Search

This paper aims to simulate diesel spray flames across a wide range of engine-like conditions using the Eulerian Stochastic Field probability density function (ESF-PDF) model. The ESF model is coupled with the Chemistry Coordinate Mapping approach to expedite the calculation. A convergence study is carried out for a number of stochastic fields at five different conditions, covering both conventional diesel combustion and low-temperature combustion regimes. Ignition delay time, flame lift-off length as well as distributions of temperature and various combustion products are used to evaluate the performance of the model. The peak values of these properties generated using thirty-two stochastic fields are found to converge, with a maximum relative difference of 27% as compared to those from a greater number of stochastic fields. The ESF-PDF model with thirty-two stochastic fields performs reasonably well in reproducing the experimental flame development, ignition delay times and lift-off lengths. The ESF-PDF model also predicts a broader hydroxyl radical distribution which resembles the experimental observation, indicating that the turbulence–chemistry interaction is captured by the ESF-PDF model. The validated model is subsequently used to investigate the flame structures under different conditions. Analyses based on flame index and formaldehyde distribution suggest that a triple flame, which consists of a rich premixed flame, a diffusion flame and a lean premixed flame, is established in the earlier stage of the combustion. As the combustion progresses, the lean premixed flame weakens and diminishes with time. Eventually, only a double-flame structure, made up of the diffusion flame and the rich premixed flame, is observed. The analyses for various ambient temperatures show that the triple-flame structure remains for a longer period of time in cases with lower ambient temperatures. The present study shows that the ESF-PDF method is a valuable alternative to Lagrangian particle PDF methods. [ABSTRACT FROM AUTHOR]

This paper reports on Large Eddy Simulation (LES) of turbulent premixed methane/air flames approaching blow-off. The study focuses on a stable flame, and on a flame just prior to blow-off, both stabilized by the Cambridge bluff-body burner. For turbulence-chemistry interaction, a model based on transported probability density function (TPDF) in conjunction with Eulerian stochastic fields is used. Velocity, species-concentration and heat release fields were first compared against experimental data showing good agreement. The results demonstrate that simulations of such complex combustion phenomena are possible and that the model is capable of reproducing the flame and the flow characteristics under both stable and close to blow-off conditions. A blow-off sequence was then examined and the results were used to evaluate some of the theories and mechanisms responsible for flame blow-off. It was found that the local extinction in the shear-layers had only minor impact on the flame blowing off and that the blow-off is a result of a series of events starting with the flame migrating into the recirculation zone. In the end, a mechanistic explanation is proposed for this series of events leading to full extinction of the flame. [ABSTRACT FROM AUTHOR]

A particle-based transported probability density function (PDF) method with a novel chemistry acceleration technique is developed in this work. The technique is based on the chemistry coordinate mapping (CCM) approach that was proposed in our previous works for accelerating direct numerical simulations (DNS) of partially premixed combustion. The method is first validated using Sandia flames D and F. It is shown that PDF-CCM results converge toward those obtained without CCM as phase-space resolution increases. PDF-CCM is then applied to simulate methane/air lifted jet flames in vitiated coflow reported in experiments by Cabra et al. (2005). It is shown that combustion is initiated in the form of auto-ignition in very fuel-lean gases where the gas velocity is low (the residence time is long) and the gas temperature is high (the ignition delay time is short). The ignition delay of the mixture below the liftoff position scales well with the liftoff height at different coflow temperature conditions. The combustion process above the liftoff height can develop into different modes depending on the coflow temperature. For high-temperature coflow, a premixed-burned combustion is formed above the liftoff height, which involves fuel-lean to fuel-rich burning modes; a triple-flame structure eventually is formed a few nozzle diameters above the liftoff position. For low-temperature coflow, the ignition delay and the liftoff height are sufficiently large to allow premixing between fuel and oxidizer before the onset of high-temperature combustion; in this case, a lean-to-stoichiometric premixed burn combustion is established downstream of the liftoff height, and no obvious triple-flame structure is formed at this condition. [ABSTRACT FROM AUTHOR]

Instantaneous structures of turbulent premixed ammonia/air flames on a piloted jet burner were investigated using simultaneous planar laser-induced fluorescence (PLIF) and Rayleigh thermometry measurements. Two-dimensional spatial distributions of temperature were simultaneously measured with those of the NH radical and the NO pollutant, respectively. Experiments were conducted for stoichiometric flames under five jet velocities. All flames are located in the distributed reaction zone regime of the Borghi-Peters diagram with the Karlovitz (Ka) number ranging from 274 to 4720. The NH PLIF images are used to characterize the fuel consumption layer of the reaction zones since the formation of NH is associated with the consumption of ammonia. The NH PLIF results show that under all flame conditions investigated, the NH layer remains thin in the proximity of the burner while it becomes progressively thickened and distorted by turbulence with increasing turbulent intensity and axial distance. For flames with Ka of 274, the NH layer essentially remains thin, while at Ka of 590 or higher, significant broadening of the NH layer is observed. Probability density functions (PDFs) of the NH layer thickness show that the NH layer can be broadened by 3 – 4 times as the flames are developed downstream. The broadening of the NH layer is considered to indicate that the flames are in the distributed reaction zone regime. The boundary between the thin-reaction zone regime and the distributed reaction zone regime occurs at a much larger Ka than that in methane/air flames. The broadening of the NH layer is due to the penetration of the turbulent eddies and the merging of flame branches. The latter occurs mainly near the flame tips. NO is shown to exist in a wide region in space, across the preheat zone, reaction zone, and postflame zone. NO formation occurs mainly in the reaction zone, however, it is transported by turbulence eddies to the preheat zone and by convection to the postflame zone. The temperature measurements indicate that the preheat zone is broadened in all flames investigated. The broadening of the preheat zone is moderately sensitive to the Ka number while it is more sensitive to the integral length scale of the flames. [ABSTRACT FROM AUTHOR]

Large-eddy simulation (LES) of multiple-injection spray combustion under engine-like conditions is performed using a coupled model of an Eulerian stochastic field transported probability density function (ESF T-PDF) and a flamelet generated manifold (FGM). This coupled model allows the use of the T-PDF methods in modeling the interaction of turbulence and chemistry at affordable computational costs for engine applications. Simulation results are compared with the available experimental data for spray flames with multiple-injection and at a high level of exhaust gas recirculation (EGR) under conditions relevant to internal combustion engines. It is shown that the T-PDF/FGM LES model is capable of reproducing not only global combustion characteristics, such as the pressure rise and ignition delay time but also replicating the evolution of liftoff length and the spray flame structure. The effects of pre-injection strategies are then investigated, by systematically varying the pre-injection duration and the dwell time between two injections, while keeping the total injected fuel mass constant. The LES results reveal different mechanisms by which a pre-injection can change the ignition delay time, the combustion mode and the emissions in spray flames, depending on the injection timing. It is shown that even an extremely short non-igniting pre-injection can substantially change the ignition and emissions characteristics of the main-injection. It is shown that the combustion mode of a single-injection can be altered by splitting the injection to pre- and main-injections. The current study also demonstrates that decreasing the dwell time, within the range that is examined here, at a given pre-injection, will potentially increase the soot oxidation rates while it does not significantly change the rate of soot formation. [ABSTRACT FROM AUTHOR]