Uniformity Principle Of Temperature Difference Field In Heat Exchanger And Its Applications

The enhancement of thermal performance has been the main subject of heat exchanger study. Extented surface, turblent promotor or applling external magnetic / electric field are adopted to enhance the heat transfer.In this paper the Uniformity Principle (UP) of Temperature Difference Field (TDF), is devloped and applied to enhance the thermal performance of heat exchanger, that is , analytical and numerical results for eleven types of heat exchangers , show that the more uniform the TDF in heat exchanger, the higher the heat exchanger effectiveness, for the given Ntu ( number of heat transfer units) and C_r (heat capacity rate ratio),and thus, the validity of the Uniformity Principle of TDF. It is found that the overall heat transfer will be increased if there are more heat transfer area in the diagonal region starting from the crosspoint of two streams for crossflow heat exchanger. Increasing the heat transfer surface in the region at large inlet fluid velocity and in large inlet fluids temperature difference may improve apparently the heat transfer rate of heat exchanger. The rearrangement of heat transfer surface in parallelflow heat exchanger according to the UP of TDF will also improve the heat transfer rate.It is found that the effectiveness of parallelflow heat exchanger could be larger than the counterflow heat exchanger under some conditions when there is a heat sink (source) on the stream .This phenomenon conform to the uniformity factor of TDF. An experimental setup for studying the relationship between the effectiveness and unifromity factor of TDF was established.The experimental results indicate that the more uniform the TDF in heat exchangers, the higher the heat exchanger effectiveness for the given Ntu and C_r.The heat transfer area for two kinds of typical air coolers can be saved 8-20% by rearranging the heat trasfer area distribution and connection between tubes.