Mass and momentum transport from a sphere in steady and oscillatory flows

Heat or mass transfer from spherical particles in oscillatory flow has important applications in combustion and spray drying. This work provides a parametric investigation of drag forces experienced by, and transport of a passive scalar from, an isolated rigid fixed sphere in steady and oscillatory axisymmetric flows. At Schmidt ~Prandtl! number of 1, oscillatory flows with Reynolds numbers in the range 1–100 and oscillation amplitudes in the range 0.05–5 sphere diameters are investigated using numerical simulation. Scalar concentration is uniform on the surface of the sphere and zero in the far field. Coefficients of peak drag for steady and oscillatory flows are presented and compared to values obtained from Basset’s analytical solution for Stokes flow, and the relative contributions of the added mass, Stokes drag, and Basset history terms are examined. At the higher Reynolds numbers and amplitudes, it is found that the time-average mass transfer rate can be more than double that for diffusion in quiescent fluid, or in Stokes flow. Time-average Sherwood ~Nusselt! numbers for oscillatory flows asymptote to the Stokes limit at low oscillation amplitude, regardless of Reynolds number. An unexpected result is that at intermediate Reynolds numbers and oscillation amplitudes, it is possible to depress the time-average mass-transfer coefficient slightly below that for Stokes flow. Within the Reynolds number range considered, Sherwood–Nusselt numbers in steady flow are found to be always higher than for an oscillatory flow of the same root-mean-square ~rms! velocity. © 2002 American Institute of Physics. @DOI: 10.1063/1.1510448#