| Popis: |
Heat pipes are critical to today's spacecraft thermal control. Ground testing of spacecraft places many heat pipes in gravity-driven reflux-mode, with condenser above evaporator. The design heat loads based on capillary-dri ven zero-gravity operation are much less than the reflux capacity. Reflux heat pipe operation with low heat input presents unique problems, e.g. start-up and cycling. In zero-g, heat pipes typically exhibit small ATs. This paper presents experiments with heat pipes in reflux that show large ATs when the pipe is driven at low heat load. In one case, heat was applied to the wall below the surface of the liquid pool in a vertical heat pipe. The results show transition from natural convection to two-phase natural convection and then to full nucleate boiling. Natural convection is shown to be sustained with 25°C superheat between the heated wall and ammonia vapor. In the second experiment, heat was applied at two sites above the liquid pool with a third heater below pool level. This test showed stable and unstable cyclic behavior and the need for a minimum heat flow to the pool to avoid unstable cyclic behavior. Models are presented to describe the exhibited behavior for heat pipe start-up and thermal cycling. |
