Heat transfer enhancement based on single phase natural circulation loops

摘要

Heat transfer between two streams of hot and cold fluids separated by a plate is an unavoidable exergy destruction process as a whole, i.e. the exergy gain from the cold fluid cannot fully cover the exergy loss of the hot. However, it is noticed that the amount of exergy destruction can be minimized by introducing the work done by the external gravity force that the natural convection of the third fluid in a closed loop driven by the hot and cold fluids themselves can enhance their heat transfer. Based on this idea, a heat transfer enhancement unit of a closed rectangular loop was constructed, the two parallel legs of which were vertically penetrated through a horizontal plate with one half on the hot side and the remained half on the cold side. In order to clarify the mechanism of heat transfer enhancement for the natural circulation loop (NCL), a numerical simulation was performed for a comparison between the heat transfer rates of the NCL and the copper fin with the same shape and size. The effects of side length, velocities of heating and cooling air, and temperature difference on the heat transfer rate were investigated in detail. The results show that the heat transfer rate of the NCL can be larger than that of the copper fin while the temperature difference is over a critical value.