Large eddy simulation of enhanced heat transfer in pulsatile turbulent channel flow

Heat transfer in pulsatile turbulent channel flow is investigated by means of Large Eddy Simulation. Incompressible flow within a periodic computational domain is driven by a pulsating axial pressure gradient at a turbulent Reynolds number of Re_τ=350. A localized dynamic sub-grid scale approach is chosen to model unclosed stress terms. A layer-averaged sub-grid model determines turbulent Prandtl numbers that depend on wall distance. Compared to the existing literature, a much wider range of oscillation parameters is studied. In particular, forcing frequencies correspond to Womersley numbers from Wo=14 to 70, while forcing amplitudes reach values that result in strongly pronounced flow reversal, i.e. reverse flow velocities up to five times larger than the mean flow velocity. At moderate pulsation amplitudes, i.e. in the range of emerging flow reversal, strong deviations of instantaneous heat transfer rates from the temporal mean are observed. Particularly at times of flow reversal, an increase in heat transfer up to 60% above non-pulsatile values is observed. However, when averaging over a complete cycle, any enhancement in mean heat transfer is only minor. On the other hand, simulations at larger pulsations amplitudes that result in strongly pronounced flow reversal, show a enhancement of mean heat transfer in excess of 100%. Note that such significant enhancement of heat transfer has not been reported previously in Large Eddy Simulation of turbulent pulsatile flow. The paper offers a physical interpretation of the results and concludes that an overall increase in turbulent transport is responsible for the observed significant enhancement of convective heat transfer.