Experimental study and optimization of pin fin shapes in flow boiling of micro pin fin heat sinks

Accepted Manuscript Research Paper Experimental study and optimization of pin fin shapes in flow boiling of micro pin fin heat sinks Wei Wan, Daxiang Deng, Qingsong Huang, Tao Zeng, Yue Huang PII: DOI: Reference:

S1359-4311(16)33672-9 http://dx.doi.org/10.1016/j.applthermaleng.2016.11.182 ATE 9593

To appear in:

Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

4 July 2016 14 September 2016 27 November 2016

Please cite this article as: W. Wan, D. Deng, Q. Huang, T. Zeng, Y. Huang, Experimental study and optimization of pin fin shapes in flow boiling of micro pin fin heat sinks, Applied Thermal Engineering (2016), doi: http:// dx.doi.org/10.1016/j.applthermaleng.2016.11.182

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Experimental study and optimization of pin fin shapes in flow boiling of micro pin fin heat sinks Wei Wan1, Daxiang Deng1,2 *, Qingsong Huang1,Tao Zeng1, Yue Huang3 1

Department of Mechanical & Electrical Engineering, Xiamen University, Xiamen, 361005, China

2

State Key Laboratory of High Performance Complex Manufacturing, Central South University, Changsha, 410083, China

3

Department of Aeronautics, Xiamen University, Xiamen 361005, China

Abstract: Micro pin fin heat sinks show their great merits of thermal management in high heat flux devices in microelectronic, energy, aerospace, and military areas. The design optimization of micro pin fin shapes is a critical issue for their applications, especially in two-phase flow boiling conditions. In this study, four types of staggered micro pin fins with different cross-section shapes, i.e., square, circular, diamond and streamline, were fabricated by a laser micromilling method, and constructed for heat sinks cooling systems. Flow boiling performance of the micro pin fin heat sinks (MPFHSs) was characterized using deionized water as coolant. The effects of cross-section shape on flow boiling characteristics of MPFHSs were examined. Test results showed that the square micro pin fins presented the best boiling heat transfer, followed by circular and streamline ones. The diamond micro pin fins performed worst in boiling heat transfer and suffered severe two-phase flow instabilities at moderate to high heat fluxes, whereas they introduced the smallest pressure drop. The streamline micro pin fins presented the largest two-phase pressure drop. The square and circular micro pin fins showed their superiority in the mitigation of two-phase flow instabilities. The square micro pin fins seem to be the optimum choice and should be selected for heat sink cooling systems. The results in this study provide critical information for the design optimization of micro pin fins heat sinks for the high heat-flux applications, and are of considerable practical importance.

Keywords: Micro pin fins; Flow boiling; laser micromilling; Heat sinks; cross-section shape

*Corresponding author. E-mail address: [email protected]

(D.X. Deng).

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1. Introduction In recent years, the miniaturization and high speed operation of devices in microelectronic, energy, aerospace and military systems have resulted in a tremendous increase in power density. It is urgent to develop highly efficient cooling techniques to meet the high heat flux dissipation needs [1]. Micro pin fin heat sinks (MPFHSs) have attracted great attentions due to their excellent heat transfer capacities, large surface area per given volume, compact dimensions and small working fluid inventory requirements [2-3]. Besides, the micro pin fins can be easily integrated in the through silicon vias (TSVs) systems in the 3D IC packaging technology applications [4]. Moreover, the micro pin fins are able to disturb the flow and break down the boundary layer, which promote the flow mixing and the redevelopment of thermal and hydraulic boundary layer [5]. Thus it is desirable for the enhancement of single-phase heat transfer. Therefore, numerous studies have been conducted to explore the convection heat transfer characteristics of MPFHSs, such as Kosar and Peles [6], Izci et al. [7] , Lee et al. [8], Duangthongsuk and Wongwises [9], and Tullius et al. [10] to name a few. The single-phase thermal and hydraulic performance of micro pin fins has been extensively explored, and the applications in microelectronic chips [11], high-power LED lighting [12], turbine blades [13] have been also implemented. From the perspective of cooling performance, two-phase flow boiling in MPFHSs is more efficient that its single-phase equivalent [14-15], as the flow boiling offers a very high heat transfer rate via the latent heat of coolant while requiring small rate of coolant flow, and simultaneously maintain the wall temperatures relatively uniform. To date, several research efforts have been devoted to exploring the flow boiling performance of MPFHSs. Krishnamurthy and Peles [16] assessed the flow boiling characteristics of water in micro circualr silicon pin fins with of a diameter of 100μm, spacing of 150μm and height of 250μm. It was found that the two-phase heat transfer coefficient was moderately dependent on the mass

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flux and independent on the heat flux. Visualization results suggested that annular flow dominated most of the experiments at high heat fluxes, while slug flow was also observed at lower heat fluxes. In their latter studies [17], a single row of inline circular pin fins was entrenched in a microchannel. The flow boiling results of dielectric coolant HFE7000 domenstrated heat transfer enhancements compared to the plain microchannels. Kosar and Peles [18] tested the flow boiling heat transfer of R-123 in an array of hydrofoil pin-fins with chord thickness of 100 μm and fin height of 243 μm. Results indictated that the heat transfer coefficient increased with increasing heat flux at low quality, while decreased with increasing heat flux at high quality. Flow patterns were identified to be bubbly, wavy intermittent, and spray-annular depending on heat flux and mass velocity. Ma et al. [19] reached considerable flow boiling heat transfer enhancement in a square micro-pin-finned chip surface compared to a smooth one in FC-72. Qu and Siu-Ho [20] studied the saturated flow boiling heat transfer of water in an array of staggered square micro pin fins with a cross-section of 200×200μm2 and a height of 670μm. They found that the heat transfer coefficient was enhanced by inlet subcooling in the low quality region, but kept fairly constant in the high quality region. They also developed a new heat transfer correlation to correlate their experimental data. Besides, Chang et al. [21] explored the subcooling flow boiling heat transfer and associated bubble characteristics of FC-72 over a micro pin fin surface flush mounted on the bottom of a horizontal rectangular channel. McNeil et al. [22] compared the flow boiling heat transfer performance between in-line square copper pin fins with a cross section of 1×1 mm and a plane channel using refrigerant R113. Slight larger heat transfer coefficients were reached for the pin fins due to the heat-transfer surface enlargement. Recently, Law and Lee [23] conducted a comparative study on flow boiling performance of copper oblique-finned microchannels and straight parallel microchannels. Significant augmentation in heat transfer and the delay of the

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onset of critical heat flux have been reached for the oblique-finned microchannels, as the oblique fins increased bubbles generation in the nucleate boiling region and induced a continuously developing thin liquid-film in the convective boiling region. As enumerated above, the current efforts were all focused on the flow boiling performance of a certain type of micro pin fins, such as circular [16], square [19], hydrofoil [17] and oblique pin fins [23]. Nevertheless, as to the best knowledge of the authors’, the information for the flow boiling performance of micro pin fins with different shapes is very scarce. Since the pin fins with different shapes may introduce different two-phase flow behaviors, they may play a significant role on the two-phase flow boiling performance. It is critical to conduct such studies to fulfill this gap for the design optimization of micro pin fins for the high heat-flux applications, which builds an aim of this study. Currently, the micro pin fins utilized for heat sink cooling applications are normally fabricated by etching [16, 18, 19], lithographie, galvanoformung und abformung (LIGA) [24] and micro end-milling [20] methods. The etching or LIGA process for fabricating micro pin

fins are mostly limited to silicon materials, which, from a heat transport point of view, are less attractive than metals such as copper or aluminum. Besides, the complicated procedures and costly machinery equipments of etching and LIGA process may also increase the fabrication cost significantly. Micro end-milling can be used to fabricate metal-based pin fins, but its long processing time for micro pin fins may hinder their wide application in micro pin fin fabrications [25]. Moreover, the micro pin fins fabricated by all the above methods are of smooth wall surfaces [19,21], which may be unfavorable for the bubble nucleation and inevitably induce a large wall superheat for the onset of nucleate boiling (ONB) [19]. To address the above issues, we in this study developed a laser micromilling method to fabricate micro pin fins on copper base. Four types of micro pin fins with different shapes, i.e., square, diamond, circular and streamline, are prepared to construct heat sinks cooling systems.

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Flow boiling tests of water were conducted to explore the boiling heat transfer coefficients, pressure drop and flow instabilities of these four types of MPFHSs for the design optimization.

2. Experiments 2.1 Fabrication of micro pin fins In this study, the micro pin fins are fabricated by a laser micromilling method using a prototype pulsed fiber laser (IPG, No: YLP-1-100-30-30- HC-RG, Russia). Laser micromilling is a non-contact and highly precise microfabrication technique, which can be used to produce microscale structures for most types of materials. During the laser micromilling process, the fiber optic bundle emits a collimated beam of white light on the surface to heat the surface material. The surface material is removed following the procedures of rapidly heating, melting, evaporation and ionization as well as evaporation spray and fluid ejection [26-27]. The cross machining route and loop multiple-pass reciprocating scanning strategy are selected to fabricate the micro pin fins, and the line spacing is set to be 5μm. Details of the fabrication process can be available in our previous study [26]. The material removal is achieved by layer-by-layer milling process around the pin fins. The laser is set to produce 100 ns pulses with a 1064nm fundamental wavelength (λ) at a repetition rate of 20kHz. The specifications of characteristic parameters of the used fiber laser system are given in Table1. The micro pin fins are fabricated at the same processing parameters, i.e., the laser output power of 27W, scanning speed of 250mm/s and scanning times of 20. Four types of staggered micro pin fins, i.e., square, diamond, circular and streamline ones, are designed, as depicted in Fig.1. All the test samples are made of pure red cooper plates (99.9% Cu) with the dimensions of 45×20×2mm (length×width×height) due to the good thermal conductivity and easy processing characteristics of copper. All the micro pin fins are fabricated in the center of copper plate with the dimensions of 17×45mm (Wht×L, Wht and L are the width

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of heat transfer area and length of heat sink, respectively). The geometric dimensions of these micro pin fins are shown in Table 2. The fin density (ε) can be calculated as follows,

ε=

NAfin

(1)

Wht L

in which N is the total number of micro pin fins, Afin is the cross-sectional area of a single micro pin fin. Fig.2 shows the SEM images of the fabricated micro pin fins. It can be noted that the processed pin fin samples are of rough bottom and wall surfaces, which may be favorable for the bubble nucleation during boiling process. After the laser micromilling fabrication, the micro pin fin samples are carefully cleaned as follows: they are firstly cleaned with kerosene oil in an ultrasonic bath for approximately an hour, and then ultrasonically cleaned with deionized water for half an hour. After such thorough cleanings, it is believed that any possible residues or contaminants during the machining process on the pin fin wall surfaces have been eliminated, as the brightness of oxygen-free copper could be visualized. Fig. 1,2 Table 1,2 2.2 Flow boiling experiments Flow boiling experiments are conducted in a closed loop, as depicted in Fig.3. It is similar to the one utilized in our previous paper [28, 29], and is described briefly for completeness. Fully

degassed coolants, driven by a magnetically coupled gear pump (Micropump, Model No.: GA-T23-DB-380B), passes a 7μm filter and a micro turbine type flowmeter (Omega, Model No.: FTB-311D). The flow meter range is 30-300 ml/min. Then the coolants are preheated using a constant temperature water bath to obtain the desired inlet subcooling. After that, coolants enter a test section and heat exchange occurs inside the microchannels. They return back to a reservoir after cooled by a coiled condenser. The test section is uniformly heated by

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ten cartridge heaters with the maximum total power of 1000W. The power are supplied by a variac with an output ranger of 0-250V and 0-5A, and monitored by a digital power meter. The inlet, outlet and steam-wise wall temperatures are monitored by type-K thermocouples, and the inlet and outlet pressure during the test are measured by two pressure transducers (Shenzhen Oriental Vanward Instrument Co., Model No.: WH-PTX7517). The temperatures, pressures and flow rates are collected by an Agilent 34970A data acquisition system. The flow behaviors during the two-phase boiling process are visualized by a microscope (Shanghai Guangmi Instrument Co., Model No.:XTL-850P) together with a high speed camera (Fastec HiSpec DVR 2F). Fig.3 The test section, as illustrated in Fig.4, consist of the MPFHSs sample, inlet and outlet plenums, a polyetheretherketone (PEEK) flow housing, a Pyrex 7740 glass cover plate, an assembling top plate, copper block heating modules and PEEK insulating components. Details of the test section construction have been given in the previous study [28] with an exploded view. The dimensions of top surface of copper block is 20mm × 45mm, which is identical to the projection area of the MPFHSs sample. The copper block consists of four vertical slots along the height direction similar to the design of Qu and Mudawar [30], which separate the heat spread along the flow direction. This ensures the vertical heat conduction inside the copper blocks, minimize the heat conduction in the horizontal direction and supply more uniform heat flux distribution. The MPFHSs sample is soldered on top of the copper block using a thin layer of solder (Pb-Sn-Ag-Sb, ks=50 W/m·K) to minimize the contact thermal resistance. The solder layer is ensured to be 0.5mm thick by the designed height difference formed between the top surfaces of copper block and the channel surface of flow housing. RTV silicon rubber is utilized in the interface of the copper block and

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flow housing to prevent any leakage of coolants. The PEEK insulating block is set around the copper block to minimize the heat loss and also holds the entire assembly in a stable position. The glass cover above the test sample provides an enclosed passage for the liquid flow, and it is sealed with an O-ring when all the cover plate, glass cover, flow housing and insulating block are clamped together by screws. Good contact between the top surfaces of pin fins with the cover plate is ensured and any bypass flow is prevented. Five type-K shielded thermocouples with diameter of 1mm are inserted in the copper block at a distance of 2.0mm under top surface of copper block to measure the stream-wise wall temperature distribution. The locations were 2.5, 12.5, 22.5, 32.5, 42.5mm, respectively, from the channel inlet position, as depicted in Fig.4. Two type-K thermocouples are set at about 5mm before and after the MPFHS sample to measure the inlet and outlet temperature. Two pressure port spaces are set in the inlet and outlet plenum to measure inlet/outlet pressure. Horizontal inlet and outlet manifold arrangements are utilized for the liquid flow and none deep inlet/outlet reservoirs are adopted to reduce the compressive upstream volume. The above means, along with the stiff stainless steel tube to connect the test section and the upstream valve, was expected to provide a means to weaken the two-phase flow instability [31]. Fig. 4 Deionized water is used as the test coolant. Experiments have been performed at atmospheric conditions. The test coolant is fully degassed via vigorous boiling for about half an hour prior to each experimental run. Effectiveness of the above degassing method is verified by visually observing the gas bubble activities in the pin fins during flow boiling tests. The bubbles formation due to trapped gases can be easily identified due to the fact that they do not appear in the same location after being moved downstream by the fluid flow, while the bubbles nucleate and detach from the pin fins walls re-appear in the same cavities. It is found that no gas/air bubbles came out of coolants in the pin fins prior to boiling incipience, and the nucleation of bubbles occurred

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inside the pin fins wall surface. This indicates that the pure coolants without dissolved gas are maintained. The above degassing method has been found to be able to prevent possible dissolved gas in the coolants by many research groups [20, 30]. The saturated temperature of water is

estimated to be 100℃. The flow boiling tests of four MPFHS samples are performed at the same mass flux of 500 kg/m2s for the purpose of direct comparison, and the inlet temperatures are maintained to be 90℃, which gives an inlet subcooling of 10℃. For each test, the flow rate and inlet fluid temperatures are kept constant throughout the test, and the heat power is increased via the variac in a small increment of 15-30 W. Once at steady state, all temperatures, pressures and flow rates are recorded at 1 s interval for 2 min. Heat transfer coefficients and pressure drop are thus determined by using the averaged values from the measured data for 2 min. 2.3 Data reduction The mass flux in micro pin fin array is referred as G and defined as G = V ρ / Ainlet

and Ainlet = Wht H fin (1 −

(2)

W fin ST

(3)

)

in which V is the volumetric flow rate, ρ is the fluid density, Ainlet is the inlet cross-section area, Hfin is the height of micro pin fin, Wfin is the width of a micro pin fin and ST is the transverse pitch of a micro pin fin. The efficient heat flux is given by q''eff = qeff / At

(4)

in which At is the top platform area of the copper block, At = W × L ,in which W and L are the width and length of MPFHS sample. Uniform heat flux is expected to be supplied for the heating surfaces due to the slots design inside the copper block as well as the excellent thermal

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conductivities of copper, which has been commonly utilized in the literatures [20, 23, 30-33]. qeff is the effective power, which can be obtained as follows, qeff = q − qloss

(5)

where q is the total input power, q = V × I , V and I are the input voltage and current readings from the digital power meter. qloss is the heat loss. To evaluate the heat loss from the test section, a set of single-phase heat transfer experiments prior to boiling tests were conducted at the same flow rate as the boiling tests. The sensible heat gain by single-phase fluid is determined as follows,  p (Tout − Tin ) qeff = mc

(6)

 denotes the mass flow rate, c p denotes the specific heat of fluid, Tin and Tout denote where m the inlet and outlet liquid temperature, respectively. The heat transfer ratio, ϕ , denoting the percentages of the sensible heat gain by fluid against the total heat power, can be obtained for each input heat flux in the single-phase region,  p (Tout − Tin ) / (VI ) ϕ = mc

(7)

It was found that ϕ in the single-phase tests ranged from 0.85 to 0.9 depending on the inlet temperature, flow rate and heat flux. Then the mean values of heat transfer ratio ( ϕ ) were utilized to calculate the effective heating power in the flow boiling experiments, qeff = ϕ q . This method to determine the effective heating power has been adopted in many previous works [20, 31-33]. The local heat transfer coefficient is determined as follows, htp ,tci =

qeff

(8)

ΔTsat ,tci Ach

and ΔTsat ,tci = Tw,tci − Tsat ,tci

(9)

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where Tw,tci = Ttci − qeff (

lCu l l + hs + s ) kCu At km At k s At

(10)

where ΔTsat ,tci is the local wall superheat of the thermocouple location(Ztci,i=1-5), Tsat ,tci is the liquid saturation temperature, which is taken corresponding to the local pressure obtained as a linear interpolation between the inlet and outlet pressure. Ttci is the local thermocouple reading(i=1-5). The wall temperature of MPFHS ( Tw,tci ) is deduced from a thermal resistance network detailed in the previous work [28], where kCu, ks, km are the thermal conductivities of copper, solder and the MPFHSs base, respectively. lCu, ls and lhs represents the distance from the thermocouple location to the top heating surface of copper block, thickness of solder, and the distance between the bottom surface of MPFHS sample to the bottom of micro pin fins surface, respectively. Ach is the total heat transfer area of MPFHS, which is given by the fin analysis method as follows, Ach = η NA fin + Aun − fin

(11)

where A fin = H fin Pfin

(12)

Aun − fin = Wht L − NA fin

(13)

where N is the numbers of micro pin fins, Pfin is the cross-section perimeter of a single micro pin fins. Aun-fin is the unfinned surface area at the bottom of the channels. For each MPFHS, local fin efficiency (η ) is used to account for the drop in temperature along the fin, similar to other flow boiling studies of micro pin fins [20, 22-23, 34]. An adiabatic fin tip condition is assumed due to the non-conductive material of the Pyrex glass cover. The corresponding fin efficiency as calculated as follows,

η=

tanh( m fin H fin )

(14)

m fin H fin

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in which m fin is the fin parameter as given by

m fin =

htp ,tci Pfin

(15)

kCu Afin

The htp can thus be evaluated employing from Eq.8 and 11-15 with an iterative scheme. The vapor quality can be obtained as follows, x=

1 qeff Li i − c p (Tsat ,tci − Tin )) ( h fg m L

(16)

where h fg is the latent heat of vaporization, Li is the distance from the inlet to the thermocouple location. The pressure drop across the MPFHS were calculated as follows,

ΔP = ( Pin − ΔPc ) − ( Pout + ΔPe )

(17)

Where ΔPc denotes the inlet contraction pressure drop, ΔPe is the outlet expansion pressure loss. They are estimated by the method detailed in Qu and Mudawar [35], and are found to be less than 5% of the total measured pressure drops, which can be thought to be negligible. The obtained pressure drop ΔP consisted of the one in the single-phase flow region and the two-phase flow pressure drop including the acceleration and friction loss terms[35]. Uncertainties in individual temperature measurements are ±0.3℃ for the type-K thermocouples. The measurement errors for the flow meter and pressure transducer are 1% and 0.1% of full scale, respectively. The supply power measured by the digital power meter yield an uncertainty of 1%. The wall temperature uncertainty comes from the thermocouple errors and from the correction from the temperature drop from the copper block as in Eq. (10). The uncertainty of the cross-section area of micro pin fin is estimated to be within 2%. Using the standard error analysis method[36], the maximum uncertainties in the vapor quality, two-phase heat transfer coefficient and pressure drop can be estimated to be 9.1%, 8.0%, and 6.3%, respectively.

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3. Results and discussion 3.1 Boiling curves

Fig.5 shows the boiling curves for four MPFHSs. The effective heat fluxes versus local wall superheat of the most downstream thermocouple location (Ztc5) are focused on. The initial low heat fluxes region is linked to the single-phase heat transfer. As the heat flux is increased, the slopes of the curves increase abruptly at wall superheat of approximately 1-3 , indicating the occurrence of the onset of nucleate boiling (ONB). The boiling hysteresis phenomena with large wall temperature excursion that is usually found at the boiling incipience for conventional microchannels [37-38] is not observed. It can be noted that the cross-section shape of micro pin fin has no significant influence on the initiation of ONB, and all the MPFHSs trigger the ONB at small wall temperature overshoots (1-3 ). After the initial stage of boiling, the q''eff − ΔTsat curves show the different slopes for four MPFHSs samples. The square micro pin fins can maintain the lowest wall temperatures than other three MPFHSs samples under the same heat flux conditions. The boiling curves of streamline micro pin fins and the circular micro pin fins are almost overlapped, which shift right compared to the square micro pin fins. The diamond micro pin fins sample produces the highest wall temperature, and it increased sharply with increasing heat flux at high heat fluxes region, indicating the fast deterioration of heat transfer. These different behaviors will be discussed in detail in the subsequent section. Fig. 5 3.2 Heat transfer performance

Fig. 6 shows the local two-phase heat transfer coefficients for MPFHS samples, in which the htp is plotted with respect to vapor quality and heat flux, respectively. The local two-phase heat transfer coefficients are determined at the most downstream thermocouple location ( Ztc 5 ) near the outlet of MPFHS as same as other studies [23, 30, 31,34, 39], which relates to the greatest amount of saturated boiling and the largest vapor quality. All the four MPFHSs

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presented high htp at the early stage of boiling. The large surface area and fabricated rough wall surfaces provided lost of nucleation sites, resulting in the fast bubble growth and movement. The latent heat of water was released rapidly after the boiling incipience. Flow visualizations results indicated that the isolated bubbles formed between the adjacent pin fins, as illustrated in Fig. 7(a). With the increase of heat fluxes and vapor qualities, a fast decline in htp can be noted for all the test samples. This can be linked to the rapid coalescence of bubbles and formation of elongated bubbles or slugs, which suppressed the nucleation behavior, and induced the early fast decrease in heat transfer. As shown in Fig. 7 (b) and (c) , the bubbles grew and coalesced to form the slugs. Nevertheless, due to the separation of pin fins, the slugs tended to depart at the head of pin fins, which served as a wrap around the pin fins. The liquid film began to emerge around the micro pin fins. The above early decrease in heat transfer is consistent with the previous water boiling results of conventional parallel microchannels [31,33, 40], and also accorded with the water results of square micro pin fins in Qu and Siu-Ho [20], and Reeser et al. [41], and FC-72 results of oblique pin fins in Law et al. [39]. Steinke & Kandikar [40] attributed it to be the unique phenomena of microchannels, and they concluded that the nucleate boiling dominated the heat transfer. The present micro pin fins accorded with this, as the heat transfer coefficients in this region are sensitive to hear flux, and bubbly flow dominated in the micro pin fins. As heat fluxes and vapor qualities increased further, the square micro pin fins tended to increase at moderate to large heat fluxes region, suggesting that the increase in heat flux and vapor qualities played a positive role on the heat transfer enhancement. On the other hand, the increasing trends of other three samples were less pronounced than the square pin fins. The heat transfer performance of these three pin fins samples increased slightly or kept fairly constant for a wide range of heat fluxes and vapor qualities. At this region, it was found that annular flow with vapor cores governed in the main pathways of micro pin fins, leaving a layer of liquid

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films around the micro pin fins, as illustrated in Fig. 7(d) and (e). The heat fluxes were conducted within the pin fins, transferred to the flowing fluid via forced convection to the liquid films, and then evaporated through the vapor cores. Thin film evaporations dominated the heat dissipation process, and rising heat transfer coefficients was induced by the thinning of the evaporating liquid film surrounding the pin fins. Therefore, the transition from nucleated boiling to convective boiling occurred, and a mild increasing or plateauing trend of heat transfer performance can be noted. With further increases in heat fluxes, the square and diamond pin fins presented a rapid decline in htp at high heat flux region. Partial dry-out in the pin fins began to occur, as shown in Fig.7(f). Thin film evaporations were hindered, and the heat transfer deteriorated quickly. It should be noted that the present transition regions of heat transfer deterioration for the square and diamond pin fins were associated to somewhat small vapor qualities (0.03-0.05), smaller than other channels with conventional sizes[42]. This may de due to that the small passages of micro pin fins played a more pronounced role on the vapor confinement and increased the flow resistance of liquid rewetting compared to those macro-scale channels. Nevertheless, the transition regions of the present square and diamond pin fins were generally consistent with the water boiling results of rectangular microchannels in Balasubramanian et al. [43], and the HFE7000 boiling results of Piranha Pin Fins in Woodcock et al. [44]. Fig. 6, 7

From Fig.6, it can be noted that the square micro pin fins exhibited the best heat transfer performance, followed by circular and streamline ones. The diamond pin fins, however, produced the worst heat transfer coefficients. The above tendency can be also verified by the elimination of the slight differences of heat transfer surface area of channel wall, as shown in Fig.8, where the htp was plotted with respect to the wall heat fluxes. The difference of heat transfer performance of these four micro pin fins can be linked to the different flow behaviors in

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micro pin fins. For the diamond micro pin fins, the formed bubbles in the upstream tended to be disrupted easily and depart into two portions quickly due to the sharp leading corners. When these separated bubbles propagated along the long edges, they tended to be form a slug easily when they met the adjacent bubbles. Besides, due to the sharp corner in the head and tail of diamond micro pin fins, the slugs were easier to wrap around the leading edge of a diamond micro pin fins and coalesce with the slugs in the secondary channel. This induced continuous vapor cores with thick liquid films, as illustrated in Fig.9. The vapor covered most of flow channels in the diamond micro pin fins and prevented liquid to wet the heat exchange surface. The bubble nucleation behaviors surrounding the pin fins were significantly suppressed, and also induced less thin film evaporation than the other pin fins. Therefore, it presented the worst heat transfer performance. The partial dry-out was earlier to occur at much smaller heat fluxes in the diamond micro pin fins. Thus the heat transfer coefficient was quickly deteriorated at moderate to large heat fluxes. On the other hand, for the square micro pin fins, the smooth and long surface at the leading edge facilitated the more stable bubble growth. Besides, the cross-links of square pin fins with four perpendicular corners hindered the continuous development of slugs and vapor cores. As shown in Fig.10, at the initial time (t0), some elongated bubbles or slugs were formed. When they moved downstream at the second time level of t0+13ms, they separated into different parts of isolated slugs. Liquid film formed around the micro pin fins. Then the slugs tended to grow bigger at the third time level of t0+26ms.They were pushed toward the downstream direction by the incoming flow in the fourth time level of t0+39ms, and the previous areas were filled with refreshed liquid. The vapor slugs were washed away completely at the fifth time level of t0+52ms, and liquid phase covered all the visualized region. At sixth time level of t0+65ms, the new slugs were formed, repeating a next cycle. The above alternation of liquid and vapor phase happened in a very short period. Therefore, the flow channels and micro pin fin walls covered by liquid film can be rewet

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quickly. This increased the bubble nucleation and slug formation behaviors, and also prevented the early occurrence of continuous vapor cores with thick liquid films. Therefore, the enhancement of heat transfer can be reached for square pin fins. For the circular and streamline micro pin fins, the circular heads of pin fins may play an intermediate role on the flow separation compared to the diamond and square pin fins, as illustrated in Fig.11. Their heat transfer performance was close to each other, which was larger than the diamond ones but smaller than the square ones. From the above, it can be noted that the square micro pin fins presented the best boiling heat transfer performance, followed by the circular and streamline ones, and the diamond micro pin fins performed the worst. The above trend generally accorded with the reported single-phase results of Zhao et al. [45] of three copper micro pin fins with circle, ellipse and diamond shapes. They also observed that the circular micro pin fins performed better than the diamond ones in single-phase heat transfer. Moreover, Tullius et al. [10] also noticed that the square micro pin fins presented the better single-phase heat transfer than the circular and diamond ones. Nevertheless, the above trend in this study somewhat differed from the single-phase forced convection heat transfer results of silicon micro pin fins in Kosar and Peles[6], and Izci et al. [7]. Izci et al. [7] experimentally accessed that the streamline micro pin fins preformed the best in single-phase heat transfer, followed by the square, diamond and circular ones. They attributed the single-phase heat transfer enhancement of streamline micro pin fins to the flow mixing induced by the sharp corners. The difference between this study and Ref. [6,7] may be due to that the streamline micro pin fins played a different role on the liquid-vapor two -phase flow compared to its single-phase counterpart, as discussed previously. Furthermore, the different base materials and fabrications procedures of micro pin fins in the present study and Ref. [6,7] may also induce the different heat transfer performance. Fig. 8,9,10,11

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3.3 Pressure drop

Fig.12 shows the pressure drop characteristics of four MPFHS samples. The initial single-phase region was associated with a small pressure drop. After the ONB, a rapid increase in ΔP can be noted with the increase in heat flux and vapor qualities. As the heat flux became larger, the amount of vapor increased in the pin fin array. The vapor traveled at a notably higher velocity than the liquid, and the acceleration of vapor flow induced a larger accelarational pressure loss. Besides, the vapor flow introduced shear forces to liquid film on the fins and caused greater friction drag along the wall, which increased the two-phase frictional pressure drop. Therefore, the total two-phase pressure drop presented an increasing tendency with increasing heat fluxes and vapor qualities. Fig. 12

Among these four pin fins, it can be found that the diamond one presented the smallest pressure drop, followed by square and circular ones. The streamline pin fins produced the largest ΔP . The cross-section shape of micro pin fins seems to play a significant role on pressure drop. For the diamond micro pin fins, their shear angles in the head of pin fin facilitated to avoid the violent collide between coolants and micro pin fins, which reduced the shear force and induced the smallest friction drag. Besides, the flow pathways were constructed by parallel edges between two opposite pin fins, which were in two directions with the small angles (30°). This helped to mitigate the flow perturbation, and reduced the two-phase pressure drop. Therefore, the smallest pressure drop can be noted for the diamond pin fins. The square micro pin fins showed a larger pressure drop than the diamond micro pin fins. For square pin fins, additional pressure losses were introduced due to the effects of cross-links, which induced the cross flow in the abrupt expansion and contraction area. The streamline and circular micro pin fins produced larger pressure drop than the square micro pin fins. This may be due to the

18

round heads of streamline and circular pin fins played a less dominated role on the separation of vapor flow, which may contribute more to the vapor acceleration and induced a larger acceleration pressure loss. This trend accorded with the reported single-phase forced convection results of circular and square pin fins by Zhao et al. [45]. While for the streamline and circular pin fins, they were of the same cross-section shapes in the head of micro pin fins, but different shapes of tails. The sharp tails of streamline pin fins may induce pronounced flow perturbation, which induced much larger shear forces to liquid film on the fins and increased the total pressure drop. Besides of the above shape effects, it is believed that the fin density of micro pin fins also have great influences on the two-phase pressure drop. As shown in Table 2, the streamline micro pin fins have the largest density(ε=0.256), and produced the largest pressure drop. The smallest pressure drop in diamond micro pin fins is also in line with the smallest fin density (ε =0.178). The fin densities of square and circular micro pin fins are 0.216 and 0.169, respectively. Both the square and circular micro pin fins performed larger pressure drop than the diamond micro pin fins but smaller pressure than the streamline micro pin fins. It seems that the pressure drop is fairly related to fin density, which is constant to the previous findings of pin fins [42]. One exceptional case is that the circular micro pin fins presented larger pressure drop than the square ones, despite that the fin density of circular pin fins was a little smaller than the square ones. This may be due to the more dominated role of pin fin shapes on the pressure drop. From the above, it can be seen that the flow across micro pin fins had a quite complex nature. Both the cross-section shape and fin density were important in determining pressure drop at two-phase flow boiling. 3.4 Flow instabilities

19

Two-phase flow instabilities are a prime concern for flow boiling in micro pin fin and microchannel heat sinks [46-48], which are detrimental to the safe and robust operation of the two-phase cooling

systems.

In

order

to

compare

the

two-phase

flow

instabilities of these four micro pin fins, the alleviation methods for two-phase instabilities as described in Section 2.3 were utilized, but the elimination methods, e.g., the installation of inlet restrictors in the upstream of test section [46], were not attempted. In this study, the standard deviations (σ) in Tin and Pin are taken as a

measurement of two-phase flow instabilities as they can provide critical information of the variations of local physical process inside the micro pin fins. Fig. 13 illustrates the temporal fluctuations of inlet temperatures and pressures at small, moderate and high heat flux conditions. Mild flow instabilities with small fluctuations of inlet pressures can be noted at small heat fluxes after the boiling incipience, and there were no distinct fluctuations in inlet temperatures. This is true for all the four pin fin samples, indicates that all the micro pin fins were able to operate fairly stably at the early stage of boiling process. At moderate and high heat fluxes, the diamond micro pin fins presented large magnitude of fluctuations in Tin and Pin, which is especially notable in the inlet temperatures, i.e., the fluctuations in Tin can reached as high as 10.4

at q''eff=435kW/m2, much larger than the other three pin fins. This suggested that the

diamond pin fins suffered much more severe flow instabilities than the other three micro pin fins in the two-phase boiling process. The diamond pin fins of the sharp corners in the head separated two flow channels in the longitudinal directions, which formed the smallest angles between these two flow channels. This induced the smallest inverse pressure gradient, and reduced the resistance of flow reversal significantly. Besides, as discussed earlier, the diamond pin fins introduced rapid formations of vapor slugs. These vapor slugs can easily grow along both upstream and downstream direction, and induced serve flow oscillations. Growing vapor

20

slugs towards the inlet plenum increased the temperature of the incoming coolant, and also blocked the channel and resulted in the increase in the inlet pressure. When the accumulations of incoming liquid were sufficient to overcome the flow resistance in the pin fins, the vapor slugs were pushed away and purged from the pin fins. With the now decreased flow resistance, a decrease in inlet pressure and temperature can be observed. The imposed heat fluxes triggered violent vapor formations again, initiating a next cycle of flow oscillation. Therefore, severe two-phase flow instabilities can be noted for diamond micro pin fins at moderate to high heat fluxes. The streamline micro pin fins also exhibited flow instabilities, but less significant than the diamond pin fins. The square and circular micro pin fins can sustain fairly stable boiling process, and the magnitude of flow fluctuations were much smaller than the diamond and streamline pin fins. This is obviously favorable for the safe operation of pin fin heat sink cooling systems. Fig. 13 3.5 Evaluation of optimum design

By the combination of the above two-phase flow boiling characteristics, it can be noted that the diamond micro pin fins performed worst in heat transfer performance, and suffered the most notable flow instabilities. But they produced the smallest pressure drop. The streamline micro pin fins exhibited better heat transfer performance than the diamond ones, and can avoid the heat transfer deterioration at high heat fluxes and vapor qualities. But they presented the largest pressure drop among four MPFHSs. Large pump power was consumed during the two-phase boiling, which may increase the operation costs of cooling systems and hinder their application in heat sink cooling systems. Furthermore, the streamline micro pin fins also exhibited somewhat large flow instabilities. The circular micro pin fins presented the second largest heat transfer performance, and were able to sustain fairly stable boiling performance for a wide range of heat fluxes and vapor qualities. They also showed a fairly good superiority in the

21

mitigation of the detrimental two-phase instabilities.

But their boiling pressure drop is

somewhat large. The square micro pin fins produced the best heat transfer performance. Besides, their pressure drop was much smaller than the streamline and circular micro pin fins, but just higher than the diamond micro pin fins. Furthermore, the square micro pin fins can also operate stably with no installation of inlet resistors. To sum up, it seems that the square micro pin fins are the optimum choice for two-phase flow boiling in these four MPFHSs. Moreover, the micro pin fin heat sinks have been demonstrated repeatedly to outperform the conventional straight microchannels in previous one-to-one comparison studies [23, 34]. Significant heat transfer enhancement as well as the mitigation of severe two-phase flow instabilities have been reached for micro pin fins [23, 34], while the pressure drop penalty were inevitably increased compared to the straight microchannels. These results indicated that the utilization of micro pin fins in heat sink cooling systems is promising for the high heat flux dissipations without considering the pump power. The obtained optimum micro pin fins in this study contributed for the design of heat sink cooling systems, and is believed to be of practical importance.

4. Conclusions In summary, four different types of micro pin fin heat sink (square, circular, diamond and streamline) were fabricated by a laser micromilling process for two-phase heat sink cooling systems. Flow boiling experiments of water were performed to characterize the two-phase boiling performance of these micro pin fin heat sink. The main conclusions can be summarized as follows: (1) The cross-section shape of micro pin fin shows no significant influence on the occurrence of ONB, and all the MPFHSs trigger the ONB at small wall temperature overshoots of 1-3℃.

22

(2) Among all the MPFHSs, the square micro pin fins presented the largest heat transfer coefficients, as the square shapes facilitated to hinder the continuous development of vapor slugs and contributed to the channel rewetting. The

diamond micro pin fins exhibited an early deterioration in heat transfer, and performed worst in the boiling heat transfer. The diamond shapes together with the sharp corners are believed to induce an early establishment of vapor cores, and played an adverse role on the surface wetting. The circular and streamline pin fins performed better than the diamond ones, but worse than the square ones. (3) The pressure drop of all the four types of micro pin fins increased with increasing heat fluxes. The diamond one presented the smallest pressure drop, followed by square and circular ones. The streamline pin fins

produced the largest pressure drop. The different shapes of micro pin fins played a significant role on the flow separation, and contribute differently to the acceleration and frictional pressure loss. The fin densities also have great influences on the two-phase pressure drop.

(4) The diamond micro pin fins suffered serve two-phase flow instabilities at moderate to high heat fluxes. The square and circular micro pin fins showed better performance in mitigating two-phase flow instabilities than the diamond and streamline micro pin fins. (5) The present experiments results suggested that the square micro pin fins performed best in general, and should be selected for micro pin fins heat sinks during two-phase flow boiling process. The circular micro pin fins are also a good choice when the pump power is not primarily concerned.

Acknowledgments The research was financially supported under the Grants of the National Nature Science Foundation of China (No. 51405407), the Fundamental Research Funds for the Central

23

Universities, Xiamen University (No. 20720150094), the Science and Technology Planning Project

for

Industry-University-Research

Cooperation

in

Huizhou

City

(Grant

No.2014B050013002), and the Open Fund of State Key Laboratory of High Performance Complex Manufacturing (No.Kfkt2015-08). Furthermore, the financial support of Collaborative Innovation Center of High-End Equipment Manufacturing in FuJian is also acknowledged. Reference

[1] B. Agostini, M. Fabbri, J. E. Park, L. Wojtan, J.R. Thome, B. Michel, State of the Art of High Heat Flux Cooling Technologies, Heat Transfer Eng. 28(2007) 258–281. [2] M. C. Riofrío, N. Caney, J.A. Gruss, State of the art of efficient pumped two-phase flow cooling technologies, Appl. Thermal Eng. 104(2016) 333-343 [3] R.S. Hamid, Investigation of Flow and Heat Transfer Characteristics in Micro Pin Fin Heat Sink with Nanofluid, Appl. Thermal Eng. 63(2014) 598–607. [4] Y. Zhang, A. Dembla, M.S. Bakir, Silicon Micropin-Fin Heat Sink With Integrated TSVs for 3-D ICs Tradeoff Analysis and Experimental Testing, IEEE Trans. Compon. Packag. Manuf. Tech. 3(2013) 1842-1850 [5] J. Zhao, S. Huang, L. Gong, Z. Huang, Numerical study and optimizing on micro square pin-fin heat sink for electronic cooling, Appl. Thermal Eng. 93 (2016) 1347–1359 [6] A. Kosar, Y. Peles, Micro Scale pin fin Heat Sinks—Parametric Performance Evaluation Study, IEEE Trans. Compon. Packag. Tech. 30(2007) 1-11. [7] T. Izci, M. Koz, and A. Kosar, The Effect of Micro Pin-Fin Shape on Thermal and Hydraulic Performance of Micro Pin-Fin Heat Sinks, Heat Transfer Eng.36(2015)1447–1457, [8] Y. J. Lee, P. K. Singh, P. S.Lee,Fluid flow and heat transfer investigations on enhanced microchannel heat sink using oblique fins with parametric study, Int. J. Heat Mass Transfer 81 (2015) 325–336 [9] W. Duangthongsuk, S. Wongwises, A comparison of the heat transfer performance and pressure drop of nanofluid-cooled heat sinks with different miniature pin fin configurations, Exp. Thermal Fluid Sci. 69(2015) 111–118 [10] J.F. Tullius , T.K. Tullius, Y. Bayazitoglu, Optimization of short micro pin fins in minichannels Int. J. Heat Mass Transfer 55 (2012 )3921-3932. [11] S.G. Kandlikar, Review and Projections of Integrated Cooling Systems for Three-Dimensional Integrated Circuits ASME J. Electronic Packag. 136(2014) 024001 [12] D. Jang, S. Yook, K. Lee, Optimum design of a radial heat sink with a fin-height profile for high-power LED lighting applications Appl. Energy 116(2014) 260–268

24

[13] C. Marques, K.W. Kelly, Fabrication and Performance of a Pin Fin Micro Heat Exchanger, ASME J. Heat Transfer 126(2004) 434-444 [14] X. Hua, G. Lin, Y. Cai, D. Wen, Experimental study of flow boiling of FC-72 in parallel minichannels under sub-atmospheric pressure, Appl. Thermal Eng. 31 (2011) 3839-3853 [15] P. Bai, T. Tang, B.Tang, Enhanced flow boiling in parallel microchannels with metallic porous coating, Appl. Thermal Eng. 58 (2013) 291-297. [16] S. Krishnamurthy, Y. Peles, Flow boiling of water in a circular staggered micro-pin fin heat sink Int. J. Heat Mass Transfer 51(2008) 1349–1364 [17] S. Krishnamurthy, Y. Peles, Flow Boiling Heat Transfer on Micro Pin Fins Entrenched in a Microchannel ASME J. Heat Transfer 132(2010) 041007 [18]A. Kosar, Y. Peles, Boiling heat transfer in a hydrofoil-based micro pin fin heat sink, Int J Heat Mass Transfer 50(2007)1018–1034 [19] A. Ma, J. Wei, M. Yuan, J. Fang, Enhanced flow boiling heat transfer of FC-72 on micro-pin-finned surfaces Int. J. Heat Mass Transfer 52(2009) 2925–2931 [20] W. Qu, A. Siu-Ho, Experimental study of saturated flow boiling heat transfer in an array of staggered micro-pin-fins Int. J. Heat Mass Transfer 52(2009) 1853-1863. [21]W.R. Chang, C.A. Chen, J.H. Ke, T.F. Lin, Subcooled flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip Int. J. Heat Mass Transfer 53(2010) 5605-5621. [22] D.A. McNeil, A.H. Raeisi, P.A. Kew, P.R. Bobbili, A comparison of flow boiling heat-transfer in in-line mini pin fin and plane channel flows Applied Thermal Engineering 30 (2010) 2412-2425 [23] M. Law, P.S. Lee , A comparative study of experimental flow boiling heat transfer and pressure characteristics in straight- and oblique-finned microchannels, Int. J. Heat Mass Transfer 85(2015) 797–810 [24]T.J. Cognata, D.K. Hollingsworth, L.C. Witte, High-speed visualization of two-phase flow in a micro-scale pin-fin heat exchanger Heat Transfer Eng. 28 (2007)861-869. [25] B.A. Jasperson, Y. Jeon, K.T. Turner, F.E. Pfefferkorn, W.L. Qu, Comparison of micro-pin-fin and microchannel heat sinks considering thermal-hydraulic performance and manufacturability IEEE Trans. Comp. Pack. Tech. 33(2010) 148-160. [26] Deng D, Wan W, Huang Q, Huang X and Zhou W. Investigations on laser micromilling of circular micro pin fins for heat sink cooling systems, Int. J. Adv. Manuf. Tech. 2016, doi:10.1007/s00170-016-8468-9 [27]

D.H.

Kam,

L.

Shah,

J.

Mazumder,

Femtosecond

laser

machining

of

multi-depthmicrochan-nel networks on to silicon J. Micromech. Microeng. 21(2011) 045027. [28] D. Deng, Y. Tang, D. Liang, H. He, S. Yang, Flow boiling characteristics in porous heat sink with reentrant microchannels, Int. J. Heat Mass Transfer 70(2014) 463–477

25

[29] D. Deng, W. Wan, H. Shao, Y. Tang, J. Feng, Zeng J, Zhou W and Liang D Effects of operation parameters on flow boiling characteristics of heat sink cooling systems with reentrant porous microchannels, Energy Conver. Manage. 96 (2015) 340–351 [30] Qu W, Mudawar I. Flow Boiling Heat Transfer in Two-Phase Micro-Channel Heat Sinks—I. Experimental Investigation and Assessment of Correlation Methods, Int. J. Heat Mass Transfer 46(2003) 2755–2771. [31] Wang G, Cheng P, Bergles AE. Effects of inlet/outlet configurations on flow boiling instability in parallel microchannels, Int J Heat Mass Transfer 51(2008) 2267–2281. [32] J.L. Xu, S. Shen, Y. Gan, Y. Li, W. Zhang, Q. Su, Transient flow pattern based microscale boiling heat transfer mechanisms J. Micromech. Microeng. 15 (2005 )1344–1361 [33] G. Hetsroni, A. Mosyak, E. Pogrebnyak,Z. Segal, Periodic boiling in parallel micro-channels at low vapor quality, Int. J. Multiphase Flow 32(2006) 1141–1159. [34] Y.K. Prajapati, M. Pathak, Mohd. K. Khan, A comparative study of flow boiling heat transfer in three different configurations of microchannels, Int J Heat Mass Transfer 85 (2015) 711–722 [35] W. Qu, I. Mudawar, Measurement and prediction of pressure drop in two-phase micro-channel heat sinks, Int. J. Heat Mass Transfer 46(2003) 2737–2753 [36] J.R. Taylor, An Introduction to Error Analysis, second ed., University Science Books, 1997. [37] T. Harirchian, S.V. Garimella, Microchannel size effects on local flow boiling heat transfer to a dielectric fluid, Int. J. Heat Mass Transfer 51(2008) 3724–3735. [38] L. Zhang, E. N. Wang, K. E. Goodson, T. W. Kenny, Phase change phenomena in silicon microchannels, Int. J. Heat Mass Transfer 48(2005) 1572–1582 [39] M. Law, P.S. Lee, K. Balasubramanian, Experimental investigation of flow boiling heat transfer in novel oblique-finned microchannels Int. J. Heat Mass Transfer 76(2014) 419-431. [40] M. E. Steinke,S.G. Kandlikar, An Experimental Investigation of Flow Boiling Characteristics of Water in Parallel Microchannels, ASME J. Heat Transfer 126(2004) 518-526 [41] A. Reeser, A. Bar-Cohen, G. Hetsroni, High quality flow boiling heat transfer and pressure drop in microgap pin fin arrays, International Journal of Heat and Mass Transfer 78 (2014) 974–985 [42] L.S. Tong, Y.S. Tang, Boiling Heat Transfer and Two-Phase Flow,1997, Second Edition, Taylor & Francis

26

[43] K. Balasubramanian, M. Jagirdar, P.S. Lee, C.J. Teo, S.K. Chou, Experimental investigation of flow boiling heat transfer and instabilities in straight microchannels, Int. J. Heat Mass Transfer 66 (2013) 655–671 [44] C. Woodcock, X.Yu, J. Plawsky, Y. Peles, Piranha Pin Fin (PPF) — Advanced flow boiling microstructures with low surface tension dielectric fluids Int. J. Heat Mass Transfer 90 (2015) 591–604 [45] H. Zhao, Z.Liu, C. Zhang, N. Guan, H. Zhao, Pressure drop and friction factor of a rectangular channel with staggered mini pin fins of different shapes, Experimental Thermal and Fluid Science 71 (2016) 57–69 [46] W. Qu, I. Mudawar. Transport Phenomena in Two-Phase Micro-Channel Heat Sinks, ASME J. Heat Transfer 126(2004) 213-224 [47] H.Y. Wu, P. Cheng, Boiling instability in parallel silicon microchannels at different heat flux, Int. J. Heat Mass Transfer 47 (2004) 3631–3641. [48] Z. Lyu, J. Xu, X. Yu, W. Jin, W. Zhang, Wavelet decomposition method decoupled boiling evaporation oscillation mechanisms over two to three timescales a study, Int. J. Multiphase Flow 72 (2015) 53–72

Rtotal

Nomenclature

Ach Afin Amin At

total heat transfer area of MPFHS, m2 cross-sectional area of a single micro pin fin, mm2 minimum transverse flow area, mm2 2

platform area of copper block, m 2

CHF

critical heat flux, kW/m

cp

specific heat of fluid, kJ/kg k

G

2

mass flux, kg/m s

SL ST

total thermal resistance along the heat conduction direction, ℃/W longitudinal pitch, mm transverse pitch, mm

Ttci

thermocouple reading (i = 1~5), ℃

Tin

inlet fluid temperature, ℃

Tout

outlet fluid temperature, ℃

Tw,tci

channel bottom wall temperature, ℃ wall superheat, ℃

Gmax

maximum mass flux, kg/m s

△Tsat,tci Tsat,tci

Hfin hfg Htp

height of micro pin fin, μm latent heat of vaporization, kJ/kg local two-phase heat transfer coefficient,

ts u W

2

27

local saturation temperature, ℃ thickness of solder paste, mm flow velocity of fluid, m/s width of MPFHS sample, mm

k kCu kf ks L Lfin Li lCu Lhs mfin m Ns Nrow N ONB Pfin qeff qin qloss q”eff RCu Rs Rhs

kW/m2 K thermal conductivity, W/m K thermal conductivity of copper block, W/m K thermal conductivity of fluid, W/m K thermal conductivity of solder paste, W/m K length of heat sink, mm length of a micro pin fin, mm distance from the inlet to thermocouple location in the stream-wise direction, m distance between the thermocouple and the top surface of copper block, m distance between heat sink bottom surface and the bottom of micro pin fin, m fin parameter mass flow rate, (kg/s) number of micro pin fins in a single row number of rows total number of micro pin fins onset of nucleation boiling perimeter of cross-section of micro pin fin,m effective heat power, W total power input, W heat loss, W effective heat flux, kW/m2 conductive thermal resistance in copper block, (℃/W) thermal resistance of solder layer, (℃/W) conductive thermal resistance in heat sink, ℃/W

Wfin Wht

width of a micro pin fin, mm width of heat transfer area, mm

Greek symbols e fin density

m r h

dynamic viscosity of fluid, Ns/m2 density of fluid, kg/m3 fin efficiency

f

heat transfer ratio

Subscripts Cu hs ht fin i in min

copper heat sink heat transfer area fin thermocouple location inlet minimum

max out s sat total

maximum outlet solder saturation total

tp

two-phase

Figure captions Fig.1. Geometry dimensions of micro pin fins. Fig.2.SEM images of the four micro pin fins: (a) square; (b) circular; (c) streamline; (d) diamond

Fig.3. Schematic of the flow boiling test loop. Fig.4. Schematic of the cross-section of test section. Fig.5. Comparison of boiling curves of four MPFHS Fig.6. Two-phase heat transfer coefficient for four MPFHSs as a function of: (a) effective heat flux; (b) vapor quality. Fig.7. Flow morphologies of square micro pin fins at the test case of •Tsub= 10•, G=500kg/m2s: (a) q’’eff = 66.7 kW/m2; (b) q’’eff = 111.4 kW/m2; (c) q’’eff = 151.7 kW/m2; (d) q’’eff = 283.1 kW/m2; (e) q’’eff = 343.8 kW/m2; (f) q’’eff = 606.7 kW/m2.

28

Fig.8. Two-phase heat transfer coefficient for four MPFHS as a function of wall heat flux Fig.9. Flow behaviors in diamond micro pin fins at successive times at q’’eff=119kW/m2, •Tsub= 10•, G=500kg/m2s Fig.10. Flow behaviors in square micro pin fins at successive times at q’’eff=129kW/m2, •Tsub= 10•, G=500kg/m2s Fig.11. Flow behaviors in circular and streamline micro pin fins: (a) circular pin fins, q’’eff=109kW/m2,•Tsub= 10•, G=500kg/m2s. (b) streamline pin fins, q’’eff=147kW/m2,•Tsub= 10•, G=500kg/m2s Fig.12. Two-phase pressure drop of four MPFHS samples Fig.13. Variation of inlet temperature and pressure of four MPFHSs samples at: (a)low heat flux, (b)moderate heat flux; (c)high heat flux. Table 1. Specifications of fabrication parameters of the laser micromilling process Table 2. Geometric parameters of four micro pin fins samples

29

Table 1 Specifications of fabrication parameters of the laser micromilling process Characteristic

Process conditions

Wavelength

1064 nm

Laser output power

27 W

Scanning speed

250mm/s

Scanning time

20

Pulse duration

100 ns

Repetition rate

20 kHz

Incident beam diameter

7 mm

Table 2 Geometric parameters of four micro pin fins samples Parameters Diamond Square Circular Streamline Longitudinal pitch, SL, (mm) 3.43 1.5 1.5 1.5 Transverse pitch, ST, (mm) 0.92 1.5 1.5 1.5 Length of a micro pin fin, Lfin, (mm) 1.5 0.5 0.5 0.5 0.4 0.5 0.5 0.5 Width of a micro pin fin, Wfin, (mm) Cross-sectional area of a single pin fin, 0.3 0.25 0.196 0.297 Afin,c (mm2) Height of micro pin fin, Hfin (mm) 0.569 0.580 0.545 0.568 Total heat transfer area of MPFHS, Ach 0.001425 0.001366 0.0012 0.00146 (m2) Fin density, ε 0.178 0.216 0.169 0.256 13 30 30 30 Number of rows, Nrow 35 22 22 22 Number of pin fins in a single row, Ns Total number of micro pin fins, N 455 660 660 660

30

Figure 1

Fig.1. Geometry dimensions of micro pin fins.

Figure 2

(a)

(c)

(b)

(d)

Fig.2 SEM images of the four micro pin fins: (a) square; (b) circular; (c) streamline; (d) diamond

Figure 3

Fig.3. Schematic of the flow boiling test loop.

Figure 4

 

lhs ls lCu

Fig.4. Schematic of the cross-section of test section.

Figure 5

900 800 700

2

q''eff (kW/m )

600

2

Tsub=10℃,G=500kg/m s

Square Circular Streamline Diamond

500 400 300 200 100 0 -20

-10

0

10

20

Tsat(℃)

30

40

50

Fig.5. Comparison of boiling curves of four MPFHSs

Figure 6

50

50

2

2

Tsub=10℃,G=500kg/m s

Square Circular streamline Diamond

30

20

10

0

Square Circular Streamline Diamond

40

htp(kW/m2℃)

40

htp(kW/m2℃)

Tsub=10℃,G=500kg/m s

30

20

10

0

100

200

300

400

500

600 2

q''eff (kW/m )

700

800

900

0 -0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

xtc5

(a) (b) Fig.6. Two-phase heat transfer coefficient for four MPFHSs as a function of: (a) effective heat flux; (b) vapor quality.

Figure 7

Liquid film

Slug

Bubbles 

Slug

Liquid film

(a) 

(b) 

(c) 

Dry‐out surface Liquid film

Vapor core (d) 

Liquid film

Vapor core (e) 

(f)  2

Fig.7. Flow morphologies of square micro pin fins at Tin=90℃, G=500 kg/m s with different heat fluxes : (a) q’’eff = 83 kW/m2; (b) q’’eff = 107 kW/m2; (c) q’’eff = 148kW/m2; (d) q’’eff = 277 kW/m2; (e) q’’eff = 336 kW/m2; (f) q’’eff = 593 kW/m2.

Figure 8

50

2

Tsub=10℃,G=500kg/m s

Square Circular streamline Diamond

htp(kW/m2℃)

40

30

20

10

0

0

50

100 150 200 250 300 350 400 450 500 550 600 650 2

q''wall(kW/m )

Fig.8. Two-phase heat transfer coefficient for four MPFHSs as a function of wall heat flux

Figure 9

 

Fig.9. Flow behaviors in diamond micro pin fins at successive times at q’’eff=119kW/m2,

△Tsub= 10℃, G=500kg/m2s  

Figure 10

 

Fig.10. Flow behaviors in square micro pin fins at successive times at △Tsub= 10℃, G=500kg/m2s  

q’’eff=129kW/m2,

Figure 11

Fig.11. Flow behaviors in circular and streamline micro pin fins:

(a) circular pin fins, q’’eff=109kW/m2,△ Tsub= 10℃, G=500kg/m2s. (b) streamline pin fins, q’’eff=147kW/m2,△ Tsub= 10℃, G=500kg/m2s

Figure 12

50

2

Tsub=10℃,G=500kg/m s

Square Circular Streamline Diamond

40

P(kpa)

30

20

10

0

0

100

200

300

400

500

600

700

800

900

2

q''eff (kW/m )

Fig.12. Two-phase pressure drop of four MPFHSs samples.

Figure 13

12

95

2s

2s

Water,Tsub=10℃,G=500kg/m 2

Square, q''eff=89 kW/m 0.05℃

Streamline, q''eff=95 kW/m 0.056℃

Circular, q''eff=89 kW/m20.038℃

Diamond, q''eff=89 kW/m20.064℃

10

93 92 91 90

Streamline, q''eff=95 kW/m20.20kPa

Circular, q''eff=89 kW/m20.10kPa

Diamond, q''eff=89kW/m20.51kPa

8 7 6 5

89 88

Square,q''eff=89 kW/m20.32kPa

9

Inlet Pressure(kpa)

Inlet Temperature(℃)

94

Water,Tsub=10℃,G=500kg/m

11

2

4 3

0

10

20

30

40

50

60

0

10

20

30

40

50

60

Times(s)

Times(s)

 

(a) 97

2

96

22

Water,Tsub=10℃,G=500kg/m2s



2

Square,q''eff=218 kW/m 0.06℃

Streamline,q''eff=227 kW/m 0.05℃

Circular,q''eff=227 kW/m20.05℃

Diamond,q''eff=225 kW/m20.55℃

20

95

Circular,q''eff=227 kW/m20.38kpa

Diamond,,q''eff=225 kW/m20.42kpa

18

Inlet Pressure(kpa)

Inlet Temperature(℃)

Water,Tsub=10℃,G=500kg/m2s Square,q''eff=218 kW/m20.48kpa Streamline,q''eff=227 kW/m20.28kpa



94 93 92 91

16 14 12 10

90 8 89

0

10

20

30

40

50

60

Times(s)

0

10

20

30

40

50

60

Times(s)

 

 

(b) 108 106 104

Square,q''eff=455 kW/m20.16℃

Streamline,,q''eff=469 kW/m20.63℃

Circular,q''eff=465kW/m20.06℃

Diamond,q''eff=435kW/m23.79℃

2s

Water,Tsub=10℃,G=500kg/m

38 36 34

Square,q''eff=455 kW/m20.59kpa

Streamline,q''eff=469 kW/m20.89kpa

Circular,q''eff=465 kW/m20.41kpa

Diamond,q''eff=435 kW/m21.35kpa

32

102

Inlet Pressure(kpa)

Inlet Temperature(℃)

40

2

Water,Tsub=10℃,G=500kg/m s

100 98 96 94

30 28 26 24 22 20 18 16

92

14

90 88

   

12 10

0

10

20

30

Times(s)

40

50

60

0

10

20

30

40

50

Times(s)

(c) Fig.13. Variation of inlet temperature and pressure of four MPFHSs samples at: (a)low heat flux, (b)moderate heat flux; (c)high heat flux.  

60

 

Manuscript’s Highlights 

Four types of micro pin fins heat sinks were developed



Flow boiling performance were accessed and compared



The optimum micro



Two-phase pressure drop and flow instabilities were explored

pin fin is the square one in flow boiling

31