SUMMARY
This work paid special attention on the laminar flame speed in a piloted methane-air in jet diffusion flame due to the strong nonlinearity of chemical reaction process where an extension for the unsteady laminar flamelet model is required. The purpose for doing that is set for two folds. The partial differential equations for solving the combustion model required the representation of laminar flame speed in it and the advanced combustion modeling which is promising step. The probability density function required the laminar flame speed to be modelled as a function in terms of the mixture fraction to perform the integration. The laminar flame speed model is discussed, and the results are compared with the experimental database with good accuracy. The model specifies the conditional laminar flame speed and the difference against the gas flow velocity.