Effects of the physical properties and chemical reactions of hydrocarbon fuel on the interaction between discrete film jets and the mainstream

The use of gaseous hydrocarbon fuel in discrete film cooling has been shown to be an efficient approach for meeting the internal thermal protection needs of scramjet engines. The film cooling process using large molecular hydrocarbon fuels differs from conventional inert cooling gases, as their physical properties and chemical reactions significantly influence the interaction between the film outflow and the main flow, thereby affecting overall film cooling effectiveness. The study aims to conduct numerical investigations to explore the influence of coolant physical parameters, chemical characteristics, and the structural configuration of discrete holes on the cooling efficiency of cylindrical discrete film cooling systems. The results show that the cooling efficiency of the inert hydrocarbon fuel discrete film surpasses that of the air discrete film under identical blowing ratio conditions, and the chemical reactions of the hydrocarbon fuel can further increase the spreading cooling range of the discrete film. The trend of secondary flow intensity within the discrete hole is correlated with the hole length-to-diameter ratio, and the air medium and hydrocarbon fuel have different sensitivities to changes in this ratio, which are affected by viscous dissipation effects. Furthermore, the chemical reactions of the hydrocarbon fuel enhance the lateral cooling range of the discrete film. The chemical reactions weaken the intensity of kidney-shaped vortex pairs within the jet, which is reflected in the spreading direction moving toward the outside of the mixing layer, promoting the expansion of the coverage range of the discrete film. Moreover, increasing the L/D ratio from L/D = 2 to L/D = 5 results in a 10 % expansion of the lateral cooling range of the hydrocarbon fuel discrete film. The length-to-diameter ratio of the discrete film hole influences the external flow characteristics of film cooling by altering the internal turbulence, thereby impacting the overall cooling performance of the discrete film.