Heat and mass transfer coefficients of viscous spheres

The problem of mass/heat transfer from a viscous droplet is solved by using a finite-difference scheme. and a dual kind of computational grid. The ste ady-state Navier-Stokes and energy equations for the flow fields inside and outside a viscous sphere in a fluid of different properties are fully solv ed numerically for Reynolds numbers (Re) ranging from 1 to 500. The corresp onding Peclet numbers (Fe) range from 1 to 1000. At high values of Re and P e a thermal and a momentum boundary layer are formed in the outside fluid. For this reason, we adopted a method of a two sub-layer concept for the com putational domain outside the sphere. The first of these computational sub- layers is positioned at the interface of the sphere and covers a thin regio n [of O(Re-1/2) for the momentum and of O(Pe(-1/2)) for the thermal boundar y layer]. The second computational layer is based on an exponential functio n and covers the rest of the domain. We utilize this numerical technique to compute the Nusselt numbers for viscous spheres at different values of Re, Pe and the viscosity ratio. The computations also show that the effect of the internal fluid density on the heat or mass transfer is negligible. (C) 2001 Elsevier Science Ltd. All rights reserved.