Maximum stretched flame speeds of laminar premixed counter-flow flames at variable Lewis number

This paper examines Lewis-number effects on stretched, laminar, premixed flames near extinction. It presents the experimental measurement of maximum stretched flame speed and extinction limit for the premixed laminar combustion of selected low, unity and high-Lewis number mixtures. Stretched, fuel-lean, laminar flames of methane with Le≅1, propane with Le>1 and hydrogen with Le≪1 are studied experimentally in a counter-flow flame configuration. Flow velocity is measured in these flames by particle tracking velocimetry. Results show that a maximum reference flame speed exists for mixtures with Le≳1 at lower flame-stretch values than the extinction stretch rate. In contrast, a continually-increasing reference flame speed is measured for Le≪1 mixtures until extinction occurs when the flame is constrained by the stagnation point. Laminar flame results are also compared to numerical simulations employing a one-dimensional stagnation flame model. The chemical-kinetic models for each respective mixture capture the important trends of a maximum in su,ref ahead of the extinction stretch rate for methane and propane, and an increasing su,ref to extinction for hydrogen. These results are important to the investigation of the leading edge theory of premixed turbulent combustion, in which maximum stretched flamelet speed is a key parameter.