Capillary Pumped Loop Performance Investigation through Flow Visualization

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Capillary pumped loops (CPLs) are two-phase thermal control devices which, like heat pipes, use surface tension forces to circulate a cooling fluid from an evaporator to a condenser. The pressure difference for fluid circulation is developed in the pores of a wick structure located in the evaporator. Unlike heat pipes, CPLs separate the liquid and vapor flows such that counterflow is eliminated. A key failure mode of a CPL is called deprime. This is where the liquid feed to the capillaries is interrupted, resulting in the loss of the meniscus from the pores of the wicking material. CPLs are used in both terrestrial and space cooling applications. Therefore performance in micro-gravity is of interest to the satellite community, particularly the recognition of events that precede CPL deprime.

Visualization in an Experimental Water Capillary Pumped Loop (VIEW-CPL) was designed and built to investigate the operation of a CPL in a micro-gravity environment through the use of a capillary evaporator with a window for flow visualization.  The experiment flew on the Middeck of the Space Shuttle Columbia in November 1996.  VIEW-CPL was instrumented with a video camcorder and sensors for measuring the loop pressure and temperature at various locations.   The data that was collected from the micro-gravity and ground tests provide significant insight into the physics of CPL operation.  

Heat transfer models of the VIEW-CPL operating modes of pressure prime and start-up were developed and compared to test data.  Video recordings of bubbles on the liquid side the capillary evaporator were collected during both ground and micro-gravity testing.  The movement of the bubbles corresponded with low frequency temperature fluctuations in the range of 0.003 to 0.01 Hz.  An analysis is presented that explains the observed phenomena through pressure changes in the evaporator resulting from a partially dry wick.   Understanding the source of the temperature oscillations is a necessary step for predicting the potential for evaporator deprime and impact on system performance.