Area-scalable high-heat-flux dissipation at low thermal resistance using a capillary-fed two-layer evaporator wick

Highlights

• A two-layer evaporator wick is fabricated by sintering and laser ablation.

• The internal structure of the two-layer wick is investigated using micro-CT scanning.

• Single-layer wicks are tested to study the effect of particle sizes on performance.

• A two-layer wick is shown to dissipate up to 485 W/cm² over 1 cm² at 0.052 K/W.

• The two-layer wick design offers a unique combination of high total heat dissipation at low thermal resistance.

Abstract

A two-layer sintered porous evaporator wick for use in vapor chambers is shown to offer very high performance in passive high-heat-flux dissipation over large areas at a low thermal resistance. The two-layer wick has an upper cap layer dedicated to capillary liquid feeding of a thin base layer below that supports boiling. An array of vertical posts bridges these two layers for liquid feeding, while vents in the cap layer provide an unimpeded pathway for vapor removal from the base wick. The two-layer wick is fabricated using a combination of sintering and laser machining processes. The thermal resistance of the wicks during boiling is characterized in a saturated environment that replicates the capillary-fed working conditions of a vapor chamber evaporator. Thermal characterization tests are first performed using conventional single-layer evaporator wicks to analyze the effect of sintered particle size on capillary-fed boiling of water. Of the particle size ranges tested, wicks sintered from 180 to 212 μm-diameter particles provided the best combination of high dryout heat flux and a low boiling resistance. A two-layer evaporator wick comprising particles of this optimal size and a 15 × 15 array of liquid feeding posts yielded a maximum heat flux dissipation of 485 W/cm² over a 1 cm² heat input area while also maintaining a low thermal resistance of only ∼0.052 K/W. The thermal performance of the two-layer wick is compared against various hybrid and biporous evaporator wicks previously investigated in the literature. While previous wick designs are typically restricted to small areas and low power levels or high surface superheats when dissipating such heat fluxes, the unique area-scalability of the two-layer wick design allows it to achieve an unprecedented combination of high total power and low-thermal-resistance heat dissipation over larger areas than were previously possible.

Introduction

The use of vapor chambers or flat heat pipes in thermal management applications is attractive due to their effective passive heat spreading capability and reliability [1], [2], [3]. Recently, there has been a focus on increasing the power dissipation limits of vapor chambers by improving the thermal-hydraulic performance of the porous wick at the location of heat input (i.e., the evaporator), where high local heat fluxes may induce boiling in the wick [4], [5]. In comparison to homogeneous or monoporous evaporator wicks, hybrid [6], [7], [8], [9] and biporous [10], [11], [12], [13] evaporator wick designs have successfully sustained higher heat fluxes by phase change during capillary feeding. The design rationale and performance tradeoffs reported in the literature, specifically for high-heat-flux dissipation during capillary-fed boiling from evaporator wicks, are reviewed in this section.

Recent developments related to porous evaporator wicks for capillary-fed boiling, all using water as the working fluid, are summarized in Table 1. This table chronologically catalogs the evaporator wick types, saturation temperature during testing, heater size, maximum heat flux, maximum total power dissipation, and the superheat and thermal resistance at the maximum power dissipation. Sintered copper porous wicks are most commonly used for capillary-fed boiling due to their high conductivity and because they offer many parallel fluid flow paths for liquid replenishment during boiling. Weibel et al. [14] studied evaporation and boiling behaviors from sintered monoporous copper wicks and reported heat flux dissipation greater than 500 W/cm². To improve on the thermal performance of monoporous wicks, different surface nanostructuring techniques and chemical modifications have been evaluated in the literature. In the case of copper wicks, growing copper oxide (CuO) nanostructures improves the wettability of the wick [15], [16]. In addition to increasing the capillary performance, the roughness of the CuO nanostructures has been shown to provide increased nucleation site densities, which enhances the heat transfer coefficients during boiling. Nam et al. [15] showed that nanostructuring copper micro-post wicks enhances the dryout heat flux by 70% compared to bare copper micro-posts, reporting dryout heat fluxes as high as ∼800 W/cm². Nevertheless, monoporous wicks have one characteristic pore size, which can be tuned to either provide a high capillary pressure or high permeability, but not a combination of both. Biporous wicks overcome this limitation, where the larger pores offer a high permeability for better liquid feeding, while the smaller pores can sustain liquid menisci for capillary feeding during high-heat-flux operation. Biporous wicks composed of patterned carbon nanotube (CNT) forests were investigated by Cai and Chen [11] and shown to dissipate extreme heat fluxes of ∼900 W/cm²; Semenic and Catton [10] used sintered copper biporous wicks to dissipate 990 W/cm².

While extremely high heat fluxes have been passively dissipated by capillary-fed boiling, Table 1 reveals that these fluxes are either limited to small hotspots (typically less than a few mm² and ∼10 s of W total power) or are attained at a very large surface superheat above the saturation temperature. Multiple studies have clearly demonstrated that there is a very strong inverse relationship between the heat input area and the dryout heat flux that can be supported by capillary-fed boiling. For example, Coso and Srinivasan [12] observed (for their biporous silicon pin fin wicks) that the maximum heat flux decreased from 733 W/cm² to 277 W/cm² when the heat input area was increased from 6.25 mm² (2.5 mm × 2.5 mm) to 100 mm² (10 mm × 10 mm). This effect can be primarily attributed to the increased fluid flow length to feed liquid to the center of the larger heated areas. Very thick wicks, on the other hand, can sustain high heat fluxes over somewhat larger areas, but the added impedance, posed by the longer vapor travel paths from the substrate through the thicker wick, induce a high surface superheat in this case. For example, the 990 W/cm² of heat flux dissipated using sintered copper biporous wicks over 32 mm² [10] was attained at a surface superheat of ∼150 K.

Effective fluid delivery throughout the evaporator region and efficient vapor removal from the wick are both necessary to enhance the dryout limits of porous wicks over larger evaporator areas (∼1 cm²). A few different hybrid sintered evaporator wick designs have been proposed to achieve this goal. Dai et al. [8] used a combination of sintered screen mesh and rectangular microchannels for the evaporator wick; the microchannels provide high-permeability pathways for liquid feeding, while the smaller pores in the screen mesh provide a high capillary pressure. These wicks demonstrated 150 W/cm² dissipated over an area of 1 cm². Hwang et al. [6] fabricated and tested a sintered copper evaporator wick with lateral converging arteries that feed a thin layer of sintered particles within the heated area. The arteries provide liquid feeding while the small thickness of the sintered layer keeps its thermal resistance to a minimum. Heat fluxes of ∼580 W/cm² were dissipated over a 1 cm² heater area, albeit at a high superheat of ∼72 K. The high superheat was attributed to local dryout occurring in the center of the heated area.

In our previous work, we designed a two-layer evaporator wick [17] to achieve high heat flux dissipation over large heater areas at a low thermal resistance. The two-layer evaporator wick, as shown in Fig. 1(a), comprises a thin base wick to support low-resistance boiling, that is supplied with liquid through an array of vertical posts attached to a thick cap layer above. In addition, vapor vents in the cap layer allow for the vapor generated by boiling in the thin base wick layer to escape. Fabrication and testing of a prototype two-layer wick [18] confirmed this working mechanism during capillary-fed boiling. The two-layer wick decoupled liquid feeding from vapor removal, thus providing an unrestricted pathway for distributed liquid supply to the base wick, without the occurrence of any local dryout that would otherwise increase the superheat. In the current study, we demonstrate high-heat-flux dissipation using the two-layer evaporator wick previously introduced. The fabrication steps and the internal structure of the two-layer wick are first described, and the effect of sintered particle size on the capillary-fed boiling behavior is then studied for a benchmark single-layer wick. Subsequently, a two-layer wick composed of the best-performing particle size is characterized and its performance compared to the literature.