Fig. 2 shows the variation of Sgu with the ratio of the hot-side heat transfer area to the total heat transfer area under
different total heat transfer area A, where the heat transfer rate is assumed to be Q ¼ 300 W, the hot-side mass flow rate
_ m h ¼ 0 :002 kg/s, and the mass flow rate ratio f = 2. From Fig. 2 ,it can be seen that for a given A, Sgu can always reach a minimum value with varying x , which means that there always exists an optimal xopt corresponding to the minimum Sgu under the constraint A. In addition, under a smaller A, the Sgu varies more sharply with x, which indicates that the allocation ratio x has a larger influence on the value of Sgu. Obviously, the optimal total heat transfer allocation ratio xopt has a smaller value under smaller A. It is found that xopt is 0.63 for the case of A