Spray Flame Research Articles

Experimental investigation on spark ignition and flame propagation of swirling kerosene spray flames

The ignition of kerosene spray in an aero-engine combustor is complex due to the fuel injection, mixing, and combustion to occur within limited time and space scales. Fundamentals of the gas–liquid two-phase ignition with realistic flow patterns remain virtually unexplored. In our study, a well-defined simplified combustor was designed to investigate the spark ignition of swirling kerosene spray flames. Non-contact optical diagnosis on flow filed and spray atomization shows that a large central recirculation zone and two small side recirculation zones, filled with small kerosene droplets below 60 μm, are formed inside the combustor with swirling air above 300 SLM. The mixture of kerosene and air in the combustor is highly sensitive to the injection pressure and air velocity. The ignition characteristic and flame behavior in four locations were compared. Ignition place at the wall where is corresponding to the maximum width of the recirculation zone is more likely to result in full flame. High-speed CH* chemiluminescence indicates that successful ignition sparks are associated with flame kernels that move upstream towards the spray nozzle. A shrinking flame kernel in failed event extinguishes after a few milliseconds. Different propagation paths after the spark depend on the ignition location, which can be understood from the flow field and the spray atomization.

Combustion performance in a cavity-based combustor under subatmospheric pressure

Combustion performance and flame stabilization mechanism of the flameholder under the subatmospheric pressure are the foundation for the design of high-altitude operating aircraft. An experimental cavity-based combustor was developed to explore the combustion performance in low pressure. Four low pressures of 0.03, 0.04, 0.05 and 0.06 MPa were validated to investigate the effect of inlet pressure on the flame stability limits, lean ignition and blowout process, combustion efficiency, and outlet temperature profiles. A unique lean spark ignition process was discovered at 0.03 MPa that the flame downstream of the V-type flameholder promotes the formation of the piloted flame in the cavity. Results indicate that the pressure reduction decreases the spray flame propagation, subsequently weakening flame stability and combustion efficiency. With the inlet pressure decreases, the pilot fuel injected into the cavity still maintains effective combustion, while the declining combustion is achieved in the mainstream fuel. This phenomenon suggests that increasing the pilot fuel proportion in the combustion chamber will enhance the combustion in ultra-low pressure.

Effects of evaporation on chemical reactions in counterflow spray flames

The role of evaporation on chemical reactions in counterflow spray flames remains a key issue due to the non-linear inter-phase interactions through mass, momentum, and energy conservation. An extended chemical explosive mode analysis to illustrate the role of evaporation (ECEMA) was previously developed by separating the evaporation source term from non-chemical terms and projecting them onto the chemical explosive mode (CEM). Evaporation was found to promote chemical reactions when the fuel supply effect dominated the evaporative cooling and to inhibit reactions otherwise. In this work, ECEMA is applied to one-dimensional laminar and three-dimensional turbulent counterflow spray flames. For laminar cases, ECEMA is applied to multi-modal spray flame solutions including the distributed, collocated, and cool flame. The analysis for the collocated and cool flame shows similar behavior, namely, that evaporation inhibits chemical reactions due to heat absorption near the fuel injection region and enhances chemistry around the zero-crossing CEM. For the distributed flame, the promotion effect is observed for most of the domain. For the turbulent counterflow flame, three CEM regions are identified, namely, a hybrid region, an inhibition region, and a promotion region. In particular, for the hybrid region near the spray injection side, the contribution of evaporation to chemical reactions changes from inhibition to promotion. For the inhibition region in the middle, significant suppressing of chemical reactions by evaporation is observed. In the promotion region near the oxidizer side, evaporation remains promoting chemical reactions. For the case investigated, the dominant combustion modes are the assisted ignition and local extinction.

Numerical prediction of the ignition probability of a lean spray burner

The optimization of the igniter position is a critical issue in modern aviation gas turbines since it can help to minimize the amount of energy required for ignition and to guarantee a fast relight in case of flameout. From a numerical perspective, several spark discharges should be simulated for each spark position, to account for different realizations due to time-dependent turbulent motions. Unfortunately, standard simulations are impractical to use for this purpose, due to the need of carrying out several unsteady simulations, leading to a huge associated computational effort. This is why low-order models have been developed, providing an affordable estimation of the local ignition probability, by sacrificing the accuracy and the physical consistency of the prediction. In the present work, a previously developed low-order design model has been implemented in ANSYS Fluent 2019R1® and used to investigate the ignition performance of a single-sector, confined spray flame, where data from laser ignition experiments are available. A non-reactive Large Eddy Simulation, which is validated against experimental data, provides the base flow needed to feed the model. If the tuning parameters of the ignition model are well calibrated, it provides quite good results. In the test case here investigated, it is shown that ignition is possible in the outer recirculation zone and very unlikely elsewhere. Later, a discussion about the effect of the most relevant tuning parameters is carried out. It is shown that the model mostly succeed to identify the area of possible ignition, even if the lack of calibration could lead to a poorer agreement with the experimental data.

Polydisperse spray flame ignition by a pulsed heat flux

A mathematical analysis of the ignition of a polydisperse spray/air mixture by an infinite surface heated in a pulsed manner is presented. In contrast to previous work in the literature, the entire history of the ignition process is accounted for starting from the flame-embryo progenitor stage, through the thermal runaway stage to the final flame propagation stage. For tractability at the current stage, the chemical kinetics is taken to be that of a single global reaction. The spray is modeled using the sectional approach and the influence of fuel spray characteristics on ignition is determined. Good agreement was found between the theoretical predictions and full numerical simulations. Delay in ignition due to the build-up of vapor from the fuel droplets as well as heat loss to the droplets for evaporation are found to play a significant role under certain operating conditions. Comparison between the critical energy flux and the initial spray polydispersity revealed small differences for larger values of the pulse duration but more significant minor differences for smaller pulse durations. Despite these seemingly minor differences, it was shown that the initial spray polydispersity can have a critical influence on whether flame ignition will occur or fail, even for sprays having the same initial SMD.

Experimental assessment of the impact of variation in jet fuel properties on spray flame liftoff height

Ensuring efficient and clean combustion performance of liquid-fueled engines requires comprehension of the influence of fuel composition and properties on flame behavior, such as flame liftoff height (LOH) and lean blowout limit (LBO). Spray flame stability is strongly affected by both the fuel reactivity and physical properties. Herein, the flame stability mechanism represented by LOH is investigated for seven jet turbine fuels, including surrogate, alternative, and conventional jet fuels, using a laboratory spray burner. Based on the experimental observations, the current work introduces a new analysis, which provides insight into the competing/complementing processes that occur in a multi-phase reacting system and highlights the key properties important in spray flame dynamics, accounting for both the fuel spray/vaporization as well as the chemical reactivity, to explain the relative differences in LOH of complex multicomponent fuels. Results show that spray flame stabilization occurs when there is a balance between the local spray burning velocity and the incoming jet velocity, which is strongly associated with laminar flame speed and the relative amount of liquid and gaseous fuel crossing the flame preheat region. Using a multicomponent droplet evaporation model, it was observed that preferential vaporization of the lighter and more reactive species of the simple surrogate fuels contribute to a shorter lifted flame as compared to fuels consisting of heavier and/or less reactive components. The LOH of real jet fuels showed strong sensitivity to droplet vaporization and mixing, which is controlled by the fuels’ volatility (i.e., boiling temperature at 50% distillation volume) and atomized droplet size, and to a lesser extent the reactivity represented by laminar flame speed. The enhanced linearity in the correlation of 50% vapor distilled compared to the other distillation cuts suggests that preferential vaporization could play an important role in defining the LOH stabilization mechanism even for more complex fuels.

Morphology, composition and optical properties of jet engine-like soot made by a spray flame

Particulate matter (PM) and soot emissions from aviation are a major source of pollution. Reference soot is needed for calibration of optical instruments used for measurements of non-volatile PM (nvPM) from jet engines with average mobility, d¯m, and primary particle, d¯p, diameters less than 70 and 20 nm, respectively, mass mobility exponent (Dfm) of 2.5±0.15, elemental to total carbon ratio (EC/TC) larger than 0.8, and mass absorption cross section (MAC) of 7.46±0.27m2/g at 532 nm. Such particles are difficult to make with gas-fueled soot generators using laminar flames with high temperature particle residence times quite different from those of jet engine combustors. Here, a flame spray pyrolysis (FSP) burner is used to generate soot agglomerates from turbulent flames made by spraying liquid jet fuel. The d¯m of FSP-made soot agglomerates is modified from less than 13 to more than 91 nm by changing common process parameters while agglomerates maintain EC/TC > 0.8. The FSP-made soot agglomerates with Dfm~ 2.52±0.2 have effective densities similar to emissions from turbofan and turboshaft engines and MAC=8.23 and 5.21(m2/g) at 532 and 870 nm, respectively, in excellent agreement with recent measurements of nvPM emissions from jet engine turbines.

Characterization of tracers for two-color laser-induced fluorescence thermometry of liquid-phase temperature in ethanol, 2-ethylhexanoic-acid/ethanol mixtures, 1-butanol, and o-xylene.

The fluorescence spectra of dye solutions change their spectral signature with temperature. This effect is frequently used for temperature imaging in liquids and sprays based on two-color laser-induced fluorescence (2cLIF) measurements by simultaneously detecting the fluorescence intensity in two separate wavelength channels resulting in a temperature-sensitive ratio. In this work, we recorded temperature-dependent absorption and fluorescence spectra of solutions of five laser dyes (coumarin 152, coumarin 153, rhodamine B, pyrromethene 597, and DCM) dissolved in ethanol, a 35/65 vol.% mixture of ethanol/2-ethylhexanoic acid, ethanol/hexamethylsiloxane, o-xylene, and 1-butanol to investigate their potential as temperature tracers in evaporating and burning sprays. The dissolved tracers were excited at either 266, 355, and 532 nm (depending on the tracer) for temperatures between 296 and 393 K (depending on the solvent) and for concentrations ranging between 0.1 and 10 mg/l. Absorption and fluorescence spectra of the tracers were investigated for their temperature dependence, the magnitude of signal re-absorption, the impact of different solvents, and varying two-component solvent compositions. Based on the measured fluorescence spectra, the tracers were analyzed for their 2cLIF temperature sensitivity in the respective solvents. Coumarin 152 showed for single-component solvents the overall best spectroscopic properties for our specific measurement situation related to temperature imaging measurements in spray-flame synthesis of nanoparticles as demonstrated previously in ethanol spray flames [Exp. Fluids61, 77 (2020)10.1007/s00348-020-2909-9].

CFD modeling of pyrolysis oil combustion using finite rate chemistry

This paper reports the first Computational Fluid Dynamics (CFD) model developed for biomass pyrolysis oil spray combustion using Finite-Rate Chemistry (FRC) approach. To make the CFD calculations feasible, a reduced mechanism for modeling the combustion of biomass Fast Pyrolysis Oil (FPO) based on the POLIMI 1412 mechanism and a model for eugenol oxidation was developed. The reduced mechanism consisted of 200 reactions and 71 species. This level of complexity was found to be a good tradeoff between predictive power and computational cost such that the reduced model could be used in CFD modeling. The predictive power of the reduced mechanism was demonstrated via 0D (adiabatic, premixed, constant pressure reactor), 1D (laminar counterflow flame) and 3D (CFD of a methane-air flat-flame piloted FPO spray flame) calculations. Results from CFD were compared against experimental data from non-intrusive optical diagnostics. The reduced model was successfully used in CFD calculations—the computational cost was approximately 2 orders of magnitude higher than that of a simplified model. Using the reduced mechanism, the concentration of pollutants, minor combustion products, and flame radicals could be predicted—this is added capability compared to already existing models. The CFD model using the reduced mechanism showed quantitative predictive power for major combustion products, flame temperature, some pollutants and temperature, and qualitative predictive power for flame radicals and soot.

Influence of angled dispersion gas on coaxial atomization, spray and flame formation in the context of spray-flame synthesis of nanoparticles

Liquid atomization determines the initial conditions for flame formation and particle synthesis. Without a stable flame, high droplet velocities and thus short droplet residence time in the flame may lead to droplets being injected into an extinguished flame, which influences synthesis and final particle output. An experimental investigation of spray formation and flame stability is performed through high-speed visualization. Targeted variation of nozzle geometry is applied to improve spray-flame interaction and compared to a standardized burner. Timescales of spray density and flame fluctuations are quantified and compared, where the latter were significantly larger and hence not correlated. Instead, dispersion gas forms a barrier between spray phase and pilot flame; hence, ignition depends on large liquid lumps with high radial momentum to break through the dispersion gas for spray ignition. Angling of dispersion gas flow increases radial shear and turbulence and leads to refined atomization and improved flame stability. To investigate the nozzle influence on particle formation, particle characteristics are examined by online and offline analytics with focus on particle structures and product purity. The modified nozzle produced smaller primary particle sizes, thus indicating a sensitivity of sintering dominance on the nozzle geometry. Impurities impact the examination of particle structures and general particle functionality. Carbon contamination was apparent in synthesized particles and also indicated sensitivity to nozzle geometry. Discrepancies to literature data are discussed regarding differences in flame activity and droplet characteristics. The report highlights, how product characteristics can differ crucially due to changes in nozzle geometry despite comparable operating conditions.Graphic abstract

An optical study on spray and combustion characteristics of ternary hydrogenated catalytic biodiesel/methanol/n-octanol blends; part П: Liquid length and in-flame soot

Methanol has been considered as a promising alternative fuel for combustion engines. However, it is quite challenging to use it directly in compressed ignition engines because of its ignition issues, especially for low load conditions. In this research, methanol was blended with hydrogenated catalytic biodiesel (HCB) using n-octanol as co-solvent. A fundamental study on spray and combustion characteristics of two ternary blends (68% HCB+17% octanol+ 15% methanol by volume; 58% HCB+17% octanol+ 25% methanol by volume) and the pure HCB was carried out within a constant volume combustion chamber equipped with a single-hole injector. As in Part Ⅰ, the spray morphology, ignition delay, and flame lift-off length have been studied in detail. This part focuses on liquid length and in-flame soot formation of reacting sprays, which were quantified utilizing a diffused back-illumination extinction imaging technique. There is an overlapping area between the fuel liquid phase and flame for all three fuels under all operating points. The reduction on liquid length after ignition is more noticeable for pure HCB than other blends, because of its shorter lift-off length. The results show that the liquid length increases with increasing fraction of methanol in the mixtures, which is mainly governed by the high latent heat of vaporization of methanol. Furthermore, extra methanol addition brings a considerable reduction on in-flame soot production because of the leaner fuel combustion at longer flame lift-off length, as well as the more oxygen content within the blends.

A Novel Piston Insulation Technique to Simultaneously Improve Both Heat Loss and Thermal Efficiency for Diesel Engines

<div class="section abstract"><div class="htmlview paragraph">This study investigates simultaneous improvement in thermal efficiency and cooling loss in the wider operating condition. To suppress the heat flux of the piston, the piston top and cavity were treated with thin thermal spraying of stainless steel. Thermal diffusivity of stainless steel (X5CrNiMo17-12-2, SUS316) is very low in comparison with the forged steel piston raw material (34CrMoS4, SCM435) to sustain local surface temperature at where spray flame directly interfered. In addition, its surface roughness was very fine finished aiming to reduce the convective heat transfer. The experimental results with the stainless-steel coated piston by utilizing a single cylinder engine showed the significant improvement in both cooling loss and thermal efficiency even in higher load operating conditions with compression ratio of 23.5:1. From the analysis of the experimental result, it is assumed that not only convective heat transfer but radiative heat transfer could play an important role in the cooling loss mechanism.</div></div>

Infrared high-speed thermography of combustion chamber wall impinged by diesel spray flame

As a demonstration of a new method to examine the extremely unsteady and spatially varying wall heat transfer phenomena on diesel engine combustion chamber wall, high-speed imaging of infrared thermal radiation from the wall surface impinged by a diesel spray flame was attempted using a high-speed infrared camera. A 35 mm-diameter chromium-coated quartz window surface was impinged by a diesel spray flame with an impinging distance of 27 mm from the nozzle orifice in a constant volume combustion chamber. The infrared thermal radiation from the back surface of the 0.6 µm thick chromium layer was successfully visualized at 10 kHz frame rate and 128 × 128 pixel resolution through the quartz window. The infrared radiation exhibited coherent and streaky structure with radial stripes extending and waving from the stagnation point. The width of the radial stripes, spatial amplitude and the period of the waving movement were comparable to the ones for turbulent heat transfer on the engine cylinder wall previously measured with a heat flux sensor, suggesting that they are resulting from the turbulent structure in the wall-impinging diesel flame. The radiation intensity was calibrated to temperature and converted to heat flux via 3-D numerical analysis of transient thermal conduction in the quartz window. The peak-to-peak variation amplitudes of temperature and heat flux among the radial stripes during the diesel spray flame impingement were about 20 K and 2.3 MW/m2, corresponding to 13% of 150 K maximum temperature swing amplitude and 18 MW/m2 maximum heat flux, respectively.

Large Eddy Simulation of an Ethanol Spray Flame with Secondary Droplet Breakup

A computational investigation of three configurations of the Delft Spray in Hot-diluted Co-flow (DSHC) is presented. The selected burner comprises a hollow cone pressure swirl atomiser, injecting an ethanol spray, located in the centre of a hot co-flow generator, with the conditions studied corresponding to Moderate or Intense Low-oxygen Dilution (MILD) combustion. The simulations are performed in the context of Large Eddy Simulation (LES) in combination with a transport equation for the joint probability density function (pdf) of the scalars, solved using the Eulerian stochastic field method. The liquid phase is simulated by the use of a Lagrangian point particle approach, where the sub-grid-scale interactions are modelled with a stochastic approach. Droplet breakup is represented by a simple primary breakup model in combination with a stochastic secondary breakup formulation. The approach requires only a minimal knowledge of the fuel injector and avoids the need to specify droplet size and velocity distributions at the injection point. The method produces satisfactory agreement with the experimental data and the velocity fields of the gas and liquid phase both averaged and ‘size-class by size-class’ are well depicted. Two widely accepted evaporation models, utilising a phase equilibrium assumption, are used to investigate the influence of evaporation on the evolution of the liquid phase and the effects on the flame. An analysis on the dynamics of stabilisation sheds light on the importance of droplet size in the three spray flames; different size droplets play different roles in the stabilisation of the flames.

Development of a Multiphase Flamelet Generated Manifold for Spray Combustion Simulations

Abstract In this study, a model flame of quasi-one-dimensional (1D) counterflow spray flame has been developed. The two-dimensional (2D) multiphase convection-diffusion-reaction equations have been simplified to one dimension using similarity reduction under the Eulerian framework. This model flame is able to directly account for nonadiabatic heat loss, preferential evaporation, as well as multiple combustion regimes present in realistic spray combustion processes. A spray flamelet library was generated based on the model flame. To retrieve data from the spray flamelet library, the enthalpy was used as an additional controlling variable to represent the interphase heat transfer, while the mixing and chemical reaction processes were mapped to the mixture fraction and the progress variable. The spray flamelet generated manifolds (SFGM) approach was validated against the results from the direct integration of finite rate chemistry as a benchmark. The SFGM approach was found to give a better performance in terms of predictions of temperature and species mass fractions.

Flame self-interactions in an open turbulent jet spray flame

A three-dimensional direct numerical simulation database of an open turbulent jet spray flame representing a laboratory-scale burner configuration has been analyzed to investigate flame self-interactions (FSIs) in the presence of flow induced shear, to the best of the authors' knowledge, for the first time. The FSI occurrences [i.e., unburned gas mixture pockets (UBGPs), tunnel formations (TFs), tunnel closures (TCs), and burned gas mixture pockets (BGPs)] have been identified across the flame at different axial locations. It has been found that the interplays between turbulence, droplet evaporation, and chemistry have a significant influence on the topological nature of the flame surface. Close to the jet exit, the FSI events are found to occur toward the burned gas side of the flame, but moving further away from the jet exit, there are significant occurrences of FSI events within the flame where increasingly fuel-rich, low Damköhler number conditions occur. In this study, the FSI events have been found to be predominantly TFs and TCs, which is consistent with previous analyses of turbulent premixed flames and combustion of droplet-laden mixtures. However, non-negligible occurrences of UBGPs and BGPs are also observed in this case. The results obtained from this analysis have important implications from a modeling perspective where flame topologies have a significant influence on the nature of the flame surface, which will, in turn, affect the flame-surface based modeling approaches. Accordingly, the findings of the current analysis may need to be accounted for during the development of flame surface-based closures in the context of turbulent spray flames.

Probability density functions for fluctuations in turbulent two-phase flames

Probability density function methods for characterizing various fluctuating quantities in the reacting gas phase of turbulent spray flames are presented, for the development of which O'Brien has been a pioneer. Balance equations for two-phase flows considered as a piecewise continuous medium are first presented with the interface conditions containing the vaporization–condensation equilibrium. From these, the equation for the joint probability density function of temperature and composition in the gas phase is derived, in which specific terms related to vaporization are identified. The terms that require modeling are then highlighted. The first one is the classical micromixing term, which has to include all the features already discussed for single phase reacting flows, but adaptation to two-phase flows has to be questioned. Mainly, several unclosed terms deal with vaporization, requiring specific attention. It is shown that proposals for their modeling can be obtained on the same basis as in the so-called Eulerian–Lagrangian models, which will directly induce changes in the shape of the probability density function. We finally discuss the model presented herein and comment on its connection with recently published literature for this subject. Specific attention is devoted to the combustion regimes and to the presence of diffusion flamelets surrounding fuel droplets or groups of droplets. The proposals presented here go one step further compared to the recent studies, and now, they have to be compared with well-defined experiments, numerical experiments with direct numerical simulations, or real experiments in conveniently controlled conditions.

An Experimental Study on Kerosene Spray Combustion Under Conventional and Hot-Diluted Conditions

ABSTRACT The combustion of kerosene spray under hot-diluted conditions and conventional conditions was experimentally investigated. By examining flame photographs, chemiluminescence images, and in-field temperature measurements, the separate effect of different variables including oxygen concentration, temperature and velocity of the co-flowing air, fuel flow rate and injection pressure, and eventually the type of spray nozzle on multiple parameters such as flame stability, structure, luminosity, temperature field, and qualitative CH radical distribution, as well as HCO and NO2 with lower precision, in the reaction region, have been studied. It was observed that an increment in injection pressure and co-flow temperature enhances the spray flame stability, while dilution exacerbates it. Also, a solid cone spray pattern with a lower spray angle has better stability than hollow cone ones with a higher spray angle. Moreover, it was noted that liquid fuels, compared to gaseous fuels, require higher preheating temperatures, for the same dilution level, to engender a stable flame. For combustion of spray in conventional conditions, a double-flame structure was observed consisting of a bluish section at the leading edge emerging into a yellowish sooting trail. An increase in co-flow velocity, as well as injection pressure, strengthens the inner flame front, whereas raising the co-flow temperature or diluting the oxidant, deteriorates the inner flame front. In the case of highly preheated air (without dilution), the flame liftoff height is reduced to as close to the atomizer as a few millimeters, forming a single flame structure similar to gaseous flames. In this case, the peak temperature was considerably higher than the conventional combustion, yet the gain was much lower than the preheating level. Combined effects of preheating and dilution alter the spray flame structure in a way that the flame volume is reduced, the temperature field has become more homogeneous, the peak temperature is limited to less than 1500 K, and temperature fluctuations have significantly decreased, seemingly approaching MILD combustion regime conditions.

Insights into near nozzle spray evolution, ignition and air/flame entrainment in high pressure spray flames

Experimental investigations were performed to study the dynamic processes related to early development of high pressure sprays and their effects on ignition, local soot formation and its entrainment as well as spray to spray interactions in a six-hole common rail diesel injector. These spray-flame measurements were performed using ultra-high speed imaging technique in an optically accessible constant volume chamber maintained under reactive high pressure environment. The acquired back scattered light from fuel droplets and the broadband natural soot luminosity images and their analysis have revealed new insights on how the radial dispersions of fuel sprays during its early development at high injection pressures affect the ignition, flame stabilisation and soot formation in diesel spray flames.Analysis of the high speed data have shown that radial spreading also called as bulge in this paper occurs during the very early stages of fuel spray development closer to nozzle. The cloud of finely dispersed fuel droplets trapped within these bulges loses its moment and evaporates quickly relative to liquid core of the spray to form a locally ignitable mixture and this initiates ignition from these locations. Local flame kernels formed from these local ignition sites tend to develop into small pockets of sooting flame, which subsequently moves counter to the main spray towards the nozzle and gets entrained into the liquid core of spray. At times, these sooting pockets are also transported to into the next spray when the bulges are large. The sooting flames that are transported towards the nozzle and into the liquid spray core are quenched upon its interaction with the core of liquid spray. Quenched flames are rich in unburnt hydrocarbon and soot particles and they have the potential to significantly impact the pollutant formation through altering the equivalence ratio within the core of fuel spray. Wide variations in the sizes were observed on the formation of bulges, and it also varied between sprays from different orifices of a multi-hole injector. These anomalies are mainly related to the complexity of nozzle sac flow affecting the early spray development.

Combustion characteristics of n-heptane spray combustion in a low temperature reform gas/air environment

This paper presents a large eddy simulation study of n-heptane spray combustion in an n-heptane low temperature reform (LTR) gas environment in a constant volume combustion chamber, under conditions relevant to single-fuel reactivity controlled compression ignition (RCCI) combustion engines. The LTR gas is made up of partially oxidized intermediate species from rich n-heptane/air mixture in an external constant temperature reformer. It is found that a higher reform temperature results in a longer ignition delay time of the n-heptane spray and a higher liftoff length, due to the chemical effect of the LTR gas and the difference in the reaction zone structures. A significantly different spray flame structure is identified in the RCCI case from that of single-fuel spray combustion. After the onset of high temperature ignition, a double-layer flame structure is established in the RCCI case, with a diffusion flame layer and a lean premixed flame layer. The lean premixed flame affects the flow field, which significantly suppresses the mixing around the spray tip. As a result, the RCCI case exhibits a lower NOx formation but a higher soot formation than the single-fuel case.