Fig. 4 gives the influence of x on the Sgu for varying heat transfer rate Q . It shows clearly that the specific entropy generation rate Sgu increases with increasing the heat transfer rate Q . Moreover, the Sgu varies more sharply with x for a larger heat transfer rate Q ,which indicates the importance of optimally allocating the total heat transfer area especially under a bigger Q . However, even for the case of Q ¼ 300 W, Sgu is also decreased noticeably by 16% from 5.69 to 4.77 W/m2K through the optimization process with x ranging from 0.3 to 0.82 as shown in Fig. 4. The optimal value of x
corresponding to minimum Sgu decreases with increasing Q . The optimal value of total heat transfer allocation ratio x
is 0.63 for Q ¼ 300 W, xopt is 0.52 for Q ¼ 320 W, and xopt is 0.48 for Q ¼ 340 W.