Comparison of the thermal performances and flow characteristics between closed-loop and closed-end micro pulsating heat pipes

https://doi.org/10.1016/j.ijheatmasstransfer.2015.12.064Get rights and content

Highlights

  • Thermal performances of the closed-loop and closed-end pulsating heat pipes were compared through experiments.

  • Visualization of flow in the pulsating heat pipes are performed through high-speed photography.

  • A contour map was suggested to provide a guideline in determining the type of pulsating heat pipe that performs better.

Abstract

The thermal performances and flow characteristics of closed-loop micro pulsating heat pipes (CLMPHPs) and closed-end micro pulsating heat pipes (CEMPHPs) are compared through experiments to determine which type of micro pulsating heat pipe (MPHP) performs better. MPHPs that have a meandering rectangular channel engraved on a silicon substrate were fabricated using MEMS techniques. The width and height of the channels are 1 mm and 0.5 mm, respectively. Pyrex glass was used as a cover, enabling the flow visualization in the MPHPs and ethanol was used as the working fluid. A series of experiments were performed at various input powers and inclination angles for the MPHPs with 5, 10, 15, and 20 turns. The flow and thermal characteristics were investigated through high-speed photography and thermometry. In the case of 20 turns, there is little difference in the thermal performances and flow characteristics between the CLMPHP and the CEMPHP, and both MPHPs have orientation-independent performance. However, in the case of 15 and 10 turns, the CEMPHP operates stably regardless of the inclination angle while the thermal performance of the CLMPHP deteriorates significantly as the orientation changes from vertical to horizontal. Specially, in the case of 10 turns, the CEMPHP has up to 1.3 times higher effective thermal conductivity in a vertical orientation and 2.5 times in a horizontal orientation than the CLMPHP. When the number of turns is 5, the thermal performance of the CLMPHP depends on the orientation, while the CEMPHP does not start up at all at any inclination angles. Finally, to ensure orientation-independent performance, the CLMPHP requires as many as 20 turns while only 10 turns is sufficient for the CEMPHP.

Introduction

With the rapid development of the semiconductor industry, the heat load of electronic devices has increased in recent years. Moreover, as modern electronic devices become smaller and more densely packed, the devices’ heat flux becomes larger [1]. Therefore, efficient thermal management of the electronic components is critical to ensuring the functionality and reliability of the electronic devices. Among various cooling devices, a heat pipe, which is two-phase passive cooling device, is widely used due to its high thermal performance. However, heat pipes have limitations in becoming thinner and more flexible as a result of their wick structure [2]. One of the new types of heat pipes developed to overcome such limitations is a pulsating heat pipe (PHP) [3]. The PHP, which was first introduced by Akachi et al. [4], is a promising cooling device that consists of a meandering tube without a wick structure.

Two primary configurations exist in PHPs. The closed-loop PHP (CLPHP) has a single closed loop connecting both ends of a serpentine channel, while the closed-end PHP (CEPHP) has both closed ends [5]. According to previous studies, some researchers have claimed that the CLPHP has better thermal performance than the CEPHP [1], [5] in a vertical orientation because only the CLPHP can have a circulating flow, which yields better thermal performance than an oscillating flow [6], [7], [8]. However, it is important to compare the thermal performances between the two not only in a vertical orientation but also at various inclination angles because most electronic devices are used in various orientations [9]. Gi et al. [10] experimentally compared the heat transfer rates between the CLPHP and the CEPHP with 10 turns at various inclination angles. The PHPs were constructed using a Teflon tube with an inner diameter of 2 mm. They reported that the CEPHP was less affected by the inclination angle than the CLPHP, even though the CLPHP had a larger heat transfer rate than the CEPHP due to the circulating flow. Although their results illustrate the possibility of a weaker orientation dependence of the CEPHP, they are limited to the CEPHP with 10 turns only. However, it is well known that the orientation dependence of a PHP is strongly coupled with the number of turns [11]. Therefore, for a more meaningful comparison, it is necessary to compare the thermal performances and orientation dependences between the CLPHP and the CEPHP with various numbers of turns.

The purpose of the present study is to determine which type of pulsating heat pipe between CLPHP and CEPHP performs better in various conditions. To answer this question, the thermal performances and flow characteristics of the CLPHP and the CEPHP were compared through experiments. Using MEMS techniques, micro pulsating heat pipes (MPHPs) that have a meandering rectangular channel engraved on a silicon substrate were fabricated. The width and height of the channels are 1 mm and 0.5 mm, respectively. To allow visualization of the internal flow behavior, the MPHPs were covered with Pyrex glass. A series of experiments were performed at various input powers and inclination angles for the MPHPs with 5, 10, 15, and 20 turns. In order to compare the thermal performances and orientation dependences between the CLMPHP and the CEMPHP, the thermal resistances and effective thermal conductivities of the MPHPs were compared in various conditions. For physical explanations of the results obtained from the experiments, the flow and thermal characteristics of the MPHPs were investigated using high-speed photography and thermometry. Finally, based on the results, a contour map is suggested to provide a guideline to assist thermal engineers in determining which type of MPHP performs better for their applications.

Section snippets

Fabrication of MPHPs

To compare the thermal performances and flow characteristics between the CLMPHP and the CEMPHP, MPHPs with 5, 10, 15, and 20 turns were fabricated using micro-electromechanical fabrication techniques [12]. The micro-channel in the MPHPs was engraved on the silicon wafer with a thickness of 1 mm using a deep reactive ion etching (DRIE) process. The width and height of the channels were 1 mm and 0.5 mm, respectively. For the flow visualization in the MPHPs, a Pyrex glass (#7740 Pyrex™) with a

Thermal performances and orientation dependences

To compare the thermal performances and orientation dependences between the CLMPHP and the CEMPHP, the thermal resistances (Rth) of the MPHPs are calculated using the following equation:Rth=T¯eva-T¯conQinput,where T¯eva, T¯con, and Qinput are the average temperatures in the evaporator and condenser sections, and the input power, respectively. The average temperatures were obtained using the arithmetic mean, and the time average was derived using the data for 20 min after reaching the

Conclusion

In the present study, experiments were conducted to determine which type of micro pulsating heat pipe would perform better between the CLMPHP and the CEMPHP. To fabricate MPHPs, meandering rectangular channels were engraved on a silicon wafer using MEMS techniques. The width and height of the channels were 1 mm and 0.5 mm, respectively. Through thermometry and high-speed photography, a series of experiments were performed at various input powers and inclination angles for the MPHPs with 5, 10,

Conflict of interest

None declared.

Acknowledgement

This work was supported by the National Research Foundation of Korea (NRF) grant which is funded by the Ministry of Education, Science and Technology (MEST) (No. 2012R1A3A2026427).

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    The visualization results showed that the closed-loop PHP had lower oscillation amplitude but higher oscillation frequency than the closed-end PHP for the acetone working fluid. The visualization experiments by Jun et al. [88] found that with fewer turns, the CEPHP did not start-up at any inclination angle, while the CLPHP was only able to start-up except horizontal conditions. This is in line with traditional conclusion.

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