Time-dependent thermal convection in dilute solutions of 3He in superfluid 4He

Abstract

Time-dependent thermal convection can occur in a unity-aspect-ratio Rayleigh-Benard convection cell containing a dilute solution of 3He in superfluid 4He when the fluid is heated from above. Results are presented primarily for a 0.24 mole % He solution at 0.925 K. Means is provided for introducing heat at the top and separately for a central plug and an outer ring such that both are at a constant temperature gDT above the bottom. A critical temperature difference δT cfor convection can be defined above which both steady and time-dependent convection occur. The time-dependent effects include a region of δT. near δT cand characterized only by excessive noise, a region of somewhat higher δT where there are intermittent major changes in the plug heating rate with a time distribution like that for random events, and a region at still higher δT where periodic but nonsinusoidal variation of the heat flow is observed. When a long enough time, several months, has elapsed after cooling down the apparatus, time-dependent states no longer occur, and the heat flow above δT cis limited to steady convection. Briefly raising the temperature of the apparatus to 77 K is sufficient to restore the possibility of time-dependent states.