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An experimental study on hydrothermal performance of microchannel heat sinks with 4-ports and offset zigzag channels
Energy Conversion and Management 152 (2017) 157–165
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
An experimental study on hydrothermal performance of microchannel heat sinks with 4-ports and offset zigzag channels
MARK
⁎
D.D. Ma, G.D. Xia , J. Wang, Y.C. Yang, Y.T. Jia, L.X. Zong Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Micro heat sink Offset zigzag microchannel 4-ports Heat transfer enhancement Flow resistance
For cooling specific chip of 2 mm∗10 mm, the 4-ports and offset zigzag microchannels are designed. The fluid flow and heat transfer characteristics of 4-ports silicon heat sinks with rectangle and zigzag microchannels have been investigated experimentally. Deionized water is employed as the cooling fluid with flow rates of 28–72 ml/ min. Results show the 4-ports heat sink can effectively reduce pressure drops and reduce temperature rising along the flow directions for the fixed flow rates. For 4-ports with rectangle microchannel, the pressure drops is decreased about 70% and average temperature also is reduced by 2.8 °C. It can be interpreted that 4-ports structures reduce the length of channel and increase channel number, which leads to the flow velocity decreased by 0.5 times and the fluid distribution more uniform. Compared with 4-ports with rectangle microchannels, for 4-ports with zigzag microchannels heat sink, the pressure drop is reduced under the lower flow rates but increased slightly under larger flow rates. And temperatures of all flow rates are reduced, which is reduced by 3.8 °C and pressure drop only increased 2.6 kPa at flow rates of 72 ml/min. It can be interpreted that zigzag cavities redevelop thermal boundary layer and enhance the fluid disturbance to make the fluid mixing better. Additional, zigzag cavities also enlarge heat transfer areas and reduce the fluid velocity by increasing flow crosssection areas. Under fixed pumping power, 4-ports with Z can meet the larger heat dissipation and smaller flow rates requirement.
1. Introduction With the fast sophistication of nano-technologies and the high heat dissipation requirement in a small area of electronic devices such as chips and processers, thermal management is considered as the key technology to ensure its operation performance and reliability. Application of microchannel heat sinks that have great potential of achieving smaller, lighter-weight, higher-performance and lower-cost dissipates the heat from power source to cold fluid. Since the classical work of Tuckerman and Pease [1], microchannel cooling has drawn more attention from researchers of liquid and boiling cooling. Compared with air, liquid cooling is more attractive for the higher specific heat and thermal conductivity. Boiling heat transfer could remove larger quantities of heat than single phase liquid cooling, but instabilities, high pressure drops and low critical heat fluxes might constitute major roadblocks for application [2]. The methods of enhancement heat transfer for given heat dissipation question can be divided into two aspects: improving transport properties of coolants and optimizing microchannel geometer. The
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dielectric fluids are preferred for electrical properties, but limited with lower specific heat and thermal conductivity [3]. The liquid metal with low melting point and high thermal conductivity was proposed as working fluid for chip cooling equipment [4]. But lower specific heat, higher density and viscosity of that could bring fluid temperature rising for fixed heat flux, which would lead to deteriorate heat transfer. Additional, the higher thermal conductivity nanoparticles are added in fluids to form a suspension nanafluids, such as different properties Al2O3 [5] and nano liquid metal [6]. However, the instabilities of nanofluids, such as agglomeration and sedimentation of nanoparticles, are the main constraints in application [7]. Increased particle concentration might cause viscosity increased, but generates a high flow resistance in turn. For coolants, the water has the best thermal properties and been used extensively to show the feasibility of liquid cooling [8]. Using deionized water, maintaining a low ionic concentration and a high electrical resistivity, is another possible option [3]. Therefore, the deionized water is used as working fluid in present paper. In order to reduce thermal contact resistance, the heat sink can be directly embedded on the back of heat source, which is usually made of silicon [9].
Corresponding author. E-mail address: [email protected] (G.D. Xia).
http://dx.doi.org/10.1016/j.enconman.2017.09.052 Received 30 June 2017; Received in revised form 25 August 2017; Accepted 17 September 2017 Available online 22 September 2017 0196-8904/ © 2017 Elsevier Ltd. All rights reserved.
Energy Conversion and Management 152 (2017) 157–165
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Nomenclature Aheater Ab As cp Dh f h hc L Nu Q Δp Pp Qv R Re Rth T u
Wc Ww Pp
area of heater surface m2 area of channel borrow m2 area of channel sidewalls m2 special heat capacity kJ/(kg·K) hydrodynamic diameter m friction factor heat transfer coefficient W/(m2·K) depth of channel m length of channel m Nusselt number heat rate W pressure drop Pa pumping power W flow rate ml/min electrical resistance K/W Reynolds number thermal resistance K/W temperature °C fluid velocity m/s
channel width m fin width m pumping power W
Greek symbols
ρ λ μ η δ
density kg/m3 thermal conductivity W/(m·K) dynamic viscosity kg/(m·s2) fin efficiency Thickness of heat sink base m
Subscript av f in max out s w
average fluid inlet maximum outlet solid channel wall
have great effect on fluid distribution and temperature uniformity. Heat sink configurations were investigated [26] and fluid distribution of Itype configuration was more uniform. Separating heat sink into several compartments could improve fluid distribution uniformity and enhance temperature uniformity [27]. And same time, increasing channel number and reducing channel length could also decrease flow resistance [28]. Based on the results described above, the novel microchannels and fin pins could enhance heat transfer albeit with larger pressure drop penalty. A new phenomenon was discovered in our team, the microchannel with offset zigzag grooves in sidewalls enhanced heat transfer and reduced flow resistance due to increasing porosity [29]. And 4ports configuration could reduce flow resistance and improve heat transfer by reducing channel length and increasing channel number [30]. Therefore, the heat transfer and flow characteristics of heat sinks with 4-ports configuration and offset zigzag microchannels (4-ports with Z) are investigated experimentally, and compared with 2-ports and 4-ports with rectangle microchannels (2-ports with R and 4-ports with R) in this paper.
Thus, the silicon is used as material of heat sink in this paper. For the optimization microchannel geometers, firstly the parameters of rectangular microchannel have been conducted by minimizing entropy generation or thermal resistance [10–12]. The flow of microchannel heat transfer is generally laminar for the low Re number. Then, based on theory of improving flow turbulence and redeveloping thermal boundary layer, the micro fins and complex microchannels were proposed. The different shapes of pin fins were studied such as hexagonal and other five shapes [13], uniform offset strip fin [14] and increasing fin density along the flow direction [15], which would improve temperature uniformity. Besides, porous media was filled in microchannel to enhance heat transfer performance. The heat transfer performance of a microchannel filled with a porous material in the slipflow regime was higher than the one in the no-slip regime [16], and the thermal model was developed in [17]. Micro pin fins and porous media could provide significant heat transfer enhancement albeit with great pressure drop penalty. Some complex microchannels are designed to redevelop thermal boundary layer, such as sinusoidal microchannel [18], converging [19] and microchannel with internal vertical Y-shaped bifurcations [20]. The heat transfer enhancement of those were more favorably over the pressure drop penalty, but those channel only change the flow direction with constant microchannel cross-sections. For further enhancing the fluid disturbance and redeveloping thermal boundary layer, the microchannels with periodic changing cross-section were studied [21–23] and optimized parameters. The hydraulic and thermal boundary layers were redeveloping in each separated zone for the interrupted channel, which shorted effective flow length, resulting in the overall heat transfer enhanced by [21,22]. Results of [23] showed the periodic triangular reentrant cavities could enhance thermal performance by interrupting boundary layer, enlarging heat transfer areas and forming vortexes to enhance mixing of cold and hot fluid. Although the pressure drop increased, the performance evaluation criteria were larger than 1. The similarly results also were found [24], which studied experimentally the microchannel heat sink with fan-shaped reentrant cavities of sidewalls. Parameters of that was optimized further by [25], cavity shapes from triangular to trapezoidal and rectangular. And optimum thermal structure was found for microchannel with trapezoidal groove with groove tip length ratio of 0.5, groove depth ratio of 0.4, groove pitch ratio of 3.334, grooves orientation ratio of 0 and Re = 100. The Nusselt number of that was improved of 51.59% and friction factor only increased by 2.35%. Additional, the inlet/outlet and channel layouts
2. Experimental procedure 2.1. Micro-fabrication and Preprocessing of heat sink The micro heat sink, partial enlarged view of microchannels and packaging are showed in Fig. 1, which is consisted with Pyrex 7740 glass of 500 μm and silicon wafer of 400 μm. Inlet and outlet of 1 mm were processed on the glass by sand blasting technology of drilling hole. And 220 μm microchannels and headers were etched, and 0.1 μm heater film was sputtered on another wall of silicon, respectively. In order to simulate the uniform heating, the three parallel serpentine strips were designed by reducing current crowing in the meander bends. And electrical leads were located on the diagonal of heat sink. Then silicon and glass were bonded anodically. The detail machining process is described in [24]. There are no significant undercuts or unexpected deformation, and the error of etching depth is ± 0.3 μm by testing. The same inlet shapes of parallel channels were designed to improve flow distribution uniformity. To reduce heat loss, adiabatic grooves were located on the outsides of channels areas. Conductive silver pulp and A/B glue were used to connect heaters and printed circuit boards, which were used to connect electrical leads and heaters. The A/B glue ensured to connecting tightly. A thin layer 158
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Fig. 1. Heat sinks and packaging test: (a) heat sink; (b) partial enlarged views of microchannel; (c) packaging of heat sink.
ml/min. And filter prevents contaminants flowing into heat sink. The temperature of inlet is controlled at 25 °C by temperature water tank and condenser. Temperatures of inlet and outlet are measured by the thermocouple with precision of 0.05 K. Pressure drops are measured by pressure senor with precision of 1%. The average temperature of heater film is calibrated by the relationship between electrical resistance and temperature, and maximum error of which is less than ± 0.31 K. Measurement data from pressure sensor, thermocouples and IR thermograph are collected by data acquisition system. Before experiment, heater of heat sink was firstly connected by printed circuit boards and preprocessed by spraying a thin layer black lacquer. And then heat sink was assembled in package with manifold holes, which were connected with inlet and outlet tubes, thermocouples, pressure sensor and DC power. During experimentation, direct voltage was supplied to the heater film after setting the required flow rates and water temperature. Then experimental data were recorded
black lacquer was sprayed on the heater film to measure temperature profile precisely using IR thermograph. The heat sink was packaged by low thermal conductivity poly methyl meth acryl ate, which included fluid connections, thermometer and pressure holes, as shown in Fig. 1(c). 2.2. Experiment procedures Fig. 2 shows the schematic diagrams of experimental apparatus. It consists of temperature water tank, plunger pump, filters, packaging heat sink, power supply, IR thermograph, data acquisition system, pressure sensor, thermocouples and condenser. Deionized water is used for all experiments. Fluid is pumped from water tank to circulator and flow rates is controlled by plunger pump. To reduce heat loss to environment, the insulation material was wrapped on the tube. Flow rates were controlled in the range of 28–72 ml/min with precision of ± 0.5 159
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Dh =
4Wc Hc 2(Wc + Hc )
(5)
where Wc, are the average width of microchannel. The resistance of heater film is equal to voltage divided by current. And average temperature of heater film also can be calculated by the relationship of temperature and resistance, as follows: 2 R = R 0 (1 + 3.9.83 × 10−3 × Tav + 5.775 × 10−7 × Tav )
(6)
where R0 is the resistance at the 0 °C. The Re number is defined as:
R e=
ρuav D h μav
(7)
where uav, μav are the average velocity of microchannel and average dynamic viscosity. The total thermal performance is evaluated by the thermal resistance and pumping power, which are defined as:
Fig. 2. Schematic diagrams of experimental apparatus.
Rth =
after signals reach steady-state, which pressure fluctuation is less 0.1%. The maximum values of uncertainty are dominated by the height of microchannel, pump, thermocouples, pressure sensor, IR thermograph and DC power. The detailed uncertainty analysis was carried by using the procedure described by Holman [31]. Due to measurement errors, the uncertainties of pressure drop, friction factor, average temperature, Reynolds number, convective heat transfer coefficient, Nusselt number and thermal resistance are 1.29%, 6.42%, 2.18%, 3.93%, 5.19%, 5.23%, 2.25%, respectively.
Tav−Tin Q
(8)
Pp = ΔpQv
(9)
Δp is pressure drop of heat sink. As the pressure transmitters are located at the manifolds, the total pressure drop measurement represents the combined losses of the frictional loss in microchannels heat sink and location losses of packaging. Losses of packaging includes abrupt contraction, expansion, bend and linear loss. Therefore, pressure drop of heat sink can be written as follow:
2.3. Data reduction
Δp = Δpt −Δpc + Δpe + Δp b + Δpl
The heat sink was heated with electric heater film to provide a constant heat flux. In order to calculate the average heat transfer coefficient, the inside wall average temperature of microchannel was calculated from one dimensional Fourier’s heat conduction equation:
where Δpt , Δpc , Δpe , Δp b and Δpl are the total pressure drop, contraction, expansion, bend and linear pressure losses, which can be calculated by follows:
A A Δpc = 0.25 ⎡ ⎛1− 2 ⎞ ρu22 + ⎛1− in ⎞ ρu in2⎤ ⎥ ⎢ ⎝ A1 ⎠ A2 ⎠ ⎝ ⎦ ⎣ ⎜
Tw,in = Tav−
Qδ Aheater λ s
(1)
Q N (Ab + As η)(Tw,in−Tf,av )
2
⎜
⎟
(12)
Δpb =
(13)
Δpl =
64 l1 64 l2 ρu12 + ρu22 Re1 d1 Re2 d2
(14)
f=
(3)
2Δpch D h 2 lρuav
(15)
Δpch = Δp−Δploss
(16)
where Δpch , Δploss are the pressure drop of microchannel and the pressure loss including inlet and outlet of heat sink losses [32].
2h λ s Ww
hav D h λf
⎟
(11)
The subscript in, 1, 2 mean heat sink inlet, bigger cross-section area and smaller cross-section area of packaging channel. The friction factor of channel can be calculated by:
3. Result and discussion
where Ab, As, Tav,f, η, Ww and Hc are areas of microchannel bottom, areas of microchannel sidewalls, average temperature of fluid, fin efficiency, average width of fin and height of channel, respectively. The average Nusselt number is calculated as:
Nuav =
⎟
0.246ρu22
(2)
th (mHc ) η= mHc m=
⎜
2
⎜
where Qv, ρ, cp, Tout, Tin are volume flow rate, fluid density, heat capacity, and outlet and inlet temperatures, respectively. Then convection heat transfer coefficient can be obtained by an iterative calculation as follows:
hav =
⎟
2 2⎤ ⎛ Ain ⎞ ⎛ A2 ⎞ Δpe = 0.5 ⎡ ⎢ 1− A2 ρu in + 1− A1 ρu2 ⎥ ⎠ ⎝ ⎠ ⎦ ⎣⎝
where Tav is the average temperature of heat sink borrow, which was measured by the IR thermograph, δ is the distance of microchannel borrow from heat sink borrow, λ s is solid thermal conduction, Aheater is area of heater film, Q is heat rate of convection heat transfer, which is calculated as follows:
Q = Q v ρc p (Tout−Tin )
(10)
3.1. Validation The adiabatic grooves were set the outside of cooling microchannel area to reduce heat loss, and low thermal conductivity glass was used as cover plate and packaging test. Besides, the insulation material was wrapped on the tube. Thus, the heat loss can be ignored. The average temperatures of inlet and outlet were measured by thermocouple experimental, and compared with the theoretic data calculated by equation (2). The average temperature of heater film was measured by IR
(4)
where λ f , Dh are the fluid thermal conduction and hydraulic diameter, which is calculated as: 160
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the length of channel is decreased about 50% and fluid velocity is decreased 50% at the same flow rate, for channel number increased by 1 times. Thus, uniformity of fluid distribution and axial heat conduction are improved. Secondly, the flow direction is changed in the middle of heat sink, which interrupts and redevelop flow and thermal boundary layer to enhance heat transfer. Besides, the offset zigzag structures enhance heat transfer by interrupting boundary layer, enhancing flow disturbance and enlarging convective heat transfer areas. From the Fig. 8, it also can be seen the average temperature difference of three heat sinks become larger with increase of flow rates, which is caused by boundary layer interrupted and redeveloped, flow disturbance and fluid mixing improved, especially for zigzag channel. The maximum temperature of heater surface for three heat sinks are presented in Fig. 9, with different flow rates at q = 200 W/cm2. The exchanging tendency of maximum temperature is similarly with average temperature. It is contributed to axial thermal conductivity enhanced, boundary layer interrupted and redeveloped, fluid mixing improved, which decrease the maximum temperature increasing. Fig. 10 shows the variation of temperature along flow direction at y = 1 mm, Qv=60 ml/min and q = 200 W/cm2. For the 2-ports with R, the temperature increasing is more obvious at channel inlet, then becomes stable with increase of x. It is because that the boundary layer is developing at channel inlet, then is developed with increase of x, which leads to temperature difference of heater surface and fluid becomes constant. For 4-ports configures, it can be seen the minimum temperature at the middle of heat sink and temperature increases along two sides, for that lower temperature fluid flows from middle to two sides. The magnitude of temperature increasing is larger than 2-ports structure. Under the same heat rate and flow rate, the temperature differences of inlet and outlet are also same for 2/4-ports. Therefore, for 4ports the fluid temperature difference of x = 5–0 mm or x = 5–10 mm is equal to that of 2-ports of x = 0–10 mm, and increasing magnitude of 4-ports is 2 times that of 2-ports. Axial heat conduction of 4-ports is enhanced to improve heat transfer. Additional, flow direction of 4-ports is changed in middle of heat sink, which redevelops boundary layer to improve entrance effects and heat transfer. Thus, even temperature of 4-ports increases more quick, that is lower than 2-ports. The thermal boundary layers of zigzag channel are interrupted and redeveloped periodically along the flow direction. When fluid flows into zigzag cavity, velocity is decreased for cross-section area enlarged. Then fluid velocity is increased for cross-section area decreased and fluid impacts contracting wall to form the vortexes and improve fluid disturbance. Besides, offset zigzag structure enlarge convection heat transfer areas. Those factors enhance heat transfer performance. Fig. 11 presents the temperature distributions of heater surface for
thermograph and compared with the theoretic value calculated by equation (6). As an important parameter of flow characteristic, the pressure drops and average temperatures of 4-ports with R were tested and compared with the simulation value, which simulation method was described as [30]. Fig. 3 shows the comparisons of the heater surface average temperature and temperature difference of inlet and outlet at different flow rate and q = 200 W/cm2 for 4-ports with Z. It can be seen that the results of experiment and theory are in good agreement with each other, and the deviations are less than 0.3 °C for temperature difference of inlet and outlet, 0.7 °C for average temperature of heater surface, respectively. Fig. 4 shows the pressure drops and average temperature of experiment and simulation of 4-ports with R at different flow rates and q = 200 W/cm2. The maximum deviation are less than 0.99 kPa and 0.7 °C. These may suggest reasonable good agreement. 3.2. Flow characteristics Fig. 5 displays the variation of pressure drops with flow rates at q = 200 W/cm2 for three heat sinks. It can be seen that the pressure drops of 4-ports configurations are reduced remarkably about 70%, compared with 2-ports, for that reduce channel length and increase channel number, resulting in fluid velocity decreased for the fixed flow rates. The pressure drops of 4-ports with Z are lower than that of 4-ports with R under the lower flow rates, but are larger under the bigger flow rates. It is because that zigzag channels interrupt and redevelop boundary layer periodically to enhance flow disturbance. And same time zigzag channels increase cross-section area and porosity to reduce the flow velocity. For the lower flow rates, the last factor plays a more important role to decrease pressure drop. But with increase of flow rates, the flow disturbance intensity is enhanced, which leads to pressure drop increased, compared with rectangle channel. The effects of heat rate on pressure drops are displayed in Fig. 6. As expected, with increase of heat rate, the pressure drops of three heat sinks all decrease slightly, and decreasing magnitude decrease. It can be contributed to that with heat rate increase, fluid temperature is increased and increasing magnitude decreases, which leads to the fluid viscosity decreasing gentler. For 4-ports configurations, the pressure drops deceasing is more slightly. It is mainly because that more uniform fluid distribution and lower flow velocity lead to more weak decreasing of boundary layer viscosity. Fig. 7 displays the variation of f with Re at q = 200 W/cm2 for three heat sinks. The friction factors decrease with increasing of Re for three heat sinks, and decreasing tendency becomes more flat. Although the pressure drops of 4-ports configurations are lower than that of 2-ports configuration, the friction factors are larger than 2-ports. For 4-ports configurations include bend loss of the channel zone. And the length of channel also are reduced by 50%, increasing entrance effect. Besides, the fluid velocity is reduced by 50% due to channel number increased by 1 time under the same flow rates. Therefore, the friction factors of 4ports configurations distribute in lower Re and are larger than 2-ports configures for whole investigated range of 28–72 ml/min. The average fluid velocity of 4-ports with Z is lower the R for cavities increasing fluid cross-section. Additional, the offset zigzag channel interrupts and redevelops boundary layer. Thus, the friction factors of 4-ports with Z is largest. 3.3. Heat transfer characteristics The heat transfer characteristics are performed by the average temperature, maximum temperature, temperature distribution of heater surface and average Nu number. The average temperatures of heater surface for three heat sinks are displayed in Fig. 8. As expected, average temperatures decrease with increase of flow rates. The 4-ports configures can enhance heat transfer and zigzag channel could further improve heat transfer performance. Firstly, for the 4-port configures,
Fig. 3. Verification of experimental and theoretic for temperature difference and average heater temperature of 4-ports with zigzag microchannel heat sink at q = 200 W/cm2.
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Fig. 4. Verification of experimental and simulation for pressure drop and average temperature of 4-ports with R at different flow rates q = 200 W/cm2.
Fig. 7. Variation of f with Re at q = 200 W/cm2.
Fig. 5. Variation of pressure drops with flow rates at q = 200 W/cm2.
Fig. 8. Variation of average temperature with flow rates at q = 200 W/cm2.
Fig. 6. Effects of heat rate on pressure drop at Qv = 60 ml/min. Fig. 9. Variation of maximum temperature with flow rates at q = 200 W/cm2.
three heat sinks at different flow rates and q = 200 W/cm . The temperatures are decreased obviously for 4-ports with Z, then 4-ports with R, compared with 2-ports with R. Temperature gradients and maximum temperatures of 4-ports with Z are lowest, and then are 4-ports with R. This means 4-ports configurations enhance axial heat conduction and 2
entrance effect to decrease temperature and improve temperature uniformity. Zigzag channel structures further enhance convection heat transfer by interrupting and redeveloping boundary layer, improving fluid disturbance and enlarging convection heat transfer areas. 162
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Fig. 10. Variation of temperature along flow direction at Qv = 60 and q = 200 W/cm2.
Fig. 12. Effects of heat rate on average temperature at Qv = 60 ml/min.
Although the heat rate has litter effects on pressure drops, that has great influence on average temperature of heater surface, as shown in Fig. 12. The average temperatures rise almost linearly with heat rate increasing for three heat sinks. But magnitude of temperature rising for 4-ports configures are decreased, and zigzag structure further decreases temperature. It means the enhanced heat transfer performance of 4ports with zigzag channel structure becomes more and more obviously with increase of heat rate, and 4-ports with Z has more promising for high heat rate cooling. The major factors are boundary redeveloped periodically and fluid disturbance improved to enhance fluid mixing of channel middle and near side wall. Fig. 13 shows the variation of Nusselt number with Re number at q = 200 W/cm2 for three heat sinks. The Nusselt number of 4-ports with Z is about 2 times than 2-ports with R under Re around 400. Channel number of 4-ports configurations are increased and average width of zigzag channel also is increased. Thus, the Re of 4-ports configures are smaller the 2-ports configures under the same flow rate, especially for 4-ports with Z. As described above, the heat transfer is enhanced and temperature of heater surface is decreased under fixed flow rates. The heat transfer enhancement is more obvious under same Re. Firstly, the flow rates of 4-ports configures are about 2 times than that of 2-ports configurations under same Re. Secondly, the convective heat transfer areas are enlarged and fin efficiency also is increased, especially for offset zigzag microchannel with thinner fin. Additional, the zigzag structures interrupt and redevelop boundary layer periodically and improve flow disturbance to enhance heat transfer performance.
Fig. 13. Variation of Nuav with Re at q = 200 W/cm2.
resistance with pumping power. And some parameters that are closely to application, such as required minimum flow rates and pumping power, also are calculated. The variations of thermal resistance with pumping power for three heat sinks at q = 200 W/cm2 are showed in Fig. 14. Thermal resistances of those all decrease with increase of pumping power, for flow rates increasing causes fluid average temperature decreased, boundary layer thinned, fluid disturbance improved and pressure drop increased. Under the same pumping power, the thermal resistance of 4-ports with Z is lowest, then is 4-ports with R. As described above, the 4-ports
3.4. Performance evaluation The total heat transfer performance is evaluated by the thermal
47ml/min 56ml/min 50ml/min 66ml/min 72ml/min 2-ports with R
4-ports with R
4-ports with Z
Fig. 11. Temperature distributions of heater surface with different flow rates at q = 200 W/cm2.
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increase with heat rate, and increasing tendency become more and more obvious. For flow rates of 28–72 ml/min, the 4-ports with Z can effectively dissipate heat for heat rate less than 60 W, but 4-ports with R only can meet heat rate less than 55 W, and 45 W for 2-ports with R. In conclusion, the 4-ports with Z can meet the larger heat dissipation with smaller flow rates requirement and lower pumping power consumption. 4. Conclusion The flow and heat transfer of 2-ports and 4-ports with rectangle and offset zigzag microchannel heat sinks have been investigated by experiment. The silicon is taken as base material and deionized water is used as cooling fluid. The conclusion can be concluded as follows: 1. 4-ports configure heat sinks can decrease pressure drop about 70%, compared with 2-ports with R, for that channel number is increased by 1 times and channel length is decreased about 50%. 2. The average temperature and maximum temperature of heater surface of 4-ports configures heat sinks are decreased obviously and temperature uniformity also improved, especially for 4-ports with offset zigzag microchannel. 3. The minimum temperature locates at the middle of 4-ports configures. Temperature rising is more quickly along the flow direction for the flow velocity is decreased, but increasing amplitude is less that of 2-ports with R for channel length is decreased and flow direction is changed to redeveloping boundary layer. 4. The mechanism of heat transfer enhancement can be concluded as follows: (1) for the 4-ports configures, axial heat conduction is enhanced to improve the uniformity fluid distribution and temperature; (2) flow direction is changed to interrupt and redevelop boundary layer; (3) the offset zigzag structures interrupt boundary layer, enhance flow disturbance to form backflow and enlarge convective heat transfer areas. 5. For 4-ports configures, compared with rectangle microchannel, the pressure drop of zigzag microchannel is decreased under lower flow rates for porosity increased, but increased under larger flow rates for boundary layer interrupted and fluid disturbance improved. 6. Compared with 2-ports and 4-ports with R, under fixed pumping power, 4-ports with Z can meet the larger heat dissipation and smaller required flow rates.
Fig. 14. Variation of thermal resistance with pumping power at q = 200 W/cm2.
Fig. 15. Variation of required flow rates and pumping power with heat rate at Tmax = 70 °C.
Acknowledgements configuration could decrease the pressure drop about 70% under the same flow rates. And same time axial heat conduction and entrance effect are enhanced to improve convection heat transfer. Thus, the flow rates of 4-ports configurations are larger than 2-ports, which further enhance heat transfer in turn and thermal resistances are lower than 2ports. Besides, compared with 4-port with R, with increase of flow rates, the pressure drop of 4-port with Z is decreased under the lower flow rates for increasing porosity to decrease velocity, but pressure drop is increased under larger flow rates for fluid disturbance improved. Thus, pumping power of 4-port with Z could be reduced under the lower flow rates, and increased under the larger flow rates. However, zigzag channels redevelop the thermal boundary layer and improve flow disturbance, enhancing convection heat transfer under given flow rates. And with increase of flow rates enhancement heat transfer of zigzag channel become more evident. Therefore, compared with 4-port with R, both of pumping power and thermal resistance of 4-port with Z all are decreased under the lower flow rates. And thermal resistance is also decreased, but pumping power are increased slightly. It means that total heat transfer enhancement by optimizing heat sink configures is better than that by channel structure. And channel structure can also further improve heat transfer performance. Fig. 15 shows the variation of required minimum flow rates and corresponding pumping power with heat rate for three heat sinks at Tmax = 70 °C. The required minimum flow rates and pumping power
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[11] Wang XD, An B, Xu JL. Optimal geometric structure for nanofluid-cooled microchannel heat sink under various constraint conditions. Energy Convers Manage 2013;65:528–38. [12] Ma DD, Xia GD, Jia YT. Multi-parameter optimization for micro-channel heat sink under different constraint conditions. Appl Therm Eng 2017;120:247–56. [13] Yang DW, Wang Y, Ding GF, Jin ZY. Numerical and experimental analysis of cooling performance of single-phase array microchannel heat sinks with different pin-fin configurations. Appl Therm Eng 2017;112:1547–56. [14] Kirsch KL, Thole KA. Pressure loss and heat transfer performance for additively and conventionally manufactured pin fin arrays. Int J Heat Mass Transf 2017;108:2502–13. [15] Yadav V, Baghel K, Kumar R. Numerical investigation of heat transfer in extended surface microchannels. Int J Heat Mass Transf 2016;93:612–22. [16] Dehghan M, Valipour MS, Saedodin S. Microchannels enhanced by porous materials: heat transfer enhancement or pressure drop increment. Energy Convers. Manage 2016;110:22–32. [17] Dehghan M, Valipour MS, Saedodin S, et al. Investigation of forced convection through entrance region of a porous-filled microchannel: an analytical study based on the scale analysi. Appl Therm Eng 2016;99:446–54. [18] Lin TY, Kandlikar SG. An experimental investigation of structured roughness effect on heat transfer during single-phase liquid flow at microscale. ASME J Heat Transf 2012;134:101701. [19] Dehghan M, Daneshipour M, Valipour MS, Rafee R, Saedodin S. Enhancing heat transfer in microchannel heat sinks using converging flow passages. Energy Convers Manage 2015;92:244–50. [20] Xie GN, Shen H, Wang CC. Parametric study on thermal performance of microchannel heat sinks with internal vertical Y-shaped bifurcations. Int J Heat Mass Transf 2015;90:948–58. [21] Xu JL, Gan YH, Zhang DC, et al. Microscale heat transfer enhancement using thermal boundary layer redeveloping concept. Int J Heat Mass Transf
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
An experimental study on hydrothermal performance of microchannel heat sinks with 4-ports and offset zigzag channels
MARK
⁎
D.D. Ma, G.D. Xia , J. Wang, Y.C. Yang, Y.T. Jia, L.X. Zong Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Micro heat sink Offset zigzag microchannel 4-ports Heat transfer enhancement Flow resistance
For cooling specific chip of 2 mm∗10 mm, the 4-ports and offset zigzag microchannels are designed. The fluid flow and heat transfer characteristics of 4-ports silicon heat sinks with rectangle and zigzag microchannels have been investigated experimentally. Deionized water is employed as the cooling fluid with flow rates of 28–72 ml/ min. Results show the 4-ports heat sink can effectively reduce pressure drops and reduce temperature rising along the flow directions for the fixed flow rates. For 4-ports with rectangle microchannel, the pressure drops is decreased about 70% and average temperature also is reduced by 2.8 °C. It can be interpreted that 4-ports structures reduce the length of channel and increase channel number, which leads to the flow velocity decreased by 0.5 times and the fluid distribution more uniform. Compared with 4-ports with rectangle microchannels, for 4-ports with zigzag microchannels heat sink, the pressure drop is reduced under the lower flow rates but increased slightly under larger flow rates. And temperatures of all flow rates are reduced, which is reduced by 3.8 °C and pressure drop only increased 2.6 kPa at flow rates of 72 ml/min. It can be interpreted that zigzag cavities redevelop thermal boundary layer and enhance the fluid disturbance to make the fluid mixing better. Additional, zigzag cavities also enlarge heat transfer areas and reduce the fluid velocity by increasing flow crosssection areas. Under fixed pumping power, 4-ports with Z can meet the larger heat dissipation and smaller flow rates requirement.
1. Introduction With the fast sophistication of nano-technologies and the high heat dissipation requirement in a small area of electronic devices such as chips and processers, thermal management is considered as the key technology to ensure its operation performance and reliability. Application of microchannel heat sinks that have great potential of achieving smaller, lighter-weight, higher-performance and lower-cost dissipates the heat from power source to cold fluid. Since the classical work of Tuckerman and Pease [1], microchannel cooling has drawn more attention from researchers of liquid and boiling cooling. Compared with air, liquid cooling is more attractive for the higher specific heat and thermal conductivity. Boiling heat transfer could remove larger quantities of heat than single phase liquid cooling, but instabilities, high pressure drops and low critical heat fluxes might constitute major roadblocks for application [2]. The methods of enhancement heat transfer for given heat dissipation question can be divided into two aspects: improving transport properties of coolants and optimizing microchannel geometer. The
⁎
dielectric fluids are preferred for electrical properties, but limited with lower specific heat and thermal conductivity [3]. The liquid metal with low melting point and high thermal conductivity was proposed as working fluid for chip cooling equipment [4]. But lower specific heat, higher density and viscosity of that could bring fluid temperature rising for fixed heat flux, which would lead to deteriorate heat transfer. Additional, the higher thermal conductivity nanoparticles are added in fluids to form a suspension nanafluids, such as different properties Al2O3 [5] and nano liquid metal [6]. However, the instabilities of nanofluids, such as agglomeration and sedimentation of nanoparticles, are the main constraints in application [7]. Increased particle concentration might cause viscosity increased, but generates a high flow resistance in turn. For coolants, the water has the best thermal properties and been used extensively to show the feasibility of liquid cooling [8]. Using deionized water, maintaining a low ionic concentration and a high electrical resistivity, is another possible option [3]. Therefore, the deionized water is used as working fluid in present paper. In order to reduce thermal contact resistance, the heat sink can be directly embedded on the back of heat source, which is usually made of silicon [9].
Corresponding author. E-mail address: [email protected] (G.D. Xia).
http://dx.doi.org/10.1016/j.enconman.2017.09.052 Received 30 June 2017; Received in revised form 25 August 2017; Accepted 17 September 2017 Available online 22 September 2017 0196-8904/ © 2017 Elsevier Ltd. All rights reserved.
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Nomenclature Aheater Ab As cp Dh f h hc L Nu Q Δp Pp Qv R Re Rth T u
Wc Ww Pp
area of heater surface m2 area of channel borrow m2 area of channel sidewalls m2 special heat capacity kJ/(kg·K) hydrodynamic diameter m friction factor heat transfer coefficient W/(m2·K) depth of channel m length of channel m Nusselt number heat rate W pressure drop Pa pumping power W flow rate ml/min electrical resistance K/W Reynolds number thermal resistance K/W temperature °C fluid velocity m/s
channel width m fin width m pumping power W
Greek symbols
ρ λ μ η δ
density kg/m3 thermal conductivity W/(m·K) dynamic viscosity kg/(m·s2) fin efficiency Thickness of heat sink base m
Subscript av f in max out s w
average fluid inlet maximum outlet solid channel wall
have great effect on fluid distribution and temperature uniformity. Heat sink configurations were investigated [26] and fluid distribution of Itype configuration was more uniform. Separating heat sink into several compartments could improve fluid distribution uniformity and enhance temperature uniformity [27]. And same time, increasing channel number and reducing channel length could also decrease flow resistance [28]. Based on the results described above, the novel microchannels and fin pins could enhance heat transfer albeit with larger pressure drop penalty. A new phenomenon was discovered in our team, the microchannel with offset zigzag grooves in sidewalls enhanced heat transfer and reduced flow resistance due to increasing porosity [29]. And 4ports configuration could reduce flow resistance and improve heat transfer by reducing channel length and increasing channel number [30]. Therefore, the heat transfer and flow characteristics of heat sinks with 4-ports configuration and offset zigzag microchannels (4-ports with Z) are investigated experimentally, and compared with 2-ports and 4-ports with rectangle microchannels (2-ports with R and 4-ports with R) in this paper.
Thus, the silicon is used as material of heat sink in this paper. For the optimization microchannel geometers, firstly the parameters of rectangular microchannel have been conducted by minimizing entropy generation or thermal resistance [10–12]. The flow of microchannel heat transfer is generally laminar for the low Re number. Then, based on theory of improving flow turbulence and redeveloping thermal boundary layer, the micro fins and complex microchannels were proposed. The different shapes of pin fins were studied such as hexagonal and other five shapes [13], uniform offset strip fin [14] and increasing fin density along the flow direction [15], which would improve temperature uniformity. Besides, porous media was filled in microchannel to enhance heat transfer performance. The heat transfer performance of a microchannel filled with a porous material in the slipflow regime was higher than the one in the no-slip regime [16], and the thermal model was developed in [17]. Micro pin fins and porous media could provide significant heat transfer enhancement albeit with great pressure drop penalty. Some complex microchannels are designed to redevelop thermal boundary layer, such as sinusoidal microchannel [18], converging [19] and microchannel with internal vertical Y-shaped bifurcations [20]. The heat transfer enhancement of those were more favorably over the pressure drop penalty, but those channel only change the flow direction with constant microchannel cross-sections. For further enhancing the fluid disturbance and redeveloping thermal boundary layer, the microchannels with periodic changing cross-section were studied [21–23] and optimized parameters. The hydraulic and thermal boundary layers were redeveloping in each separated zone for the interrupted channel, which shorted effective flow length, resulting in the overall heat transfer enhanced by [21,22]. Results of [23] showed the periodic triangular reentrant cavities could enhance thermal performance by interrupting boundary layer, enlarging heat transfer areas and forming vortexes to enhance mixing of cold and hot fluid. Although the pressure drop increased, the performance evaluation criteria were larger than 1. The similarly results also were found [24], which studied experimentally the microchannel heat sink with fan-shaped reentrant cavities of sidewalls. Parameters of that was optimized further by [25], cavity shapes from triangular to trapezoidal and rectangular. And optimum thermal structure was found for microchannel with trapezoidal groove with groove tip length ratio of 0.5, groove depth ratio of 0.4, groove pitch ratio of 3.334, grooves orientation ratio of 0 and Re = 100. The Nusselt number of that was improved of 51.59% and friction factor only increased by 2.35%. Additional, the inlet/outlet and channel layouts
2. Experimental procedure 2.1. Micro-fabrication and Preprocessing of heat sink The micro heat sink, partial enlarged view of microchannels and packaging are showed in Fig. 1, which is consisted with Pyrex 7740 glass of 500 μm and silicon wafer of 400 μm. Inlet and outlet of 1 mm were processed on the glass by sand blasting technology of drilling hole. And 220 μm microchannels and headers were etched, and 0.1 μm heater film was sputtered on another wall of silicon, respectively. In order to simulate the uniform heating, the three parallel serpentine strips were designed by reducing current crowing in the meander bends. And electrical leads were located on the diagonal of heat sink. Then silicon and glass were bonded anodically. The detail machining process is described in [24]. There are no significant undercuts or unexpected deformation, and the error of etching depth is ± 0.3 μm by testing. The same inlet shapes of parallel channels were designed to improve flow distribution uniformity. To reduce heat loss, adiabatic grooves were located on the outsides of channels areas. Conductive silver pulp and A/B glue were used to connect heaters and printed circuit boards, which were used to connect electrical leads and heaters. The A/B glue ensured to connecting tightly. A thin layer 158
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Fig. 1. Heat sinks and packaging test: (a) heat sink; (b) partial enlarged views of microchannel; (c) packaging of heat sink.
ml/min. And filter prevents contaminants flowing into heat sink. The temperature of inlet is controlled at 25 °C by temperature water tank and condenser. Temperatures of inlet and outlet are measured by the thermocouple with precision of 0.05 K. Pressure drops are measured by pressure senor with precision of 1%. The average temperature of heater film is calibrated by the relationship between electrical resistance and temperature, and maximum error of which is less than ± 0.31 K. Measurement data from pressure sensor, thermocouples and IR thermograph are collected by data acquisition system. Before experiment, heater of heat sink was firstly connected by printed circuit boards and preprocessed by spraying a thin layer black lacquer. And then heat sink was assembled in package with manifold holes, which were connected with inlet and outlet tubes, thermocouples, pressure sensor and DC power. During experimentation, direct voltage was supplied to the heater film after setting the required flow rates and water temperature. Then experimental data were recorded
black lacquer was sprayed on the heater film to measure temperature profile precisely using IR thermograph. The heat sink was packaged by low thermal conductivity poly methyl meth acryl ate, which included fluid connections, thermometer and pressure holes, as shown in Fig. 1(c). 2.2. Experiment procedures Fig. 2 shows the schematic diagrams of experimental apparatus. It consists of temperature water tank, plunger pump, filters, packaging heat sink, power supply, IR thermograph, data acquisition system, pressure sensor, thermocouples and condenser. Deionized water is used for all experiments. Fluid is pumped from water tank to circulator and flow rates is controlled by plunger pump. To reduce heat loss to environment, the insulation material was wrapped on the tube. Flow rates were controlled in the range of 28–72 ml/min with precision of ± 0.5 159
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Dh =
4Wc Hc 2(Wc + Hc )
(5)
where Wc, are the average width of microchannel. The resistance of heater film is equal to voltage divided by current. And average temperature of heater film also can be calculated by the relationship of temperature and resistance, as follows: 2 R = R 0 (1 + 3.9.83 × 10−3 × Tav + 5.775 × 10−7 × Tav )
(6)
where R0 is the resistance at the 0 °C. The Re number is defined as:
R e=
ρuav D h μav
(7)
where uav, μav are the average velocity of microchannel and average dynamic viscosity. The total thermal performance is evaluated by the thermal resistance and pumping power, which are defined as:
Fig. 2. Schematic diagrams of experimental apparatus.
Rth =
after signals reach steady-state, which pressure fluctuation is less 0.1%. The maximum values of uncertainty are dominated by the height of microchannel, pump, thermocouples, pressure sensor, IR thermograph and DC power. The detailed uncertainty analysis was carried by using the procedure described by Holman [31]. Due to measurement errors, the uncertainties of pressure drop, friction factor, average temperature, Reynolds number, convective heat transfer coefficient, Nusselt number and thermal resistance are 1.29%, 6.42%, 2.18%, 3.93%, 5.19%, 5.23%, 2.25%, respectively.
Tav−Tin Q
(8)
Pp = ΔpQv
(9)
Δp is pressure drop of heat sink. As the pressure transmitters are located at the manifolds, the total pressure drop measurement represents the combined losses of the frictional loss in microchannels heat sink and location losses of packaging. Losses of packaging includes abrupt contraction, expansion, bend and linear loss. Therefore, pressure drop of heat sink can be written as follow:
2.3. Data reduction
Δp = Δpt −Δpc + Δpe + Δp b + Δpl
The heat sink was heated with electric heater film to provide a constant heat flux. In order to calculate the average heat transfer coefficient, the inside wall average temperature of microchannel was calculated from one dimensional Fourier’s heat conduction equation:
where Δpt , Δpc , Δpe , Δp b and Δpl are the total pressure drop, contraction, expansion, bend and linear pressure losses, which can be calculated by follows:
A A Δpc = 0.25 ⎡ ⎛1− 2 ⎞ ρu22 + ⎛1− in ⎞ ρu in2⎤ ⎥ ⎢ ⎝ A1 ⎠ A2 ⎠ ⎝ ⎦ ⎣ ⎜
Tw,in = Tav−
Qδ Aheater λ s
(1)
Q N (Ab + As η)(Tw,in−Tf,av )
2
⎜
⎟
(12)
Δpb =
(13)
Δpl =
64 l1 64 l2 ρu12 + ρu22 Re1 d1 Re2 d2
(14)
f=
(3)
2Δpch D h 2 lρuav
(15)
Δpch = Δp−Δploss
(16)
where Δpch , Δploss are the pressure drop of microchannel and the pressure loss including inlet and outlet of heat sink losses [32].
2h λ s Ww
hav D h λf
⎟
(11)
The subscript in, 1, 2 mean heat sink inlet, bigger cross-section area and smaller cross-section area of packaging channel. The friction factor of channel can be calculated by:
3. Result and discussion
where Ab, As, Tav,f, η, Ww and Hc are areas of microchannel bottom, areas of microchannel sidewalls, average temperature of fluid, fin efficiency, average width of fin and height of channel, respectively. The average Nusselt number is calculated as:
Nuav =
⎟
0.246ρu22
(2)
th (mHc ) η= mHc m=
⎜
2
⎜
where Qv, ρ, cp, Tout, Tin are volume flow rate, fluid density, heat capacity, and outlet and inlet temperatures, respectively. Then convection heat transfer coefficient can be obtained by an iterative calculation as follows:
hav =
⎟
2 2⎤ ⎛ Ain ⎞ ⎛ A2 ⎞ Δpe = 0.5 ⎡ ⎢ 1− A2 ρu in + 1− A1 ρu2 ⎥ ⎠ ⎝ ⎠ ⎦ ⎣⎝
where Tav is the average temperature of heat sink borrow, which was measured by the IR thermograph, δ is the distance of microchannel borrow from heat sink borrow, λ s is solid thermal conduction, Aheater is area of heater film, Q is heat rate of convection heat transfer, which is calculated as follows:
Q = Q v ρc p (Tout−Tin )
(10)
3.1. Validation The adiabatic grooves were set the outside of cooling microchannel area to reduce heat loss, and low thermal conductivity glass was used as cover plate and packaging test. Besides, the insulation material was wrapped on the tube. Thus, the heat loss can be ignored. The average temperatures of inlet and outlet were measured by thermocouple experimental, and compared with the theoretic data calculated by equation (2). The average temperature of heater film was measured by IR
(4)
where λ f , Dh are the fluid thermal conduction and hydraulic diameter, which is calculated as: 160
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the length of channel is decreased about 50% and fluid velocity is decreased 50% at the same flow rate, for channel number increased by 1 times. Thus, uniformity of fluid distribution and axial heat conduction are improved. Secondly, the flow direction is changed in the middle of heat sink, which interrupts and redevelop flow and thermal boundary layer to enhance heat transfer. Besides, the offset zigzag structures enhance heat transfer by interrupting boundary layer, enhancing flow disturbance and enlarging convective heat transfer areas. From the Fig. 8, it also can be seen the average temperature difference of three heat sinks become larger with increase of flow rates, which is caused by boundary layer interrupted and redeveloped, flow disturbance and fluid mixing improved, especially for zigzag channel. The maximum temperature of heater surface for three heat sinks are presented in Fig. 9, with different flow rates at q = 200 W/cm2. The exchanging tendency of maximum temperature is similarly with average temperature. It is contributed to axial thermal conductivity enhanced, boundary layer interrupted and redeveloped, fluid mixing improved, which decrease the maximum temperature increasing. Fig. 10 shows the variation of temperature along flow direction at y = 1 mm, Qv=60 ml/min and q = 200 W/cm2. For the 2-ports with R, the temperature increasing is more obvious at channel inlet, then becomes stable with increase of x. It is because that the boundary layer is developing at channel inlet, then is developed with increase of x, which leads to temperature difference of heater surface and fluid becomes constant. For 4-ports configures, it can be seen the minimum temperature at the middle of heat sink and temperature increases along two sides, for that lower temperature fluid flows from middle to two sides. The magnitude of temperature increasing is larger than 2-ports structure. Under the same heat rate and flow rate, the temperature differences of inlet and outlet are also same for 2/4-ports. Therefore, for 4ports the fluid temperature difference of x = 5–0 mm or x = 5–10 mm is equal to that of 2-ports of x = 0–10 mm, and increasing magnitude of 4-ports is 2 times that of 2-ports. Axial heat conduction of 4-ports is enhanced to improve heat transfer. Additional, flow direction of 4-ports is changed in middle of heat sink, which redevelops boundary layer to improve entrance effects and heat transfer. Thus, even temperature of 4-ports increases more quick, that is lower than 2-ports. The thermal boundary layers of zigzag channel are interrupted and redeveloped periodically along the flow direction. When fluid flows into zigzag cavity, velocity is decreased for cross-section area enlarged. Then fluid velocity is increased for cross-section area decreased and fluid impacts contracting wall to form the vortexes and improve fluid disturbance. Besides, offset zigzag structure enlarge convection heat transfer areas. Those factors enhance heat transfer performance. Fig. 11 presents the temperature distributions of heater surface for
thermograph and compared with the theoretic value calculated by equation (6). As an important parameter of flow characteristic, the pressure drops and average temperatures of 4-ports with R were tested and compared with the simulation value, which simulation method was described as [30]. Fig. 3 shows the comparisons of the heater surface average temperature and temperature difference of inlet and outlet at different flow rate and q = 200 W/cm2 for 4-ports with Z. It can be seen that the results of experiment and theory are in good agreement with each other, and the deviations are less than 0.3 °C for temperature difference of inlet and outlet, 0.7 °C for average temperature of heater surface, respectively. Fig. 4 shows the pressure drops and average temperature of experiment and simulation of 4-ports with R at different flow rates and q = 200 W/cm2. The maximum deviation are less than 0.99 kPa and 0.7 °C. These may suggest reasonable good agreement. 3.2. Flow characteristics Fig. 5 displays the variation of pressure drops with flow rates at q = 200 W/cm2 for three heat sinks. It can be seen that the pressure drops of 4-ports configurations are reduced remarkably about 70%, compared with 2-ports, for that reduce channel length and increase channel number, resulting in fluid velocity decreased for the fixed flow rates. The pressure drops of 4-ports with Z are lower than that of 4-ports with R under the lower flow rates, but are larger under the bigger flow rates. It is because that zigzag channels interrupt and redevelop boundary layer periodically to enhance flow disturbance. And same time zigzag channels increase cross-section area and porosity to reduce the flow velocity. For the lower flow rates, the last factor plays a more important role to decrease pressure drop. But with increase of flow rates, the flow disturbance intensity is enhanced, which leads to pressure drop increased, compared with rectangle channel. The effects of heat rate on pressure drops are displayed in Fig. 6. As expected, with increase of heat rate, the pressure drops of three heat sinks all decrease slightly, and decreasing magnitude decrease. It can be contributed to that with heat rate increase, fluid temperature is increased and increasing magnitude decreases, which leads to the fluid viscosity decreasing gentler. For 4-ports configurations, the pressure drops deceasing is more slightly. It is mainly because that more uniform fluid distribution and lower flow velocity lead to more weak decreasing of boundary layer viscosity. Fig. 7 displays the variation of f with Re at q = 200 W/cm2 for three heat sinks. The friction factors decrease with increasing of Re for three heat sinks, and decreasing tendency becomes more flat. Although the pressure drops of 4-ports configurations are lower than that of 2-ports configuration, the friction factors are larger than 2-ports. For 4-ports configurations include bend loss of the channel zone. And the length of channel also are reduced by 50%, increasing entrance effect. Besides, the fluid velocity is reduced by 50% due to channel number increased by 1 time under the same flow rates. Therefore, the friction factors of 4ports configurations distribute in lower Re and are larger than 2-ports configures for whole investigated range of 28–72 ml/min. The average fluid velocity of 4-ports with Z is lower the R for cavities increasing fluid cross-section. Additional, the offset zigzag channel interrupts and redevelops boundary layer. Thus, the friction factors of 4-ports with Z is largest. 3.3. Heat transfer characteristics The heat transfer characteristics are performed by the average temperature, maximum temperature, temperature distribution of heater surface and average Nu number. The average temperatures of heater surface for three heat sinks are displayed in Fig. 8. As expected, average temperatures decrease with increase of flow rates. The 4-ports configures can enhance heat transfer and zigzag channel could further improve heat transfer performance. Firstly, for the 4-port configures,
Fig. 3. Verification of experimental and theoretic for temperature difference and average heater temperature of 4-ports with zigzag microchannel heat sink at q = 200 W/cm2.
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Fig. 4. Verification of experimental and simulation for pressure drop and average temperature of 4-ports with R at different flow rates q = 200 W/cm2.
Fig. 7. Variation of f with Re at q = 200 W/cm2.
Fig. 5. Variation of pressure drops with flow rates at q = 200 W/cm2.
Fig. 8. Variation of average temperature with flow rates at q = 200 W/cm2.
Fig. 6. Effects of heat rate on pressure drop at Qv = 60 ml/min. Fig. 9. Variation of maximum temperature with flow rates at q = 200 W/cm2.
three heat sinks at different flow rates and q = 200 W/cm . The temperatures are decreased obviously for 4-ports with Z, then 4-ports with R, compared with 2-ports with R. Temperature gradients and maximum temperatures of 4-ports with Z are lowest, and then are 4-ports with R. This means 4-ports configurations enhance axial heat conduction and 2
entrance effect to decrease temperature and improve temperature uniformity. Zigzag channel structures further enhance convection heat transfer by interrupting and redeveloping boundary layer, improving fluid disturbance and enlarging convection heat transfer areas. 162
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Fig. 10. Variation of temperature along flow direction at Qv = 60 and q = 200 W/cm2.
Fig. 12. Effects of heat rate on average temperature at Qv = 60 ml/min.
Although the heat rate has litter effects on pressure drops, that has great influence on average temperature of heater surface, as shown in Fig. 12. The average temperatures rise almost linearly with heat rate increasing for three heat sinks. But magnitude of temperature rising for 4-ports configures are decreased, and zigzag structure further decreases temperature. It means the enhanced heat transfer performance of 4ports with zigzag channel structure becomes more and more obviously with increase of heat rate, and 4-ports with Z has more promising for high heat rate cooling. The major factors are boundary redeveloped periodically and fluid disturbance improved to enhance fluid mixing of channel middle and near side wall. Fig. 13 shows the variation of Nusselt number with Re number at q = 200 W/cm2 for three heat sinks. The Nusselt number of 4-ports with Z is about 2 times than 2-ports with R under Re around 400. Channel number of 4-ports configurations are increased and average width of zigzag channel also is increased. Thus, the Re of 4-ports configures are smaller the 2-ports configures under the same flow rate, especially for 4-ports with Z. As described above, the heat transfer is enhanced and temperature of heater surface is decreased under fixed flow rates. The heat transfer enhancement is more obvious under same Re. Firstly, the flow rates of 4-ports configures are about 2 times than that of 2-ports configurations under same Re. Secondly, the convective heat transfer areas are enlarged and fin efficiency also is increased, especially for offset zigzag microchannel with thinner fin. Additional, the zigzag structures interrupt and redevelop boundary layer periodically and improve flow disturbance to enhance heat transfer performance.
Fig. 13. Variation of Nuav with Re at q = 200 W/cm2.
resistance with pumping power. And some parameters that are closely to application, such as required minimum flow rates and pumping power, also are calculated. The variations of thermal resistance with pumping power for three heat sinks at q = 200 W/cm2 are showed in Fig. 14. Thermal resistances of those all decrease with increase of pumping power, for flow rates increasing causes fluid average temperature decreased, boundary layer thinned, fluid disturbance improved and pressure drop increased. Under the same pumping power, the thermal resistance of 4-ports with Z is lowest, then is 4-ports with R. As described above, the 4-ports
3.4. Performance evaluation The total heat transfer performance is evaluated by the thermal
47ml/min 56ml/min 50ml/min 66ml/min 72ml/min 2-ports with R
4-ports with R
4-ports with Z
Fig. 11. Temperature distributions of heater surface with different flow rates at q = 200 W/cm2.
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increase with heat rate, and increasing tendency become more and more obvious. For flow rates of 28–72 ml/min, the 4-ports with Z can effectively dissipate heat for heat rate less than 60 W, but 4-ports with R only can meet heat rate less than 55 W, and 45 W for 2-ports with R. In conclusion, the 4-ports with Z can meet the larger heat dissipation with smaller flow rates requirement and lower pumping power consumption. 4. Conclusion The flow and heat transfer of 2-ports and 4-ports with rectangle and offset zigzag microchannel heat sinks have been investigated by experiment. The silicon is taken as base material and deionized water is used as cooling fluid. The conclusion can be concluded as follows: 1. 4-ports configure heat sinks can decrease pressure drop about 70%, compared with 2-ports with R, for that channel number is increased by 1 times and channel length is decreased about 50%. 2. The average temperature and maximum temperature of heater surface of 4-ports configures heat sinks are decreased obviously and temperature uniformity also improved, especially for 4-ports with offset zigzag microchannel. 3. The minimum temperature locates at the middle of 4-ports configures. Temperature rising is more quickly along the flow direction for the flow velocity is decreased, but increasing amplitude is less that of 2-ports with R for channel length is decreased and flow direction is changed to redeveloping boundary layer. 4. The mechanism of heat transfer enhancement can be concluded as follows: (1) for the 4-ports configures, axial heat conduction is enhanced to improve the uniformity fluid distribution and temperature; (2) flow direction is changed to interrupt and redevelop boundary layer; (3) the offset zigzag structures interrupt boundary layer, enhance flow disturbance to form backflow and enlarge convective heat transfer areas. 5. For 4-ports configures, compared with rectangle microchannel, the pressure drop of zigzag microchannel is decreased under lower flow rates for porosity increased, but increased under larger flow rates for boundary layer interrupted and fluid disturbance improved. 6. Compared with 2-ports and 4-ports with R, under fixed pumping power, 4-ports with Z can meet the larger heat dissipation and smaller required flow rates.
Fig. 14. Variation of thermal resistance with pumping power at q = 200 W/cm2.
Fig. 15. Variation of required flow rates and pumping power with heat rate at Tmax = 70 °C.
Acknowledgements configuration could decrease the pressure drop about 70% under the same flow rates. And same time axial heat conduction and entrance effect are enhanced to improve convection heat transfer. Thus, the flow rates of 4-ports configurations are larger than 2-ports, which further enhance heat transfer in turn and thermal resistances are lower than 2ports. Besides, compared with 4-port with R, with increase of flow rates, the pressure drop of 4-port with Z is decreased under the lower flow rates for increasing porosity to decrease velocity, but pressure drop is increased under larger flow rates for fluid disturbance improved. Thus, pumping power of 4-port with Z could be reduced under the lower flow rates, and increased under the larger flow rates. However, zigzag channels redevelop the thermal boundary layer and improve flow disturbance, enhancing convection heat transfer under given flow rates. And with increase of flow rates enhancement heat transfer of zigzag channel become more evident. Therefore, compared with 4-port with R, both of pumping power and thermal resistance of 4-port with Z all are decreased under the lower flow rates. And thermal resistance is also decreased, but pumping power are increased slightly. It means that total heat transfer enhancement by optimizing heat sink configures is better than that by channel structure. And channel structure can also further improve heat transfer performance. Fig. 15 shows the variation of required minimum flow rates and corresponding pumping power with heat rate for three heat sinks at Tmax = 70 °C. The required minimum flow rates and pumping power
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