Numerical Investigation of Entropy Generation in the Case of a Finned Oval Tube With a Punched Longitudinal Vortex Generator in Form of Delta Winglet

Abstract

In order to improve the performance of compact heat exchangers engineers and researchers explore different passive techniques of flow manipulation. Among them one can find the delta wing shaped longitudinal vortex generator. In order to assess the optimal shape of finned tube heat exchangers, the engineer has at disposal many coefficients, such as the Colburn factor j and friction factor f. In the present paper the second law of thermodynamics is introduced to explore the flow and thermal field generated by punched longitudinal vortex generators shaped as winglets around an oval tube. The winglets are in common flow up configuration near the leading edge of the fin. Not only the heat transfer and fluid flow characteristics are studied, but also the local irreversibility methodology is applied to predict the two components of entropy generation rate: the one caused by direct dissipation and the other due to heat transfer. The flow velocity and temperature are numerically determined by solving the Navier-Stokes and energy equations with a finite volume method. The local entropy production is then calculated with the use of available information from the solved flow and thermal fields. This paper is based on Chen works. He has studied the flow characteristics for such a geometrical configuration. But here the fin efficiency is supposed to be equal to the unity. The influence of the angle of attack of winglets on the entropy production is studied. Three elemental configurations are displayed. Each one corresponds respectively to an angle of attack β equal to 20°, 30° and 45°. The minimal entropy principle is adopted to evaluate a global heat exchanger build up as a pile up of elemental component.