Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.
Heat transfer in laminar pulsating flow past a heated flat plate is investigated. In contrast to previous investigations, the response of the wall heat flux to both harmonic velocity and temperature fluctuations is considered over a wide range of pulsation amplitudes and Strouhal numbers. The reason for choosing the flat plate configuration is that for one given oscillation frequency, both low- and high-frequency regimes can be observed in a single simulation. Due to the spatial development of the hydrodynamic and thermal boundary layers along the length of the plate, the Stokes length is first larger (upstream section), then (downstream section) smaller than the local boundary layer thickness. Firstly, the case with constant wall temperature is studied, where fluid temperature fluctuations are locally generated by velocity perturbations in the presence of mean flow gradients. Depending on the local Strouhal number, which controls boundary layer phase lags, the local wall heat flux can be enhanced as well as decreased. This effect is due to a nonlinear interaction between velocity and temperature fluctuations. Secondly, periodic temperature fluctuations of the wall temperature are imposed too, and the deviations from the stationary case are stronger. At high velocity amplitudes, secondary flows induced by viscous forces near stagnation points come into play and can either support or impede the enhancement of heat transfer.