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Review Article

Flow Boiling in Microgaps for Thermal Management of High Heat Flux MicrosystemsOPEN ACCESS

[+] Author and Article Information
Xuefei Han

George W. Woodruff School of
Mechanical Engineering,
Georgia Institute of Technology,
771 Ferst Drive,
Atlanta, GA 30332-0405
e-mail: xhan40@gatech.edu

Andrei Fedorov

George W. Woodruff School of
Mechanical Engineering,
Georgia Institute of Technology,
771 Ferst Drive,
Atlanta, GA 30332-0405
e-mail: andrei.fedorov@me.gatech.edu

Yogendra Joshi

Fellow ASME
George W. Woodruff School of
Mechanical Engineering,
Georgia Institute of Technology,
771 Ferst Drive,
Atlanta, GA 30332-0405
e-mail: yogendra.joshi@me.gatech.edu

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received January 16, 2016; final manuscript received July 21, 2016; published online August 24, 2016. Assoc. Editor: Eric Wong.

J. Electron. Packag 138(4), 040801 (Aug 24, 2016) (12 pages) Paper No: EP-16-1013; doi: 10.1115/1.4034317 History: Received January 16, 2016; Revised July 21, 2016

Abstract

In the first part of this paper, a review of fundamental experimental studies on flow boiling in plain and surface enhanced microgaps is presented. In the second part, complimentary to the literature review, new results of subcooled flow boiling of water through a micropin-fin array heat sink with outlet pressure below atmospheric are presented. A 200 μm high microgap device design was tested, with a longitudinal pin pitch of 225 $μm$, a transverse pitch of 135 $μm$, and a diameter of 90 $μm$, respectively. Tested mass fluxes ranged from 1351 to 1784 , and effective heat flux ranged from 198 to 444 W/cm2 based on the footprint surface area. The inlet temperature varied from 6 to 12 °C, and outlet pressure ranged from 24 to 36 kPa. The two-phase heat transfer coefficient showed a decreasing trend with increasing heat flux. High-speed visualizations of flow patterns revealed a triangular wake after bubble nucleation. Flow oscillations were seen and discussed.

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Introduction

Flow boiling in microscale systems was studied intensively in the past two decades and is still attracting the interest of many researchers [15]. Possible improvement in heat transfer over single-phase flow at the same mass flux due to latent heat of vaporization makes it a promising thermal management method for high power dissipation electronics. Moreover, two-phase flow may offer better temperature uniformity than single-phase flow, if dryout and instabilities can be mitigated. A block diagram of a closed-loop two-phase microfluidic cooling system for electronic devices is shown in Fig. 1. The system generally consists of a pump to drive fluid flow, microscale heat sink for heat removal via two-phase flow, heat exchanger to condense the vapor, and reservoir to supply sufficient fluid for circulation. Filters with microscale pore size and pressure regulating valves are also common components in these systems.

Thermodynamic properties of six typical coolants, FC-72, water, HFE-7000, R-123, R-134 a, and R-245fa, all of which have been frequently used for flow boiling experiments, are listed in Table 1 [6]. Water is one of the most attractive coolants because of its large-specific heat and latent heat of vaporization, which enable absorption of considerable amount of heat during both sensible heating and boiling. However, saturation temperature of water at atmospheric pressure is 100 °C, which may be unacceptably high for continuous operation of complementary metal oxide semiconductor (CMOS) electronic devices. Water is also not as chemically and electrically inert as other coolants in Table 1, but is the most environmental friendly. The liquid to vapor density ratio of water is also the largest among the six coolants, which can result in very large void fraction at relatively low vapor quality. Void fraction is the fraction of the cross-sectional area of the channel that is occupied by the vapor, and quality is the vapor mass fraction. The high density ratio also results in relatively large accelerational pressure drop associated with convective boiling flow in microgeometries.

Dielectric coolants, FC-72 and refrigerants, are of great interest because they are chemically and electrically inert and have low saturation temperature, making them more suitable for electronics cooling. Some refrigerants, such as R-134a, need to operate at high saturation pressures at the temperatures relevant to operating electronics devices, adding extra structural strength requirement to the flow system. Some dielectric coolants contribute to ozone depletion and global warming. Hydrofluoroethers (HFEs) are a better option due to their nearly zero stratospheric ozone depletion and relatively low global warming potential [7]. HFEs (HFE-7000, for instance) also have lower boiling point compared to water.

Reviews of flow boiling in microchannels (single microchannel and parallel microchannels) have been performed by Garimella and Sobhan [1], Thome [2], and recently by Tibirica and Ribatski [3]. To compliment these works, a review of experimental work on flow boiling in plain and surface enhanced microgaps is presented in Sec. 2. Representative geometries are shown in Fig. 2. The details of geometry, experimental conditions, and studied parameters in each reviewed paper are summarized in Table 2. Survey of relevant literature revealed that flow boiling of water in microscale system at subatmospheric pressure was rarely studied previously. Lower system pressure reduces saturation temperature to compensate the drawback of high boiling point of water, thus improving its utility for electronics thermal management applications. In Sec. 3, subcooled flow boiling of water in a staggered micropin-fin array heat sink with outlet pressure below atmospheric was studied. The two-phase heat transfer coefficient showed a decreasing trend with increasing heat flux. Reversed flow and instabilities were observed and briefly discussed.

Review of Experimental Studies on Flow Boiling in Microgap, and Microchannels/Microgaps With Surface Enhancement

Plain Microgaps.

Lee and Lee [8] and Yang and Fujita [9] studied flow boiling of R-113 in minigap and microgap with gap heights ranging from 2 mm to 0.2 mm. Lee and Lee [8] developed a two-phase heat transfer coefficient correlation for minigap and microgap flows. Their pressure drop data agreed well with a correlation obtained with adiabatic water air two-phase flow in a microgap [40]. Flow patterns such as bubbly, intermittent, wavy, and annular were observed, and gap height was found to impact flow patterns and heat transfer characteristics [9]. As gap height decreased, annular flow was dominant, and intermittent and wavy flow diminished.

Sheehan et al. investigated wall temperature during flow boiling of FC-72 in a microgap using infrared (IR) imaging techniques [14,20]. Wall temperature fluctuations were reported and ascribed to local dryout and rewetting during film evaporation. They found that both the flow regime and heat flux influence the wall temperature fluctuations. Kim et al. studied subcooled flow boiling of FC-72 in a microgap as a cooling strategy for high power light-emitting diodes (LEDs) [18,41]. The LEDs were mounted directly on the microgap cooler, and the peak heat fluxes measured was 20 W/cm2. Three microgap heights, 110 $μm$, 210 $μm$, and 500 $μm$, were studied for both single-phase and two-phase flows. The two-phase heat transfer coefficients were found to be higher than for single-phase flow. Generally, the two-phase heat transfer coefficients were higher in shorter microgaps than in taller ones, ranging from 10 kW/m2 K to 7.5 kW/m2 K for 110 $μm$ and 500 $μm$ microgaps, respectively. Averaged two-phase heat transfer coefficients were also compared with correlations of Chen [42] and Shah [43]. Closer agreement with Shah's correlation at lower quality and with Chen's prediction at higher quality was found.

Alam et al. studied local flow boiling heat transfer and pressure drop characteristics in a silicon microgap heat sink using de-ionized (DI) water [24]. The microgap had an area of 1.27 cm $×$ 1.27 cm, and an array of 5 $×$ 5 heating elements and temperature sensors. Three gap heights were investigated: 180 $μm$, 285 $μm$, and 381 $μm$. The experiments were conducted with three mass fluxes, 420 kg/m2 s, 690 kg/m2 s, and 970 kg/m2 s, and effective heat flux ranging from 0 to 110 W/cm2. Confined slug flow/annular flow were observed after onset of nucleate boiling as heat flux increased. At fixed mass flux and heat flux, smaller gap tended to have confined annular flow while larger gap tended to have confined slug flow. Thin liquid film evaporation was the main heat transfer mechanism in confined annular flow, resulting in higher heat transfer coefficients in smaller gap than in larger gap. This also agreed with the work of Kim et al. [18,41]. Pressure drop increased with heat flux in smaller gap, but was independent of heat flux in larger gap. Wall temperatures were almost uniform along flow direction after boiling occurred for all the gap heights.

Alam et al. studied the effects of surface roughness on flow boiling in microgap [29]. The device also had an area of 1.27 cm $×$ 1.27 cm, and an array of 5 $×$ 5 heating elements and temperature sensors. They tested three gap heights of 500 $μm$, 300 $μm$, and 200 $μm$ and three surface roughness levels of 0.6 $μm$, 1.0 $μm$, and 1.6 $μm$. Lower wall superheat was sufficient to initiate boiling in microgap with higher surface roughness. Rougher surface also increased nucleation density, wall temperature uniformity, and local two-phase heat transfer coefficients. No significant effect on pressure drop was observed in microgap with different surface roughnesses. However, increased surface roughness showed an adverse effect on pressure instability, and higher amplitude in pressure oscillations was observed. Using the same test setup, Alam et al. compared the ability of minimizing temperature gradient and mitigating hotspot of microgap and microchannel [21,30]. Tested microgap height was from 200 $μm$ to 400 $μm$, and the 200 $μm$ high microgap was compared with a microchannel with a channel pitch of 200 $μm$ in Ref. [21]. Tested microgap height was 190 $μm$, microchannel width was 208 $μm$, and height was 386 $μm$ in Ref. [30]. For uniform heating, at the same mass flux of 690 kg/m2 s and heat flux range from 0 to 60 W/cm2, microgap cooled device demonstrated a smaller temperature gradient and smaller amplitude of pressure and temperature oscillation than microchannel [21]. Reducing gap heights suppressed flow oscillation as well. When a hotspot was activated, microgap also showed better temperature uniformity than microchannel, and smaller gap height lowered wall temperature compared to higher gap height. Microgap gave better heat transfer performance at high heat flux due to confined slug/annular flow, which was dominant, and microchannel performed better at low heat flux due to early occurrence of slug/annular flow [30]. At lower mass flux, microgap outperformed microchannel as well.

Alam et al. further studied the effects of microgap heights on two-phase flow regimes, heat transfer coefficient, and pressure drop [25]. They studied microgap heights from 80 $μm$ to 1000 $μm$. They found that for microgap heights smaller than 500 $μm$, confined slug flow was the dominant flow pattern at low heat flux, while confined annular flow was the dominant flow pattern at higher heat flux; for microgap heights larger than 700 $μm$, bubbly flow was dominant at lower heat flux while slug/annular flow was dominant at higher heat flux, which agreed with the findings in Ref. [4]. Thus, they concluded that confinement occurred in microgap heights smaller than 500 $μm$, and effect of confinement was negligible for microgap heights larger than 700 $μm$. The microgap of heights from 100 $μm$ to 500 $μm$ among the tested height range presented best performance in terms of maintaining uniform and low wall temperatures and achieving high heat transfer coefficients. Smaller microgap heights assisted to suppress pressure oscillation and wall temperature oscillation as well. Morshed et al. compared two-phase heat transfer in a microgap with bare copper base surface, and copper base surface with electrochemically grown nanowires [22] using DI water. They studied mass fluxes ranging from 45.9 $kg/m2s$ to 143.8 $kg/m2s$, heat fluxes ranging from 0 to 60 $W/cm2$, and inlet temperatures ranging from 22 °C to 80 °C. Their results indicated that the microgap with nanowires improved two-phase heat transfer coefficient by up to 56%, with a pressure drop increase of 20%, and improved single-phase heat transfer coefficient by up to 25%. The nanowires also reduced the wall superheat by up to 12 °C to initiate boiling. Morshed et al. also compared a microgap with bare copper surface with four square cross-grooves of 0.5 mm $×$ 0.5 mm, and copper surface with four nanoparticle deposited cross-grooves [26]. The grooves increased both single-phase and two-phase heat transfer coefficients by up to 50%, increased critical heat flux (CHF) by 15%, and lowered the boiling incipience temperature. The nanoparticles deposited grooves could further lower boiling incipience temperature, however, showed no evidence of heat transfer coefficient improvement.

Microgaps With Micropin-Fin Surface Enhancement.

Kosar and Peles studied flow boiling of R-123 from a staggered circular micropin-fin array [10] and staggered hydrofoil-shaped micropin-fin array [11] within a microchannel of 1.8 mm width, 1 cm length, and 243 $μm$ height. While not a microgap, this configuration involves the use of surface enhancement features. The diameter of circular pins in Ref. [10] was 99.5 $μm$. The near zero contact angle of R-123a on silicon resulted in deactivation of large nucleation sites, and high superheat as boiling initiated. At certain conditions, a slight increase in heat flux caused a sudden increase in boiling and flow oscillations, as well as pressure and temperature oscillations. The microchannel in Ref. [11] contained an array of 20 × 12 or 13 (in tandem) staggered hydrofoil pin fins with a wetted perimeter of 1030 $μm$ and chord thickness of 10 $μm$. They tested heat flux range from 19 to 312 W/cm2, and a mass flux range from 976 to 2349 kg/m2 s. The heat transfer coefficient was found to increase with increasing heat flux until a maximum was reached and then decreased monotonically with increasing heat flux until CHF. The increasing trend at low quality was ascribed to nucleate boiling, and the decreasing trend at high quality to dominance of convective boiling heat transfer mechanism. Krishnamurthy et al. studied flow boiling of water in a 1.8 mm wide, 1 cm long, and 250 $μm$ deep microchannel with staggered circular pin-fins of diameter of 100 $μm$ and pitch-to-diameter ratio of 1.5 [13]. The authors tested heat flux ranged from 20 W/cm2 to 350 W/cm2 and mass flux ranged from 346 kg/m2 s to 794 kg/m2 s. The outlet of the device was maintained at atmospheric pressure. They found that two-phase heat transfer coefficient was moderately dependent on mass flux, and independent of heat flux, for the range of mass flux and heat flux tested. They developed a correlation to predict heat transfer coefficient using a superposition model based on Reynolds analogy. They also constructed a flow pattern map using gas and liquid Reynolds numbers defined using superficial velocities and found good agreement with the flow map constructed for adiabatic microscale systems developed using DI water and nitrogen [44]. Lie et al. investigated heat transfer coefficients and bubble characteristics in flow boiling of FC-72 in a microgap with inline square pin-fins with pin side length of 200 $μm$ and 100 $μm$ pin height of 70 $μm$, and both transverse and longitudinal pitches same as pin side length [12]. The flow pattern was mostly bubbly flow. Two-phase heat transfer coefficients were found to be relatively independent of mass flux and to increase with heat flux. The micropin fins improved bubble departure frequency and two-phase heat transfer coefficients. Bubble departure diameter and active nucleation site density decreased with mass flux, while bubble departure frequency increased with mass flux. The departing bubbles at higher heat flux were significantly larger than those at lower heat flux.

Qu and Abel studied flow boiling heat transfer of water in an array of staggered square micropin fins [15]. The pin fins covered an area of 3.38 cm length by 1 cm width. The cross section area of a single pin was 200 $μm$  × 200 $μm$, and height was 670 $μm$. Their tests were performed for inlet temperatures of 90 °C, 60 °C, and 30 °C, with six mass fluxes in the range from 183 to 420 $kg/m2s$ for each inlet temperature. The outlet pressure ranged from 103 to 108 kPa, and heat flux ranged from 23.7 to 248.5 W/cm2. They observed that the two-phase heat transfer coefficient decreased with increasing heat flux at low quality and was fairly constant at quality greater than 0.15. The two-phase heat transfer coefficient was also independent of mass flux at quality greater than 0.15. The mean absolute error of their results was beyond $±30%$ when compared with the previously mentioned correlations developed by Krishnamurthy and Peles [13].

Subcooled flow boiling of FC-72 on a micropin-finned silicon chip was studied by Ma et al. [16] and Yuan et al. [17], respectively. The micropin fins of sizes less than 100 $μm$ were relatively small compared to channel width 30 mm and channel height 5 mm in both studies. Micropin-finned surface showed significant improvement in heat transfer, compared to smooth surface in terms of heat transfer coefficient and CHF. CHF was also increased at higher subcooling, and the enhancement was more noticeable over the micropin-finned surfaces. With a similar test setup, Guo et al. conducted subcooled flow boiling of FC-72 on a micropin-fin silicon chip with jet impingement [23] to achieve enhanced heat transfer and delayed CHF. As jet velocity increased, increase in mixing and turbulence in stagnation area improved heat transfer significantly.

Isaacs et al. [27,28] investigated flow boiling of R-245fa in staggered circular pin-fin enhanced microgap. The fined area was 1 cm $×$ 1 cm, and the circular pin had a pin diameter, height, and pitch of 150 μm, 200 μm, and 225 μm, respectively. The tested mass flux ranged from 598 W/cm2 to 1639 W/cm2, heat flux up to 40 W/cm2, and inlet subcooling of 10 °C and 13 °C. By flow visualization, triangular-shaped vapor wakes were observed after nucleation points.

A passive flow separation technique was proposed and tested by Dai et al. using a copper microgap of 5 mm (W) $×$ 26 mm (L) $×$ 0.34 mm (H) [31]. A two-layer copper mesh of thickness 160 $μm$ was present on copper surface. A portion of incoming fluid was routed to an opening of diameter 0.8 mm located in the center of microgap and entered microgap from the opening. It was a passive flow separation because the fraction of liquid going through the center opening was purely determined by pressure force balance between microgap inlet and the opening during fluid flow. This flow separation was found to enhance mixing and reduce temperature gradient in single-phase flow, as well as to suppress bubble growth and flow instabilities in two-phase flow.

Reeser et al. recently studied heat transfer and pressure drop characteristics of HFE-7200 and DI water in inline and staggered micropin-fin arrays [32]. The arrays had a 0.96 $×$ 2.88 cm footprint area and square pin fin width and height of 153 and 305 $μm$. For HFE-7200 and DI water, the mass flux ranged from 200 to 600 $kg/m2$ s and 400 to 1300 $kg/m2$ s, and heat fluxes ranged from 1 to 36 $W/cm2$ and 10 to 110 $W/cm2$, respectively. They achieved high exit quality up to 0.9 for HFE-7200. Heat transfer coefficients behavior differed significantly for HFE-7200 and DI water due to different material properties of both working fluids. They also found that pressure drop correlation developed by Qu and Abel [15] and heat transfer coefficient correlation developed by Krishnamurthy and Peles [13] showed poor accuracy in prediction for their work and these correlations needed to be modified.

Ong et al. studied flow boiling of R1234ze in a radial hierarchical fluid network [34]. The concept was to introduce fluid inlet at the center of test device, and fluid was then directed radially to outlets located on the edge of test device. They utilized different sizes of orifices at inlet to distribute fluid flow to subsection of the test devices. The radial quadrant microgap with circular pin-fin mitigated the pressure gradients and reduced temperature gradient as well. They also studied microgap with staggered pin-fin (27 deg and 45 deg), but the results were not compared with radial quadrant test device. The observed two-phase flow instabilities and believed two-phase flow instabilities were related to the degree of inlet subcooling. Schultz and coworkers also tested a radial microgap with embedded pin arrays using the same fluid [35,36]. The test device size was 20.25 mm $×$ 20.25 mm and had eight core heaters and 16 hotspot heaters, and they studied the effects of local hotspot. They found that 50% increase in mass flow rate only resulted in 8% in two-phase heat transfer coefficients. Increase in mass flow rate did not necessarily help to mitigate temperature nonuniformity. Using R-134 a in an open-loop, David et al. studied effects of transient heat load on two-phase heat transfer coefficients in a microgap with staggered square pin fins [33]. Their results indicated that temperature was maintained near uniform under both steady state and transient heating. Higher heat transfer coefficient was achieved under transient heating than steady-state heating. Heat transfer coefficient varied with vapor quality, and a peak was observed for vapor quality of 0.55.

Tamanna et al. investigated the effect of expanding the microgap height on flow boiling heat transfer and pressure drop characteristics [37,38]. The microgap was formed with silicon base and polycarbonate cover with the inlet height of 200 $μm$ for all the gaps, and the outlet height increases from 200 $μm$ to 300 $μm$ and to 460 $μm$. A delay of partial dryout was observed in the 200–460 $μm$ microgap at a heat flux of 79 W/cm2, compared to the straight 200–200 $μm$ microgap at a heat flux of 61 W/cm2. The expanding microgap with outlet height of 300 $μm$ gives the smallest pressure drop, by providing room for the vapor expansion without excessive flow acceleration, and best wall temperature uniformity of all the three tested heights. Further expansion of outlet height 460 $μm$ increased pressure drop due to unstable boiling and vapor acceleration. The fluctuations in temperature caused by unstable boiling in microgap were found to be independent of fluid quality and heat fluxes [20].

Woodcock et al. developed a piranha pin fin (PPF) structure in a microgap [39]. They investigated flow boiling of HFE-7000 in PPF-enhanced microchannel and achieved heat flux as high as 700 W/cm2. Tested mass fluxes ranged from 1200 kg/m2 s to 7000 kg/m2 s. A staggered array PPF, each of diameter 150 μm and with a 300 μm long tail, was used. The PPFs had open mouths on leading edges, with PPFs wall thickness of 30 μm, and fluid flow could come inside the PPFs and be extracted from the bottom fluid passage of each PPF. This way heat transfer was significantly enhanced in single-phase and two-phase conditions.

In summary, the review on flow boiling in plain microgap and microgap with pin fin surface enhancement revealed the ability of this promising strategy as thermal management method for high heat flux removal. However, fundamental research is still needed to understand the physics of boiling in microgap and especially in microgap with pin fin surface enhancement. The dependences of boiling mode, two-phase heat transfer coefficient and pressure drop on inlet temperature, heat flux, and mass flux are still unclear and require further study.

Experimental Study of Water Flow Boiling in Micropin-Fin Heat Sink at Reduced Pressure

Experimental Setup and Procedure.

Here, we describe new flow boiling experiments performed with DI water for pin fin enhanced microgaps. The experiments were performed in a closed flow loop shown schematically in Fig. 3, consisting of gear pump, filter, flow meter, preheater, test section, heat exchanger, and fluid reservoir. The gear pump (Cole-Parmer EW-07002-27) could deliver volumetric flow rates from 4.2 mL/min to 420 mL/min, with maximum differential pressure of 40 psi. The filter (Swagelok inline particulate filter B-4F-7) has a pore size of 7 $μm$. The flow meter (McMillan S-114) is used to measure volumetric flow rate of fluid in the range of 50–500 $mL/min$. The nickel ribbon resistance wire preheater is wrapped on the outer surface of the section of tubing upstream of test section to elevate fluid temperature at test sample inlet close to saturation condition. Two-phase flow from the exit of test section is condensed in the heat exchanger (LYTRON LL520G12), which is cooled by a thermostatic bath circulator (LabCompanion RW-1025 G). The capacity of stainless steel fluid reservoir (Swagelok 316 L-HDF4-300) is 300 $mL$. Pressure and temperature are measured at multiple locations in the loop, as marked in Fig. 3. Pressure transducers (Omega PX219-300 G-5V) and T-type thermocouples (Omega HTQSS-116 G-12) are utilized for these measurements.

Figure 4 illustrates the schematic of the test section. The staggered micropin-fin heat sink was microfabricated from silicon and was anodically bonded to Pyrex to form a microgap. The transparency of Pyrex enables flow visualization. The device is sandwiched between the printed circuit board (PCB) and package, which are affixed with screws. The device is electrically connected to the PCB by wire bonding. The package is designed to connect the device to the flow loop and to enable local pressure and temperature measurements. Thermocouples are inserted into inlet and outlet fluid plenums inside package, so inlet and outlet fluid temperatures can be measured at locations that are only 8 mm away from the device. O-rings are used to seal between the device and the package.

A schematic of the test device is shown in Fig. 5. The staggered pin fins cover an area of 1 cm $×$ 1 cm in the center of device. There is one row of pins both upstream and downstream of the staggered pin-fin arrays to redistribute fluid flow. Pressure taps are placed between the staggered pin-fin arrays and inlet and outlet flow redistribution pins, as seen in Fig. 5(a). Figure 5(b) shows the back side of the device. Four platinum resistance heaters are deposited on the back side, also covering an area of 1 cm $×$ 1 cm directly underneath the pin-fin arrays. The heaters also work as resistance temperature detectors (RTDs) due to the near linear dependence of platinum resistance on temperature. The dimensions of tested device are shown in Fig. 6. The pin height is 200 $μm$, which is the same as the gap height. The overall device size is 28 mm × 13.5 mm, and the total thickness is 500 μm.

Before testing, the heaters were calibrated in an oven to obtain the resistance–temperature curve for each heater. The resistances showed nearly linear dependence on temperature from 20 °C to 145 °C. The calibrated device was sealed in package which was then connected to the closed flow loop. The system was evacuated, and then charged with DI water. The DI water was boiled for 30 mins to remove dissolved gas before charging the flow system.

Data Reduction.

Before conducting flow boiling measurements, single-phase heat transfer tests were performed. The power required to increase water temperature from inlet to outlet was calculated and subtracted from the total power supplied to the heaters to estimate heat loss. Heat loss is estimated from single-phase measurements by Display Formula

(1)$Qloss=Ptotal−m˙Cp(Tout−Tin)$

where $m˙$ is the mass flow rate of water. The average heat loss was 11% of the total supplied power. These heat loss estimates were used to calculate effective heat fluxes for the two-phase data.

Mass flux is defined as Display Formula

(2)$G=m˙Ac,min$

$Ac,min$ is the minimum cross section area of the microgap, and thus Display Formula

(3)$Ac,min=(Wch−WchSTD)Hch$

The average net heat flux is calculated accounting for heat losses Display Formula

(4)$qeff″=Ptotal−QlossAh$

where $Ah$ is the total heated area and equals to 1 cm2. Water comes in the device at a significant subcooling condition, and boiling only occurs in the finned area close to microgap exit. Thus, local two-phase heat transfer coefficient $htp$ was evaluated in a unit cell area containing a single pin using local $Tw$ from Display Formula

(5)$qeff″Auc=htp(Auc−Ac)(Tw−Tsat)+htpηfAf(Tw−Tsat)$

where $Auc$ is the base area of a unit cell, $Af$ is the surface area of a single pin fin, and $Ac$ is the cross section area of a single pin fin. Thus, Display Formula

(6)$Auc=STSL$
Display Formula
(7)$Af=PfHf$
Display Formula
(8)$Ac=πD24$

where $Pf=πD$ is the pin perimeter, and $Hf$ is the pin height. Assuming that fin tip is insulated, fin efficiency $ηf$ can be calculated using Display Formula

(9)$ηf= tan h(mHf)mHf$

where Display Formula

(10)$m=htpPfksAc$

where $ks$ is the solid material thermal conductivity, and $Tw$ is the pin fin base temperature. Assuming one-dimensional conduction in heat sink base Display Formula

(11)$qeff″=ksTh−Twt$

where $t$ is the distance from the pin fin base to heaters, and $Th$ is the heater temperature.

The exit quality was calculated from Display Formula

(12)$x=[qeff″Ah−m˙Cp(Tsat−Tin)hvap]/m˙$

where $Tsat$ and $hvap$ are the water saturation temperature and latent heat of vaporization, both of which are evaluated at device exit pressure.

The estimated measurement uncertainties in absolute scale are listed in Table 3.

Results and Discussion.

The operating conditions for each test are presented in Table 4. The inlet temperature varied slightly due to the fluctuation in temperature at the condenser heat exchanger, which affected the reservoir fluid temperature. The fluctuation in condenser temperature was caused by the limited ability of the thermostatic bath circulator to control bath coolant temperature. The fluctuation in condenser temperature also caused saturation pressure at condenser to fluctuate and also the device outlet pressure. Device outlet pressure also increased as fluid quality increased, because increasing quality would increase pressure drop across the tubing from device exit to condenser. For the same mass flux, the boiling area started at the rear portion of the microgap, close to exit and eventually moved forward into the pin fin array. Two-phase heat transfer coefficients for conditions where boiling was in the pin fin array are shown in Fig. 7(a), and similar trends were reported earlier [15]. Other works found two-phase heat transfer coefficient slightly increased with heat fluxes [13,32], and all these works used different device designs and different experimental conditions, such as inlet temperatures and mass fluxes. The decreasing trend of two-phase heat transfer coefficient becomes less dependent on heat flux as heat flux increases. Two-phase heat transfer coefficient also showed dependence on mass flux due to different degrees of mixing at different mass fluxes. This dependence on mass flux was also observed by other researchers [13,15,32]. The wall superheat and estimated exit quality increase almost linearly with increasing heat flux as shown in Figs. 7(b) and 7(d). At heat flux of 444 W/cm2, wall superheat was as high as 60 °C. The high wall superheat was because of large degree of inlet subcooling. The pressure drop increases with increasing heat flux, more rapidly at higher heat fluxes as seen in Fig. 7(c), because increase in pressure drop due to vapor phase was more prominent than decrease in pressure drop in single-phase region due to decrease in viscosity.

Flow instabilities were observed at high heat flux. The wall temperatures and pressure drop, for the mass flux 1351 kg/m2 s at 267 W/cm2, fluctuated over a large range, as seen in Fig. 8. This happened when vapor pressure increased enough to overcome inlet pressure and induced reverse flow. Liquid was pushed backward and not enough liquid flow occurred in the heat sink, causing dramatic increase in temperature. As liquid accumulated at inlet, and pressure was sufficient to overcome pressure drop across the pin-fin arrays, the liquid was forced into the microheat sink again, and the temperature decreased. Two-phase flow instabilities in microchannels have been studied by many other researchers, and pressure restrictor at inlet was recommended to stabilize two-phase flow [4548].

High-speed images taken at a frame rate of 4200 fps at multiple values of heat flux for test 1 are shown in Fig. 9. Boiling initiation started in the vicinity of the outlet inside the device and moved toward inlet, as supplied heat flux increased. The dotted area in Fig. 10 indicates two-phase area. The majority of finned area was in single phase due to inlet subcooling. Once vapor bubbles formed, they expanded rapidly into a triangular-shaped vapor wake for all the heat fluxes. This agrees with observation reported in Refs. [27,28]. Due to the substantially subcooled inlet condition, vapor phase only existed in a few rows of pins close to exit before reverse flow and oscillations occurred.

Conclusions.

In this study, subcooled flow boiling of water across staggered circular micropin-fin array heat sink was investigated. The main conclusions from the new experiments are as follows:

1. (1)Two-phase heat transfer coefficient decreased with increasing heat flux, and the dependence became less pronounced at higher heat fluxes. The highest heat transfer coefficient achieved was 48 kW/m2 K, and the lowest was 25 kW/m2 K.
2. (2)Wall superheat and quality increased almost linearly with heat flux. The wall superheat was as high as 60 °C at effective heat flux of 450 W/cm2.
3. (3)The pressure drop ranged from 60 kPa to 130 kPa for the tested mass flux and heat flux ranges and increased rapidly at higher heat flux due to accelerational vapor flow contributions.
4. (4)Instead of traditional two-phase flow patterns such as bubbly flow and slug flow, triangular-shaped vapor wakes were observed after bubble nucleation sites.
5. (5)Reverse flow would cause pressure and temperature to fluctuate dramatically and degrade the effectiveness of cooling significantly if no provisions made for suppressing instabilities using flow restrictors or other means.

For future work, flow restrictors at the inlet are recommended to prevent reverse flow. Elevated inlet temperatures could be used to reduce wall superheat and to achieve higher vapor quality. Effects of nonuniform heating should be studied since it will mimic a real microelectronic device better due to local hot spots.

Acknowledgements

The authors acknowledge the support from DARPA ICECool Fundamentals Program for this work. In addition, they acknowledge Professor Muhannad S. Bakir and Thomas E. Sarvey, who provided the device.

Nomenclature

• $Ac$ =

single pin fin cross section area, m2

• $Ac,min$ =

minimum channel cross section area, m2

• $Af$ =

single pin fin surface area, m2

• $Ah$ =

total heated area, m2

• $Auc$ =

unit cell area associated with single pin, m2

• $Cp$ =

specific heat capacity, kJ/kg K

• $D$ =

circular pin diameter

• $G$ =

mass flux, kg/m2 s

• $H$ =

microgap height, $μm$

• $htp$ =

local two-phase heat transfer coefficient, W/m2 K

• $hvap$ =

latent heat of vaporization, kJ/kg

• $Hf$ =

fin height, m

• $HCH$ =

channel height, m

• $ks$ =

thermal conductivity of solid material, W/m K

• $m$ =

fin efficiency parameter

• $m˙$ =

mass flow rate, kg/s

• $P$ =

pressure, kPa

• $Pf$ =

fin perimeter, m

• $Pout$ =

outlet pressure, kPa

• $Psat$ =

saturation pressure, kPa

• $Ptotal$ =

total power supplied to device, W

• $Qloss$ =

heat loss, W

• $qeff"$ =

effective heat flux, W/cm2

• $R$ =

surface roughness, $μm$

• $S$ =

square pin-fin side length, $μm$

• $SL$ =

longitudinal pitch, m

• $ST$ =

transverse pitch, m

• $t$ =

heat sink base thickness, m

• $Th$ =

heater temperature,  °C

• $Tw$ =

wall temperature,  °C

• =

maximum wall temperature,  °C

• $Tin$ =

inlet fluid temperature,  °C

• $Tout$ =

outlet fluid temperature,  °C

• $Tsat$ =

saturation temperature,  °C

• $Tsub$ =

inlet subcooling,  °C

• $WCH$ =

channel width, m

• $x$ =

exit quality

• $δ$ =

liquid film thickness, $μm$

• $ΔP$ =

pressure drop, kPa

• $ηf$ =

fin efficiency

• $ρl$ =

liquid density, kg/m3

• $ρv$ =

vapor density, kg/m3

References

Garimella, S. V. , and Sobhan, C. , 2003, “ Transport in Microchannels: A Critical Review,” Annu. Rev. Heat Transfer, 13(13), pp. 1–50.
Thome, J. R. , 2004, “ Boiling in Microchannels: A Review of Experiment and Theory,” Int. J. Heat Fluid Flow, 25(2), pp. 128–139.
Tibirica, C. B. , and Ribatski, G. , 2013, “ Flow Boiling in Micro-Scale Channels–Synthesized Literature Review,” Int. J. Refrig., 36(2), pp. 301–324.
Bar-Cohen, A. , and Rahim, E. , 2007, “ Modeling and Prediction of Two-Phase Refrigerant Flow Regimes and Heat Transfer Characteristics in Microgap Channels,” ASME Paper No. ICNMM2007-30216.
Bar-Cohen, A. , Sheehan, J. R. , and Rahim, E. , 2012, “ Two-Phase Thermal Transport in Microgap Channels—Theory, Experimental Results, and Predictive Relations,” Microgravity Sci. Technol., 24(1), pp. 1–15.
Klein, S. A. , 2015, “ Engineering Equation Solver Academic Professional Software.”
Tsai, W.-T. , 2005, “ Environmental Risk Assessment of Hydrofluoroethers (HFEs),” J. Hazard. Mater., 119(1), pp. 69–78. [PubMed]
Lee, H. J. , and Lee, S. Y. , 2001, “ Heat Transfer Correlation for Boiling Flows in Small Rectangular Horizontal Channels With Low Aspect Ratios,” Int. J. Multiphase Flow, 27(12), pp. 2043–2062.
Yang, Y. , and Fujita, Y. , 2004, “ Flow Boiling Heat Transfer and Flow Pattern in Rectangular Channel of Mini-Gap,” ASME Paper No. ICMM2004-2383.
Koşar, A. , and Peles, Y. , 2006, “ Convective Flow of Refrigerant (R-123) Across a Bank of Micro Pin Fins,” Int. J. Heat Mass Transfer, 49(17), pp. 3142–3155.
Koşar, A. , and Peles, Y. , 2007, “ Boiling Heat Transfer in a Hydrofoil-Based Micro Pin Fin Heat Sink,” Int. J. Heat Mass Transfer, 50(5–6), pp. 1018–1034.
Lie, Y. , Ke, J. , Chang, W. , Cheng, T. , and Lin, T. , 2007, “ Saturated Flow Boiling Heat Transfer and Associated Bubble Characteristics of FC-72 on a Heated Micro-Pin-Finned Silicon Chip,” Int. J. Heat Mass Transfer, 50(19), pp. 3862–3876.
Krishnamurthy, S. , and Peles, Y. , 2008, “ Flow Boiling of Water in a Circular Staggered Micro-Pin Fin Heat Sink,” Int. J. Heat Mass Transfer, 51(5), pp. 1349–1364.
Sheehan, J. , Kim, D. W. , and Bar-Cohen, A. , 2009, “ Thermal Imaging of Two-Phase Cooled Microgap Channel Wall,” ASME Paper No. InterPACK2009-89238.
Qu, W. , and Abel, S.-H. , 2009, “ Experimental Study of Saturated Flow Boiling Heat Transfer in an Array of Staggered Micro-Pin-Fins,” Int. J. Heat Mass Transfer, 52(7–8), pp. 1853–1863.
Ma, A. , Wei, J. , Yuan, M. , and Fang, J. , 2009, “ Enhanced Flow Boiling Heat Transfer of FC-72 on Micro-Pin-Finned Surfaces,” Int. J. Heat Mass Transfer, 52(13), pp. 2925–2931.
Yuan, M. , Wei, J. , Xue, Y. , and Fang, J. , 2009, “ Subcooled Flow Boiling Heat Transfer of FC-72 From Silicon Chips Fabricated With Micro-Pin-Fins,” Int. J. Therm. Sci., 48(7), pp. 1416–1422.
Dae-Whan, K. , Rahim, E. , Bar-Cohen, A. , and Han, B. , 2010, “ Direct Submount Cooling of High-Power LEDs,” IEEE Trans. Compon. Packag. Technol., 33(4), pp. 698–712.
Kim, D. W. , Rahim, E. , Bar-Cohen, A. , and Han, B. , 2008, “ Thermofluid Characteristics of Two-Phase Flow in Micro-Gap Channels,” 11th Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITHERM 2008), Orlando, FL, May 28–31, pp. 979–992.
Sheehan, J. , and Bar-Cohen, A. , 2010, “ Spatial and Temporal Wall Temperature Fluctuations in Two-Phase Flow in Microgap Coolers,” ASME Paper No. IMECE2010-40227.
Alam, T. , Lee, P. S. , Yap, C. R. , and Jin, L. , 2011, “ Experimental Investigation of Microgap Cooling Technology for Minimizing Temperature Gradient and Mitigating Hotspots in Electronic Devices,” 2011 IEEE 13th Electronics Packaging Technology Conference (EPTC), Singapore, Dec. 7–9, pp. 530–535.
Morshed, A. K. M. M. , Yang, F. , Ali, M. Y. , Khan, J. A. , and Li, C. , 2012, “ Enhanced Flow Boiling in a Microchannel With Integration of Nanowires,” Appl. Therm. Eng., 32, pp. 68–75.
Guo, D. , Wei, J. , and Zhang, Y. , 2011, “ Enhanced Flow Boiling Heat Transfer With Jet Impingement on Micro-Pin-Finned Surfaces,” Appl. Therm. Eng., 31(11), pp. 2042–2051.
Alam, T. , Lee, P. S. , Yap, C. R. , and Jin, L. , 2012, “ Experimental Investigation of Local Flow Boiling Heat Transfer and Pressure Drop Characteristics in Microgap Channel,” Int. J. Multiphase Flow, 42, pp. 164–174.
Alam, T. , Lee, P. S. , Yap, C. R. , Jin, L. , and Balasubramanian, K. , 2012, “ Experimental Investigation and Flow Visualization to Determine the Optimum Dimension Range of Microgap Heat Sinks,” Int. J. Heat Mass Transfer, 55(25), pp. 7623–7634.
Morshed, A. , Paul, T. C. , and Khan, J. A. , 2012, “ Effect of Cross Groove on Flow Boiling in a Microgap,” ASME Paper No. HT2012-58305.
Isaacs, S. , Kim, Y. J. , McNamara, A. J. , Joshi, Y. , Zhang, Y. , and Bakir, M. S. , 2012, “ Two-Phase Flow and Heat Transfer in Pin-Fin Enhanced Micro-Gaps,” 2012 13th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), San Diego, CA, May 30–June 1, pp. 1084–1089.
Isaacs, S. A. , Joshi, Y. , Zhang, Y. , Bakir, M. S. , and Kim, Y. J. , 2013, “ Two-Phase Flow and Heat Transfer in Pin-Fin Enhanced Micro-Gaps With Non-Uniform Heating,” ASME Paper No. MNHMT2013-22124.
Alam, T. , Lee, P. S. , and Yap, C. R. , 2013, “ Effects of Surface Roughness on Flow Boiling in Silicon Microgap Heat Sinks,” Int. J. Heat Mass Transfer, 64, pp. 28–41.
Alam, T. , Lee, P. S. , Yap, C. R. , and Jin, L. , 2013, “ A Comparative Study of Flow Boiling Heat Transfer and Pressure Drop Characteristics in Microgap and Microchannel Heat Sink and an Evaluation of Microgap Heat Sink for Hotspot Mitigation,” Int. J. Heat Mass Transfer, 58(1), pp. 335–347.
Dai, X. , Yang, F. , Fang, R. , Yemame, T. , Khan, J. A. , and Li, C. , 2013, “ Enhanced Single-and Two-Phase Transport Phenomena Using Flow Separation in a Microgap With Copper Woven Mesh Coatings,” Appl. Therm. Eng., 54(1), pp. 281–288.
Reeser, A. , Bar-Cohen, A. , and Hetsroni, G. , 2014, “ High Quality Flow Boiling Heat Transfer and Pressure Drop in Microgap Pin Fin Arrays,” Int. J. Heat Mass Transfer, 78, pp. 974–985.
David, T. , Mendler, D. , Mosyak, A. , Bar-Cohen, A. , and Hetsroni, G. , 2014, “ Thermal Management of Time-Varying High Heat Flux Electronic Devices,” ASME J. Electron. Packag., 136(2), p. 021003.
Ong, C. L. , Paredes, S. , Sridhar, A. , Michel, B. , and Brunschwiler, T. , 2014, “ Radial Hierarchical Microfluidic Evaporative Cooling for 3-D Integrated Microprocessors,” 4th European Conference on Microfluidics, Limerick, Ireland, Dec. 10–12.
Yang, F. , Schultz, M. , Parida, P. , Colgan, E. , Polastre, R. , Dang, B. , Tsang, C. , Gaynes, M. , Knickerbocker, J. , and Chainer, T. , 2015, “ Local Measurements of Flow Boiling Heat Transfer on Hot Spots in 3D Compatible Radial Microchannels,” ASME Paper No. IPACK2015-48341.
Schultz, M. , Yang, F. , Colgan, E. , Polastre, R. , Dang, B. , Tsang, C. , Gaynes, M. , Parida, P. , Knickerbocker, J. , and Chainer, T. , 2015, “ Embedded Two-Phase Cooling of Large 3D Compatible Chips With Radial Channels,” ASME Paper No. IPACK2015-48348.
Tamanna, A. , and Lee, P. S. , 2015, “ Flow Boiling Heat Transfer and Pressure Drop Characteristics in Expanding Silicon Microgap Heat Sink,” Int. J. Heat Mass Transfer, 82, pp. 1–15.
Tamanna, A. , and Lee, P. S. , 2014, “ Investigation of Flow Boiling Characteristics in Expanding Silicon Microgap Heat Sink,” IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), Orlando, FL, May 27–30, pp. 458–465.
Woodcock, C. , Yu, X. , Plawsky, J. , and Peles, Y. , 2015, “ Piranha Pin Fin (PPF)—Advanced Flow Boiling Microstructures With Low Surface Tension Dielectric Fluids,” Int. J. Heat Mass Transfer, 90, pp. 591–604.
Lee, H. J. , and Lee, S. Y. , 2001, “ Pressure Drop Correlations for Two-Phase Flow Within Horizontal Rectangular Channels With Small Heights,” Int. J. Multiphase Flow, 27(5), pp. 783–796.
Whan, K. D. , Bar-Cohen, A. , Rahim, E. , and Han, B. , 2008, “ Thermofluid Characteristics of Two-Phase Flow in Microgap Channels,” IEEE ITHERM Conference, Orlando, FL, May 28–31.
Chen, J. C. , 1966, “ Correlation for Boiling Heat Transfer to Saturated Fluids in Convective Flow,” Ind. Eng. Chem. Process Des. Dev., 5(3), pp. 322–329.
Shah, M. , 1982, “ Chart Correlation for Saturated Boiling Heat Transfer: Equations and Further Study,” ASHRAE Trans., 88(1), pp. 185–196.
Kawahara, A. , Chung, P.-Y. , and Kawaji, M. , 2002, “ Investigation of Two-Phase Flow Pattern, Void Fraction and Pressure Drop in a Microchannel,” Int. J. Multiphase Flow, 28(9), pp. 1411–1435.
Bogojevic, D. , Sefiane, K. , Walton, A. J. , Lin, H. , and Cummins, G. , 2009, “ Two-Phase Flow Instabilities in a Silicon Microchannels Heat Sink,” Int. J. Heat Fluid Flow, 30(5), pp. 854–867.
Chang, K. H. , and Pan, C. , 2007, “ Two-Phase Flow Instability for Boiling in a Microchannel Heat Sink,” Int. J. Heat Mass Transfer, 50(11–12), pp. 2078–2088.
Wu, H. Y. , and Cheng, P. , 2004, “ Boiling Instability in Parallel Silicon Microchannels at Different Heat Flux,” Int. J. Heat Mass Transfer, 47(17–18), pp. 3631–3641.
Kandlikar, S. G. , Kuan, W. K. , Willistein, D. A. , and Borrelli, J. , 2006, “ Stabilization of Flow Boiling in Microchannels Using Pressure Drop Elements and Fabricated Nucleation Sites,” ASME J. Heat Transfer, 128(4), pp. 389–396.
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References

Garimella, S. V. , and Sobhan, C. , 2003, “ Transport in Microchannels: A Critical Review,” Annu. Rev. Heat Transfer, 13(13), pp. 1–50.
Thome, J. R. , 2004, “ Boiling in Microchannels: A Review of Experiment and Theory,” Int. J. Heat Fluid Flow, 25(2), pp. 128–139.
Tibirica, C. B. , and Ribatski, G. , 2013, “ Flow Boiling in Micro-Scale Channels–Synthesized Literature Review,” Int. J. Refrig., 36(2), pp. 301–324.
Bar-Cohen, A. , and Rahim, E. , 2007, “ Modeling and Prediction of Two-Phase Refrigerant Flow Regimes and Heat Transfer Characteristics in Microgap Channels,” ASME Paper No. ICNMM2007-30216.
Bar-Cohen, A. , Sheehan, J. R. , and Rahim, E. , 2012, “ Two-Phase Thermal Transport in Microgap Channels—Theory, Experimental Results, and Predictive Relations,” Microgravity Sci. Technol., 24(1), pp. 1–15.
Klein, S. A. , 2015, “ Engineering Equation Solver Academic Professional Software.”
Tsai, W.-T. , 2005, “ Environmental Risk Assessment of Hydrofluoroethers (HFEs),” J. Hazard. Mater., 119(1), pp. 69–78. [PubMed]
Lee, H. J. , and Lee, S. Y. , 2001, “ Heat Transfer Correlation for Boiling Flows in Small Rectangular Horizontal Channels With Low Aspect Ratios,” Int. J. Multiphase Flow, 27(12), pp. 2043–2062.
Yang, Y. , and Fujita, Y. , 2004, “ Flow Boiling Heat Transfer and Flow Pattern in Rectangular Channel of Mini-Gap,” ASME Paper No. ICMM2004-2383.
Koşar, A. , and Peles, Y. , 2006, “ Convective Flow of Refrigerant (R-123) Across a Bank of Micro Pin Fins,” Int. J. Heat Mass Transfer, 49(17), pp. 3142–3155.
Koşar, A. , and Peles, Y. , 2007, “ Boiling Heat Transfer in a Hydrofoil-Based Micro Pin Fin Heat Sink,” Int. J. Heat Mass Transfer, 50(5–6), pp. 1018–1034.
Lie, Y. , Ke, J. , Chang, W. , Cheng, T. , and Lin, T. , 2007, “ Saturated Flow Boiling Heat Transfer and Associated Bubble Characteristics of FC-72 on a Heated Micro-Pin-Finned Silicon Chip,” Int. J. Heat Mass Transfer, 50(19), pp. 3862–3876.
Krishnamurthy, S. , and Peles, Y. , 2008, “ Flow Boiling of Water in a Circular Staggered Micro-Pin Fin Heat Sink,” Int. J. Heat Mass Transfer, 51(5), pp. 1349–1364.
Sheehan, J. , Kim, D. W. , and Bar-Cohen, A. , 2009, “ Thermal Imaging of Two-Phase Cooled Microgap Channel Wall,” ASME Paper No. InterPACK2009-89238.
Qu, W. , and Abel, S.-H. , 2009, “ Experimental Study of Saturated Flow Boiling Heat Transfer in an Array of Staggered Micro-Pin-Fins,” Int. J. Heat Mass Transfer, 52(7–8), pp. 1853–1863.
Ma, A. , Wei, J. , Yuan, M. , and Fang, J. , 2009, “ Enhanced Flow Boiling Heat Transfer of FC-72 on Micro-Pin-Finned Surfaces,” Int. J. Heat Mass Transfer, 52(13), pp. 2925–2931.
Yuan, M. , Wei, J. , Xue, Y. , and Fang, J. , 2009, “ Subcooled Flow Boiling Heat Transfer of FC-72 From Silicon Chips Fabricated With Micro-Pin-Fins,” Int. J. Therm. Sci., 48(7), pp. 1416–1422.
Dae-Whan, K. , Rahim, E. , Bar-Cohen, A. , and Han, B. , 2010, “ Direct Submount Cooling of High-Power LEDs,” IEEE Trans. Compon. Packag. Technol., 33(4), pp. 698–712.
Kim, D. W. , Rahim, E. , Bar-Cohen, A. , and Han, B. , 2008, “ Thermofluid Characteristics of Two-Phase Flow in Micro-Gap Channels,” 11th Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITHERM 2008), Orlando, FL, May 28–31, pp. 979–992.
Sheehan, J. , and Bar-Cohen, A. , 2010, “ Spatial and Temporal Wall Temperature Fluctuations in Two-Phase Flow in Microgap Coolers,” ASME Paper No. IMECE2010-40227.
Alam, T. , Lee, P. S. , Yap, C. R. , and Jin, L. , 2011, “ Experimental Investigation of Microgap Cooling Technology for Minimizing Temperature Gradient and Mitigating Hotspots in Electronic Devices,” 2011 IEEE 13th Electronics Packaging Technology Conference (EPTC), Singapore, Dec. 7–9, pp. 530–535.
Morshed, A. K. M. M. , Yang, F. , Ali, M. Y. , Khan, J. A. , and Li, C. , 2012, “ Enhanced Flow Boiling in a Microchannel With Integration of Nanowires,” Appl. Therm. Eng., 32, pp. 68–75.
Guo, D. , Wei, J. , and Zhang, Y. , 2011, “ Enhanced Flow Boiling Heat Transfer With Jet Impingement on Micro-Pin-Finned Surfaces,” Appl. Therm. Eng., 31(11), pp. 2042–2051.
Alam, T. , Lee, P. S. , Yap, C. R. , and Jin, L. , 2012, “ Experimental Investigation of Local Flow Boiling Heat Transfer and Pressure Drop Characteristics in Microgap Channel,” Int. J. Multiphase Flow, 42, pp. 164–174.
Alam, T. , Lee, P. S. , Yap, C. R. , Jin, L. , and Balasubramanian, K. , 2012, “ Experimental Investigation and Flow Visualization to Determine the Optimum Dimension Range of Microgap Heat Sinks,” Int. J. Heat Mass Transfer, 55(25), pp. 7623–7634.
Morshed, A. , Paul, T. C. , and Khan, J. A. , 2012, “ Effect of Cross Groove on Flow Boiling in a Microgap,” ASME Paper No. HT2012-58305.
Isaacs, S. , Kim, Y. J. , McNamara, A. J. , Joshi, Y. , Zhang, Y. , and Bakir, M. S. , 2012, “ Two-Phase Flow and Heat Transfer in Pin-Fin Enhanced Micro-Gaps,” 2012 13th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), San Diego, CA, May 30–June 1, pp. 1084–1089.
Isaacs, S. A. , Joshi, Y. , Zhang, Y. , Bakir, M. S. , and Kim, Y. J. , 2013, “ Two-Phase Flow and Heat Transfer in Pin-Fin Enhanced Micro-Gaps With Non-Uniform Heating,” ASME Paper No. MNHMT2013-22124.
Alam, T. , Lee, P. S. , and Yap, C. R. , 2013, “ Effects of Surface Roughness on Flow Boiling in Silicon Microgap Heat Sinks,” Int. J. Heat Mass Transfer, 64, pp. 28–41.
Alam, T. , Lee, P. S. , Yap, C. R. , and Jin, L. , 2013, “ A Comparative Study of Flow Boiling Heat Transfer and Pressure Drop Characteristics in Microgap and Microchannel Heat Sink and an Evaluation of Microgap Heat Sink for Hotspot Mitigation,” Int. J. Heat Mass Transfer, 58(1), pp. 335–347.
Dai, X. , Yang, F. , Fang, R. , Yemame, T. , Khan, J. A. , and Li, C. , 2013, “ Enhanced Single-and Two-Phase Transport Phenomena Using Flow Separation in a Microgap With Copper Woven Mesh Coatings,” Appl. Therm. Eng., 54(1), pp. 281–288.
Reeser, A. , Bar-Cohen, A. , and Hetsroni, G. , 2014, “ High Quality Flow Boiling Heat Transfer and Pressure Drop in Microgap Pin Fin Arrays,” Int. J. Heat Mass Transfer, 78, pp. 974–985.
David, T. , Mendler, D. , Mosyak, A. , Bar-Cohen, A. , and Hetsroni, G. , 2014, “ Thermal Management of Time-Varying High Heat Flux Electronic Devices,” ASME J. Electron. Packag., 136(2), p. 021003.
Ong, C. L. , Paredes, S. , Sridhar, A. , Michel, B. , and Brunschwiler, T. , 2014, “ Radial Hierarchical Microfluidic Evaporative Cooling for 3-D Integrated Microprocessors,” 4th European Conference on Microfluidics, Limerick, Ireland, Dec. 10–12.
Yang, F. , Schultz, M. , Parida, P. , Colgan, E. , Polastre, R. , Dang, B. , Tsang, C. , Gaynes, M. , Knickerbocker, J. , and Chainer, T. , 2015, “ Local Measurements of Flow Boiling Heat Transfer on Hot Spots in 3D Compatible Radial Microchannels,” ASME Paper No. IPACK2015-48341.
Schultz, M. , Yang, F. , Colgan, E. , Polastre, R. , Dang, B. , Tsang, C. , Gaynes, M. , Parida, P. , Knickerbocker, J. , and Chainer, T. , 2015, “ Embedded Two-Phase Cooling of Large 3D Compatible Chips With Radial Channels,” ASME Paper No. IPACK2015-48348.
Tamanna, A. , and Lee, P. S. , 2015, “ Flow Boiling Heat Transfer and Pressure Drop Characteristics in Expanding Silicon Microgap Heat Sink,” Int. J. Heat Mass Transfer, 82, pp. 1–15.
Tamanna, A. , and Lee, P. S. , 2014, “ Investigation of Flow Boiling Characteristics in Expanding Silicon Microgap Heat Sink,” IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), Orlando, FL, May 27–30, pp. 458–465.
Woodcock, C. , Yu, X. , Plawsky, J. , and Peles, Y. , 2015, “ Piranha Pin Fin (PPF)—Advanced Flow Boiling Microstructures With Low Surface Tension Dielectric Fluids,” Int. J. Heat Mass Transfer, 90, pp. 591–604.
Lee, H. J. , and Lee, S. Y. , 2001, “ Pressure Drop Correlations for Two-Phase Flow Within Horizontal Rectangular Channels With Small Heights,” Int. J. Multiphase Flow, 27(5), pp. 783–796.
Whan, K. D. , Bar-Cohen, A. , Rahim, E. , and Han, B. , 2008, “ Thermofluid Characteristics of Two-Phase Flow in Microgap Channels,” IEEE ITHERM Conference, Orlando, FL, May 28–31.
Chen, J. C. , 1966, “ Correlation for Boiling Heat Transfer to Saturated Fluids in Convective Flow,” Ind. Eng. Chem. Process Des. Dev., 5(3), pp. 322–329.
Shah, M. , 1982, “ Chart Correlation for Saturated Boiling Heat Transfer: Equations and Further Study,” ASHRAE Trans., 88(1), pp. 185–196.
Kawahara, A. , Chung, P.-Y. , and Kawaji, M. , 2002, “ Investigation of Two-Phase Flow Pattern, Void Fraction and Pressure Drop in a Microchannel,” Int. J. Multiphase Flow, 28(9), pp. 1411–1435.
Bogojevic, D. , Sefiane, K. , Walton, A. J. , Lin, H. , and Cummins, G. , 2009, “ Two-Phase Flow Instabilities in a Silicon Microchannels Heat Sink,” Int. J. Heat Fluid Flow, 30(5), pp. 854–867.
Chang, K. H. , and Pan, C. , 2007, “ Two-Phase Flow Instability for Boiling in a Microchannel Heat Sink,” Int. J. Heat Mass Transfer, 50(11–12), pp. 2078–2088.
Wu, H. Y. , and Cheng, P. , 2004, “ Boiling Instability in Parallel Silicon Microchannels at Different Heat Flux,” Int. J. Heat Mass Transfer, 47(17–18), pp. 3631–3641.
Kandlikar, S. G. , Kuan, W. K. , Willistein, D. A. , and Borrelli, J. , 2006, “ Stabilization of Flow Boiling in Microchannels Using Pressure Drop Elements and Fabricated Nucleation Sites,” ASME J. Heat Transfer, 128(4), pp. 389–396.

Figures

Fig. 1

Schematic of closed-loop microfluidic cooling system

Fig. 2

(a) Plain microgap and (b) microgap with pin fin surface enhancement

Fig. 3

Flow loop schematic

Fig. 4

Test section

Fig. 5

Device schematic: (a) micropin-fin heat sink side and (b) heater side

Fig. 6

Device dimensions

Fig. 7

(a) Two-phase heat transfer coefficient versus effective heat flux, (b) boiling curve, (c) pressure drop across staggered pin-fin arrays versus effective heat flux, and (d) exit quality versus effective heat flux

Fig. 8

(a) Temperature oscillations at G = 1351 kg/m2 s and qeff″ = 267 W/cm2 and (b) pressure drop at G = 1351 kg/m2 s and qeff″ = 267 W/cm2

Fig. 9

High-speed images for G = 1784 kg/m2 s at (a) 256 W/cm2, (b) 342 W/cm2; (c) 381 W/cm2, and (d) 444 W/cm2

Fig. 10

Schematic of two-phase area for G = 1784 kg/m2 s at (a) 256 W/cm2, (b) 342 W/cm2, (c) 381 W/cm2, and (d) 444 W/cm2

Tables

Table 1 Sample flow boiling coolant thermodynamic properties
FC-72, water, HFE-7000, and R-123 properties evaluated at $Tsat$ and R-134 a and R-245fa properties evaluated at $Psat$.
Table 2 Descriptions of flow boiling in microgap and microgap with surface enhancement
Table 3 Measurement and data reduction uncertainty
Table 4 Performed tests parameters

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