Elsevier

Energy Conversion and Management

Volume 180, 15 January 2019, Pages 769-783
Energy Conversion and Management

A novel ultra-thin flattened heat pipe with biporous spiral woven mesh wick for cooling electronic devices

https://doi.org/10.1016/j.enconman.2018.11.031Get rights and content

Highlights

  • An ultra-thin flattened heat pipe with biporous spiral woven mesh wick is proposed.

  • The biporous wick is hybrid woven using 0.05 and 0.04 mm diameter copper wires.

  • The biporous wick has advantages of high permeability and large capillary force.

  • The effects of wick parameters on the performance of heat pipe are investigated.

  • The biporous wick realizes the demands of low cost and high thermal performance.

Abstract

In this work, a novel biporous spiral woven mesh wick is developed to enhance the thermal performance of an ultra-thin flattened heat pipe for cooling high heat flux electronic devices. The biporous wick with different sized pores is hybrid woven using 0.05 and 0.04 mm diameter copper wires in every strand. Three different structures are designed to study the effect of the characteristic parameters of the wick on the thermal performance of the ultra-thin flattened heat pipe. The working fluid flow characteristics of the wick are analyzed theoretically. The capillary rate-of-rise experiment with deionized water using the infrared camera method is carried out to characterize the capillary performance of the wick. The thermal performance of the ultra-thin flattened heat pipe is experimentally investigated. The results indicate that the biporous wick combines the advantages of high permeability due to the large pores and large capillary force due to the small pores. The optimal biporous wick has 22% fewer copper wires than the monoporous wick, but the maximum heat transport capacity of the ultra-thin flattened heat pipe is able to approach 24 W, which realizes the demands of both low production cost and high thermal performance using the biporous wick.

Introduction

With the rapid development of high-performance and lightweight devices, the heat flux of electronics has continued to increase as they have become much smaller; therefore, heat dissipation is more difficult. These developments have brought about many challenges in the design of cooling modules for compact electronic devices. For example, Tang et al. [1] introduced that the main requirements of the cooling module for a laptop are that the heat dissipation power is higher than 20 W, and the thicknesses is thinner than 2 mm. To solve the heat dissipation problem in continued miniaturization and high-performance electronic devices, Jang and Lee [2] investigated the flow characteristics of a dual piezoelectric cooling jet for cooling ultra-slim electronics. Murshed and Castro [3] reviewed the cooling mechanism, cooling techniques, and coolants for high performance electronics products. Therefore, the most economical and effective method to solve the heat issue is by using a high thermal conductivity component to evenly spread the heat and then quickly dissipate it. Heat pipes are considered to be superior thermal conductors and are characterized by high thermal conductivity, good start-up performance, high reliability and no need for extra energy, as summarized by Yang et al. [4]. To solve the problem of fluid flow and heat transfer of the heat pipe for cooling high-power-density semiconductor devices, Zhang et al. [5] proposed a numerical model to analysis the evaporation and boiling of a liquid thin film on a wick structure. Sun et al. [6] proposed using a gravity-assisted heat pipe on the thermoelectric cooler hot side to improve its cooling performance. Zhou et al. [7] developed a loop heat pipe for mobile electronics cooling. Yang et al. [8] developed a micro flat heat pipe for high-power concentrated heat sources (e.g. LED), and the micro flat heat pipe was fabricated using MEMS based on silicon technology. To reduce the thickness of heat pipes to accommodate the limited space available for heat dissipation, the use of flattened cylindrical heat pipes to make ultra-thin flattened heat pipes (UTHPs) is now a common practice. Li et al. [9] proposed a new method by heating a micro-grooved heat pipe to eliminate its collapse during the flattening process. Jiang et al. [10] analyzed theoretically and experimentally a phase change flattening process of grooved-sintered heat pipes. The results showed that the flattening collapse of the heat pipe can be well eliminated at an operating temperature of 480 K. These experiments have shown that the flattening process requires simple equipment and has a high production efficiency. Therefore, the effective enhancement of the heat transfer performance of UTHP has become a key issue in addressing heat dissipation in electronics. The menisci that exist on the surface and inside of a wick are the locations where the working fluid evaporates and absorbs heat from the heat source. Moreover, the wick provides capillary forces and flow paths to assist the condensation fluid backflow to the evaporator section. Hence, the wick structure is a critical factor that influences the thermal performance of the UTHP.

There are many types of wicks, such as grooved, powder sintered, foam metal, ordinary mesh (orthotropic woven mesh) and their composite structures (including the conventional biporous wick). Li et al. [11] analyzed theoretically the heat transfer performance of heat pipes with different types of groove wick structures based on the capillary limit models. Li et al. [12] designed a novel sintered wick structure to enhance the heat transfer performance of UTHPs. The wick was sintered at the middle of UTHP as the fluid path and the vapor flow paths are located on both sides. Dhanabal et al. [13] investigated experimentally the effect of different parameters on the thermal performance of a flat heat pipe with a copper foam metal wick. Mahdavi et al. [14] analyzed experimentally the thermal performance of a cylindrical heat pipe with a four-layer copper mesh wick under various operating conditions. The wicks in the above studies were all homogeneous wicks. A homogeneous wick is difficult to achieve in practice since the desired outcomes of high permeability and large capillary forces are contrasting demands. However, composite wicks can often combine the advantages of each component, thus increasing the overall permeability and capillary forces of the wick [15]. Many experiments have shown that the heat transfer performance of heat pipes can be enhanced by designing and optimizing their wick structures. Tang et al. [16] designed a two-phase heat transfer devices with a composite wick of sintered copper powder on micro V-grooves and investigated the effect of the wick parameters on its capillary performance. Jiang et al. [17] developed a flattened heat pipe with a porous crack composite wick for electronic device cooling. Sintered powder and micro-crack grooved structures were used to fabricate the composite wick. Zhou et al. [18] investigated the thermal performance of a UTHP with sintered copper foam-mesh wick structures. The thickness and maximum heat transport capacity of this UTHP were 0.8 mm and 5 W, respectively. Li et al. [19] analyzed theoretically and experimentally the heat transfer in the evaporator and condenser sections of a heat pipe with a sintered-grooved composite wick. Li et al. [20] studied the effects of vacuuming process parameters on the thermal performance of a heat pipe with a composite wick structure. Deng et al. [21] fabricated a two-phase heat transfer devices with a composite mesh-grooved wick and investigated the capillary performance. Li et al. [22] designed three types of composite wicks (single arch-shaped sintered-grooved wick, bilateral arch-shaped sintered-grooved wick, and mesh-grooved wick) and investigated the thermal performance of UTHPs with the wick structures. The experimental results showed that the composite wicks are very conducive to the enhancement of the thermal performance of the heat pipes. Huang and Franchi [23] fabricated a type of heat pipe with a hybrid bimodal wick structure, which had a higher evaporating meniscus density and larger capillary force; thus, the effective thermal conductivity was increased by up to 400% relative to that of a monolithic wick. Semenic and Catton [24] studied the heat transfer performance of biporous and monoporous wicks based on copper powder sintered structures, and the results indicated that the biporous wick had more evaporating menisci and could reach a higher critical heat flux than the monoporous wick. Wu et al. [25] designed a type of loop heat pipe with a double-layer wick (biporous outer layer and monoporous inner layer) and investigated the effect of the wick thickness ratio on its thermal performance. Wu et al. [26] studied the effect of powder-mixing parameters in biporous wicks on the thermal performance of loop heat pipes (LHPs) and optimized the powder sizes of the wicks. The biporous wicks were sintered using high polymer PMMA and nickel powder with different mixing ratios. This experiment showed that the thermal resistance of the LHP with the biporous wick was reduced compared to that of a previously reported monoporous wick. Wang et al. [27] designed a type of evaporator with two biporous wicks for an LHP to improve the operating performance. The results showed that the biporous wick could improve the startup performance of the LHP at a low heat load of 10 W.

As previously indicated, considerable research has been conducted on composited wicks based on grooved or powder sintered approaches to improve the performance of heat pipes. However, the UTHP with a grooved wick is not suitable for cooling the electronic devices with a working directional requirement, given the poor anti-gravity performance of the grooved wick. The filling consistency of copper powder is poor during the manufacturing process of the sintered UTHP, and limited by the total thickness of the UTHP, the powder sintered layer is thin, which results in a low production yield and a poor heat transfer performance of the UTHP with the powder sintered wick. The spiral woven mesh (SWM) is a new type of wick whose structure is soft, strong, and has good elasticity, and it is well suited for manufacturing UTHPs. There are a few studies on the SWM wick structure. In 2016, Li and Zhou et al. [28] proposed a type of UTHP with an SWM wick structure for heat dissipation of mobile phones, and disclosed an economical manufacturing process for UTHPs. Tang et al. [29] studied the capillary force of a type of copper mesh wick at the different sintering temperatures and chemical deposition times. The wick was woven using copper fiber with an outer diameter (OD) of 0.05 mm. Yang et al. [30] fabricated a kind of UTHP with mono and composite braided wire wicks. The mono wick was woven using 0.1 mm copper wire, while the composite wick was woven with a structure of 0.05 mm copper wires covered with 0.1 mm copper wires in the core. This experiment showed that the maximum heat transport capacity of the composite wick UTHP with oxidization manipulation was 20 W, which was more than 32.5% higher than the mono wick UTHP. The capillary pressure for the heat pipe with oxidization was estimated to increase approximately 30–40%. Zhou et al. [31] designed a type of UTHP with an SWM wick structure for smartphone cooling. The SWM was woven using 96 copper wires with an OD of 0.04 mm. The maximum heat transport capacity and the thickness of the UTHP were 3.6 W, and 0.4 mm, respectively. Ahamed et al. [32] developed a type of UTHP with a center fiber wick and fabricated a module made of the UTHP and a metal sheet for electronics cooling. The monoporous wick was woven using copper fiber with an OD of 0.05–0.1 mm. The woven wire mesh wicks in the above studies were all monoporous wicks. Although the woven mesh proposed by Yang et al. [30] was woven using two different OD copper wires, the wires were respectively in the inside and outside layers. The majority of the pores in the wick were still of a single size. Therefore, the wick was still a monoporous wick.

In the present study, a novel type of biporous SWM wick is proposed to enhance the thermal performance of UTHP for cooling high heat flux electronic devices. The new design of the SWM was hybrid woven using 0.05 and 0.04 mm diameter copper wires in every strand and has a multilayered structure. The large- and small-diameter wires were spiral woven to form different sized pores. The hybrid-sized pores inside the SWM wick helped to enhance the thermal performance and resulted in a combination of low flow resistance due to the large pores and high capillary forces due to the small pores. The capillary rate-of-rise experiment with deionized water using the IR camera method was carried out to characterize the capillary performance of the SWM wicks. The low-cost mass production process for the SWM wick and the UTHP was investigated, and the comprehensive thermal performance of the SWM UTHPs with different characteristic parameters was studied via experiments, including heat load, filling ratio and wick structure.

Section snippets

Specifications of ultra-thin flattened heat pipe

In this section, the structure of SWM and the fabrication of UTHP are presented. At first, the SWMs with monolayered and multilayered structures, the monoporous SWM and the biporous SWM are introduced, and then the manufacturing process of UTHPs are presented.

Flow characteristics analysis of spiral woven mesh

Two important characteristic parameters of the wick, namely, the permeability and the capillary pressure limit are theoretically investigated. The internal micro structure and the pore characteristics of the biporous wick are observed by using SEM.

Experimental setup

In this section, two experiment systems are set up for testing the capillary performance of the SWM wicks and the heat transfer performance of the UTHP samples, respectively.

Results and discussion

The experimental results of the capillary performance of the SWM wick and the heat transfer performance of the UTHP are presented and discussed.

Conclusions

A UTHP with a novel biporous SWM wick was developed and investigated. Three different structures (FOW, FIW, and SIW) were designed to study the effect of the characteristic parameters of the SWM wick on UTHP thermal performance. The flow characteristics of the SWM were analyzed theoretically, the capillary performance of the wicks was characterized by capillary rate-of-rise experiments using the IR camera method, and the thermal performance of the SWM UTHPs was experimentally investigated. The

Acknowledgements

This research was supported by the National Natural Science Foundation of China (Grant No. 51675185), the Guangdong Natural Science Foundation (Grant No. 2018B030311043), the Project of the Guangzhou Science and Technology Plan (Grant Nos. 201807010074 and 201707010071), the Project of Tianhe District Science and Technology Plan (Grant No. 201705YX263) and the Fundamental Research Funds for the Central Universities, SCUT.

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