Mass transfer from small spheroids suspended in a turbulent fluid

By coupling direct numerical simulation of homogeneous isotropic turbulence with a localised solution of the convection-diffusion equation, we model the rate of transfer of a solute (mass transfer) from the surface of small, neutrally buoyant, axisymmetric, ellipsoidal particles (spheroids) in dilute suspension within a turbulent fluid at large Peclet number, Pe. We observe that, at Pe = O(10), the average transfer rate for prolate spheroids is larger than that of spheres with equivalent surface area, whereas oblate spheroids experience a lower average transfer rate. However, as the Peclet number is increased, oblate spheroids can experience an enhancement in mass transfer relative to spheres near an optimal aspect ratio lambda approximate to 1/4. Furthermore, we observe that, for spherical particles, the Sherwood number Sh scales approximately as Pe(0.26) over Pe = 1.4 x 10(1) to 1.4 x 10(4), which is below the Pe(1/3) scaling observed for inertial particles but consistent with available experimental data for tracer-like particles. The discrepancy is attributed to the diffusion-limited temporal response of the concentration boundary layer to turbulent strain fluctuations. A simple model, the quasi-steady flux model, captures both of these phenomena and shows good quantitative agreement with our numerical simulations.

Automated summary

In turbulent fluid, particles with optimal aspect ratios can enhance mass transfer, while the Sherwood number for spherical particles scales approximately as Pe(0.26), consistent with experimental data.