Mass transfer at the base of a cylindrical cavity recessed in the floor of a flat duct

Experiments involving either sublimation or evaporation at the base surface of a cylindrical cavity were performed respectively using naphthalene and water as the transferred vapor. The cavity was mounted in the lower wall of a flat rectangular duct through which turbulent air was passed. Supplementary experiments were also carried out using a cavity with a constricted opening (i.e., a Helmholtz resonator). The base surface mass transfer did not decrease monotonically with increasing cavity depth. Rather, there were two local maxima, respectively at about 0.06D and 0.5D (D = cavity diameter). Of these, the first is due to the reattachment of the shear layer on the cavity base. For the second, both fluid-elastic and fluid-resonant oscillations were ruled out as causes on the basis of definitive experimental data, leaving fluid-dynamic oscillations as a possible cause. The base surface Sherwood number was well correlated by power-law dependences on the Reynolds and Schmidt numbers. Fluid flow experiments encompassing oil-lampblack flow visualization, helium bubble flow visualization, and spectral analysis of the pressure field at the cavity base were also carried out.