Development of Transient Flamelet Library Based Combustion Models

Abstract: Three different methods for Reynolds-averaged Navier-Stokes computational fluid dynamics modeling of non-premixed ignition and combustion using tabulated chemistry have been developed. All methods make use of flamelet libraries, where the flamelet auto-ignition process is parameterized using a progress variable. The progress variable parameterization of the auto-ignition chemistry allows for using arbitrarily large chemical mechanisms, at constant computational cost; and for modeling of turbulence-chemistry interactions. In the first method, a coordinate transform from time and space to a space described by mixture fraction and progress variable is made. The method was shown to be capable of predicting the response of injection pressure and nozzle diameter on lift-off length. It was shown that it was possible to apply the method for use in computational fluid dynamics simulations of compression-ignited engine combustion. In the second method, the transient flamelet libraries were directly used in an interactive flamelet setting. It was investigated if it was possible to generate tables by computing homogeneous adiabatic constant-pressure reactors instead of igniting flamelets. It was found that omitting the effect of scalar dissipation rate during the tabulation process leads to an error in prediction of ignition delay. In the third method a simplified conditional moment closure approach was developed. By using tabulated chemistry, and by making the conditional moment closure for the progress variable only, it was possible to use the same computational grid as used by the flow solver for the spatial transport of the conditionally averaged scalars. This method was tested for a simple auto-igniting spray configuration and it was found that it was able of capturing the response on ignition delay and lift-off length due to changed ambient oxygen level. Software technical improvements from the transient flamelet library based approaches were carried over to the stationary flamelet library based soot source term model, and further model updates yielded a model capable of predicting soot emissions for a light-duty diesel engine.