Spray Flame Structure Research Articles

Study of methanol spray flame structure and combustion stability mechanisms by optical phenomenology and chemical kinetics

Methanol is a very promising clean alternative fuel, and the study on pure methanol spray flames has not been found before to authors' knowledge. In this study, the flame structure, soot formation, and combustion stability of spray flame for methanol and conventional fuels were investigated at the ambient pressure of 4 MPa and the ambient temperature of 950 K by the means of optical phenomenology, theoretical analysis and chemical kinetic simulations. Optical images reveal that the methanol spray auto-ignition location and the established steady lifted flame is away from the injector nozzle compared to that of isooctane and n-heptane. The results of the RGB two-color method show that the KL factor in the methanol spray flame is only 0.04–0.05, which is one twentieth of n-heptane and isooctane. The combustion stability of methanol increases with increasing injection mass. Methanol spray flame is the most unstable which is 13.33% and 151.58% less than that of the spray flame of isooctane and n-heptane respectively. In chemical kinetic simulations, the appropriate region for methanol high temperature reactions is outside the relative dense region. The residence time of the rich mixture is short and may not come into contact with the supporting peripheral hot product pool, which leads to poor combustion stability.

Multiple structures and transition mechanisms of laminar fuel-rich ethanol/air counterflowing spray flames

Structures of laminar non-premixed ethanol/air spray flames in the axisymmetric counterflow configuration are studied under fuel-rich conditions by means of numerical simulations. The monodisperse ethanol spray is carried by air and directed against an air stream. Both streams enter at 300 K, and the system is at atmospheric pressure. Up to three different structures of these flames for identical boundary and initial conditions are identified, and regime diagrams are presented that show their conditions of existence in terms of the gas strain rate on the spray side of the configuration, [Formula: see text], starting from 55/s at an initial spray velocity of 0.44 m/s. The equivalence ratio on the spray side, [Formula: see text], is varied between 1.1 and 1.6, and initial droplet radii, [Formula: see text], from 10 to 50 [Formula: see text]m are considered. The most stable spray flame structure is characterized by two chemical reaction zones. For some conditions, single chemical reaction zones on either side of the counterflow configuration are found. Conditions under which these different flame structures exist are analyzed. Previous studies identified only two different structures for non-identical boundary conditions, and in this study, three different structures are presented for the first time. Moreover, the transition mechanisms of one structure to another are analyzed. The competition between the energy-consuming spray evaporation and the exothermic chemical reaction rates as well as the location of the spray determines the existence of the different flame structures. This transition of the different flame structures may explain spray flame characteristics such as flame pulsation or flame instabilities.

Numerical simulation of TTIP/[formula omitted]-xylene precursor solution spray flames in the counterflow configuration

Sprays of precursor solutions are used in flame spray pyrolysis (FSP) to produce functional nanoparticles with specific properties. The present study concerns the precursor titanium(IV) isopropoxide (TTIP) in p-xylene. A dilute monodisperse spray carried by an air stream is directed against an air stream to establish a counterflowing laminar spray flame. The complex thermophysical properties of the bi-component droplets are incorporated into the code as well as the detailed chemical reactions of the system with 213 chemical reactions among 52 species; the chemical reaction mechanism includes thermal decomposition of the TTIP and the combustion of p-xylene. The droplets are modeled using the multicomponent evaporation model. Drag and gravity forces are considered in the equation of droplet motion. Monodisperse spray flame structures are presented depending on the gas strain rate, the initial droplet size, the precursor mass loading as well as the equivalence ratio. The preferential evaporation of the more volatile p-xylene provides the gaseous fuel for combustion, and the thermal decomposition of the precursor TTIP considerably reduces the flame temperature.

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.

Large Eddy Simulations of complex multicomponent swirling spray flames in a realistic gas turbine combustor

Large Eddy Simulations of the realistic liquid fueled gas turbine combustor LOTAR operated at ONERA are performed for two fuels; a conventional JetA-1 and an alternative alcohol to jet fuel At-J, each modeled by a 3-component formulation. JetA-1 is composed of n-dodecane, methyl-cyclohexane and xylene each corresponding to the major hydrocarbon families found in real fuel. At-J is a synthetic drop in fuel composed of only branched chain alkanes, iso-octane, iso-dodecane and iso-hexadecane. Analytically Reduced Chemistry and multicomponent spray evaporation model coupled to the dynamic thickened flame turbulent combustion model are employed to understand the processes involved in turbulent spray flames in the LOTAR configuration. The objectives are to predict and understand the potential effects of staged vaporisation and consumption of the fuel components, and their impact on the spray flame structures. Simulations confirm the role of preferential evaporation in establishing and stabilising the reaction zone. JetA-1 evaporation zones extend deep into the rich burnt gasses resulting in a combustion regime with the possibility of droplet clusters burning individually. At-J which is more volatile, leads to complete combustion with the majority occurring due to the premixed lean reactions of the smaller pyrolysed components. The need to further include models capable of identifying and handling combustion regimes encountered in such spray flames is hence highlighted. This work is intended as a starting point for improving multicomponent spray modelling and requires additional experimental data for validation.

A priori and a posteriori studies of a novel spray flamelet tabulation methodology considering evaporation effects

Turbulent spray flames are complicated combustion configurations involving many coupled processes including atomization, evaporation, mixing, chemical reactions, etc. Different flamelet models with intrinsically high computational efficiency have been proposed and extended to model spray combustion. One popular approach for flamelet modeling is based on the gaseous flamelet library, but the accuracy is not guaranteed for combustion with gas–liquid two-phase flows. The other approach is based on the spray flamelet library that takes into account the effects of evaporation on the flamelet structure, but problems related to the non-monotonicity of mixture fraction occur. We develop a novel tabulated spray flamelet/progress variable (TSFPV) model in the present work. An additional mixture fraction Zcarr (named carrier mixture fraction) is defined to describe the mixing process, and it proves to avoid the problem of non-monotonicity of the spray flame structure. The empirical formulas of mass and energy evaporation source terms are obtained and then applied to one-dimensional flames in order to create the flamelet library considering evaporation effects. The developed TSFPV model is validated via simulations of laminar spray counterflow flame and turbulent spray reactive jet in cross-flow, where strong interactions between spray droplets and flame occur and the traditional Flamelet/Progress Variable (FPV) model fails. The a priori and a posteriori studies of TSFPV model are conducted, and the model shows to achieve good agreement with the detailed chemistry solutions. The double-reaction spray flame structure can be captured correctly. The TSFPV model can also give sound quantitative predictions for temperature and minor intermediate products (such as OH, C2H2, and C2H4).

Multiple structures of laminar fuel-rich spray flames in the counterflow configuration

Multiple structures of laminar non-premixed ethanol/air spray flames in the counterflow configuration are presented which exist under fuel-rich conditions and moderate gas strain rates. A mono-disperse liquid fuel spray with carrier gas air is directed against an air stream. Two different spray flame structures are found for identical boundary and initial conditions: one of these flame structures consists of two different chemical reaction zones which reside on either side of the gas stagnation plane. In the second case, the spray-sided flame does not exist and combustion occurs entirely on the gas side of the configuration with largely distinct evaporation and combustion zones. Parametric studies of equivalence ratio between 1.1 and 1.8 and gas strain rates on the spray side of the configuration from 55/s as well as initial droplet size between 10 µm and 50 µm are performed. A regime diagram is presented to display the range of existence of the multiple/single structures as well as the flame extinction. Physical reasons for the existence and breakdown of the multiple structures are explored and discussed. The spray flame structure with a single chemical reaction zone breaks down with increased strain rate when the chemical time scales of OH and fuel vapor are equal at the crossover temperature (Tcross=1104 K) of the reaction rate constants of H + O2⇌ OH + O and H + O2 + M →HO2 + M.

Dilute spray flames of ethanol and [formula omitted]-heptane in the transition to mild combustion

The structure and stabilisation mechanisms of ethanol and n-heptane spray flames are investigated in this study. The burner configuration involves a dilute spray of dispersed droplets which is produced and transported via a carrier gas stream of air to the reaction zone, where the flames are stabilised by a hot coflow of combustion products. A range of coflow conditions were implemented for the different flame cases, allowing the effects of the coflow oxygen (O2) concentration and temperature to be examined independently. The resulting flames were analysed using three simultaneous laser diagnostic techniques, enabling the combined planar imaging of the hydroxyl (OH) and formaldehyde (CH2O) radicals, along with the location of droplets. For both fuel types, a noticeable shift in stabilisation behaviour was observed with a variation in the coflow O2 concentration from 11% to 3%, while the coflow temperature was not seen to have a significant impact. These flames also show an interesting departure from the typical behaviour observed for gaseous and prevaporised flames in a similar configuration, particularly for coflow conditions that are typically associated with the transition to the mild combustion regime.

Turbulent spray flames of kerosene issuing from a hybrid electrohydrodynamic-air-blast atomiser

This paper presents an experimental study of turbulent spray flames of kerosene atomised using a new hybrid air-blast/electrohydrodynamic atomiser. Electrohydrodynamic (EHD), or charge injection atomization applies to electrically insulating fuels where the liquid is a dielectric and does not require conductive additives. The study focuses on the influence of electric charge on the global stability and downstream flow-field and reaction zone structure of turbulent spray flames. Stability characteristics for the burner are examined with and without electric charge, and cover sprays over a broad range of aerodynamic Weber numbers, We. The velocity and droplet fields are measured using laser/phase Doppler anemometry, and results indicate a notable increase in particle concentration with the application of charge for both reacting and non-reacting sprays. With the addition of electric charge, it is found that the mean and rms velocity fields are not significantly influenced but a significant rise in particle concentration is noted and this is more pronounced for the lower We number cases. Planar laser induced fluorescence of OH shows clear influence of charge on the reaction zone structures pushing them to outer radial locations, whilst also reducing the intermittency in the area of OH fields over time. The burner provides a new experimental platform that enables the study of hybrid atomization as a means to modify spray and flame behaviour.

Numerical Study of the Influence of Turbulence–Chemistry Interaction on URANS Simulations of Diesel Spray Flame Structures under Marine Engine-like Conditions

The present work performs unsteady Reynolds-averaged Navier-Stokes simulations to study the effect of turbulence-chemistry interaction (TCI) on diesel spray flames. Three nozzle diameters (d0) of 100, 180, and 363 μm are considered in the present study. The Eulerian stochastic fields (ESF) method (with the TCI effect) and well-stirred reactor (WSR) model (without the TCI effect) are considered in the present work. The model evaluation is carried out for ambient gas densities (ρam) of 30.0 and 58.5 kg/m3. The ESF method is demonstrated to be able to reproduce the ignition delay time (IDT) and lift-off length (LOL) with an improved accuracy than that from the WSR method. Furthermore, TCI has relatively more influence on LOL than on IDT. A normalized LOL (LOL*) is introduced, which considers the effect of d0, and its subsequent effect on the fuel-richness in the rich premixed core region is analyzed. The RO2 distribution is less influenced by the TCI effect as ambient density increases. The ESF model generally predicts a longer and wider CH2O distribution. The difference in the spatial distribution of CH2O between the ESF and WSR model diminishes as d0 increases. At ρam = 30.0 kg/m3, the ESF method results in a broader region of OH with lower peak OH values than in the WSR case. However, at ρam = 58.5 kg/m3, the variation of the peak OH value is less susceptible to the increase in d0 and the presence of the TCI model. Furthermore, the influence of TCI on the total OH mass decreases as d0 increases. The total NOx mass qualitatively follows the same trend as the total OH mass. This present work clearly shows that the influence of TCI on the global spray and combustion characteristics becomes less prominent when d0 increases. (Less)

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.

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.

Numerical study of multicomponent spray flame propagation

A computational study of one dimensional multicomponent laminar Jet-A/air spray flames is presented. The objective is to understand the effect of various spray parameters (diameter, droplet velocity, liquid loading) on the spray flame structure and propagation. Simulation of the Eulerian gas phase is coupled with a Lagrangian tracking of the dispersed liquid phase. Jet-A surrogate of n-dodecane, methyl-cyclohexane and xylene is considered. A discrete multicomponent model for spray vapourisation is used along with an analytically reduced chemistry for computing the gas phase reactions. Both overall lean and rich cases are examined and compared with existing literature for single component spray flames. The preferential evaporation effect, unique to multicomponent fuels causes a variation of fuel vapour composition on both sides of the flamefront and this has a direct impact on the spray flame structure and propagation speed. In the rich cases, multiple flame structures exist due to the staged release of vapours across the reactive zone. Spray flame speed correlations proposed for single component fuels are extended to the multicomponent case, for both zero and high relative velocity between the liquid and the gas. The correlations are able to accurately predict the effective equivalence ratio at which the flame burns and hence the laminar spray flame speeds of multicomponent fuels for all cases studied in this work.

Experimental study on flame characteristics and air entrainment of diesel horizontal spray burners at two different atmospheric pressures

The application of small horizontal burners in plateau environments has become an important problem in the field of energy conversion and heating equipment, particularly in the Qinghai-Tibet plateau. A low-pressure chamber was used to investigate the diesel spray flame at different equivalence ratios under two atmospheric pressures (0.08 and 0.05 MPa). The shape, fluctuation, and the characteristics of flame bending upward were analyzed using the method of multi-frame flame probability statistics. The results show that the flame bending upward characteristic are mainly affected by turbulence intensity when the equivalence ratio is less than 1.4, and otherwise controlled by atmospheric pressure. The dimensionless fluctuation parameter β differentiates the control regions of chemical and turbulence intensity at different equivalence ratios. The air entrainment coefficient increases with the decrease of atmospheric pressure and equivalence ratio, which results in a slight decrease of the temperature in the buoyancy plume flame region, and inhibits the uplifted buoyancy. In addition, the explicit equation of the flame trajectory is derived, and results show the calculated value is in good agreement with the experimental value. This work provides new insights into the evolution of horizontal spray flame structure at low pressure and supplies theoretical guidance for the design and use of plateau burners.

Structural analysis and regime diagrams of laminar counterflow spray flames with low-temperature chemistry

The understanding of spray combustion is a complex scientific problem, as it involves the interaction between evaporation, convection, diffusion, and chemistry. The focus of this study is to examine the effects of chemistry, in particular the low-temperature chemistry, on the structure of canonical counterflow spray flames at relatively low strain rate conditions. Parametric studies using a Eulerian formulation are carried out for monodisperse n-dodecane counterflow spray flames, in order to analyze the sensitivity of the flame structure to strain rate and droplet diameter over a wide range of equivalence ratios. These results reveal complex and multi-modal spray-flame structures that exhibit collocated, distributed, and cool flames. For the collocated flame, the presence of single and multiple low-temperature reaction zones is identified, which results from the evaporation and low-temperature chemistry interaction. The impact of low-temperature chemistry on the distributed flame is not pronounced. For low strain-rate conditions, the presence of a weak cool flame without high-temperature reaction zones is reported. Regime diagrams with respect to Stokes number and equivalence ratio are constructed, identifying distinctly different flame structures. By independently varying the strain rate and changing the droplet diameter, qualitatively similar but quantitatively different regime diagrams are obtained due to the lack of intrinsic self-similarity of the spray flamelet solution.

Heat release zones in turbulent, moderately dense spray flames of ethanol and biodiesel

This paper extends the application of laser-induced fluorescence imaging of OH & CH2O (LIF-OH-CH2O) to provide a measure of heat release zones in turbulent, moderately dense spray flames of ethanol and biodiesel fuels. A custom filter is used to remove interference from stimulated Raman scattering arising from the liquid fragments. Mie scattering is measured jointly on a separate camera to co-locate the fluid fragments with respect to the reaction zones. It is found that while the overall spray flame structure is similar to that of a turbulent gaseous diffusion flame, additional complexities arise due to the continuous release of fresh fuel vapor from the evaporating fragments. Structures referred to as burning rings of different sizes are observed, and these are ignited by interfacing with a hot edge before they grow, propagate, and burnout. Statistics on the occurrence of these rings, with and without clouds of liquid fragments within them, are presented. These findings are common to both ethanol and biodiesel, although measurements in the latter were affected by strong fluorescence interference from the excited components in the biodiesel blend.

Analysis of the spray flame structure in a lab-scale burner using Large Eddy Simulation and Discrete Particle Simulation

The numerical study of an academic lab-scale spray burner using Large Eddy Simulation coupled with a Discrete Particle Simulation is presented. The objectives are first, to validate current turbulent combustion modeling approach for two-phase flames, and second, to bring new insight on two-phase flame structure in a complex flow, representative of real configurations. The comparison with the experiment shows a good quantitative prediction of the velocity field of the gas and the liquid phases, in both non-reacting and reacting cases. Experimental and numerical results of the spray flame are also in good agreement. The detailed study of the interaction between the flame front and the droplets shows that both single droplet and group combustion regimes occur in the present configuration. These regimes are investigated from the numerical and physical points of view, highlighting the necessity to further investigate their possible importance for the modeling of two-phase combustion.

A comparative numerical investigation of reactivity controlled compression ignition combustion using Large Eddy Simulation and Reynolds-Averaged Navier-Stokes approaches

In this work, a comparative numerical study of Reactivity Controlled Compression Ignition (RCCI) combustion was conducted using both the Large Eddy Simulation (LES) and Reynolds-Averaged Navier-Stokes (RANS) approaches. In order to clarify the associated chemical kinetic processes, a data processing method that is able to calculate average reaction pathways, and identify representative reactions for the species creation, consumption, and heat release was adopted. The LES case yielded a wrinkled spray-flame structure, in contrast to the smoother and concentrated structure predicted by the RANS case. However, despite the different distribution features, both cases demonstrated similar reaction pathways and heat release characteristics. Modeling results showed that the low-temperature heat releases in RCCI combustion were controlled by the n-heptane spray-combustion process and dominated by the reactions R8 (C7H15O2 − 2 = C7H15 − 2 + O2), R95 (CH3COCH2 + O2 = CH3COCH2O2), and R285 (HCO + O2 = CO + HO2). In addition, low-temperature consumption pathways of n-heptane led to the substantial formation of a hydroxyl radical, which enhanced the consumption of iso-octane. As the combustion process progressed, high-temperature ignition pockets were found near the piston rim edge and squish regions, where heat release was dominated by the reaction R285. Intense high-temperature heat release was observed instantly from the ignition pockets, resulting in a rapid propagation of the reactive surfaces.

Instantaneous planar measurements of nitric oxide concentration in a turbulent n-heptane spray flame

Instantaneous and mean nitric oxide (NO) concentrations are measured in a turbulent, lifted, n-heptane jet spray flame at an atmospheric pressure using planar laser-induced fluorescence (PLIF). The flame-front is simultaneously located using OH-PLIF. The NO excitation line with the least temperature-quenching dependence is selected, and the associated NO-LIF dependence is corrected. LIF optimization and data reduction are based on previous work [Mulla et al. Combust. Flame 203 (2019) 217–229] where the NO-LIF signal was simulated at the spray flame condition (temperature and gas composition from LES) using a spectroscopic model. In the present work, to reduce interferences from polycyclic aromatic hydrocarbons (PAH) and soot, the detection bandwidth is progressively reduced from broadband (F6), intermediate (F3), to narrowband (F1). F6 results in a significant interference, while F1 contains the least interference. Instantaneous NO mole fraction (χNO) is deduced from F1 dataset using a novel instant-wise mapped interference subtraction approach (F1-Inst). The conditional mean NO concentrations derived from F6, F3, F1, and F1-Inst are in agreement (within uncertainty limits); however, F1-Inst appears to be the most reliable. Instantaneous flame-structure (OH) and NO concentration fields are analyzed. The spray flame structure consists of the inner (B1) and outer (B2) flame branches. Local extinctions are observed along B1. Small-scale (<1 mm) extinctions do not alter the NO concentration, whereas large-scale extinctions result in a sharp (50–100%) χNO drop, possibly due to a reduced temperature. Instantaneous χNO along B2 shows a sharp growth until 55 mm height above burner (HAB), beyond which the growth saturates. The deceleration of NO growth and χNO reduction near the sooting region is most likely due to the soot-radiation induced temperature drop. Mean χNO along B1 measures ∼ 25 ppm with a small axial variation (20–30 ppm). Expected lower temperature and nearly constant χNO with residence time suggest the nitrous oxide and prompt NO formations routes. Mean χNO in the outer diffusion branch (B2) increases from 35 ppm at the flame-base to 75 ppm at 60 mm HAB. The NO growth and potentially high temperature in B2 indicate the thermal route of NO formation.

Investigation of inversed-delta injection rate shaping diesel spray flame structure and combustion characteristics towards thermal efficiency improvement

Reduction of the late combustion is of interest for further diesel engine thermal efficiency improvement. In this work, “inversed-delta” (i.e., progressive reduction of injection rate during injection period) and conventional “rectangle”-injected diesel spray flames are investigated from simultaneous high-speed imaging and in-cylinder pressure trace in a constant volume combustion chamber CVCC at modern diesel engine conditions utilizing a novel injection rate shaping TAIZAC – TAndem Injectors Zapping ACtivation injector. Optically thinner spray tip rich mixture, shorter spray tip penetration and more relatively even UV emissions distributions are noticed in 175 MPa inversed-delta injections compared to than that of 175 MPa rectangle injection profiles, probably due to much even fuel-gas mixture distribution owing to following spray velocity reduction. However, in near-zero end-of-injection EOI pressure reduction condition as in 135 MPa inversed-delta injection profile, optically thicker and broader spray mixture accompanied with intense UV emissions and long soot residence are observed, suggesting poor atomization, mixing and soot oxidation processes. Based on pressure derived apparent heat release rate AHRR, inversed-delta injection exhibits slightly faster combustion than that of rectangle injection in fixed injection duration cases. However, for constant initial injection pressure comparison cases, combustion duration is faster in rectangle injection profiles. In similar initial injection pressure cases, inversed-delta injection combustion exhibits a distinct “shoulder”-like shape, where its AHRR departs from that of rectangle injection, results in lower peak pressure but longer combustion duration. Initial similarity and departing timing of AHRR shoulder are highly identical to that of the spray tip penetration, suggesting that the inversed-delta AHRR progress is restrained from the shoulder formation timing, possibly due to smaller volumetric reactive zone at the spray periphery.