High-Speed Visual Analysis of Fluid Flow and Heat Transfer in Oscillating Heat Pipes with Different Diameters

Xiangdong Liu,Qing Sun,Chengbin Zhang,Liangyu Wu

doi:10.3390/app6110321

Abstract

The oscillating heat pipe (OHP) is a new member in the family of heat pipes, and it has great potential applications in energy conservation. However, the fluid flow and heat transfer in the OHP as well as the fundamental effects of inner diameter on them have not been fully understood, which are essential to the design and optimization of the OHP in real applications. Therefore, by combining the high-speed visualization method and infrared thermal imaging technique, the fluid flow and thermal performance in the OHPs with inner diameters of 1, 2 and 3 mm are presented and analyzed. The results indicate that three fluid flow motions, including small oscillation, bulk oscillation and circulation, coexist or, respectively, exist alone with the increasing heating load under different inner diameters, with three flow patterns occurring in the OHPs, viz. bubbly flow, slug flow and annular flow. These fluid flow motions are closely correlated with the heat and mass transfer performance in the OHPs, which can be reflected by the characteristics of infrared thermal images of condensers. The decrease in the inner diameter increases the frictional flow resistance and capillary instability while restricting the nucleate boiling in OHPs, which leads to a smaller proportion of bubbly flow, a larger proportion of short slug flow, a poorer thermal performance, and easier dry-out of working fluid. In addition, when compared with the 2 mm OHP, the increasing role of gravity induces the thermosyphon effect and weakens the ‘bubble pumping’ action, which results in a little smaller and bigger thermal resistances of 3 mm OHP under small and bulk oscillation of working fluid, respectively.

Highlights

With rapid increase in energy consumption, in order to realize the sustainable development of energy and environment, it is necessary to improve the efficiency of energy transfer, especially that of thermal energy transfer [1,2,3,4,5]
After the start-up process [45], a quasi-steady steady operation of the oscillating heat pipe (OHP), it can be seen from Figure 2 that the fluid flow inside the OHPs with stationthree is achieved in theunder
From the comparison among the changes of the thermal resistance and fluid flow motions versus heating load under different inner diameters of the OHPs, it can be seen that only the oscillation motions of working fluid occur in the major heating loads range (Q ≤ 110 W) under Di * = 0.65 (Di = 1 mm), due to the large frictional flow resistance

Summary

Introduction

With rapid increase in energy consumption, in order to realize the sustainable development of energy and environment, it is necessary to improve the efficiency of energy transfer, especially that of thermal energy transfer [1,2,3,4,5]. As one of several types of high efficiency heat transfer elements, the heat pipe has been widely used for improving the efficiency of thermal energy transfer and reducing the environmental impact during the heat exchanging process in real applications [6,7,8,9]. OHP is fabricated by a sealed wickless capillary partially filled with the working fluid, which is arranged in an interconnected meandering manner. Compared with the conventional wick heat pipe, OHP has its own special operation mechanism due to its unique wickless structure [12]. In the OHP, owing to the sufficient small scale of the meandering capillary, working fluid

Methods

Results

Conclusion