Free convection in low-temperature gaseous helium

Abstract

Free convection has been studied in gaseous helium at low temperatures in a cylindrical vessel of aspect ratio (diameterlheight) 2·5. Compared with measurements in fluids at room temperature the present arrangement has the advantages of small size, a short time constant and improved accuracy. As the Rayleigh number was varied from 60 to 2 × 10⁹, the Nusselt number rose from 1 to 69, obeying the relationNu= 0·173Ra^{0·2800±0·0005}over the upper four decades ofRa.The critical Rayleigh number was 1630, but the conditions of the experiment did not allow reliable measurements at such low values ofRa.The very high sensitivity within a given experiment showed the presence of several ‘discrete transitions’, which were often step like and not merely a change of gradient as reported by other workers. The largest of these, atRa= 3 · 10⁵, involved a drop in heat flux of some 6% and was somewhat hysteretic. The temperature fluctuations increased markedly as the step was crossed.