Natural convection heat transfer in a rib-roughened asymmetrically heated channel

Abstract

A two dimensional numerical simulation is carried out to investigate the effect of two adiabatic square ribs on laminar flow and heat transfer in an asymmetrically heated channel. The two ribs are symmetrically located on each wall, exactly above the heating zone. The adiabatic square ribs could be an effortless and cheap means that can be incorporated in natural ventilation systems (double skin facades, trombe walls…) to control the mass flow rate and heat transfer. The computational procedure is made by solving the unsteady bi-dimensional continuity, momentum and energy equations with the finite volume method. The investigations focused more specifically on the influence of the ribs sizes RS (0, b/18, 2b/18, 3b/18, 4b/18, 5b/18 and 6b/18) on the flow structure and heat transfer enhancement. The numerical study is carried out for a fixed modified Rayleigh number Ra* = 2.86× 106 and for a fixed aspect ratio of the heated part Rf= 5.2. The modified Rayleigh number based on the heat flux density (φ) and the thickness of the channel (b) also takes into account the aspect ratio of the heated part Rf. Concerning the value of the heated part aspect ratios Rf, it is within the range of those found for horizontally divided double-skin facades of high-rise buildings. The results showed that the variation of the ribs sizes significantly alters the heat transfer and fluid flow distribution along the channel, especially in the vicinity of protrusions. Also, the results show that streamlines, isotherms, and the number, sizes and formation of vortex structures inside the channel strongly depend on the size of protrusions. The changes in heat transfer parameters have also been presented. The present numerical results are compared with experimental data and a good agreement was found.