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Abstract A flamelet tabulation method for partially premixed flames is proposed, in which partially premixed flamelets are incorporated as the archetypal flamelet elements. This method considers triple flame structures with both the partial premixing of fuel in the oxidizer side and the partial premixing of oxidizer in the fuel side, by replacing the pure-air and pure-fuel in the counterflow diffusion flame with a range of fuel-lean and -rich mixtures, respectively. The thermo-chemical quantities in the partially premixed flamelet are stored in a four-dimensional flamelet library as a function of the mixture fraction Z , describing the mixing process, the reaction progress variable Y PV , describing the progress of reactions, and the trajectory variables Y F and Y O , characterizing the partial premixings of fuel and oxidizer, respectively. The performance of the proposed partially premixed flamelet tabulation (PPFT) method is evaluated through both a priori and a posteriori tests on laminar tribrachial flames with different mixture fraction gradients. The PPFT results are compared with those from a premixed flamelet tabulation (PFT) method and a diffusion flamelet tabulation (DFT) method. It is found that the combustion-mode-sensitive species such as CO and H 2 can be accurately predicted by the PPFT method for both the low and high mixture fraction gradient flame cases, which cannot be well predicted by the PFT and DFT methods. [ABSTRACT FROM AUTHOR]

Highlights • High quality experiment of injection rate profile of common rail injection system was conducted. • Systematic quantification of the impact of injector parameters on the injection characteristics was investigated. • A new injection rate model was developed for common rail injection system. • Systematic evaluations of the injection model against injection rate and diesel engine experiments was done. • The injection rate model provides a powerful tool for CFD study of DI engines. Abstract Fuel injection is one of the most important processes in compression-ignition internal combustion engines owing to its significant impact on the exhaust emissions and thermal efficiency. In this study, experiments were carried out to investigate the influence of injection pressure and injection timing on the temporal evolution of the injection rate and injection duration in a specially designed experiment rig equipped with a common rail injection system. It is well known that the injection signal from the electronic control unit (ECU) of the injection system, which is often the only injection information available in engine operation and experiments, gives little information about the actual injection rate profile. It is shown in the present experiments that the actual injection duration is usually longer than the energizing time (ET). The time delay between the actual injection of the fuel and the ECU signal is about 0.3–0.4 ms, and the time delay appears to be insensitive to the injector geometry and injection pressure condition. The injection process can be characterized as five stages, a fast injector valve opening stage, a slow valve opening stage, a valve fully open stage, followed by a slow valve closing stage, and finally a rapid valve closing stage. It is found that the first stage, the fast valve opening stage, is insensitive to the injection pressure and injector nozzle diameter; however, the peak injection rate is a strong function of these parameters. The second and the third stage may not appear with a short injection duration. A new injection model was developed for the common rail injection system, which was capable of simulating the instantaneous fuel injection rate and injection duration for a range of injection pressure and injection duration. The model was shown to be able to replicate the experimental injection rate profile of the present experiments and experiments found in the literature for common rail injection system. The new injection model was applied to predict the effect of injection pressure and injection duration on the performance of a diesel engine under various engine speed and load conditions. The new injection model was shown to be able to describe the injection mass flow rate, which eventually leads to a reasonably good prediction of the variations of the spray development, in-cylinder pressure, heat release rate, and emissions. [ABSTRACT FROM AUTHOR]

Low temperature combustion (LTC) of high-octane number fuels in compression ignition engines offers an opportunity to simultaneously achieve high engine thermal efficiency and low emissions of NO x and particulate matter without using expensive after-treatment technologies. LTC engines are known to be sensitive to the operation conditions and combustor geometry. It is important to understand the fundamental flow and combustion physics in order to develop the technology further for commercial application. A joint numerical and experimental investigation was conducted in a heavy-duty compression ignition engine using a primary reference fuel with an octane number of 81 to investigate the effects of injection timing, piston geometry, and compression ratio (CR) on the fuel/air mixing and combustion covering different regimes of LTC engines, homogeneous charge compression ignition (HCCI), partially premixed combustion (PPC), and the transition regime from HCCI to PPC. The results show that with the same combustion timing, a higher CR leads to a lower NO x , but a higher emission of UHC and CO. The piston geometry shows a significant impact on the combustion and emission process in the transition regime while it has minor influence in the HCCI and PPC regimes. It is found that high engine efficiency and low emissions of NO x , CO and UHC can be achieved in the earlier PPC regime and later transition regime. The fundamental reason behind this is the stratification of the mixture in composition, temperature and reactivity, which is dictated by the interaction between the spray and the cylinder/piston walls. • Engine performance with different SOI, compression ratio, piston geometry is studied. • Three regimes (homogeneous charge compression ignition, partially premixed combustion and transition) during the SOI variation are investigated. • Piston bowl shows significant impact on the charge stratification in transition regime. • Compression ratio shows a tradeoff of NOx reduction with unburned hydrocarbons and carbon monoxide emissions. • Optimal SOI window range from -50 to -42 oCA with the stepped-lip piston is found. [ABSTRACT FROM AUTHOR]

Abstract A numerical and experimental study on the devolatilization of a large biomass particle is carried out to quantify the effect of homogeneous volatile combustion on the conversion of the particle and on the temperature and species distribution at the particle vicinity. A global chemical kinetic mechanism and a detailed reaction mechanism are considered in a one dimensional numerical model that takes into account preferential diffusivity and a detailed composition of tar species. An adaptive moving mesh is employed to capture the changes in the domain due to particle shrinkage. The effect of gas phase reactions on pyrolysis time, temperature and species distribution close to the particle is studied and compared to experiments. Online in situ measurements of average H 2 O mole fraction and gas temperature above a softwood pellet are conducted in a reactor using tunable diode laser absorption spectroscopy (TDLAS) while recording the particle mass loss. The results show that the volatile combustion plays an important role in the prediction of biomass conversion during the devolatilization stage. It is shown that the global reaction mechanism predicts a thin flame front in the vicinity of the particle deviating from the measured temperature and H 2 O distribution over different heights above the particle. A better agreement between numerical and experimental results is obtained using the detailed reaction mechanism, which predicts a wider reaction zone. [ABSTRACT FROM AUTHOR]

We report micrometric imaging measurements of the ignition of micron-sized aluminum (Al) wires and the consequent combustion processes of single micron-sized Al droplets in hot steam and steam with oxygen. The specially designed flames were anchored on a modified burner where a small central H 2 /O 2 diffusion flame in the center jet provided a hot flow (>2330 K) for Al ignition, and the diffusion flame together with a laminar co-flow of premixed H 2 /N 2 /O 2 flame provided the vitiated flow of desired combustion environment. As soon as the horizontally arranged Al wire was moved to the center hot jet, a high-speed color camera was triggered to record the transient event of the ignition and combustion process. The high imaging quality and spatial resolution allow direct observation of the structure of the burning particles, i.e., the liquid core, the bright flame front encapsulating the center droplet, and the coalescent alumina agglomerates downstream above the burning Al droplets. A distinct thin condense sheet was observed defining the flame front in all symmetric and asymmetric combustion modes, indicating that homogeneous oxidation dominates the heat-release reactions as a diffusion flame. A color ratio RGB pyrometry was calibrated to provide temperature distribution over the burning particles. In the steam condition, the center droplet temperature in the symmetric phase was analyzed to be ∼2650 K and the temperature of the flame front ∼3400 K, while in the oxygenated environments, the droplet temperature was evaluated to be ∼2790 K, and the temperature of the flame front ∼4150 K. The transient temperature evolution of different parts of the burning elements was recorded and analyzed. To the authors' awareness, the current study represents the first structure-resolved temperature measurement of the combustion of single aluminum particles in high-temperature steam. These measurements can provide unprecedented spatiotemporally resolved information on Al combustion for improved insight into the governing mechanism. [ABSTRACT FROM AUTHOR]

Autoignition and stabilization of n -Dodecane spray combustion under diesel engine like conditions are investigated using large eddy simulation and detailed chemical kinetics. The Spray A cases of Engine Combustion Network (ECN) with ambient temperatures of 900 K and 1000 K are considered. Two-stage ignition behavior is predicted in the studied conditions. It is found that the first-stage ignition occurs on the fuel-lean mixture, whereas the second-stage ignition starts on the fuel-rich mixture. The first stage ignition in the fuel-lean mixture promotes the first and the second stage ignition in the fuel-rich mixture owing to rapid turbulent mixing. Two mechanisms, autoignition and flame propagation coupling with the low temperature ignition, are used to explain the lift-off position and stabilization of the combustion process. They compete with each other, and their relative importance depends on the ambient temperature. The ambient temperature is shown to affect the soot emission in the flame through its influences on the lift-off length and the reaction zone structure. Higher ambient temperature results in a shorter lift-off length, which gives rise to higher soot emission due to the lower air entrainment to the fuel-rich zone in front of the flame. In the lower temperature case, the flame is stabilized by an autoignition induced flame front where a considerable amount of fuel is oxidized to CO at the leading front of the flame. Consequently, it reduces the soot formation in the flame. [ABSTRACT FROM AUTHOR]

In the afterburner assembled with an air-cooled bluff-body flameholder, cooling air is directly injected into the recirculation zone behind the bluff-body, which can reduce the local temperature and increase the oxygen concentration of the gas mixture in the wake of the bluff-body, thereby affecting the total pressure loss and combustion characteristics. To better understand the flow and combustion process of the system, the exhaust gas temperature, cold and hot total pressure losses in a rectangular premixed combustor are investigated under different cooling air jet conditions. Experimental results show that the added cooling air could improve the combustion efficiency and widen the blowout limit, whereas it could also give rise to an extra total pressure loss. However, when the cooling air flow rate was higher than a critical value, i.e., after the blowing ratio reached 2.5, the recirculation zone could be blown away, resulting in a failed ignition in the afterburner. Notably, the decreased temperature difference between the mainstream and the cooling air could improve the combustion efficiency and reduce the thermal resistance loss but enlarge the cold flow loss and hot total pressure loss. Moreover, since the oxygen content declined and autoignition appeared after the mainstream temperature reached 1100 K, the exhaust gas temperature and combustion efficiency declined rapidly, and the hot total pressure loss also decreased. In addition, with the fuel-gas ratio increasing, the combustion efficiency significantly dropped, the exhaust gas temperature and thermal resistance loss firstly increased to a peak value (at the equivalence ratio of 1.14) and then decreased for excessively fuel-rich combustion. [ABSTRACT FROM AUTHOR]

Methanol (CH 3 OH) is an attractive alternative fuel that can reduce net carbon release and decrease pollutant emissions. In this study, methanol and n -dodecane spray flames were investigated using Large-Eddy Simulation (LES) and direct coupling with finite-rate chemistry. The selected ambient conditions are relevant to engines and were previously unreported for numerical methanol spray studies, i.e. high pressure (60 bar) and temperature (900 – 1200 K) with high injection pressure (1500 bar). The Engine Combustion Network (ECN) Spray A case was used to validate the n -dodecane spray flame. For methanol, a modified ECN Spray A condition was used with a high initial ambient temperature (1100 K-1200 K) to ensure fast enough ignition relevant to engine time scales. The performed homogeneous reactor (0D) simulations revealed a new phenomenon of a two-stage ignition process for methanol, confirmed by the 3D LES at high pressure, high temperature, and lean conditions. The present numerical results also show that: 1) there is a strong ambient temperature sensitivity for methanol ignition delay time (IDT) with a five-fold decrease in IDT (ID T 1100 K / ID T 1200 K = 5) and a factor of 2.6 decrease in the flame lift-off length (FLO L 1100 K / FLO L 1200 K = 2.6) as the ambient temperature is increased from 1100 K to 1200 K, 2) methanol spray ignition takes place at a very lean mixture (ϕ M R ≈ 0.2) consistent with the 0D predicted most reactive mixture fraction (Z M R) , 3) on average, methanol sprays are significantly leaner than n -dodecane sprays at quasi-steady-state (ϕ m e o h , a v e ≈ 0.2 vs ϕ n d o d , a v e ≈ 0.7) , implying very low soot emissions, and 4) the methanol spray flames could have similar temperatures as the n -dodecane sprays depending on the initial conditions, thus a similar level of NO x emissions. [ABSTRACT FROM AUTHOR]

Metal powder is a promising carbon-free and recyclable energy carrier. Direct combustion of the micron-sized iron particles involves complex physical and chemical processes, such as heat transfer, surface reaction, and phase change. In this work, computational modelling of these processes is investigated and validated against experiments. A single iron particle combustion and phase change model is proposed in an Eulerian–Lagrangian framework. The new phenomenological model considers five stages, i.e., solid phase oxidation, melting of iron oxides and raw iron, liquid phase oxidation, cooling of liquid iron oxides, and solidification of super-cooled liquid iron oxides. The proposed model is first validated and then adopted in simulations of micron-sized iron particle combustion in premixed CH 4 /O 2 /N 2 flames to study the effects of ambient temperature and oxygen concentration on single iron combustion. Results show that the new model is capable of replicating the melting, heterogeneous surface reaction, cooling, and solidification processes. Two-stage solidification is observed in experiments and modelled in simulations. This two-stage solidification includes a fast solidification with a significant temperature rise (∼ 150–200 K) and a thermal equilibrium solidification featuring a constant temperature and a slight particle radiant intensity decrease. In addition, a diffusion-controlled mechanism is identified during the melting process, in which the oxygen concentration dominates the melting time and the subsequent burning time. Furthermore, it is found that the reaction between iron and CH 4 /O 2 /N 2 flame products, such as CO 2 and H 2 O, plays a non-negligible role in the iron combustion process. [ABSTRACT FROM AUTHOR]

In this paper, numerical simulations were carried out to identify the mode of ignition and ignition delay time of pulverized biomass particles in hot flue gas produced by a methane/air flame. In the experiments, it was observed that for most biomass residues the dominant combustion mode was the staged gas-phase ignition in the surrounding gas followed by surface ignition at the char surface. There were some exceptions to this general trend, e.g. wheat straw particles, which ignited at the surface of the particle under some temperature conditions. Moreover, temporally and spectrally resolved images of the single burning particles were obtained in the experiments and CH* chemiluminescence at different stages of biomass conversion was recorded. In this study, by means of a detailed numerical model for conversion of biomass particles and employing detailed gas chemistry mechanism, the ignition mode and ignition delay time of the particles are studied. The model is able to distinguish between different ignition modes of the particles in agreement with the experimental data. The underlying physics behind shifting ignition mode from homogeneous ignition to heterogeneous ignition for wheat straw are discussed. The ignition delay times for different biomass sources at different conditions are calculated and the results are in good agreement with the experimental data. Apart from the detailed model, CFD simulations are performed to assess the flow and combustion process (temperature, O 2 concentration and velocity difference between the ambient gas and the particle) around the particle. The CFD results show similar trends compared with the CH* chemiluminescence from the particle at different times during the devolatilization stage. [ABSTRACT FROM AUTHOR]

Transient mixing and ignition mechanisms in a simplified pre-chamber/main-chamber system are investigated using direct numerical simulation (DNS) with detailed chemical kinetics. Full ignition and flame propagation processes in the premixed methane/air mixtures are simulated. Ignition, the progress and topology of flame evolution, and the mean burning velocity in the main chamber are analyzed in detail. Four important phases in the ignition and flame propagation processes are identified based on the flame structure development in the main chamber, the pressure and velocity evolution at typical points in both the pre-chamber and main chamber. Results show that the intermediate species OH, CH 2 O, and HO 2 are critical for flame stabilization and propagation in the main chamber due to their high reactivity. This is sorted as the chemical effect that the pre-chamber jet acts on the main chamber. The high temperature jet also brings heat and unburned fuel into the main chamber, which are sorted as thermal and enrichment effect, respectively. The heat release rate is found to be approximately proportional to the product of CH 2 O and OH mass fractions, which could be regarded as a reliable and effective marker for the heat release rate of methane/air mixtures. It is found that the mean burning velocity in the main chamber can be elevated up to 30 times under the condition investigated. [ABSTRACT FROM AUTHOR]

The combustion process in a hydrogen fueled scramjet combustor with a rearwall-expansion cavity was investigated numerically under inflow conditions of Ma =2.52 with stagnation pressure P 0 =1.6 Mpa and stagnation temperature T 0 =1486 K. The numerical solver was first evaluated for supersonic reactive flows in a similar combustor configuration where experimental data is available. Wall-pressure distribution was compared with the experiments, and grid independency analysis and chemical mechanism comparison were conducted. The numerical results showed fairly good agreements with the available experimental data under supersonic combustion conditions. Then the numerical solver was used to study the effects of combustor geometry, fuel injection scheme and injection equivalence ratio on the combustion process. It was found that under the same fuel injection condition, the combustor configuration with a rearwall-expansion cavity is in favor of the supersonic combustion mode and present better ability of thermal choking prevention than the other combustor configurations. For the rearwall-expansion cavity combustor, the supersonic flow field was found to be sensitive to the injector position and injection scheme, but not highly sensitive to the injection pressure. Besides, rearwall-expansion cavity with the combined fuel injection scheme (with an injection upstream the cavity and a direct injection on the rear wall) is an optimized injection scheme during the flame stabilization process. [ABSTRACT FROM AUTHOR]

Wall-type pilot flameholder has been used in aero-engines to effectively create a stable ignition source and achieve an excellent ignition performance in high-velocity flows. The ignition delay, flame propagation, and flame boundary are crucial characteristics to evaluate the performance of pilot flameholders. Previous work displayed the significantly changed ignition performance and flow features of the wall flameholder when jet cooling schemes were adopted, indicating that the ignition process and the flame features strongly depend on the cooling jet and the optimal design of jet cooling schemes needs further investigation. Therefore, in this study, two practical image processing algorithms are developed to characterize the flame development of the ignition process, reveal the effect of cooling schemes on the ignition delay time, and quantitatively analyze the flame area and expansion ratio of the wall flameholder during stable combustion. Two image processing methods proposed based on MATLAB algorithms exhibit effectiveness in studying the ignition process and flame expansion characteristics. The ignition process is divided into four phases: kernel generation, kernel propagation, flame growth, and stable combustion. Meanwhile, the ignition delay time is identified by counting the total pixels of the flame projected region during the ignition process. The edge detection and pixel statistics of the flame projected region in the binary luminosity image show the flame boundary and expansion ratio under various working conditions. It was found that the cooling jet angle has a significant impact on the ignition process, ignition delay time, flame propagation, and steady flame features. The pressure-driven gas jet cooling scheme with a smaller jet angle and the external-inhaled air cooling scheme with a larger jet angle present a better capability of ignition and flame expansion and propagation. [ABSTRACT FROM AUTHOR]

Eulerian stochastic fields (ESF) based transported PDF method has evolved as one of the general purpose methods for the numerical simulation of complex chemically reactive turbulent flows in which the effect of turbulence on the chemical reaction rates can be computed directly without the need of modeling. It is found that ESF method may suffer from violation of the element mass conservation law due to the stochastic Wiener process and the highly non-linear dependence of the chemical reaction rates on the stochastic fields. This paper presents a modified ESF method to remove the error in the element mass conservation. The method is evaluated in numerical simulation of two turbulent flames, (i) Reynolds averaged Navier-Stokes (RANS) simulation of a turbulent non-premixed methane/air counterflow flame under stationary burning and transient flame extinction conditions, (ii) large eddy simulation of swirling turbulent methane/air non-premixed flames under local extinction and re-ignition conditions. The original ESF method violates the element mass conservation in both flames, and the element mass error would not disappear even if a large number of stochastic fields were used. The new method yields a satisfactory prediction of the element mass conservation even with a small number of stochastic fields. The new and original methods predict similar stationary flame structures but the results under flame extinction and re-ignition conditions are rather different. [ABSTRACT FROM AUTHOR]

Simultaneous two-species imaging using single-shot planar laser-induced fluorescence have been performed to record high quality image pairs of CH/OH, CH/CH 2 O and OH/CH 2 O to visualize the flame front structures in swirl-stabilized lean premixed methane/air flames. The results show that the investigated flames exhibit various flame front structures distinctly in space, which covers: (1) the corrugated flamelet at the leading front; (2) the thin reaction-zone layer with distorted preheat zone in the shear-layer downstream; and (3) quenching, re-ignition and distributed reactions further downstream. The large variation of the flame characteristics in space stems from the entrainment of ambient cold air to the flame that results in flame quenching at the trailing edge of the flame. Thereafter, the unburned fuel/air mixture in the downstream region mixes with the entrained air and the hot combustion products from the upstream leading flame front, leading to reignition with distributed reactions. The current results provide a direct experimental evidence that distributed reactions can be a common combustion mode along with the results (Ref. [1], Zhou et al., 2015) recently reported in the highly turbulent premixed jet flames. [ABSTRACT FROM AUTHOR]

Gasoline and methanol are highly recommended for low temperature combustion (LTC) engines due to their high research octane number. However, engines fuelled with these fuels suffer from high unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions at low loads, which can be improved by controlling the level of mixture stratification. This paper reports on the experimental and numerical comparison of the emission characteristics and engine performance of methanol and iso-octane in a heavy-duty direct-injection compression ignition engine operating in HCCI and PPC regimes. Overall, methanol requires a higher intake temperature, compared with iso-octane, to counter the high heat of vaporization. The onset of ignition is from the relatively fuel-lean regions resulting in lower CO emissions. This is due to the methanol ignition delay times being less sensitive to the fuel/air mixture and the larger temperature gradient of the equivalence ratio. Methanol achieves lower charge stratification because of its low stoichiometric A/F ratio, which extends the SOI window with ultra-low NO x emissions. However, methanol suffers from higher UHC emissions in the HCCI regime due to fuel trapped in the crevice region. Three cases include HCCI, PPC with low and high mixture stratification were detailed investigated. Of these cases, PPC with low mixture stratification case shows the highest engine thermal efficiency and the lowest emissions. Methanol has more advantages operating in the PPC regime in the aspect of emissions. • The overall trend of the intake temperature for both fuels shows 'spoon' shape. • Methanol requires a higher intake temperature to counter the high heat of vaporization. • Ignition of methanol is from the relatively fuel-lean regions resulting in lower CO emissions. • Methanol achieves lower charge stratification because of its low stoichiometric A/F ratio. • Methanol has more advantages operating in the PPC regime in the aspect of emissions. [ABSTRACT FROM AUTHOR]

A three-dimensional reactive multi-phase particle-in-cell (MP-PIC) model is employed to investigate biomass combustion and gasification in fluidized bed furnaces. The MP-PIC model considered here is based on a coarse grain method (CGM) which clusters fuel and sand particles into parcels. CGM is computationally efficient, however, it can cause numerical instability if the clustered parcels are passing through small computational cells, resulting in over-loading of solid particles in the cells. To overcome this problem, in this study, a distribution kernel method (DKM) is proposed and implemented in an open-source CFD code, OpenFOAM. In DKM, a redistribution procedure is employed to spread the solid volume and source terms of the particles in the parcel to the domain in which the particles are clustered. The numerical stiffness problem caused by the CGM clustering can be remedied by this method. Validation of the model was performed using data from different lab-scale reactors. The model was shown to be able to capture the transient heat transfer process in a lab-scale bubbling fluidized bed reactor under varying fluidization velocities and loads of sand. Then, the model was used to study the combustion/gasification process in a bubbling fluidized bed reactor under varying ambient temperatures, equivalent air ratios, and steam-to-biomass ratios. The performance of DKM was shown to improve the accuracy and the robustness of the model. [ABSTRACT FROM AUTHOR]

This paper presents a numerical investigation of the ignition and combustion process of a primary reference fuel in a partially premixed light-duty internal combustion (PPC) engine. Partially pre-mixed combustion is achieved by employing a multiple injection strategy with three short injection events of fuel pulses. The timing of the first two fuel pulses, 48 and 22 crank angle degrees before top dead center, are chosen with the purpose to stratify the fuel and air charge, whereas the third injection, at five crank angle degrees before top dead center, serves as an actuator of the main heat release. In addition to this baseline injection, three alternative injection strategies are studied, including a split-fuel two-injection strategy and modified triple-injection strategies. Large eddy simulations are employed utilizing a skeletal chemical kinetic mechanism for primary reference fuel capable of capturing the low-temperature ignition and the high temperature combustion. The large eddy simulation (LES) results are compared with experiments in an optical accessible engine. The results indicate that the first ignition sites are in the bowl region where the temperature is relatively higher, and the reaction fronts thereafter propagate in the swirl direction and towards the centerline of the cylinder. The charge from the first two injections initially undergoes low-temperature reactions and thereafter high-temperature reservoirs are formed in the bowl region. The main heat-release is initiated in the engine when the fuel from the third injection reaches the high-temperature reservoirs. Finally, the remaining fuel in the lean mixtures from the first two injections is oxidized. By variation of the injection strategy, two trends are identified: (1) by removing the second injection a higher intake temperature is required to enable the ignition of the charge, and (2) by retarding second injection, a longer ignition delay is identified. Both can be explained by the stratification of fuel and air mixture, and the resulting reactivity in various equivalence ratio and temperature ranges. The LES results reveal the details of the charge stratification and the subsequent heat release process. The present results indicate a rather high sensitivity of partially premixed combustion process to the injection strategies. [ABSTRACT FROM AUTHOR]