The Effects of Temperature and Pressure on Stretched, Freely Propagating, Premixed, Laminar Methane-Air Flame 2006-01-0494

Flame stretch arises due to strain and change in flame curvature and is extremely important in spark ignition (SI) engine combustion. It can significantly alter the flame speed and hence, the burning duration. This, in turn, can have serious influence on engine performance and exhaust emission. A good understanding of stretch effect can thus lead to improved fuel efficiency, reduced cyclic variations, and better emission control for SI engines. In this study, we analyzed initial temperature and pressure effects on the stretched flame speed of a laminar, premixed, freely propagating, spherical methane-air flame in accordance with the Markstein theory, which postulates a linear relationship between stretched and unstretched flame speed. The unstretched flame speed was computed using CHEMKIN GUI 4.0.1 with GRI Mech 3.0 reaction mechanism. Analytical expressions relating stretched and unstretched flame speed were employed to derive the stretched flame speed. Fuel/air equivalence ratio was varied from 0.6 to 1.4, while initial temperature and pressure were altered from 300 to 500 K and 1 to 3 atm, respectively. It is observed that in this particular type of flame, stretch always decreases the flame speed. Further, the stretch effect is minimal near the stoichiometric composition and is enhanced for rich and lean mixtures, by around 10 times with respect to the stoichiometric mixture under atmospheric conditions. The stretch effect also decreases with increase in temperature and/or pressure, over the range of conditions considered.