Evaluation of an Unsteady Flamelet Progress Variable Model for Autoignition and Flame Lift-Off in Diesel Jets

An unsteady flamelet progress variable (UFPV) model is evaluated for modeling autoignition and flame lift-off in diesel jets. Changes in injection pressure, orifice diameter, ambient temperature, density, and O-2 concentration are considered. In implementing the model in a Reynolds-averaged NavierStokes (RANS) code, a look-up table of reaction source terms is generated as a function of mixture fraction Z, stoichiometric scalar dissipation rate (st) and progress variable C-st by solving the unsteady flamelet equations. It is assumed that the probability density functions (pdfs) of Z, (st), and C-st are statistically independent, and presumed functions are employed for the pdfs. Comparisons with experimental results show that the model is able to predict ignition delay and flame lift-off with reasonable accuracy in the RANS simulations. The quantitative agreement between computed and measured results depends on the definitions employed to quantify autoignition time and lift-off height, but, in general, the agreement is within 25%.