Heat transfer enhancement, flow visualization and friction characteristics in rib-roughened channels

Abstract

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.In the last two decades thermochromic liquid crystal (TLS), true-color digital image processing and laser anemometry (PIV) have been successfully used in non-intrusive, technical and heat and mass transfer studies and applications. The aim of the paper is to asses the state of the art in the exploration of heat transfer control by transverse vortex generators. They are able to increase heat transfer by several hundred percent. Prior to the use of vortices to influence heat transfer it must be known how different vortices are generated and controlled and how they interact with the original or base flow and temperature field. Liquid crystals were used to determinate the distribution of surface temperature and then evaluation of heat transfer coefficient or the Nusselt number. The flow pattern produced by transverse vortex generators (rib-roughened passages) was visualized using a planar beam of double-impulse laser tailored by a cylindrical lens and oil particles. Sequential images of particles in the cross sectional plane taken with CCD video camera from the downstream side the flow were stored on a personal computer to obtain distributions of velocity vectors by means of the PIV method. Local and average Nusselt numbers, friction factors and velocity fields are presented for rectangular channels with an aspect ratio of 6.35 and five types of transverse vortex generators which were brought to required temperatures by hot film method. The pitch-to-height-ratio of the ribs was 11.