Three-dimensional buoyancy-driven convection in horizontal cylindrical annuli.

摘要

Natural convection in the annular space between concentric horizontal cylinders has been of interest to many researchers, owing to its relevance to a number of practical applications. The published studies have focused on two-dimensional flow in a long annulus, which occurs mainly at low Rayleigh numbers. Relatively little is known about three-dimensional flow, which arises owing to the presence of the end walls or the onset of thermal instabilities at higher Rayleigh numbers, and is prominently present over wide ranges of parameters.;A numerical investigation of three-dimensional buoyancy-driven convection in horizontal cylindrical annuli is performed. Results obtained using the present numerical scheme are shown to be in excellent agreement with results of experimental and previous numerical studies, thereby verifying the accuracy of this scheme. It is shown for the first time that three-dimensional supercritical states of natural convection occur in a narrowgap annulus, and the existence of five such states is established. The present work provides the first quantitative three-dimensional descriptions of spiral convection within a moderate-gap annulus containing air, flow structures preceding oscillation in a large-gap annulus for air, and unicellular flow development in a large-gap annulus for large Prandtl number fluids. Factors influencing the occurrence and structure of the flows are studied, and the local and average heat transfer characteristics of the flows are determined. Buoyancy-induced convection between concentric horizontal cylinders resulting from g-jitter under microgravity is investigated for the first time. The results show that low frequency g-jitter considerably enhances heat transfer in a space environment, and that g-jitter has a stabilizing effect on supercritical flows that arise in moderate- and narrow-gap annuli. The present work lends insight into the fundamental transport processes associated with buoyancy-induced convection in horizontal cylindrical annuli, and provides practical information for design purposes.