Metal materials are used widely in industry for piping, as they provide desirable material properties for use in harsh environments. In this work, we explore different flexible pipe joint designs that enable fluid transport in flexible heat transfer devices. Flexible joints would allow heat transfer devices to fit in variable cooling areas. The proposed flexible joints will be 3D printed in metal with the goal of maximizing their bending angles, while minimizing volume. The objective of this research is to develop flexible pipes to connect parts involving fluid flow, and we propose three types of flexible joint designs: 1) helical coil spring, 2) torsional coil spring, and 3) serpentine structure. Then, we introduce a low-cost design process to improve these joint designs. Metal 3D printing offers increased design flexibility in comparison to traditional metal manufacturing methods. However, metal 3D printing can be labor intensive and costly, thus the number of design iterations using metal printing should be minimized during the design process. To reduce the frequency of metal prototyping, we used plastic 3D prints for rapid prototyping and evaluation. The flexible joint design considered the effects of pipe shape, pipe size, pipe diameter, free length, and total pipe length. Although mathematical models exist for the three types of flexible designs, they tend to be complex, difficult to implement, and specific to certain types of boundary conditions. In our experience, it is faster and more meaningful to perform experimental parametric studies using plastic prints due to accessibility and cost-effectiveness of plastic 3D printers. However, analytical models do offer significant insights during the design process. Thus, we developed simple analytical models for each of the three types of flexible joint designs which can be used in the initial design phase. Tests were performed to characterize the designs. Then, finite element simulations were performed for plastic prints and these simulations were validated against experimental data. We show that the results from analytical models, finite element simulations, and testing results for plastic prints are consistent, and these tools can be used to predict performance. In this paper, we discuss the following findings: 1) development of simple analytical models, including successes, limitations, and challenges; 2) the role of finite element simulations in the design process; 3) testing results from the three types of joint design; 4) quantification, interpretation, and discussion of testing results.
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.
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