ENHANCEMENT OF HEAT-TRANSFER WITH SWIRLING FLOWS ISSUED INTO A DIVERGENT PIPE

Experiments have been performed to study the heat transfer process of swirling flow issued into a heated divergent pipe with a divergent ang le of 7 deg with respect to the pipe axis. A pat vane swirler situated at the entrance of the pipe is used to generate the swirling flow. Th e heat transfer results along the pipe wall demonstrate that at low Re ynolds numbers without the swirl, the separation point of the recircul ation flow caused by the divergence of the pipe makes a minimum in the heat transfer. The presence of the swirler itself can have a signific ant effect on the heat transfer. The minimum disappears and the heat t ransfer results approach the case of a constant diameter pipe when the how is swirled. The enhancement of the heat transfer is small and is mostly caused by the swirl velocity. At high Reynolds numbers, the lar ge turbulence intensity generated by the impingement of the flow strea m on the swirl vane can significantly enhance the heat transfer. A qua litative flow structure can be inferred from the heat transfer measure ments, During the experiments, the Reynolds number ranges from 10(4) t o 2 x 10(5), and the swirl number from 0 to 0.65. Because the increase of the heat transfer with the swirl number is monotonic, correlations of the Nusselt number in terms of the swirl number and the Reynolds n umber are obtained.