Convective Heat Transfer Enhancement: Mechanisms, Techniques, and Performance Evaluation

In this chapter, the existing mechanisms for enhancing single-phase convective heat transfer are reviewed and the fundamental mechanism, that is, to reduce the intersection angle between fluid velocity and temperature gradient, is presented in detail. This basic idea is called the field synergy principle (FSP). A great number of examples are provided to demonstrate the validity of the FSP. Some typical convective heat transfer phenomena are analyzed and found that their characteristics can be well understood by the FSP. An effective way for improving convective heat transfer performance of an existing heat transfer structure is to reveal the locations with a bad synergy (i.e., large local synergy angle) and improve the performance by changing the local structure of the surface. Examples of new enhanced surfaces are provided which are developed under the guidance of the FSP. It is demonstrated that for the best synergy case where fluid velocity coincides with temperature gradient, the exponent in Nu∞Re^m reaches its maximum value of 1. Then, the thermohydraulic performance comparisons of the enhanced configurations with the reference one are discussed under three constraints: identical pumping power, identical pressure drop, and identical flow rate. All the three constraints can be unified in a picture with log(f_e/f_o) and log(Nu_e/Nu₀) as abscissa and ordinate, respectively. The entire plane is divided into four quadrants by the two coordinates, and the first quadrant is the most frequently encountered. An enhanced technique can be represented in this plot and the constraint under which heat transfer is enhanced can be clearly identified.