Transitional heat transfer and turbulent characteristics of drag-reducing flow through a contracted channel

Accompanying the significant reduction of drag of turbulent flow when it dr ag-reducing surfactant is used, there is a serious reduction of heat transf er, typically 80%. Since the drag reduction can be terminated by a large wa ll shear stress, the drag-reducing flow of surfactant solution has a specia l property of tl-le diameter effect. Based on this, a contracted section wa s designed in a two-dimensional channel to control the drag-reducing flow o f a surfactant solution so as to produce turbulence for heat transfer purpo ses when it passes through the contracted part at certain Reynolds numbers. The same level of turbulence intensity as that of water Row is approached when the surfactant solution passes through the contracted section at certa in Reynolds numbers. The heat transfer coefficient thus achieves 70 to 80 p ercent of that of water flow in the contracted section. In the non-contract ed 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 a round 30 degreesC with the concentration ranging from 30 ppm to 60 ppm.