Experimental Investigation of Evaporator and Condenser Placement Configuration for Oscillating Heat Pipes

Abstract In this work, we explore two different Oscillating Heat Pipe (OHP) evaporator-condenser placement configurations, and investigate and quantify the effects on thermal performance. The proposed study focuses on two different OHP evaporator-condenser configurations with a change in adiabatic length. One of the challenges in OHP literature is the variety of experiment setups, (e.g. varying condenser, evaporator, and adiabatic lengths, heat input) which makes it difficult to compare results directly. To quantitatively compare thermal performance, a (or a set of) standardized metric(s) must be used. Therefore, we define a standardizing metric to quantify OHP’s ability to conduct heat that can be used across multiple experiments and setups. This study was conducted on an additively manufactured flat-plate AlSi10Mg OHP which has a channel diameter of 1.4 mm, 22 turns, and a plate size of 200 mm × 90 mm × 4 mm. Both the evaporator and condenser are rectangles with contact areas of 46 mm by 78 mm. The OHP was charged with R134a with a 45% filling ratio. In these tests, the location of the evaporator was fixed, while the placement of the condenser is varied such that the adiabatic length ranged between 4 mm to 94 mm. The condenser temperature was maintained between 10°C to 25°C and the heat input ranged between 20W to 50W. The results showed that a reduction in adiabatic length increased the thermal conductivity. To quantify the thermal performance, the thermal conductivities of an empty and charged OHP were determined for each placement configuration, then a thermal conductivity ratio of charged and empty OHP can be determined to quantify the improved performance. For an adiabatic length of 4 mm, we observed that the OHP’s ability to conduct heat was 40 times more effective when compared to an empty OHP. It was also observed that the OHP’s ability to conduct heat was 9 times more effective when compared to an empty OHP for an adiabatic length of 94 mm. We conclude that the area outside the evaporator-condenser that is neither heated nor cooled, called the reservoir, significantly influenced the thermal performance. The OHP with a shorter adiabatic length increased the reservoir in the condenser region which showed higher thermal performance. In this placement configuration, the reservoir essentially acted as an extension of the condenser. This is a favorable condition where the subcooled liquid slugs re-enter the condenser section which affects heat transfer drastically. Thus, the placement of the evaporator-condenser will influence OHP performance due to the reservoir and warrants future work.