Spray Combustion Research Articles

Boron-loaded gel fuel as potential fuel for ramjet and scramjet engines- recent advancements

Boron, a high-energy metalloid has been recognized as a potential secondary energy source for liquid hydrocarbon fuel to power volume-limited propulsion devices such as ramjet and scramjet. The higher volumetric and gravimetric energetic content makes boron a more suitable candidate than other energetic particles. The stability of the particle is one of the significant drawbacks of adopting metal particles in liquid fuel, which gel fuels can overshoot. Though several works have been published on metalized and non-metalized gel fuels, studies focusing on boron-loaded gel fuels are scarce. The present work comprehensively reviews recent advancements and challenges in adopting boron-loaded gel fuel for practical ramjet and scramjet propulsion devices. It mainly encompasses the formulation of boron-loaded gel fuel, rheological behavior and its influencing factors, single droplet combustion characteristic, spray characteristics, spray combustion characteristics, and the effect of boron-loaded gel fuel on the performance of practical ramjet and scramjet combustors.

Experimental study on the effect of injection schemes on fuel spray and combustion characteristics in a compact combustor

Fuel injection schemes based on close-coupled injection are usually used in compact combustor design. In this paper, an experimental method is adopted to study the fuel spray Sauter mean diameter (SMD) and combustion characteristics when different fuel injection schemes, strut cavity structures, and fuel/air jet momentum flux ratios are applied. Four fuel injection schemes based on close-coupled injection schemes and premixed injection schemes are proposed for use in a compact integrated combustor to compare the effects of the fuel injection schemes. Three struts are designed to study the influence of the cavity on the close-coupled fuel injection scheme. The results show that the spray and combustion characteristics of the close-coupled injection scheme differ from those of premixed injection schemes. Specifically, the SMD in the close-coupled scheme is in the range of 70-150 μm, which is larger and more uneven than that of the premixed scheme (15-40 μm), and the close-coupled scheme has better ignition performance at a temperature of 450 K. The flame structure of the close-coupled injection schemes with reverse mainstream inside the cavity has better radial uniformity, but more fuel is attached to the strut surface, resulting in a lower flame intensity and increased diffusion flame. The cavity on the strut sidewall can effectively promote fuel atomization by approximately 25% when the jet momentum flux ratio is low, make the fuel more uniform in the strut radial direction, increase the flame intensity, and reduce diffusion flame zone. In the close-coupled injection scheme, the large jet momentum ratio will lead to the collision and fusion of multiple fuel streams downstream of the struts, thus increasing the fuel droplet size and limiting the promoting effect of the strut wall structure. In summary, this paper provides further understanding of the combustor spray and combustion characteristics of different fuel injection schemes.

Large eddy simulation of OME3 and OME4 spray combustion under heavy-duty conditions

The synthetic diesel fuel (Poly-)oxymethylene ether (OME) has various interesting properties for the transport sector. OME reduces emissions in direct injection (DI) diesel engines, e.g., soot and NOx, and is considered CO2 neutral. While most studies have examined a mixture of OMEs with different chain lengths, recent studies have focused on neat OME3 and OME4. In the literature similar chemical ignition delay times were shown under homogeneous conditions and significant differences were reported in their thermophysical properties. Furthermore, differences between OME3 and OME4 in terms of the mixture formation were shown on a heavy-duty injector under non-reactive conditions. The question of how this mixture formation affects the ignition and flame characteristics of neat OME3 and OME4 spray flames remains unresolved. To address this topic, this study conducts a combined experimental and numerical investigation under inert and reactive spray conditions for OME3 and OME4 on a heavy-duty injector. Large Eddy Simulations (LESs) and high-pressure, high-temperature spray chamber measurements are conducted. The distinct differences between neat OME3 and OME4 are analyzed along the cause–effect chain comprising the mixture formation, ignition behavior and its effects on the overall heat release rate. Based on the differences in the thermophysical properties, e.g., density and vapor pressure, the investigations in inert conditions reveal differences in the liquid penetration and mixture formation. The reactive spray flame measurements and simulations show a significant offset in the ignition delay time for OME3 and OME4. An explanation is found in the significantly different distribution of the scalar dissipation rate due to the differences in the mixture formation. Despite the differing ignition delay times, similar local flame structures are found for both fuels.

A Numerical Study of Aero Engine Sub-idle Operation: From a Realistic Representation of Spray Injection to Detailed Chemistry LES-CMC

High altitude relight is a matter of increasing importance for aero engine manufacturers, in which combustion plays literally a vital role. In this paper we want to evaluate the predictive capability of a combined Smoothed Particle Hydrodynamics (SPH) and Large Eddy Simulation with Conditional Moment Closure (LES-CMC) approach for a spray combustion process at these extreme conditions. The focus is on the SPH modelling of the kerosene primary atomization, the extraction of realistic spray boundary conditions for LES-CMC and the effect of the spray on combustion. Interestingly, it will be demonstrated that the fragment size distributions resulting from the airblast atomization are characterized by bimodal behaviour during the relight process and that small and large fragments differ significantly in their dynamical behavior. This is shown to affect the combustion in the Central Recirculation Zone (CRZ). Very large fragments are even able to supersede the flame from the CRZ, such that endothermic pyrolysis becomes dominant, but simultaneously essential to sustain and stabilize the remaining flame with reactive pyrolysis species. The study proves the ability of our methodology for extreme operating conditions, in which experimental insights are hardly possible.

Toward Elimination of Soot Emissions from Jet Fuel Combustion.

Soot from jet fuel combustion in aircraft engines contributes to global warming through the formation of contrail cirrus clouds that make up to 56% of the total radiative forcing from aviation. Here, the elimination of such emissions is explored through N2 injection (containing 0-25 vol % O2) at the exhaust of enclosed spray combustion of jet fuel that nicely emulates aircraft soot emissions. It is shown that injecting N2 containing 5 vol % of O2 enhances the formation of polyaromatic hydrocarbons (PAHs) that adsorb on the surface of soot. This increases soot number density and volume fraction by 25 and 80%, respectively. However, further increasing the O2 concentration to 20 or 25 vol % enhances oxidation and nearly eliminates soot emissions from jet fuel spray combustion, reducing the soot number density and volume fraction by 87.3 or 95.4 and 98.3 or 99.6%, respectively. So, a judicious injection of air just after the aircraft engine exhaust can drastically reduce soot emissions and halve the radiative forcing due to aviation, as shown by soot mobility, X-ray diffraction, Raman spectroscopy, nitrogen adsorption, microscopy, and thermogravimetric analysis (for the organic to total carbon ratio) measurements.

Simulations of ammonia spray evaporation, cooling, mixture formation and combustion in a direct injection compression ignition engine

This study investigates the novel approach of direct liquid ammonia injection into the combustion chamber of a compression ignition engine. Unlike traditional gaseous ammonia port injection, this method shows promise in reducing ammonia slip and emissions. To simulate and analyze the behavior of the liquid ammonia spray, we employed numerical models based on existing literature on flashing and non-flashing spray simulations, as well as recent experimental studies on ammonia spray characteristics using a gasoline direct injection (GDI) injector. Our approach utilized a Lagrangian discrete phase modelling technique to perform a series of three-dimensional computational fluid dynamics (CFD) simulations. The primary objectives of this research were to evaluate the evaporation process of ammonia and its cooling effect, as well as to gain insights into mixture formation within the engine cylinder under different operating conditions. The simulations encompassed the entire engine cycle, considering a multi-hole GDI ammonia spray in conjunction with pilot n-heptane, a diesel surrogate, in equal proportions on an energy basis. Three distinct injection timings for ammonia were examined, leading to varying thermodynamic conditions within the combustion chamber during each spray formation. Consequently, different modelling parameters and initial conditions were necessary to accurately replicate the spray behavior. To address this, we proposed a systematic procedure for assigning pre-defined spray angles. The non-reacting, evaporating spray simulations yielded valuable insights into the mixture formation process, which guided our analysis. Finally, reacting conditions were considered to evaluate engine performance and resulting exhaust gas emissions. This study provides a comprehensive overview of critical aspects related to direct ammonia injection in internal combustion engine (ICE) systems. Moreover, it uncovers performance trends that can serve as a foundation for future investigations in this field.

A hydrogen-based burner concept for pilot-scale spray-flame synthesis of nanoparticles: Investigation of flames and iron oxide product materials

Nanoparticle synthesis in spray flames is a flexible method to produce materials with a wide range of compositions, morphologies, and properties. On the road to industrial application, the transfer from laboratory to pilot scale is an important intermediate step. In the present paper, nanoparticle synthesis based on spray combustion combined with a novel burner concept based on a fuel/air pilot flame ignited by an electrical heat ring is presented. When operating with H2, the burner concept allows nanoparticle production with a sustainable fuel. The temperature profile in the flame is one of the key factors determining the kinetics of precursor decomposition, particle formation, and growth. In this work, we have studied the gas-phase temperature in the reactive zone using non-intrusive multi-line NO-LIF temperature imaging. A solution of iron nitrate nonahydrate dissolved in ethanol was used as nanoparticle precursor mixture, atomized by a commercial two-fluid nozzle, and ignited by the premixed flame to synthesize iron-oxide nanoparticles. The burner can be operated at different conditions to direct the properties of the nanoparticles produced. To this goal, process conditions were varied in a targeted manner. In addition to variations in fuel gas and flow rates, the use of compressed air instead of pure O2 as a dispersion gas has also been investigated. The effects of these variations on temperature distribution and materials properties have been investigated. It has been determined that the dispersion gas mass flow has relatively small influence on the materials properties, while higher flame temperatures are advantageous to suppress the often-undesired liquid-to-particle synthesis pathway.

A population balance method for simulation of particle-induced droplet breakup in spray flame synthesis and suspension spray combustion

A population balance method based on weighted Monte-Carlo droplets is used to investigate breakup phenomena in single droplet combustion and spray flame synthesis (SFS). Particle shell formation in conjunction with superheating of liquid components is shown to be a plausible cause for droplet breakup. The breakage rate is calculated based on temperature and particle concentration profiles inside individual droplets, whereas the breakage function was determined using shadowgraphy imaging of droplets and image analysis. This enabled the simulation of consecutive breakup events in single droplet combustion and SFS. Furthermore, in the context of population balance simulations, an adaptive grid method is introduced for the calculation of particle accumulation at the droplet surface. This method is applicable to quickly evaporating droplets where particle transport by advection is fast compared to transport by radial diffusion. The new adaptive grid method is evaluated by comparison with seven single droplet combustion experiments from literature. More than that, parallel algorithms for a computationally efficient implementation using graphics processing units (GPUs) are discussed.

Role of Altitude in Influencing the Spray Combustion Characteristics of a Heavy-Duty Diesel Engine in a Constant Volume Combustion Chamber. Part I: Free Diesel Jet

Heavy-duty diesel engines operating in plateau regions experience deteriorated combustion. However, the lack of up-to-date information on the spray-combustion process limits the fundamental understanding of the role of altitude. In this work, the in-cylinder thermodynamic conditions of a real diesel engine operating under different altitudes were reproduced in a constant-volume combustion chamber (CVCC). The liquid spray, ignition, and combustion processes were visualized in detail using different optical diagnostics. Apart from predictable results, some interesting new findings were obtained to improve the understanding of free spray-combustion processes with different altitudes. The spatial distributions of ignition kernels provided direct evidence of higher peak pressure rise rates for high-altitude diesel engines. The percent of stoichiometric air was calculated to confirm that the net effect of altitude was an increase in the amount of air-entrained upstream of the lifted flame; therefore, the soot levels deduced from flame images were inconsistent with those from real engines, revealing that accelerating the soot oxidation process could effectively reduce engine soot emissions in plateau regions. Finally, a novel schematic diagram of the spray flame structure was proposed to phenomenologically describe the role of altitude in influencing the spray-combustion process of a free jet.

Visualization and analysis of the impinging spray and combustion characteristics in a lateral swirl combustion system (LSCS)

A lateral swirl combustion system (LSCS) was proposed to improve the fuel/air mixing and combustion processes, and previous results have validated its excellent combustion performance in direct injection diesel engines. To further explore the fuel/air mixture mechanism of the LSCS, the effects of convex edge height ha = 2–8 mm (corresponding to interferential angle δ = 133°-14°) on the impinging spray and flame were investigated visually in a constant volume combustion system employing high-speed shadowgraph, direct photography and two-color methods. The experimental results suggested that the combustion chamber with ha = 6 mm (δ = 48°) exhibited superior results in terms of total spray area and combustion. Regardless of whether ha increased or decreased from 6 mm, all of which impaired the synthesis result of the wall-flow-guided and interferential effects. Specifically, when the ha was 2 mm, the wall-flow-guided effect was decreased, and the excessive δ corresponding to 133° caused the neighboring wall jets to approximate a head-on collision, which inevitably resulted in the part of fuel accumulation. Modified ha to 8 mm (δ = 14°), the neighboring wall jets developed nearly in the same direction, diminishing the interferential capacity. Similar higher-temperature and higher-soot areas were observed during the burn-out process for ha = 2 and 8 mm compared to what was obtained at ha = 6 mm, except that the bright area of ha = 2 mm was located near the convex edge at the bottom, while ha = 8 mm appeared in a higher position. Therefore, the LSCS, through exerting the comprehensive effects of wall-flow-guided and interferential interactions, obtains the optimal fuel/air mixing and combustion effects.

Experimental study on wall heat transfer from diesel spray flame in two-dimensional combustion chamber operated with rapid compression and expansion machine (RCEM)

Heat transfer phenomenon in internal combustion diesel engine is one of problems to be solved. This study was conducted to investigate the relationship between diesel spray impingement, combustion processes, and heat loss using a rapid compression and expansion machine (RCEM). Furthermore, spray combustion and heat transfer processes were investigated using multiple injection strategies with a 6-holes injector. The amounts of diesel fuel used in the pilot, pre-injection, and main injection sprays were 3.964, 0.747, and 11.205 mg, respectively. Variations in common rail pressure conditions (30, 120, and 180 MPa) were applied to properly investigate spray evaporation, air–fuel mixture formation, and heat transfer in the combustion chamber. In addition, a local wall heat flux sensor was installed in the cylinder (head and liner) and piston cavity (upper lip, lip, and bottom) to measure the heat flux distribution, and a diffused backlight illumination method was used to visualize the spray and flame behavior in the combustion chamber and 2D piston cavity. The results showed that the impingement flame surrounding the cylinder head contributed to the highest magnitude and longest duration of the heat flux on the cylinder side owing to the spray flame velocity, impingement, and residence time. Meanwhile, the highest different temperature at cavity bottom induced more heat flux on the cavity side than that on the squish side. In addition, the higher injection pressure increased the spray momentum and spray tip penetration which led to a better air–fuel mixture, better evaporation rate, and less flame residence that contributed to reduce the accumulated heat loss in combustion chamber. Furthermore, a low common rail pressure was found to induce a greater difference in the accumulated heat flux ratio than a high common rail pressure which is owing to longer flame residence time in the combustion chamber. They were approximately 14% and 6% greater than that compared to the baseline common rail pressure which mean that a high common rail pressure contributes for the improving thermal efficiency in spray combustion.

Large eddy simulation of combustion instability in a subcritical hydrogen peroxide/kerosene liquid rocket engine: Intermittency route to period-2 thermoacoustic instability

This paper presents the first numerical evidence of an intermittency route to period-2 thermoacoustic instability in a subcritical single-element liquid rocket engine burning hydrogen peroxide/kerosene as we decrease the equivalence ratio (ϕ) from fuel-rich to fuel-lean. To achieve this, three-dimensional compressible large eddy simulation algorithms combined with the Euler–Lagrangian framework are used. A one-equation eddy sub-grid turbulence model with a partially stirred reactor sub-grid combustion model is employed to simulate the spray turbulent combustion process in a high-pressure liquid-fueled combustor based on open-source platform OpenFOAM. This paper focuses on examining the transition process of the dynamical states in the thermoacoustic system and the synchronization between multiple subsystems. The results indicate that, as the equivalence ratio reduces continuously (1.5 ≤ ϕ ≤ 0.5), the system dynamics shift from period-1 oscillations (ϕ = 1.5) to period-2 oscillations (ϕ = 0.5) via intermittency (1.3 ≤ ϕ ≤ 0.9). Under the equivalence ratio of 0.7 (ϕ = 0.7), a transient mode switching between period-1 and period-2 was also observed. The synchronization processes between the pressure and combustion subsystems in terms of phase-locking and frequency-locking are responsible for the emergence of complex dynamical states. The cycle snapshots analysis also provides more details on the synchronization processes between the pressure and the multiple subsystems, such as vortex dynamics, mixture fraction, and combustion heat release. In summary, this paper sheds light on the complex non-linear thermoacoustic oscillations and the underlying physical mechanisms related to the two-phase flow of spray combustion in liquid rocket engines using three-dimensional large eddy simulations, paving the way for developing passive or active control methods.

Structure and similarity properties of the laminar counterflow spray flame

The spray flame in a laminar counterflow is influenced by various setup conditions, for instance the strain rate and liquid droplet parameters, including the initial temperature, size distribution, hydrodynamics and the vaporisation history. With some reasonable simplification, the two phase governing equations can be reformulated as equations in the single gas phase with reconstructed spray-related source terms. Numerical solutions assume interesting similarity features, such as the (almost) independence of the evaporation path after mapping onto a newly defined quantity R, constructed from sensible enthalpy and mixture fraction. In this regard, the dimensionality of free parameters describing the structure of the spray flame can be hopefully reduced, which then provides a different scenario to understand the spray combustion physics. The flame regime diagram is also elaborated in the present framework.

Spray Flamelet Modeling of Turbulent Two-Phase Reacting Flows with Multi-Component Fuel in a Lean Direct Injection Combustor

ABSTRACT Towards the numerical evaluation of the performance in aero-engine combustors, implementing detailed reaction kinetics into the large eddy simulation (LES) cost-effectively is highly desirable. In this work, a newly-developed spray flamelet model is combined with a four-component jet fuel surrogate model to allow integration of the detailed kinetics in the open-source CFD code. The spray flamelet approach (SFGM) can take into account the non-adiabatic effect caused by the evaporating spray and can directly reflect the preferential evaporation, as well as multiple combustion regimes present in complex spray combustion systems. The coupling of the SFGM-LES approach is assessed by comparison with the experimental data from a Lean Direct Injection (LDI) combustor, where the real aviation kerosene, Jet-A, was utilized. The SFGM-LES approach well predicts the experimental statistics in both gas and liquid phases. It is capable of capturing the rich physics and complex flame structures, including the preferential evaporation effect and the multi-regime combustion phenomenon. The present work demonstrates that the complex nature of multi-components and the multiple-combustion regimes in the two-phase combustion can be directly included in the spray flamelet approach, and thus this model can provide new insights into the turbulent spray combustion phenomena.

Experimental study on the interactions of wall temperature and impingement distances and their effects on the impinged diesel spray ignition and combustion characteristics

This study examines the interactions of wall temperature and impingement distances and their effects on the impinged diesel spray ignition and combustion characteristics experimentally in a modified constant-volume combustion chamber. Four wall temperatures of 570 K, 620 K, 700 K, and 800 K, and four impingement distances that correspond to severe wall-wetting, slight wall-wetting, critical wall-wetting, and non-wall-wetting separately are tested. The results indicate increasing wall temperature can promote auto-ignition and the following combustion process, denoted by shorter ignition delays and higher flame area (FA) and spatially integrated natural luminosity (SINL) values, but the promotional effect generally weakens with the increase of the impingement distance. When the wall temperature is high enough, the spray/wall impingement can accelerate fuel evaporating and mixing with the entrained air, in which case decreasing the impingement distance is positive to auto-ignition and combustion. This trend is basically the opposite in low-wall temperature cases where the cooling effect plays a leading role. Besides, the normal distances from the ignition points to the wall decrease with the increment of the wall temperature, and the changes are more remarkable at lower impingement distances, which means that increasing the wall temperature may enhance auto-ignition and combustion in the near-wall region, probably leading to an enlarged temperature gradient in the boundary layer and strengthened heat transfer.

Influence of Ambient Temperature on the Thermal Safety of Fireworks Products in Civil Aviation Transportation

Fireworks products are energy-containing materials and are hazardous during production, storage, transportation, and use. By analyzing the range of civil aviation ground ambient temperature and civil aviation cabin ambient temperature in storage and ground operation as well as establishing a spontaneous combustion mathematical model for cylindrical fireworks products based on the spontaneous combustion theory, we identified the critical temperature for spontaneous combustion of a single spray and analyzed the thermal safety of fireworks products under the civil aviation ambient temperature by example to provide theoretical support for the feasibility study of transporting fireworks products by civil aviation.

Analysis on roles of thermal radiation to evaporation and combustion of fuel droplets

Thermal radiation is a significant mode of energy transmission besides conduction and convection in most combustion systems. Accurate treatment of thermal radiation is a crucial element for predicting the flame structure and pollutant formation. However, thermal radiation to fuel droplet evaporation is often neglected in the numerical simulations of two-phase spray combustion systems. In this paper, computational analysis on the thermal radiation to evaporation and combustion of fuel droplets in two-phase spray combustion is conducted. The influence of flame radiation on droplet evaporation and combustion is studied by adding radiation source term to the energy equation describing droplet evaporation process. Radiation transfer equation is solved, in which the radiation of CO2, H2O and soot is imposed to consider radiation effect of the flame. The partially premixed combustion model is employed, in which radiation heat transfer between droplet and ambient gases is taken into account. A reduced chemical reaction mechanism for n-heptane consisting of 76 species and 349 reactions is used. Four different numerical setups have been used and compared, and the simulation results coincide with the experiment data in terms of spray statistics and flame shapes. The peak flame temperature increases by 28 K when the radiative heat source term in the evaporation process is considered. The droplet evaporation is significantly enhanced by thermal radiation, leading to a promotion of 34.8% in C7H16 concentration in the center region of combustor. The concentration of combustion products, such as H2O and CO2, increases by 13.2% in the reaction region.

Development of a low-cost pseudo-fuel spray combustion chamber for gas injection analysis of two-stroke dual-fuel engines

The present work is aimed to model, design, build and experimentally characterize a low-cost pseudo-fuel spray combustion chamber (SCC) for gas injection analysis of two-stroke dual fuel engines. The novel SCC, operating at low pressure and with a pseudo-fuel, was capable to reproduce, under equivalent fluid-dynamic conditions, the operating behavior of a full-scale engine combustion chamber. A mathematical model, based on AVL Boost®, was developed to simulate pressure, mass flow rate, velocity and momentum rate within the system. Then, according to these numerical results, the SCC was built and experimentally characterized. Its main element was a velocimetric chamber provided with four optical accesses to perform imaging analysis (PIV and LIF) on pseudo-fuel sprays. In conclusion, this article may be a useful guide to build a new type of low-cost pseudo-fuel spray combustion chamber for gas injection analysis of two-stroke dual-fuel engines, avoiding dangerous (because of high pressures and temperatures involved) and expensive full-scale studies.

The general formulation of the energy equation and the impact of enthalpy diffusion on multi-component droplet heat and mass transfer

A key aspect of modeling the heat and mass transport of droplets is the description of diffusion transport, and multiple approaches exist to address it. While many contributions focus on the description of the mass transfer, details about the energy formulation to describe the droplet’s heat transfer are seldom developed in the literature. This work demonstrates that a general gas-phase energy formulation can be obtained independently from the species diffusion transport modeling while incorporating multi-species enthalpy diffusion terms with a simple expression. Considering the ideal gas simplification, it is shown that a simple choice of specific heats is sufficient to characterize the inclusion or not of multi-species enthalpy diffusion effects. Furthermore, it is also demonstrated that the final energy expression degenerates in the single-component limit to the same energy formulation as in the model of (Abramzon and Sirignano, Int. J. Heat and Mass Trans, Vol.32(9), pp.1605–1618, 1989), considered as a reference in the literature. Simulations are conducted for droplets of multiple fuel compositions in varied atmospheric conditions to represent spray combustion scenarios and evaluate the impact of incorporating or not the enthalpy diffusion term, and conclude on the need and ease of use of the proposed energy formulation.

Four-step global kinetics mechanism for diluted combustion fueled with kerosene

The aim of the present study was to develop a global kinetics mechanism capable of predicting characteristics of the combustion of kerosene diluted with combustion gases. A four-step model of the Aachen mechanism was developed via modification of a mechanism for lower molecular weight hydrocarbons. A simplified fuel was used to simulate kerosene, the thermodynamic properties of which were estimated based on the surrogate fuel in the Aachen mechanism. Combustion of this model fuel in a perfectly stirred reactor (PSR) operating under high-temperature air combustion (HiTAC) conditions was simulated using inflow mixtures preheated and diluted with nitrogen. In this new mechanism, the activation energy for the fuel consumption reaction and the pre-exponential factor for the hydrogen oxidation reaction were adjusted. The resulting model accurately predicted variations in temperature and major species over time. The four-step mechanism was also validated under conventional flame and moderate or intense low oxygen dilution (MILD) combustion conditions. The PSR simulations with the four-step mechanism accurately predicted variations in temperature and major species under MILD combustion conditions. In addition, this four-step mechanism was confirmed to be superior to other global mechanisms with regard to predicting the mole fractions of various species. Experimental trials were carried out with diluted kerosene flames formed above parallel jet burners composed of a fuel spray nozzle and an oxidizer nozzle, and it was found that NOx emission decreased with increasing distance between nozzles even for spray combustion. These diluted flames were numerically simulated using the eddy dissipation concept model combined with the four-step mechanism. The temperature distributions obtained from the simulations showed good agreement with the experimental data in the downstream region.