Suppression of buoyancy-driven vortex flow resulting from a low speed jet impinging onto a heated disk in a vertical cylinder by cylinder top tilting

摘要

Buoyancy-driven vortex flow resulting from a low speed round gas jet impinging vertically downwards onto a heated horizontal circular disk confined in an adiabatic vertical cylindrical chamber can be strong and even unstable as the buoyancy-to-inertia ratio exceeds certain critical level. An experiment combining flow visualization and temperature measurement is conducted in the present study to explore the suppression of the above buoyancy-driven vortex gas flow by inclining the top of the cylindrical chamber. The chamber top is chosen to incline linearly in the radial direction so that the mean flow in the wall-jet region is accelerated and meanwhile the effective buoyancy signified by the local Rayleigh number reduces in that direction. Specifically, in the present experiment the disk-to-chamber top separation distance decreases from 20.0 mm at the jet axis to 10.0 mm at the chamber side. Tests are conducted for the jet flow rate varied from 1.0 to 5.0 slpm (standard liter per minute) and the jet-to-disk temperature difference varied from 0 to 35.0℃ for two injection pipes with diameter 10.0 and 22.1 mm for the chambers with horizontal and inclined tops. The results from the flow visualization indicate that the chamber top inclination can effectively suppress the unstable buoyancy-induced vortex roll and the temporal flow oscillation at high buoyancy-to-inertia ratios. However, the effects of the chamber top inclination on the inertia-driven rolls are much milder. The non-monotonic air temperature distribution in the radial direction is found to result from the unique vortex flow structure in the chamber. A universal criterion based on the local flow and thermal conditions in the wall-jet region 'for the onset of the buoyancy-driven roll is proposed. To quantify the characteristics of the vortex flow in the chamber with the inclined top, empirical correlations have been proposed for the size and location of the vortex rolls.