The abnormal heat transfer behavior of supercritical n-decane flowing in a horizontal tube under regenerative cooling for scramjet engines

This paper aims to describe the reasons why abnormal heat transfer occurs under the condition of regenerative cooling applied in scramjet engines. Supercritical n-decane flowing in a horizontal tube under various mass flow rates (G = 0.001, 0.002, 0.003, 0.004 kg/s) and heat fluxes (q_w = 100, 200, 300, 400 kW/m²) is investigated by numerical simulations. The mechanisms of the abnormal heat transfer were explained from the perspective of thermal diffusivity, velocity and the process of vortex evolution. The great temperature difference generated by the extreme operating condition (low mass flow rate and large heat flux) contributes to the higher buoyancy force inside the tube. Several interesting phenomena are found due to the existence of the vortices and a very special one is the slight downturn when the wall temperature reaches the first maximum point. In addition, the basic principles of the abnormal heat transfer are represented by the regularity of the distribution of thermal diffusivity, and mainstream velocity. An important conclusion is that the thinner boundary layer of the flow generated by the high velocity in the mainstream direction is benefit for restraining the abnormal heat transfer.