Rixin Yu

Comparison of LES Models Applied to a Bluff Body Stabilized Flame

Present-day demands on combustion equipment are increasing the need for improved understanding and prediction of turbulent combustion. Large Eddy Simulation (LES), in which the large-scale flow is resolved on the grid, leaving only the small-scale flow to be modeled, provides a natural framework for combustion calculations as the transient nature of the flow is resolved. In most situations, however, the flame is thinner than the LES grid, and subgrid modeling is required to handle the turbulence-chemistry interaction. Here, we examine the predictive capabilities and the theoretical links between LES flamelet models, such as the G-equation model (G-LES), and LES finite rate chemistry models, such as the Thickened Flame Model (TFM-LES), the Partially Stirred Reactor model (PaSR-LES), the Eddy Dissipation Concept (EDC-LES) model, a Presumed Probability Density Function (PPDF-LES) model and the Implicit LES (QL-LES) model. The models are described, and theoretical links between these are discussed in terms of the turbulent flame speed and flame thickness. In addition, the different models are used to study a bluff-body stabilized flame and the resulting predictions are compared with experimental data for two operating conditions.

Effect of Turbulence on HCCI Combustion

This paper presents large eddy simulation (LES) and experimental studies of the combustion process of ethanol/air mixture in an experimental optical HCCI engine. The fuel is injected to the intake port manifolds to generate uniform fuel/air mixture in the cylinder. Two different piston shapes, one with a flat disc and one with a square bowl, were employed to generate different in-cylinder turbulence and temperature field prior to autoignition. The aim of this study was to scrutinize the effect of in-cylinder turbulence on the temperature field and on the combustion process. The fuel tracer, acetone, is measured using laser-induced fluorescence (LIF) to characterize the reaction fronts, and chemiluminescence images were recorded using a high-speed camera, with a 0.25 crank angle degree resolution, to further illustrate the combustion process. Pressure in the cylinder is recorded in the experiments. Spatial and temporal resolved LES was used to gain information on the turbulence mixing, heat transfer and combustion process. It was shown that gas temperature in the piston bowl is generally higher than that in the squish, leading to an earlier ignition in the bowl. Compared to the disc engine, the square bowl engine has a higher temperature inhomogeneity owing to the turbulence wall heat transfer. The experimentally observed higher combustion duration and slower pressure rise rate in the square bowl engine as compared to the disc engine can be explained by the higher temperature inhomogeneity in the square bowl engine.

Numerical and Experimental Investigation of Turbulent Flows in a Diesel Engine

This paper presents a study of the turbulence field in an optical diesel engine operated under motored conditions using both large eddy simulation (LES) and Particle Image Velocimetry (PIV). The study was performed in a laboratory optical diesel engine based on a recent production engine from VOLVO Car. PIV is used to study the flow field in the cylinder, particularly inside the piston bowl that is also optical accessible. LES is used to investigate in detail the structure of the turbulence, the vortex cores, and the temperature field in the entire engine, all within a single engine cycle. The LES results are compared with the PIV measurements in a 40 x 28 mm domain ranging from the nozzle tip to the cylinder wall. The LES grid consists of 1283 cells. The grid dynamically adjusts itself as the piston moves in the cylinder so that the engine cylinder, including the piston bowl, is described by the grid. In the intake phase the large-scale swirling and tumbling flow streams are shown to be responsible for the generation of large-scale vortex pipes which break down to small-scale turbulent eddies. In the later phase of compression turbulence is mainly produced in the engine bowl. The bore wall and the piston bowl wall heat the fluid near the walls. Turbulence and the large-scale coherent vortex shedding due to the Kelvin-Helmholtz instability are responsible for the enhanced heat transfer between the bulk flow and the walls. A temperature inhomogeneity of about 50 - 60 K can be generated in the cylinder. (Less)

Large Eddy Simulation and Experiments of the Auto-Ignition Process of Lean Ethanol/Air Mixture in HCCI Engines

Recent experiments and numerical studies have showed that piston geometry has a significant effect on the homogeneous charge compression ignition (HCCI) process. There are two effects generated by the combustor geometry: the geometry affects the flow/turbulence in the cylinder; the geometry also affects the temperature stratification. The temperature stratification is more directly responsible for the observed alteration of the auto-ignition process. To clarify this issue further we present in this paper a study of two engines with the same geometry but difference ways of cooling. Measurement of the two engines~a metal engine and quartz piston engine, both with the same piston bowl geometry~is carried out. Large eddy simulation (LES) is used to simulate the flow, the temperature field and the auto-ignition process in the two engines. The fuel is ethanol with a relative air/fuel ratio of 3.3. It is found that lower temperature stratification is established in the metal engine under similar conditions as the optical quartz engine due to the more effective cooling of the piston in the metal engine configuration. The combustion phasing in the two engines is the same by controlling the intake temperature. Both measurements and LES show a more rapid auto-ignition in the metal engine than in the optical engine with the same piston geometry. This confirms the conclusion that large temperature stratification can decrease the pressure-rise-rate and thereby increase the load of HCCI engines. The dependence of temperature stratification on the wall temperature and intake temperature is systematically studied using LES. (Less)

Effect of Turbulence and Initial Temperature Inhomogeneity on Homogeneous Charge Compression Ignition Combustion

A 0.5-liter optical HCCI engine firing a mixture of n-heptane (50%) and iso-octane (50%) with air/fuel ratio of 3 is studied using large eddy simulation (LES) and laser diagnostics. Formaldehyde and OH LIF and in-cylinder pressure were measured in the experiments to characterize the ignition process. The LES made use of a detailed chemical kinetic mechanism that consists of 233 species and 2019 reactions. The auto-ignition simulation is coupled with LES by the use of a renormalized reaction progress variable. Systematic LES study on the effect of initial temperature inhomogeneity and turbulence intensity has been carried out to delineate their effect on the ignition process. It was shown that the charge under the present experimental condition would not be ignited without initial temperature inhomogeneity. Increasing temperature inhomogeneity leads to earlier ignition whereas increasing turbulence intensity would retard the ignition. This is mostly due to the effect of turbulence on the bulk flow that turbulence tends to decrease the temperature inhomogeneity by enhanced eddy heat transfer. The LES results suggest that desirable ignition timing could be achieved by controlling the turbulence intensity and temperature inhomogeneity.

Effect of Temperature Stratification on the Auto-ignition of Lean Ethanol/air Mixture in HCCI Engine

It has been known from multi-zone simulations that HCCI combustion can be significantly affected by temperature stratification of the in-cylinder gas. With the same combustion timing (i.e., crank angles at 50% heat release, denoted as CA50), large temperature stratification tends to prolong the combustion duration and lower down the in-cylinder pressure-rise-rate. With low pressure-rise-rate HCCI engines can be operated at high load, therefore it is of practical importance to look into more details about how temperature stratification affects the auto-ignition process. It has been realized that multi-zone simulations can not account for the effects of spatial structures of the stratified temperature field, i.e., how the size of the hot and cold spots in the temperature field could affect the auto-ignition process. This question is investigated in the present work by large eddy simulation (LES) method which is capable of resolving the in-cylinder turbulence field in space and time. The initial temperature field for LES is presumed as the superimposition of a mean temperature and a sine-function fluctuating temperature. The engine runs on ethanol with a relative air/fuel ratio of 3.3. The LES results show that the initial shape of hot/cold spots is quickly modified by turbulence. A particular hot/cold spot size on the order of large eddy integral scale is found at which the combustion duration tends to be shorter. This reveals the fact that not only the magnitude of the temperature stratification but also the spatial structure of the stratification could affect the auto-ignition process. (Less)

Large Eddy Simulation of Turbulent Combustion in a Spark-Assisted Homogenous Charge Compression Ignition Engine

A large eddy simuation (LES) model is presented for simulation of spark-assisted homogeneous charge compression ignition combustion. The model is based on tabulated chemical kinetic rate for ignition and flame surface density for flame propagation, taking into account interaction between flame propagation introduced by the spark and auto-ignition due to charge compression. The model is used to simulate the combustion process in an experimental HCCI engine, with operation conditions ranging from spark-ignition controlled flame propagation to auto-ignition controlled HCCI combustion. The model is shown to be able to predict the combustion behavior observed in previous engine experiments. With low initial temperature, the SI flame mode prevails; with high initial temperature, the HCCI mode prevails. With moderate initial temperature, the SI flame and HCCI ignition interact more closely, which results in higher sensitivity to the initial temperature and turbulence conditions. This may be the reason of the observed high cyclic variation in the experiments.

Flow and Temperature Distribution in an Experimental Engine: LES Studies and Thermographic Imaging

Temperature stratification plays an important role in HCCI combustion. The onsets of auto-ignition and combustion duration are sensitive to the temperature field in the engine cylinder. Numerical simulations of HCCI engine combustion are affected by the use of wall boundary conditions, especially the temperature condition at the cylinder and piston walls. This paper reports on numerical studies and experiments of the temperature field in an optical experimental engine in motored run conditions aiming at improved understanding of the evolution of temperature stratification in the cylinder. The simulations were based on Large-Eddy-Simulation approach which resolves the unsteady energetic large eddy and large scale swirl and tumble structures. Two dimensional temperature experiments were carried out using laser induced phosphorescence with thermographic phosphors seeded to the gas in the cylinder. The results revealed different mechanisms for the development of temperature stratification: intake gas and residual gas mixing, heat transfer in the wall boundary layer, compression of the charge, and large scale flow transport. The sensitivity of LES results to different wall boundary conditions and inflow conditions was analyzed. (Less)

Effects of Negative Valve Overlap on the Autoignition Process of Lean Ethanol/Air Mixture in HCCI-Engines

This paper presents a computational study of the effects of fuel and thermal stratifications on homogenous charge compression ignition (HCCI) combustion process in a personal car sized internal combustion engine. Stratified HCCI conditions are generated using a negative valve overlap (NVO) technique. The aims of this study are to improve the understanding of the flow dynamics, the heat and mass transfer process and the onset of auto-ignition in stratified charges under different internal EGR rate and NVO conditions. The fuel is ethanol supplied through port-fuel injection; the fuel/air mixture is assumed to be homogenous before discharging to the cylinder. Large eddy simulation (LES) is used to resolve in detailed level the flow structures, and the mixing and heat transfer between the residual gas and fresh fuel/air mixtures in the intake and compression strokes. Multi-Zone model based on a detailed chemical kinetic mechanism is then used to simulate the onset of auto-ignition in the combustion stroke near TDC, based on the mixtures predicted in LES. It is found that for low and moderate EGR rates (low and moderate NVO) the onset of ignition is more sensitive to the temperature of the mixture than to the fuel concentration. For the high EGR rate and large NVO case, there is a preferred mixture and temperature at which the first ignition occurs. Under similar operating conditions the moderate NVO and EGR rate case is found to have the earliest ignition, whereas the longest combustion duration is found in the lowest EGR rate and the lowest NVO case. (Less)

Large eddy simulation of turbulent flows in a laboratory reciprocating engine

Large eddy simulation (LES) of turbulent flows in an experimental reciprocating internal combustion engine was carried out. The engine had a rectangular shaped combustor geometry and rectangular channel intake and exhaust manifolds with a large optical window to allow for detailed two-dimensional velocity field measurement in the entire combustion chamber. The objectives of this work were to study the structures of the tumble flow and turbulence in the combustion chamber and to examine different approaches for characterizing the incylinder turbulent flows. LES were performed for two different engine configurations, one with the intake channel included in the simulation and one with intake flow modeled as a simple plug flow at the exit of the intake channel, to investigate the effect of intake flow on the tumble flow and turbulence. The convergence of cycle-averaged statistics was investigated. It was found that for the ensemble averaged mean flow field 10 cycles LES could give reasonably converged mean velocity; however, more than 60 cycles were needed to generate converged rms of velocity fluctuation. A global turbulence intensity defined based on single cycle LES or PIV data was analyzed. This quantity was shown to characterize the overall turbulence intensity in the cylinder reasonably well.