Effect of CH4, Pressure, and Initial Temperature on the Laminar Flame Speed of an NH3–Air Mixture

Ammonia (NH3) is not only expected to be used as a hydrogen energy carrier but also expected to become a carbon-free fuel. Methane (CH4) can be used as a combustion enhancer for improving the combustion intensity of NH3. In addition, it is important to understand the flame characteristics of NH3–air at elevated pressures and temperatures. The laminar flame speed of NH3–CH4–air is numerically investigated, where the mole fraction of CH4 ranges from 0 to 50% in binary fuels and the pressure and initial temperature are up to 10 atm and 1000 K, respectively. The calculated value from the Okafor mechanism is in excellent agreement with experimental data. The CH4 in the fuel affects the flame speed by changing the main species of free radicals in the flame; the high pressure not only increases the rate-limiting reaction rate in the flame but also reduces the amount of H, O, and OH radicals in the flame, so as to restrain the propagation of the flame. At a higher initial temperature, the faster flame speed is mainly due to the higher adiabatic flame temperature. The laminar flame speed correlation equation has a consistent trend with the simulation results, though with a slight underestimation at higher pressures and temperatures. It is a more effective way to calculate the laminar flame speeds of NH3–air for a given pressure and temperature.