Experimental study on heat and momentum transfer for turbulent flow in rectangular channels with a sharp turn

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Rectangular cross-sectioned serpentine channels with a sharp 180-degree turn are often used in thermal equipment such as compact heat exchangers and internal cooling passages in gas turbine blade. In such channels with a 180-degree turn, the secondary flow is induced by the centrifugal force in flowing around the turn section, and the separation and reattachment of the flow also occur in and after the turn section. Thus, the turbulent flow characteristics in the channel become very complex with a three-dimensional structure, and the local heat transfer rates are expected to change steeply over small distances on the heat transfer surface.In this study, detailed characteristics of heat transfer for turbulent flow in the rectangular cross-sectioned serpentine channels with a sharp turn have been examined experimentally using the naphthalene sublimation technique. Among many possible experimental parameters, main attention has been directed to the influence of the following for parameters on the heat (mass) transfer characteristics in the channel ; (i) Turn clearance, (ii) Reynolds number, (iii) Flow-inlet condition at the channel entrance, (iv) Inclination angle of the partition wall. The local heat (mass) transfer characteristics on all the walls of the channels and distributions of the local wall pressure have been measured under the condition of turbulent flow.Next, the flow characteristics in the channels have been measured by LDV and PIV, directing a special attention to the influences of the turn clearance, which is the most important geometrical parameter in this type of channel, on the flow structure in and after the turn section. Then, by comparing the distributions of the local heat (mass) transfer rates on the spanwise centerline on the channel walls with those of the mean velocities and turbulence intensities near the walls, the mechanism of heat (mass) transfer from the channel walls have been examined.