The mechanism of heat transfer enhancement using longitudinal vortex generators in a laminar channel flow with uniform wall temperature

Abstract

Heat transfer enhancements are used extensively to improve the performance of heat exchangers and to reduce their running cost. Longitudinal vortices can enhance heat transfer greatly. To enrich the mechanism of heat transfer enhancement produced by longitudinal vortices, this paper addresses the mechanisms of heat transfer enhancement enforced by longitudinal vortices generated by vortex generators (VGs) from another view point. The view point is based on the convective transport equation of heat flux, a description of convective heat transfer process in the process parameter of heat flux q. Based on this view point, the heat transfer enhancement mechanism is explained by the contributions of the combinations of flux with velocity gradient and velocity with flux gradient to the heat transfer. To find these contributions the heat transfer in a channel with and without VGs is studied numerically, and the differences of these contributions under conditions with and without VGs are compared in the frame of the convective transport equation of heat flux. The results show that longitudinal vortices greatly enhance the local convection contribution terms that promote the local transport of heat flux in span direction, which enforces the transports of heat flux along the main flow direction and heat flux along the normal direction by enlargements of the values of the corresponding convection contribution terms. The later results in the increase of the heat flux per unit temperature difference on the top and the bottom surfaces, and hence the heat transfer enhancement is enforced by longitudinal vortices. The local terms intensify the local transport of heat flux in span direction, but do not do so in an average measure in the presently studied case.