EXPERIMENTS ON EFFECT OF JET IMPINGEMENT ON HEAT TRANSFER IN ANNULAR CHANNELS WITH PARALLEL AND COUNTER FLOWS

摘要

This experimental research examines, for turbulent parallel and counter flows of air through an annular channel, the effects of varying the geometries of the channel and the array of holes along the inner tube on the heat transfer distribution on the inner surface of the outer tube. Each hole array has 5 or 6 inline or staggered circular holes around the circumference of the inner tube at 10 axial stations along the inner tube. Heat transfer experiments are performed for three inner tube diameters, two hole diameters, and Reynolds numbers of 5,000, 12,250 and 30,000, to determine the distribution of the regional average Nusselt numbers along the outer tube, as a result of the jets impinging on its inner surface. Pressure measurements give the overall pressure drops, and the pressure distributions along the inner tube and the annular channel between the inner and outer tubes. The pressure data is needed to determine the mass flow rates of the impinging jets along the inner tube. The jets along the inner tube enhance the regional heat transfer on the inner surface of the outer tube by up to eight times when compared with the heat transfer for fully developed turbulent flow through an annular channel. Heat transfer enhancement is higher for a smaller inner tube and a lower Reynolds number. In the parallel flow case, the heat transfer coefficient on the outer wall of the annular channel is higher near the downstream end of the annular channel, while in the counter flow case, the heat transfer coefficient is higher near the upstream end of the annular channel. For both parallel and counter flows, the heat transfer coefficient is higher in a channel with a larger inner tube. With smaller holes along the inner tube, the heat transfer coefficient along the outer tube is higher and more uniform. Smaller holes, however, cause a higher overall pressure drop across the annular channel, resulting in a lower thermal performance. Increasing the total number of holes lowers the heat transfer, and causes a lower overall pressure drop. The hole mass flow rate increases along the main flow direction in the annular channel for parallel flow, and decreases for counter flow. The variation of the hole mass transfer is smaller for a larger inner tube diameter.