Sherif H. El Tahry

Probability density function approach for multidimensional turbulentflow calculations with application to in-cylinder flows in reciprocating engines

The probability density function (pdf) method is extended to two- and three-dimensional transient turbulent flows. The numerical approach couples a Lagrangian Monte Carlo method to solve for the joint pdf of velocity and scalar compositions with an Eulerian finite-volume algorithm to calculate mean pressure and a turbulence time scale. A general approach applicable to variable-density chemically reacting flows has been taken; the present results are for nearly uniform density inert flows.

Structure and propagation speeds of turbulent premixed flames—A numerical study

Abstract The aim of the work was to gain understanding of turbulent premixed flames, particularly the local flame speed and structure. Full numerical simulations were made for flames (with simple reactions and Lewis number unity) propagating in a constant density, isotropic turbulent flow; the Reynolds and Damkohler numbers were varied. The range of Damkohler numbers allowed investigation of both the unstrained and strained flame regimes. The latter included cases in which local flame extinction was observed. The results showed that the local structure of weakly strained flames, i.e., those at high Damkohler numbers, is similar to that of an unstrained, one-dimensional, steady, planar laminar flame. Moreover, the local propagation speed had a narrow probability density function peaked at the speed of the laminar flame. The results support the validity of flamelet modeling concepts which are used in current combustion models. However, at small values of the Damkohler number, the local flame structure is complex and flamelet models may not be appropriate.

FULL NUMERICAL SIMULATIONS AND MODELING OF TURBULENT PREMIXED FLAMES

Premixed flame propagation in homogeneous isotropic turbulence is studied using full numerical simulations. Low heat release (constant density), low Mach number, and single step Arrhenius kinetics are assumed. Effects of the turbulence on the flame are studied with results focusing on the global aspects as characterized by the turbulent flame speed. Correlations of the flame speed with the turbulent fluctuating velocity and the turbulent Reynolds number are made; good agreement is found with experimental correlations in the literature. The flame exhibits a transient response to the turbulence as evidenced by the flame speed which first increases and then decreases. The increase occurs despite the fact that the turbulence is decaying. A concept of flame-turbulence equilibrium is introduced to account for this behavior. Equilibrium is defined as occurring when the flame speed begins to decay along with the turbulence. The time to achieve equilibrium was found to be slightly longer than the turbulence integral time scale. Probability density functions of the principal curvatures of the flame are examined. Results indicate the flame responds to all turbulent length scales, and that positive and negative curvatures are equally likely. Insights gained from the simulations are brought together in a model which tracks the evolution of the flame speed and curvature. Agreement of the model with simulation data is very good indicating promise for use of a similar model in more complex flows.