Variable Viscosity Effect on the Laminar Water Boundary Layer on Heated Cones

Abstract

Heat transfer and shear stress may be significantly affected by buoyancy-forced and associated free-convection motions in many forced-convection flows. A crossflow is induced when a uniform, horizontal stream passes along a heated, axisymmetric slender body. The crossflow effects on heat transfer and shear stress grow as the fluid flows downstream, and eventually become one of the dominant mechanisms even for a moderate-speed forced-convection flow. Early study of the longitudinal cylinder flow showed that the crossflow may destabilize the boundary layer and degrade the heat transfer over the upper half of the body. On the other hand, heating can be used to stabilize the water boundary layer due to its temperature-dependent viscosity, since (dμ/dT) of water is negative. However, the influences of the pressure gradient on the destabilizing crossflow effect and on the stabilizing variable-viscosity effect have never been studied before. It is important to know the interaction of the buoyancy-force effect and the variable-viscosity effect under the non-zero pressure gradient conditions in stabilizing the boundary layer by heating. In this paper a similarity solution is presented for a three-dimensional boundary layer on a heated cone to stimulate the water flow past the forward part of an axisymmetric slender body. The numerical solutions of the ordinary differential equations reduced by the similarity transformation are presented in the region near the vertex of the cone. The results indicate that the crossflow grows as the fluid flows downstream for the cone of its half angle less than 66.25°. For a cone of its half angle larger than 66.25°, the magnitude of the crossflow is about the same order as that of the axial flow in the neighborhood of the cone vertex and is suppressed by the favorable pressure gradient as the fluid moves downstream. The effect of the temperature-dependent water viscosity has been shown to enhance the favorable pressure-gradient effects and to counterbalance the crossflow effects.