Stretch in Premixed Laminar Flames Under IC Engine Conditions

Abstract

Flamelet models for turbulent combustion provide an approach (via the laminar flame) to include detailed flame chemistry into fluid dynamic simulations of IC engines. The flamelet model postulates that a turbulent flame is a statistical distribution of premixed, laminar flames. However. turbulence affects the laminar flames through strain (a) and curvature. The effect of positive strains (outflow condition) on counter-flow flames is evaluated by integrating the species destruction (or formation) through the flame.This integral is the net reaction per area of the flame and is compared to the integral for the unstrained flame. Strain reduces the net reaction rate per unit area. Flame stretch is a measure of the reduction in fuel consumption due to turbulent strain. Flame stretch is a property tending to slow turbulent flame speeds by the reduction in fuel consumption.

In a strained flame, reactant and product reaction rates are changed by different amounts indicating that the internal structure of the flame is affected by strain. Analysis of engine conditions suggests that the average strain varies from 1000-120,000sec_-1 dependent on rpm and intake produced turbulence. A strained laminar flame with a = 50,000sec= under conditions typical of a motored engine at TDC loses 50% of the net formation rate of CO₂ compared to the unstrained flame. This flame can tolerate strains over a range of engine conditions, but not without distortion. Reducing φ to 0.6 drastically decreases the tolerance of the flame to strain. The effects of strain on these flames due to cylinder residuals and EGR have not yet been evaluated. This analysis of strain from engine turbulence on a laminar flame suggests that flame stretch should be included in turbulent combustion models in homogeneous charge engines.