Passive transport in steady Rayleigh–Bénard convection

Abstract

Enhancement of the diffusive transport of impurities by two‐dimensional, time‐independent Rayleigh–Bénard convection is studied experimentally. Two impurities are used: a molecular dye (methylene blue) and a particulate impurity (latex spheres). The convective flow is characterized by laser Doppler velocimetry, and the transport is monitored by optical absorption techniques. It is found that the transport can be modeled as a diffusive process on long space and time scales, with an effective diffusion coefficient D* whose absolute magnitude and variation with the velocity amplitude W of the flow are in good agreement with recent theoretical predictions. The enhancement factor D*/D scales with the Peclet number approximately as Pe^1/2≡(Wd/πD)^1/2, where D is the diffusion coefficient and d the layer depth. Several subtle problems that complicate the study of transport phenomena in cellular hydrodynamic flows are discussed.