A hybrid Closed Loop Thermosyphon/Pulsating Heat Pipe with an inner diameter bigger than the capillary threshold is tested both on ground and in hyper/microgravity conditions. The device, partially filled up with FC-72, consists of an aluminum tube (inner diameter: 3 mm) bent into a planar serpentine with five curves at the evaporator. A transparent section closes the loop in the condenser zone, allowing fluid flow visualization. Five heaters are mounted alternatively on the branches, just above the turns and controlled independently, in order to investigate the effect of non-uniform heating configurations. On ground, where the device works as a thermosyphon, the non-uniform heating configurations promote the fluid net circulation in a preferential direction, increasing the thermal performance with respect to the homogeneous heating. Parabolic flights point out that during the 20 seconds of microgravity, the sudden absence of the buoyancy force activates an oscillating slug/plug flow regime, typical of the Pulsating Heat Pipes, allowing the device to work also without the assistance of gravity. Furthermore, peculiar heating configurations can shorten the stop-over periods and stabilize the pulsating two-phase flow motion.
The prediction of flow pattern transitions is extremely important to understand the coupling of thermal and fluid dynamic phenomena in two phase systems and it contributes to the optimum design of heat exchangers. Two phase flow regimes have been extensively studied under controlled mass flow rate and velocity. On the other hand, less effort has been spent in the literature on the cases where the flow motion is purely thermally induced and consequently the mass flow rate or the velocity of the phases are not known a priori. In the present work, flow pattern transitions and bubble break-up and coalescence events have been investigated in a passive two phase wickless capillary loop, where the mass flow rate is intrinsically not controllable. Modified Froude, Weber and Bond numbers have been introduced, considering the actual acceleration of the fluid and the length of the bubble as merit parameters for the transitions. The proposed nondimensional investigation was developed by analysing experimental data obtained with ethanol and FC-72, as working fluids, different heat input levels (from 9 to 24 W) as well as three different gravity levels (through a parabolic flight campaign). A new empirical diabatic flow pattern map for accelerated two-phase capillary flows is presented, together with quantitative criteria for the calculation of the flow regime transitions, defining the physic limits for the bubble coalescence and break-up. This kind of new regime maps will be useful to the further development of comprehensive designing tools for passive two-phase wickless heat transfer devices.
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