Circular tubes are common heat transfer units used in tube-andshell heat exchangers. Fig. 1 shows the model of a circular tube whose diameter D is 20 mm and length L is 1700 mm. As considering the inlet and outlet effect, the tube is divided into three parts which are inlet region, stable region and outlet region. The length of inlet region L1 is 1200 mm. Stable region is the calculation domain of heat transfer optimization and its length L2 is 300 mm. The length of outlet region L3 is 200 mm. Thus water flows with an inlet Reynolds number and achieves fully development in the end of inlet region, then the additional force affect the flow in the stable region so as to get the optimum velocity field.