Effects of the dimensions of upward- and downward-facing vertical cavities on natural convection

摘要

The mass transfer due to natural convection in open upward- and downward-facing vertical cylindrical cavities was measured experimentally for Rayleigh numbers (Ra_(Lw)) ranging from 2.57 × 10~8 to 1.04 × 10~(10) for various diameters and heights. Mass transfer was measured using a sulfuric acid-copper sulfate electroplating system and the limiting current technique. The results of preliminary natural convection experiments using horizontal circular disks were in good agreement with predictions based on existing correlations for horizontal surfaces. The Sherwood numbers (Sh_(Lw)) for upward-facing cylindrical cavities were higher than those for downward-facing ones. Cavity dimension affected mass transfer rate. Total mass transfer rates measured in cavities were compared with ones calculated using horizontal and vertical correlations for the same areas to delineate flow interactions in the cavities. For both cavity orientations, diameter had a stronger effect for lower cavity heights. In upward-facing cavities, the total mass transfer rate was increased due to the secondary flow generated by the negative pressure formed by two ascending flows from the bottom plate and the vertical inner wall. In downward-facing cavities, flow interaction impaired the mass transfer rate. Empirical correlations for upward- and downward-facing vertical cylindrical cavities were derived based on the test results.