droplets for improved combustion, it can damage injector orifices. Optimizing the orifice design, such as by adopting conical orifices over cylindrical ones, can significantly reduce cavitation and its adverse effects. Furthermore, innovations such as swirling fuel–air premixing within injectors enhance combustion efficiency and lower emissions by improving fuel–air mixing. However, spray characteristics, particularly the Sauter mean diameter (SMD), remain critical for predicting combustion performance. Further investigations into spray fineness and its impact on combustion dynamics are essential for advancing emission control and performance optimization. Full article

Glycerol, a byproduct of biodiesel, has moderate energy but high viscosity, making clean combustion challenging. Quickly evaporating fine fuel sprays mix well with air and burn cleanly and efficiently. Unlike conventional air-blast atomizers discharging a jet core/film, a newly developed swirl burst (SB) injector generates fine sprays at the injector’s immediate exit, even for high-viscosity fuels, without preheating, using a unique two-phase atomization mechanism. It thus resulted in ultra-clean combustion for glycerol/methanol (G/M) blends, with complete combustion for G/M of 50/50 ratios by heat release rate (HRR). Lower combustion efficiencies were observed for G/M 60/40 and 70/30, representing crude glycerol. Hence, this study investigates the effect of premixed methane amount from 0–3 kW, and the effect of atomizing gas to liquid mass ratio (ALR) on the dual-fuel combustion efficiency of G/M 60/40-methane in a 7-kW lab-scale swirl-stabilized gas turbine combustor to facilitate crude glycerol use. Results show that more methane and increased ALR cause varying flame lift-off height, length, and gas product temperature. Regardless, mainly lean-premixed combustion, near-zero CO and NOx emissions (≤2 ppm), and ~100% combustion efficiency are enabled for all the cases by SB atomization with the assistance of a small amount of methane. Full article

This paper examines the atomization characteristics of liquid hydrogen fuel in a premixing tube under different operating conditions. Hydrogen fuel’s unique injection morphology and atomization behavior were analyzed using the Volume of Fluid-to-Discrete Particle Model (VOF to DPM) approach, coupled with the SST $k-\omega$ turbulence model and adaptive mesh refinement. The study revealed that the breakup and transformation of liquid surfaces into particles are significantly impacted by varying air velocities and injection pressure. Specifically, higher air velocities caused the liquid sheet to lengthen and narrow due to intensified vortices. However, the breakup was delayed at higher velocities, occurring at distances of 0.037 m and 0.043 m for air velocities of 10 m/s and 20 m/s, respectively. The research also highlights the significant role that injection pressure plays in fluid sheet breakup. Higher pressures promote better atomization and fuel–lair mixing, resulting in more particles with increased diameters. Notably, the fluid sheet exhibited a small angle of about 43.79° when using the velocity component corresponding to p1 = 0.5 MPa. Similarly, for p2 = 1 MPa and p3 = 2 MPa, the angles were measured to be approximately 47.5° and 49.5°, respectively. Additionally, the study observed that the injection expands in length and diameter as time progresses, indicating fuel dispersion. These insights have significant implications for the design principles of injectors in power generation technologies that utilize liquid hydrogen fuel. Full article

The objective of this study was to investigate the effect of a premixed ethanol ratio based on the same total heating value in a cylinder on the equivalence ratio distributions and the injected fuel droplet behavior in the cylinder of an RCCI engine. The spray simulation was conducted in two parts. First, we carried out spray validation simulations to determine the spray-influenced factor of the test injector. Next, engine simulations were performed with the spray-influenced factor obtained from the spray validation simulations to investigate the effect of the premixed ethanol ratio based on the same total heating value in a cylinder on the injected fuel atomization and the equivalence ratio distributions. The introduced total heating value was fixed at 595 J based on the lower heating value of diesel, 14 mg. The heating value of the premixed ethanol ratio varied from 0% to 40% based on the same total heating value in the cylinder in steps of 10%. It was revealed that when the premixed ethanol ratio based on the same total heating value in the cylinder was increased, the spray tip penetration value was reduced after 4 deg of diesel was injected because of the short injection duration and the small amount of diesel fuel used. The SMD value was also increased up to 32.58% with an increasing premixed ethanol ratio because of the low kinetic energy of the injected fuel, the short injection duration, the slow evaporation of the injected fuel and the low cylinder temperature. Full article

In three-dimensional (3D) computational fluid dynamics (CFD) simulations, the effects of injection timing and spray inclusion angle (SIA) on performance and emissions of diffusion-controlled and Premixed Charge Compression Ignition (PCCI) combustion in part load for a heavy-duty direct injection (HDDI) diesel engine are studied. The start of injection (SOI) of a 146° SIA injector is varied between −70 and −10 °crank angle (°CA) after top dead center (ATDC). For −50 °CA ATDC SOI with various SIAs between 80° and 146°, PCCI combustion reduces mono-nitrogen oxide (NO_x) emissions significantly compared to conventional diesel combustion (CDC). Due to incomplete combustion in rich zones formed by droplet–cylinder wall interaction, early wide SIA injection deteriorates combustion efficiency (CE) and Indicated Mean Effective Pressure (IMEP) and increases soot and carbon monoxide (CO) emissions. Narrow-angle sprays interacting with the piston bowl elevate CE and IMEP and decrease soot and CO emissions but increases NO_x emissions. Optimal combustion is achieved by avoiding fuel droplet–cylinder wall interaction. By spray-targeting at the stepped lip of the piston bowl, 100° SIA and −50 °CA ATDC SOI yield, respectively, the highest CE and IMEP: 97.8% and 3.37 bar and the lowest soot and CO emissions: 33.5 and 2.2 ppm, with acceptable NO_x emissions. Full article

The target of net-zero emissions set by the 2015 Paris Agreement has strongly commissioned the energy production sector to promote decarbonization, renewable sources exploitation, and systems efficiency. In this framework, the utilization of hydrogen as a long-term energy carrier has great potential. This paper is concerned with the combustion characterization in a non-premixed gas turbine burner, originally designed for natural gas, when it is fed with NG-H2 blends featuring hydrogen content from 0 to 50% in volume. The final aim is to retrofit a 40 MW gas turbine. Starting from the operational data of the engine, a CFD model of the steady-state combustion process has been developed, with reference to the base load NG conditions, by reducing the fuel mass-flow rate by up to 17% to target the baseline turbine inlet temperature. When the fuel is blended with hydrogen, for a given temperature at turbine inlet, an increase in the peak temperature up to 800 K is obtained, if no countermeasures are taken. Furthermore, the flame results are more intense and closer to the injector in the case of hydrogen blending. The results of this work hint at the necessity of carefully analyzing the possible NOx compensation strategies, as well as the increased thermal stresses on the injector. Full article

Experimental investigations are conducted to determine the mechanism and characteristics of a jet in an L-shape crossflow simulating the radial swirl injector of a lean premixed-prevaporized (LPP) combustor. To simplify the radial flow of the actual injector while ignoring the centrifugal effect, the L-shaped 2D-channel is used for the crossflow, and water is used as a fuel simulant. The jet breakup is captured using a high-speed camera, and the density gradient magnitude is post-processed to clarify the spray. The Sauter mean diameter (SMD) of the spray is measured via a laser diffraction method with a helium–neon laser optical system (HELOS). The characteristics of the jet in the L-shape crossflow are compared with the characteristics of the jet in a typical crossflow through the flat channel. The results for different outlet heights of the L-shape channel ($H/d_0$) and different injector positions ($L/d_0$) are presented. A dimensionless number ($\tau$) consisting of a time ratio is introduced to describe the jet characteristics. In a previous work, the spraying tendency was demonstrated for different injector positions. In addition, the effect of the recirculation area on $H/d_0$ was empirically shown. $H/d_0$ determines the size of the recirculation area, and the range of τ determines the jet breakup mechanism inside the L-shape channel. The results of this study present the breakup mechanism of the jet in the L-shape channel flow, which simulates a jet in a radial swirler injector for gas turbine engines. It is expected that these results can be used to assist in designing gas turbine engines with more combustion efficiency. Full article

Natural gas (NG) direct injection (DI) technology benefits the engine with high efficiency and clean emissions, and the high-pressure gas fuel injection process causes crucial effects on the combustion. This study presents an optical experimental investigation on the high-pressure methane single-hole direct injection and premixed ignition combustion based on a visualization cuboid constant volume bomb (CVB) test rig. The experimental results show that the methane jet process is divided into two stages. The methane gas jet travels at a faster speed during the unstable stage I than that during the stable stage II. The injection pressure causes more influence on both the jet penetration distance and the jet cone area during stage II. The methane jet premixed flame is a stable flame with a nearly spherical shape, and its equivalent radius linearly increases. The methane jet premixed flame area also increases while the flame stretch rate declines. The methane jet premixed flame velocity rises as both the standing time and equivalent ratio increase. The methane jet premixed flame is a partial premixed flame, and the peak of the methane jet premixed flame occurs at greater equivalence ratio ϕ, i.e., ϕ > 2. As the injection pressure rises, the jet premixed flame equivalent radius increases, and the flame velocity linearly increases. The higher the methane injection pressure, the faster the jet premixed flame velocity. Full article

The tire pyrolysis oil is a waste-derived fuel with a lower cetane number and higher den-sity than diesel fuel, but this is a promising waste-based fuel for compression ignition en-gines. In the European Union, it is necessary to increase the bio-share of fuels, and the second-generation waste-derived blend components are essential for achieving the 2030 goals. The injection characteristics of tire pyrolysis oil and diesel oil were investigated on a Bosch solenoid type common rail (CR) injector. Six different premixed ratios were investi-gated, including in a low volume percentage 250 ppm and higher 10%, 20%, and 100% pyrolysis oil and 100% diesel oil. The simulation investigation was done in the AVL Fire software, the experimental investigations were done on a LDX CR injection test bench, and the videos were taken on an Olympus Ispeed 3 camera. The scope of the research was to record the flow pattern of the fuel mixture, flowing out of the high-pressure injector, from which the mixing with air and the quality of the resulting combustion can be deduced, which has a significant effect on the emissions. Full article

Supersonic combustion experiments were performed using three different hydrogen fuel-injection configurations in a cavity-based model scramjet combustor with various global fuel–air equivalence ratios. The configurations tested were angled injection at 15° to the flow direction upstream of the cavity, parallel injection from the front step, and upstream injection from the rear ramp. Planar laser-induced fluorescence of the hydroxyl radical and time-resolved pressure measurements were used to investigate the flow characteristics. Angled injection generated a weak bow shock in front of the injector and recirculation zone to maintain the combustion as the equivalence ratio increased. Parallel and upstream injections both showed similar flame structure over the cavity at low equivalence ratio. Upstream injection enhanced the fuel diffusion and enabled ignition with a shorter delay length than with parallel injection. The presence of a flame near the cavity was determined while varying the fuel injection location, the equivalence ratio, and total enthalpy of the air flow. The flame characteristics agreed with the correlation plot for the stable flame limit of non-premixed conditions. The pressure increase in the cavity for reacting flow compared to non-reacting flow was almost identical for all three configurations. More than 300 mm downstream of the duct entrance, averaged pressure ratios at low global equivalence ratio were similar for all three injection configurations. Full article

Flame pulsation has a significant effect on combustion, and understanding its oscillatory behavior is important to the combustion community. An experiment was performed to analyze the pulsation characteristics of a swirl non-premixed flame under various parameters. The results showed that as fuel mass flow rate increased, the puffing frequency increased due to the decreased flame radiation fraction, and the puffing amplitude became smaller resulting in a more stable flame. With an increase in combustor pressure, the flickering frequency declined because of the increasing soot radiation, while the flickering amplitude uniformly increased, leading to more deteriorative flame stability. With an increment in mass flow rate of primary air, the puffing frequency decreased due to the enhanced mixing between fuel and primary air. Also, the puffing amplitude had an oscillating relationship with the mass flow rate of primary air. When the exit velocity of the injector was increased, the flickering frequency diminished nearly linearly because of the improving swirl intensity, and the flickering amplitude was approximately unaffected by injector exit velocity. Moreover, the measured puffing frequencies summarized over all cases varied within the range of 3–22 Hz, the predicted values from theoretical models based on non-swirl flame also fell within this range. The puffing frequency of swirl combustion was more sensitive to the variation in operating conditions than that of non-swirl combustion. Additionally, the obtained correlations indicated that the Strouhal number St was proportional to Fr^−1.4 (the Froude number) and Re^−2.9 (the Reynolds number), respectively. Full article

In opposed-piston folded-cranktrain diesel engines, the relative movement rules of opposed-pistons, combustion chamber components and injector position are different from those of conventional diesel engines. The combustion and heat release characteristics of an opposed-piston folded-cranktrain diesel engine under different operating conditions were investigated. Four phases: ignition delay, premixed combustion, diffusion combustion and after combustion are used to describe the heat release process of the engine. Load changing has a small effect on premixed combustion duration while it influences diffusion combustion duration significantly. The heat release process has more significant isochoric and isobaric combustion which differs from the conventional diesel engine situation, except at high exhaust pressure and temperature, due to its two-stroke and uniflow scavenging characteristics. Meanwhile, a relatively high-quality exhaust heat energy is produced in opposed-piston folded-cranktrain diesel engines. Full article

A multi-pulse injection strategy for premixed charge compression ignition (PCCI) combustion was investigated in a four-valve, direct-injection diesel engine by a computational fluid dynamics (CFD) simulation using KIVA-3V code coupled with detailed chemistry. The effects of fuel splitting proportion, injection timing, spray angles, and injection velocity were examined. The mixing process and formation of soot and nitrogen oxide (NO_x) emissions were investigated as the focus of the research. The results show that the fuel splitting proportion and the injection timing impacted the combustion and emissions significantly due to the considerable changes of the mixing process and fuel distribution in the cylinder. While the spray, inclusion angle and injection velocity at the injector exit, can be adjusted to improve mixing, combustion and emissions, appropriate injection timing and fuel splitting proportion must be jointly considered for optimum combustion performance. Full article