09. Heat and Mass Transfer

This study deals with the feasibility of active temperature control with a pulsating flow for various industrial applications which operate under thermal unsteady state conditions such as reciprocating heat engines, metal castings and so on. The authors focus on the heat transfer characteristics on the flat plate installed in a pulsating duct flow. Experiments aiming to make clear the effect of flow conditions (time-averaged flow rate, pulsating frequency and amplitude) on heat transfer in pulsating flows with two different flow-rate-fluctuation modes (continuous sinusoidal wave and intermittent pulse wave). In the past experiments, regardless of pulsation mode, no difference in heat transfer between steady flow and pulsating flow was obtained when flow was supplied with uniform velocity distribution at inlet. On the other hand, heat transfer enhancement by flow pulsation was confirmed when flow was supplied with non-uniform velocity distribution at inlet, regardless of pulsation mode. This tendency was confirmed in the both flow regimes of laminar and turbulent flows. To elucidate the mechanisms of such heat transfer enhancement in the cases with non-uniform inflow conditions, local velocity measurements of its temporal fluctuation and time-averaged value in span-wise direction were carried out. Time-averaged velocity profile was obtained by traversing a hot wire in span-wise direction. Measurements were conducted for four cross sections in the flow direction. The results showed that time-averaged velocity profile changes along the flow direction from non-uniform to uniform one. This tendency was observed more clearly with increasing pulsating frequency. Momentum exchange in span-wise direction seems to be enhanced by flow pulsation, which causes local flow rate increase near the flat plate. In addition, local turbulence was also observed in the temporal velocity fluctuation under high frequency conditions. This local turbulence may contribute the heat transfer enhancement as well as the one by facilitating momentum exchange.