Heat Release Rate Markers for Highly Stretched Premixed CH<sub>4</sub>/Air and CH<sub>4</sub>/H<sub>2</sub>/Air Flames
Xinyi Chen, Yiqing Wang, Thorsten Zirwes, Feichi Zhang, H. Bockhorn, Zheng Chen
Abstract
Accurate quantification of heat release rate (HRR) profiles is useful for identifying important combustion processes in practical engines. This study aims to show whether the conventional HRR markers can still be used for highly stretched premixed methane/air flames with and without hydrogen addition. The correlation of formaldehyde-based molar concentrations, [H][CH2O] and [OH][CH2O], with HRR is numerically studied for highly stretched premixed CH4/air and CH4/H2/air flames. Two types of premixed flames are considered: the spherically expanding flame (SEF) starting from a highly positively stretched ignition kernel and the Bunsen flame with a high negative stretch rate at the flame tip. It is found that both [H][CH2O] and [OH][CH2O] can qualitatively describe the spatial distribution of HRR profiles in these two types of flames. In comparison to [H][CH2O], [OH][CH2O] is slightly better because it has a higher correlation coefficient and weaker sensitivity to the stoichiometry. However, during the ignition-influenced regime of SEFs, the magnitude of HRR cannot be accurately correlated by the concentrations of CH2O, OH, and H. Besides, hydrogen addition does not change the good spatial reconstruction qualities of these two HRR markers when its volume fraction in methane/hydrogen binary fuel blends is below 70%. The present results demonstrate that the conventional HRR markers, [OH][CH2O] and [H][CH2O], can be used to quantify the shape of HRR profiles even for highly stretched premixed flames with a small amount of hydrogen addition. Nevertheless, the magnitude of HRR cannot be accurately correlated by these two HRR markers for a highly stretched ignition kernel and hydrogen-dominated binary fuel blends.