Free convection in vertical gaps

Abstract

Free convective;flow was investigated experimentally in a variety of slender vertical gaps of large horizontal extent. Temperature fields were visualized by holographic real-time interferometry, and local temperatures measured by thermocouples at the lower and upper boundaries of the gap as well as in the fluid. The critical Rayleigh number at the onset of convection was determined for different gap geometries (aspect ratios) and different thermal properties of the sidewalls and the fluid. For supercritical Rayleigh numbers, bounds of stability of steady-state two-dimensional convection were determined for transient and oscillatory states of the flow. The oscillatory flow is caused by an instability of the thermal boundary layers at the lower and upper boundaries, as evidenced by direct interferometric observation and by the measured period of oscillation depending on the Rayleigh number. The oscillations of the flow exhibit a periodic behaviour at the threshold from steady to unsteady flow. However, the periodic character of the oscillations is superseded by stochastic features im- mediately beyond the threshold Rayleigh number.