Numerical Investigations on Enhancement of Heat Transfer in a Compact Fin-and-Tube Heat Exchanger Using Delta Winglet Type Vortex Generators

Abstract

Three dimensional flow and heat transfer in a compact fin-tube cross flow heat exchanger have been modeled as the flow in a rectangular channel with built-in cylindrical obstacles. The flow past a tube bank has been simulated by a spacewise periodic fully developed flow in a computational domain consisting of a channel element with one tube. Flow and temperature fields in this element and in another element with two in-line tubes have been computed from the numerical solution of three dimensional Navier-Stokes and energy equations. Comparison of results show that at low Reynolds number (∼ 400) the Nusselt number in the neighborhood of the second tube of a two tube in-line configuration is close to the Nusselt number for spacewise periodic fully developed flow. Computational results also show poor heat transfer in the wake region. In the presence of the winglet type longitudinal vortex generators in the wake behind the cylinder, heat transfer in this region is enhanced significantly. Results show increases in average Nusselt number due to the vortex generators can be between 20 and 50 percent in the Reynolds number range of 400 and 1200. A factor, defined as the ratio of increase in Nusselt number to increase in friction factor, of 0.65 to 0.78 can be obtained, over the range of Reynolds number mentioned above.