NUMERICAL AND EXPERIMENTAL INVESTIGATION OF THERMAL-CONVECTION FOR A THERMODEPENDENT HERSCHEL-BULKLEY FLUID IN AN ANNULAR DUCT WITH ROTATING INNER CYLINDER

Thermal convection for an incompressible Herschel-Bulkley fluid along an annular duct, whose inner cylinder is rotating and outer is at rest , is analyzed numerically and experimentally. The outer cylinder is he ated at constant heat flux density and the inner one is assumed adiaba tic. The first part of this study deals with the effect of the rheolog ical behavior of the fluid and that of the rotation of the inner cylin der on the flow field and heat transfer coefficient. All the physical properties are assumed constant and the flow is assumed fully develope d. The critical Rossby number Ro(c) = (R-1 Omega/U-d)(c), for which th e dimension of the plug flow is reduced to zero is determined with res pect to the Bow behavior index, the radius ratio and the Herschel-Bulk ley number for axial flow. The rotation of the inner cylinder induces a decrease of the axial velocity gradient at the outer cylinder thereb y reducing the heat transfer between the heated wall and the fluid. Th e second part of this study introduces the variation of the consistenc y K with temperature and analyzes the evolution of the Bow pattern and heat transfer coefficient along the heating zone. Two cases are disti nguished depending on the Rossby number: (i) Ro < Ro(c), the plug Bow dimension increases along the heating zone; (ii) Po < Ro(c), the decre ase of K with temperature leads to the reappearance of the plug Bow. F or high angular velocities, it is possible to have a plug zone attache d to the outer cylinder. Finally, a correlation is proposed for the Nu sselt number. It shows clearly that the effect of thermodependency of K on the heat transfer becomes more important with increasing rotation al velocity of the inner cylinder. (C) Elsevier, Paris.