LAUSR

Abstract Gravity and/or flight acceleration would lead to significant buoyancy effect on heat transfer of a hydrocarbon fuel in the propulsion system. Large eddy simulations have been conducted to study upward flows and heat transfer of n-decane in a vertical tube at supercritical pressures. Detailed data analyses indicate that the fluid flow and heat transfer process with buoyancy effect can be divided into three stages. In the first stage, owing to the relatively low inlet Reynolds number, flow and heat transfer are initially in the laminar state. Large density reduction under strong heating at a supercritical pressure leads to flow acceleration, which is mainly due to buoyancy and helps improve heat transfer. Further downstream, buoyant acceleration results in an M shaped velocity profile, which promotes the shear production of turbulence. Turbulence is also strongly generated by buoyant production at this stage. In addition, viscous dissipation decreases at high temperature. These factors eventually cause turbulence transition. Heat transfer is thus significantly enhanced, and the wall temperature is drastically reduced. Comparing to the cases of forced convection, buoyancy could promote or delay turbulence transition under different surface heat fluxes. In the third stage, fluid flows and heat transfer enter the fully developed turbulence region, which is maintained mainly by the shear production mechanism. Numerical analyses herein would provide fundamental understanding of buoyancy effect on supercritical-pressure heat transfer of hydrocarbon fuel in the regenerative engine cooling application.