Numerical solution for laminar mixed convection in a horizontal annular duct: temperature-dependent viscosity effect

摘要

The influence of free convection and variable viscosity on forced laminar flow of a Newtonian fluid in a horizontal annular duct is investigated. The inner and outer cylinders are subjected to a constant heat flux density. At the entrance of theannular duct, the flow is fully developed and the temperature profile is uniform. The Prandtl and Boussinesq hypothesis were adopted. The continuity equation and the three-dimensional parabolized form of the momentum and energy equations are solvednumerically using finite differences. Near the entrance section, forced convection is the dominant mechanism. Further downstream, the fluid heats up and buoyancy becomes more important. The fluid near the walls is warmer, and therefore lighter than thefluid in the central part of the annular space; it therefore flows upward along the walls. A continuity requires a downflow of the heavier fluid between the two cylinders. This secondary flow modifies the structure of the dynamic and thermal fields. Thenumerical results show that a decrease in fluid viscosity with temperature leads to: (i) an increase in axial velocity in the upper part of the annular duct and a decrease in the lower part; (ii) a rise in the secondary flow intensity; (iii) a reductionin temperature difference between the upper and lower parts of a cylinder; (iv) an increase of the overall heat transfer coefficient.