Gilles Desrayaud

Laminar natural convection in a vertical isothermal channel with symmetric surface-mounted rectangular ribs

Abstract In this paper, a numerical investigation of laminar natural convective flows in a vertical isothermal channel with two rectangular ribs, symmetrically located on each wall, is carried out. The governing elliptic equations are solved in a two-dimensional domain using a control volume method and the SIMPLER algorithm for the velocity–pressure coupling is employed. Special emphasis is given in the systematic analysis to detail the effects of the location of the isothermal ribs on the flow structure and isotherms pattern. The profiles of the local Nusselt number are presented for three different locations of the obstructions, near the inlet, at the center and near the outlet of the channel. The influence of the rib conductivity is also considered, the ribs being either perfectly conducting or adiabatic. The variations in the mean Nusselt number and inlet flow rate versus the channel Rayleigh number are investigated. Finally, for centered ribs extra computations are performed for various sizes (width and length) of the ribs and for two aspect ratios to demonstrated the significant effects on the flow and heat transfer characteristics.

Thermoconvective instabilities in a narrow horizontal air-filled annulus

Abstract This paper reports a numerical investigation of natural convection flows between horizontal concentric annuli with the inner cylinder isothermally heated. Thermal and hydrodynamic instabilities for air-filled annuli of small radius ratios are discussed. At fairly low Rayleigh numbers (Ra⩽3000), thermal instabilities develop at the upper part of the annulus as steady cells. The results show the existence of an imperfect bifurcation, which could explain the discrepancies between the solutions reported previously in the literature. Locations of the bifurcation points are determined numerically for various radius ratios. At higher Rayleigh numbers, unsteady hydrodynamic instabilities are demonstrated in the vertical portions of an annulus with a radius ratio R =1.14. It is also shown that a reverse transition from multicellular to unicellular base flow patterns occurs when further increasing the Rayleigh number.

Numerical Analysis of General Trends in Single-Phase Natural Circulation in a 2D-Annular Loop

The aim of this paper is to address fluid flow behavior of natural circulation in a 2D-annular loop filled with water. A two-dimensional, numerical analysis of natural convection in a 2D-annular closed-loop thermosyphon has been performed for various radius ratios from 1.2 to 2.0, the loop being heated at a constant flux over the bottom half and cooled at a constant temperature over the top half. It has been numerically shown that natural convection in a 2D-annular closed-loop thermosyphon is capable of showing pseudoconductive regime at pitchfork bifurcation, stationary convective regimes without and with recirculating regions occurring at the entrance of the exchangers, oscillatory convection at Hopf bifurcation and Lorenz-like chaotic flow. The complexity of the dynamic properties experimentally encountered in toroidal or rectangular loops is thus also found here.

Buoyant plane plumes from heated horizontal confined wires and cylinders

Two-dimensional computations are reported for time-dependent laminar buoyancy-induced flows above a horizontal heated source immersed in an air-filled vessel. Two kinds of heated source were considered: a line heat source, modelled as a heat source term in the energy equation, and a heat-flux cylinder of small diameter. First, comparisons are presented for the results obtained for these two heated sources. Rather large discrepencies between the velocity fields appeared in the conduction regime due to the weak plume motion, while close agreements were found in the boundary layer regime. Nevertheless, same types of bifurcations occur with almost identical frequencies, whatever the Rayleigh number. It is concluded that for dimensions of the enclosures, which largely compared with the cylinder radius, the heat source term model is a promising way to study the behaviour of unsteady plumes owing to its simplicity, flexibility, and low computational costs. Second, transitions to unsteady flows were studied through direct flow simulations for various depths of immersion of a line heat source in the central vertical plane of a vessel. Different routes to chaos were shown to occur according to the aspect ratio of the vessel and the depth of immersion of the line source. Three distinct regimes were detected with different underlying physical mechanisms called natural swaying motion, penetrative convection and Rayleigh-Benard-like convection. The first bifurcations associated with these regimes are supercritical Hopf bifurcation, pitchfork bifurcation and subcritical Hopf bifurcation. Comparisons with experimental results of confined buoyant plumes above heated wires show very good agreement with laminar frequency correlations.