Transitional Heat Transfer and Turbulent Characteristics of Drag-reducing Flow Through a Contracted Channel

Abstract

Accompanying the significant reduction of drag of turbulent flow when a drag-reducing surfactant is used, there is a serious reduction of heat transfer, typically 80%. Since the drag reduction can be terminated by a large wall shear stress, the drag-reducing flow of surfactant solution has a special property of the diameter effect. Based on this, a contracted section was designed in a two-dimensional channel to control the drag-reducing flow of a surfactant solution so as to produce turbulence for heat transfer purposes when it passes through the contracted part at certain Reynolds numbers. The same level of turbulence intensity as that of water flow is approached when the surfactant solution passes through the contracted section at certain Reynolds numbers. The heat transfer coefficient thus achieves 70 to 80 percent of that of water flow in the contracted section. In the non-contracted section the friction drag keeps the low level as drag-reducing flow. It is shown that the termination of the drag-reducing effect of the surfactant solution within a heat exchanger is possible by using the present method. The studied surfactant solution is CTAC/NaSal/Water at the temperature of around 30°C with the concentration ranging from 30 ppm to 60 ppm.