Vortex heat transfer enhancement in the narrow plane-parallel channel with the oval-trench dimple of fixed depth and spot area

Abstract

The article is devoted to the analysis of vortex heat transfer enhancement due to the use of oval-trench dimples. The main role in the understanding of this process is associated with the application of the technologies developed in engineering practice thanks to design decisions. As a result, prevailing interest has been paid to a spherical dimple when a spot diameter is chosen as a characteristic size, whereas hydraulic losses depend on the dimple-to-device size ratio. Progress in vortex heat transfer enhancement due to the use of oval dimples is connected with the explanation of the mechanism of generation of both vortex structures in dimples and spiral vortices behind them. An abrupt increase of heat transfer in the vicinity of the spherical dimple due to the restructuring of the flow structure in the dimple with two vortices to that in the dimple with one spiral vortex made it possible to propose a new shape of a surface vortex generator – an oval dimple located at an angle of inclination to the incoming flow and consisting of two spherical dimple halves separated by a cylindrical insert. The generation of vortex structure in this case is rather stable and intense in comparison to spherical dimple. The numerical results for vortex heat transfer enhancement in the turbulent water flow in the rectangular narrow channel with spherical, 10°-truncated conical and oval dimples of the same spot area and depth at the heated wall are presented. In the article, central attention is given to the mechanism of secondary flow restructuring and heat transfer enhancement due to increase in a relative length and width of an oval dimple followed by the formation of a long spiral vortex in it. The change in the length of the oval dimple (in terms of its width) from 1 to 6.78 allowed one to rationally mount spiral vortex surface generators in the narrow channel with high thermal and thermal-hydraulic efficiencies, significantly exceeding the identical characteristics of channels with spherical and conical dimples. In this case, moderate hydraulic losses in the channel with an oval-trench dimple, when its length is increased to 6.78, are comparable to those in the channel with a basic spherical dimple.