Numerical study on the temporal variations and physics of heat transfer coefficient on a flat plate with unsteady thermal boundary conditions

Numerical simulations with unsteady RANS method have been performed to study the effect of unsteady thermal boundary conditions on the temporal behavior of heat transfer coefficient in unsteady heat transfer processes. Steady flow velocity condition of U = 21 m/s was used in the computational models, including conjugated model and unconjugated models, to focus on the effects of unsteady flow temperature and thermal boundary conditions on the wall. Especially for the unsteady heat flux boundary condition, analysis has been performed on the effects of pulsation frequency, mean value and amplitude which range from 0 to 8 Hz, 1000-4000W/m2 and 500-1000W/m2 respectively. Temporal behavior of two kinds of heat transfer coefficient with different reference temperature has been analyzed. The results show that the conventionally defined heat transfer coefficient hTg(τ) with the mainstream temperature pulsates intensely for the conditions with unsteady flow temperature. When the local adiabatic wall temperature Taw(τ), which has phase shift from local mainstream temperature Tg(τ), is used to define the heat transfer coefficient hTaw(τ), it can exclude the influence of unsteady flow temperature and make the hTaw(τ) be a good invariant descriptor in heat transfer under unsteady flow temperatures. Unsteady heat flux boundary condition, which can cause unsteady hTaw(τ) under flow conditions with steady velocity and temperature, is an important reason for the temporal variations of heat transfer coefficient in unsteady heat transfer processes. However, the temporal behavior of heat transfer coefficient keeps the same under different unsteady heat flux boundary conditions which are temporally similar. The reason for the effects of adiabatic wall temperature and heat flux boundary condition on the temporal behavior of heat transfer coefficient has been revealed with an analysis method on the spatial distribution of the “Unsteady Term” ∂T(y)/∂τ.