An investigation into gravity independent oscillating heat pipes
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] A number of oscillating heat pipes (OHPs) with unbalanced structures were investigated in an effort to develop methods enabling OHPs to be gravity independent. The unbalanced structures investigated herein include the effects of uneven turn and check valves. Two 2-D and two 3-D tubular uneven-turn OHPs, four miniature uneven-turn OHPs, and one OHP with check-valves were investigated. At the same time, the theoretical analysis of the maximum channel diameter was conducted in order to find the primary factor affecting the channel size in an OHP. A model was developed that attempts to determine the maximum channel diameter by considering the contact angle effect. It is found that the contact angle significantly affects the maximum channel diameter of an OHP. In order to verify that the uneven-turn structure can generate the oscillating motion in an OHP, a heat pipe with 3 turns in the condenser and 6 turns in the evaporator was first tested. The heat pipe with uneven turns can generate and maintain oscillating motion. When the turn number increases with 16 turns in the condenser and 20 turns in the evaporator, the heat transfer performance can be further increased. An experimental investigation of a new 3-D OHP with uneven turn design was conducted in order to further develop a gravity independent OHP. Experimental results show that the uneven turn OHP developed herein can start the oscillating motion in the negative vertical position (the evaporator being above the condenser) and demonstrate that the uneven turn OHPs can significantly reduce the effect of gravity on the heat transport capability in an OHP. Three miniature OHPs (18-turn acetone OHP, 18-turn water OHP, and 20-turn acetone OHP) were developed and tested to determine whether the uneven-turn OHPs can function in a high-g environment. Experimental results demonstrate that these miniaturized uneven turn designs are extremely capable in high gravity environments and will operate effectively in any orientation. An OHP with check valves has been successfully developed and tested to determine the check valve effect on the oscillating motion and heat transfer performance in an OHP. Experimental results show that the OHP with check valves can function well in both the inverted and vertical positions with little variation between the two positions in performance once startup occurred, while the control OHP without check valves which had the same channel layout was not able to achieve startup in the inverted position. This shows that the check valves allowed the OHP to operate in the inverted position first achieving startup and then maintain oscillating motion.
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