Author(s)

O. Kholai, M. Kadja & T. H. Mai

Abstract

The paper presents the results of a numerical study on transient mixed laminar convection for the case of an ascending flow inside a vertical tube in which the wall conduction is significant. The outer surface of the tube is submitted to a convective heat exchange. The transient regime is provoked by a step type perturbation in temperature at entry of the tube. The governing equations are solved using a finite volume method. Results are presented for water (Pr=5) and for two values of Grashof numbers of 104 and 105. This method shows that no matter what the value of the imposed step is, it provokes the birth of a big recirculation cell next to the wall, and gives rise to an instability situated between the cool and the hot regions of the flow. Keywords: transient convection, temperature step, convective heat exchange, finite volume method, instabilities. 1 Introduction Conjugate heat transfer (mixed convection + conduction) in transient regime in internal flows is present in many industrial installations: such as compact heat exchangers, solar collectors, cooling of electronic components etc. As a matter of fact, the importance and complexity of this phenomenon and the consequences have stimulated the interest of many researchers: so that a very important number of studies have been done in this field. Nguyen et al. [1] performed a numerical study on transient mixed convection in vertical tubes having a wall, which is submitted to variable heat flux. Their

Keywords

transient convection, temperature step, convective heat exchange,finite volume method, instabilities.