Effects of radiation reabsorption on the laminar burning velocity of methane/air and methane/hydrogen/air flames at elevated pressures

Due to the development of high-load and high-pressure combustion equipment, it is crucial to increase the accuracy of laminar burning velocity prediction under elevated pressures. However, the radiation ability of methane under high pressures is ignored, which may cause large discrepancies and fluctuations in the simulation of burning velocity. In this study, the HITRAN 2016 database was employed to build the Statistical Narrow-Band (SNB) model of methane at elevated pressures. The effects of radiation reabsorption on the laminar burning velocity were investigated in methane/air and methane/hydrogen/air flames using different radiation models. The simulation was conducted within a wide equivalence ratio (phi = 0.6-1.4) and the calculated pressures were up to 20 atm. Comparing the results of different radiation models revealed that the laminar burning velocity was promoted by radiation reabsorption. For varying equivalence ratios at P = 5 atm, in the case of phi = 0.6-1.2, the preheat-induced chemical effect by radiation reabsorption was the major factor affecting the burning velocity, controlled by R290 CH2 + O-2 <=greater than 2H + CO2 and R86 2OH <=> O + H2O. And the direct radiation effect played the key role with the equivalence ratio between 1.2 and 1.4, enhanced by increasing concentration of methane. For methane/air flames under stoichiometric conditions, the effects of radiation reabsorption increased with pressure. The preheat-induced chemical effect, related to the consumption of H radical through R35 H + O-2 + H2O<=>HO2 + H2O and R43 H + OH + M<=>H2O + M, was significant at P = 1-10 atm. And the direct radiation effect was the dominant factor above 10 atm by increasing optical thickness. Besides, the change of preheat-induced chemical effect by radiation reabsorption caused by H-2 addition was examined. The results clarified that the dominant reactions remained the same, while the range of pressures where each reaction dominated changed as 20% H-2 was added, owing to the higher sensitivity to H-2 addition for R43 comparing to R35.