Experimental Study on the Heat Release of Spherical Premixed Flames

Laminar flames have been thoroughly investigated, with detailed descriptions of the flame structure and mechanisms that drive the flame propagation process. Typically, the flame structure is described as unburned zone, preheat zone, reaction zone, and burned zone. Models based on this flame structure were developed to investigate the fundamental combustion mechanisms. Theoretically, an assumed outwardly propagating spherical flame has been used to determine the flame propagation speed, especially under elevated pressures, while the heat release of spherical premixed flame is less discussed but is important to real-world applications. In this paper, the heat release of spherical premixed flame is investigated via a combination of chamber pressure, shadowgraph imaging, direct imaging, and chemiluminescence of the flame. Three types of fuels, including hydrogen, methane, and propane were used to generate stoichiometric mixtures with three types of inert gases including nitrogen, argon, and helium gas. It is observed that the spherical flame kernel demonstrates a two-stage combustion behavior, with the first stage of flame propagation with little chemiluminescence emissions, and a second stage after flame occupies most of the chamber volume with high chemiluminescence emissions. This phenomenon is more obvious under higher background densities. By cross-referencing the test results from various test methods, it is discovered that the profile of light intensity generated by chemiluminescence agrees with chamber pressure the most, reflecting the actual heat release of the air–fuel mixture. Whereas the volume of the spherical flame kernel increases without causing significant changes in chamber pressure in the flame propagation stage. 3-D simulation was also performed using detailed chemical kinetics, and the test results support the empirical behavior of a delayed heat release as compared to the flame front propagation.