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Heat transfer characteristics of hybrid microjet – Microchannel cooling module
Applied Thermal Engineering xxx (2015) 1e7
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Heat transfer characteristics of hybrid microjet e Microchannel cooling module Tomasz Muszynski*, Rafal Andrzejczyk Gdansk University of Technology, Faculty of Mechanical Engineering, Department of Energy and Industrial Apparatus, Narutowicza 11/12, 80-233 Gdansk, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2015 Received in revised form 10 July 2015 Accepted 28 August 2015 Available online xxx
The paper presents experimental investigation of heat transfer intensification in a microjet emicrochannel cooling module. Applied technology takes benefits from two very attractive heat removal techniques. When jets are impinging on the surface, they have a very high kinetic energy at the stagnation point, also in microchannels boundary layer is very thin allowing to obtain very high heat fluxes. Main objective of this paper was to experimentally investigate the performance of a microjet emicrochannel cooling module. Intense heat transfer in the test section has been examined and described with precise measurements of thermal and flow conditions. Reported tests were conducted under steady state conditions for single phase liquid cooling. Obtained database of experimental data were compared to standard cooling techniques, and compared with superposed semi-empirical models for minichannels and microjet cooling. Gathered data with analytical solutions and numerical computer simulation allows the rational design and calculation of hybrid modules and optimum performance of these modules for various industrial applications. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Microjets Heat transfer intensification Microchannels
1. Introduction The paper presents experimental investigation of heat transfer intensification in a microjetemicrochannel cooling module. Applied technology takes benefits from two very attractive heat removal techniques. When jets are impinging on the surface, they have a very high kinetic energy at the stagnation point, also in microchannels boundary layer is very thin allowing to obtain very high heat fluxes. Accurate control of cooling parameters is required in ever wider range of technical applications. Achieving of high heat fluxes can be obtained by means of numerous technologies. Using liquids such as water, boiling is likely to occur when the surface temperature exceeds the coolant saturation temperature. The high convection of jet impingement commonly prolongs the heat flux range where partial boiling is available. Partial boiling of a water jet may cover a heat flux range of 30e300 W/cm2, which corresponds to the heat generation densities of many industrial applications where jet impingement boiling can be applied as an effective cooling method. * Corresponding author. E-mail addresses: [email protected] [email protected] (R. Andrzejczyk).
(T.
Muszynski),
Rafal.
Boiling is associated with large rates of heat transfer because of the latent heat of evaporation and because of the enhancement of the level of turbulence between the liquid and the solid surface. This enhancement is due to the mixing action associated with the cyclic nucleation, growth, and departure or collapse of vapor bubbles on the surface. Utaka et al. [1] measured the thickness of the micro-layer that forms on a heating surface by vapor growth using the laser extinction method for mini/micro-channel gap sizes of 0.5, 0.3 and 0.15 mm, in order to clarify the heat transfer characteristics of boiling in a mini/micro-channel vapor generator. Furthermore, the heat transfer characteristics in narrow gap mini/micro-channel boiling were investigated for the micro-layer dominant region. It was shown that the heat transfer is enhanced due to the microlayer evaporation. Alam et al. [2] carried out a comparative study to investigate the heat transfer and pressure drop characteristics of deionized water (DI) in microgap heat sink and compare results with similar data obtained for microchannel heat sink. These studies were carried out with the inlet DI water temperatures 86 C at different mass fluxes ranging from 400 to 1000 kg/m2 s, for effective heat flux 0e85 W/cm2. In the case of heat transfer enhancement techniques, such as impinging microjets and microchannels, significant rates of heat
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T. Muszynski, R. Andrzejczyk / Applied Thermal Engineering xxx (2015) 1e7
transfer can be achieved [3]. In the past years, many reports concerning the heat transfer and pressure drop behavior inside circular and noncircular microchannels have been published for example: Choi et al. [4], Yu et al. [5], Adams et al. [6,7], Mala and Li [8], Celata et al. [9]. But there are only a few papers on the subject undertaken. It is known that reducing the dimensions of the size of nozzle leads to an increase in the economy of cooling and improves its quality [10]. Space restrictions in many applications of impinging jets, due to installation limitations, results in inevitable confinement. Jet confinement in the stagnation or wall jet regions may affect the flow field and heat transfer to be different from those of the unconfined jet as reviewed by Garimella [11]. This is also the case when heat transfer intensification techniques are fabricated on the target surface, thus necessitating a thermal analysis that considers the degree of confinement. Also, jet impingement boiling can be seriously deteriorated by confinement due to considerable volumetric expansion by phase change, which increases the size of the pressure drop and can decrease the maximum heat flux. Although there exists great potential for applying partial boiling to cooling applications, few investigations of the matter have been reported in the open literatures. In study of Wu and Du [12] a miniature Vapor Compression Refrigeration system is designed, built and tested. The system is incorporating microchannel technology for electronics cooling. In case of jet impingement, high fluid velocities substantially extends the heat flux range corresponding to the partial boiling regime, thus covering the diversity of the heat removal rates required in industrial applications. For single-phase convection, local measurements of heat transfer are commonly employed as module-averaged ones, but the installation of as many sensors as required to obtain fine resolution data sets can be difficult when high-heat flux is involved. Numerous research papers are contributing to the jet impingement heat transfer enhancement. Li et al. [13] determined the thermal performance of heat sinks with various geometries, and with confined impingement cooling. The effects of the impinging Reynolds number, the width and the height of the fins, the distance between the nozzle and the tip of the fins on the thermal resistance were investigated. Geedipalli et al. [14] simulated the combination heating of food using microwave and jet impingement by coupling Maxwell's equations. Sung and Mudawar [15] studied the jet impingement single-phase and two-phase heat transfer characteristics. Naphon and Wongwises [16] studied the jet impingement single-phase cooling in personal computers central processing units. As mentioned earlier, numerous papers presented the study on heat and fluid flow in mini- and micro-channel or jet impingement heat transfer on flat plate. Almost all the works reported on the study of air jet impingement heat transfer. Heat transfer capability is limited by the working fluid transport properties. Present study describes results of research related to the design and construction of the hybrid microjet e microchannel, which may be applied as a cooling module in electronics.
Fig. 1. The schematic diagram of test section.
valve. In order to reduce pressure drop necessary to create a steady laminar jet, nozzles were created as a slot jet in a 1 mm thick aluminum plate. Detailed view of test module microchannel plate is presented on Fig. 2. The water jets impinged on the bottom of 0.7 mm high, 0.5 mm wide and 20 mm long microchannels. Every channel vas supplied by single water slot microjet, placed at the entrance of the microchannel. Slot microjet forms a rectangular microjet of 500 mm width. The cooled surface was made from brass with 20 20 mm
2. Experimental setup Present study shows results of steady state heat transfer experiments, conducted for single phase cooling in order to obtain wall temperature and heat fluxes. Fig. 1 shows the view of the test section diagram with data acquisition apparatus. It consisted of the module, fluid supplying system, the measuring devices and DC power supply. Working fluid was fed by a pulsation free gear pump from a supply tank. Desired fluid flow rate was obtained by means of power inverter and flow control
Fig. 2. Hybrid microjet e microchannel module construction.
Please cite this article in press as: T. Muszynski, R. Andrzejczyk, Heat transfer characteristics of hybrid microjet e Microchannel cooling module, Applied Thermal Engineering (2015), http://dx.doi.org/10.1016/j.applthermaleng.2015.08.085
T. Muszynski, R. Andrzejczyk / Applied Thermal Engineering xxx (2015) 1e7
bottom surface. Temperature was measured at the bottom surface of hybrid module by single T-type thermocouple. All thermocouples are pre-calibrated with a dry box temperature calibrator. Heat is supplied by a kanthal heater mounted at the bottom of the module. They are connected to the National Instruments data acquisition set. The signal from thermocouples was processed with the aid of the LabVIEW application. Heater is powered by a DC power supply and the total power input is determined by measuring current intensity and voltage. During tests heater was capable of dissipating up to 240 W. The whole set was thermally insulated. Heater operating power values are precisely controlled and measured. The applied power losses through conduction into the insulation and radiation to the surroundings are accurately calculated and accounted for in all tests according to the following procedure. Air side heat transfer coefficient was determined for case of natural convection, [17].
aair
l ¼ 1; 18 air $ d
1 3 Tair $g$d n2
!1 8
ðTw Tair ÞPr
(1)
The characteristic dimension d was taken as the casing insulation width. The thermodynamic properties of air were determined for ambient temperature. The influences of radiation heat transfer was omitted in calculations. The total thermal resistance was calculated as:
Ri ¼
1 d þ i aair li
3
To allow continuous operation of the cooling system, heat is removed by efficient microchannel heat sink. Fig. 4 presents view of full test facility, close-up on test module is presented on Fig. 5. The test pressure was kept constant by a diaphragm tank. Heat flux in all experiments covered range of the single-phase convection. The wall temperature was measured from the thermocouples welded to the backside of the heater, temperature drop along microchannel plate thickness was calculated assuming a onedimensional conduction through the plate. When a solid body is being heated by the hotter fluid surrounding it, heat is first convected to the body and subsequently conducted within the body. The Biot number is the ratio of the internal resistance of a body to heat conduction to its external resistance to heat convection. Therefore, a small Biot number represents small resistance to heat conduction, and thus small temperature gradients within the body. In this article it was assumed that experimental module behaves like lumped system, in order to simplify measuring system by use of single thermocouple to measure wall temperature. Heat transfer analysis that utilizes this idealization of heat transfer problems without much sacrifice from accuracy, in this case:
Bi ¼
a$Lc < 0:01 lb
(4)
(2)
where the thermal conductivity of insulation is li ¼ 0.03 W/m-K. Finally the total loss was calculated as:
1 Q_ loss ¼ $A$ðTw Tair Þ Ri
(3)
In all cases the total heat loses were smaller than 5% of heater power. Data are taken from a steady state measuring points in order to exclude heat capacity of the installation. Due to low volumetric flow rate of coolant it was measured by means of a coriolis flow meter. Test module construction allows to modify both channel and jets dimensions, Fig. 3 presents one of created microchannel test plates. The surface roughness of the mirrochannel plate was determined in the Ship Design and Research Center in Gdansk. For the measurements Mitutoyo Surftest 211 gauge was used. Before measurements the instrument was calibrated using a sample roughness standard Mitutoyo 178e601. The measurements, determined roughness parameters as Ra ¼ 0.57 mm, Rz ¼ 3.46 mm.
Fig. 3. Microchannel plate view.
Fig. 4. Test section view.
Please cite this article in press as: T. Muszynski, R. Andrzejczyk, Heat transfer characteristics of hybrid microjet e Microchannel cooling module, Applied Thermal Engineering (2015), http://dx.doi.org/10.1016/j.applthermaleng.2015.08.085
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T. Muszynski, R. Andrzejczyk / Applied Thermal Engineering xxx (2015) 1e7 Table 1 Measurement uncertainties. Variable
Uncertainty (%)
Accuracy
m_ (TwTo) Vjet Pp q_
0.5% 2.1% 1.41% 1% 1.0% 2.5% 2.5%
±0.5 g/s ±1.44 K ±0.02 m/s ±0.1 mW ±4 kW/m2 ±0.05 kW/m2K ±18
a Re
Fig. 5. Test module view.
or jet impingement heat transfer on the flat plate. Almost all the works reported on the study of jet air impingement heat transfer. However, heat transfer capability is limited by the working fluid transport properties. At present there is limited data available for jet liquid impingement heat transfer especially for the central processing unit (CPU) of personal computers (PC). Based on the literature review selected were the correlations dedicated to determining the heat transfer coefficient during laminar single-phase flow:
Nu ¼ 0:023$Re0:8 $Pr0:4 where, a is heat transfer coefficient inside channel, Lc is brass plate thickness as can be seen on Fig. 6, and lb is brass heat conductivity. Lumped system analysis is exact when Bi ¼ 0. It is generally accepted that lumped system analysis is applicable if Bi 0:1. If so the variation of temperature with location within the body is slight and can reasonably be approximated as being uniform. In order to determine the reliability of the experimental results, an uncertainty analysis was conducted on all measured quantities as well as the quantities calculated from the measurement results. Uncertainties were estimated according to the standard procedures described by NIST [18]. The combined standard uncertainty is used to express the uncertainty of measurement results. The uncertainty that defines an interval about the measurement result y within which the value of the measurand Y is confidently believed to lie. The measure of uncertainty intended to meet this requirement is termed expanded uncertainty U, and is obtained by multiplying uc(y) by a coverage factor k. Thus U ¼ k$uc(y) and it is confidently believed that y U Y y þ U, which is commonly written as Y ¼ y ± U. Overall, the uncertainty in the calculated Nusselt number is lower than 30%. Uncertainties of the other calculated variables, are shown in Table 1. 3. Working fluid selection As mentioned earlier, the numerous papers presented the study on heat transfer and pressure drop in the mini- and micro-channel
(5)
and authors own correlation [10]for heat transfer coefficient in microjet impingement:
1:897 D Nu ¼ 0:9871$Re0:8843 $Pr1:776 $ d
(6)
where D[m] is jet orifice diameter and d[m] is surface diameter on which jet impinges. Then, using the method of superposition it is assumed that the total value of the transfer coefficient is equal to the sum coefficients resulting from the contribution of individual phenomena. Also considered is the impact of flow resistance, represented as the value of the dimensionless Euler's number described as:
Eu ¼
DP r$w2
(7)
In Eq. (7) predicted pressure drop DP is the sum of the frictional pressure drop in the microchannel, pressure loss originating from the contraction at the minichannel inlet and expansion at its outlet. Hydrostatics pressure drop is neglected in the study as the error incurred can be neglected. Such assumption is acceptable and stems from authors experience as well as literature review. In open literature there are many papers, for example Owhaib [19]. The measured pressure drop reads:
DP ¼ DPF þ DPexpþcontr
(8)
The pressure drop associated with the contraction at the entrance to the minichannel has been calculated in line with the formula by Hewitt et al. [20]:
DPcontr ¼
G2 $ 2$rl
1 1 þ 1 g2 Cc
(9)
In Eq. (7) g represents the ratio of the test tube cross-section and the inlet plenum cross-section (Asect./Ainlet). The Cc coefficient is calculated from the Chisholm equation [21]:
Cc ¼
Fig. 6. Microchannel plate schematic.
1 0:639$ð1 gÞ0:5 þ 1
(10)
For the pressure loss as associated with the expansion at the outlet from the section the Hewitt et al. formula [20] is applied:
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T. Muszynski, R. Andrzejczyk / Applied Thermal Engineering xxx (2015) 1e7
DPexp ¼
G2 $g$ð1 gÞ rl
(11)
Individual substances were then compared in terms of Nusselt number ratio as a function of the heat transfer coefficient, and the number of Euler, as a function of resistance. Fig. 7 presents this comparison in function of Reynolds number. Certainly it can be said that both the thermodynamic properties, cost-effectiveness and safety related to the implementation of environmental policy, the best agents dedicated to the use with the hybrid module is HFE 7100 and water. In case of this study ethanol was discarded due to its flammability, water was chosen as a coolant because of its good characteristics and availability, as described in Table 2. In course of future investigations HFE 7100 is believed to be a best working fluid due to its compatibility with electronic equipment.
4. Correlation of heat transfer data In the analysis of experimental results the simple superpositioning technique is used to correlate single-phase heat transfer data. The schematic of microjet impingement in a channel is presented in Fig. 8. The channel portion on which the microjet heat transfer mechanisms are dominant can be determined from the energy balance on the element of the cooled channel. The water jets were examined with the average coolant temperature at nozzle exit, Tin ¼ 20 C. Both the bulk velocity at orifice exit (V) and volumetric flow rate (Q) conditions were considered to be the same in all channels. The superimposing technique consists of applying different correlations of general form to the different surface regions. The surface consists of an impingement region and channel flow region. Assuming surface temperature is uniform along entire microchannel length, overall heat transfer coefficient can be written as:
hL L ¼ hjet Ljet þ hmch L Ljet
(12)
Heat transfer in microchannel hmch can be found from
5
DittuseBoelter correlation for laminar flow Eq. (5), impingement heat transfer is taken from Eq. (6). The Reynolds numbers used to characterize heat transfer are defined as:
Rejet ¼
Gjet Djet m
(13)
and
Remch ¼
Gmch Dmch m
(14)
5. Experimental results In the course of experiments data of transferred heat amount along with surface temperatures have been recorded. Data were taken for water as a test fluid. Hydraulic characteristics of evaluated geometry of the microjet e microchannel module, is presented on Fig. 9. In order to allow economic operation of presented module, data were taken for laminar region. Which is obtained by setting flow rate so that Rejet < 1000 and Remch < 1000. Fig. 10 shows the heat transfer coefficient (HTC) obtained with water, for varying applied heat flux. In every case increased fluid inlet velocity resulted in higher heat fluxes removal. HTC is obtained by total heat removed by coolant, divided by wetted surface area (Aw), and temperature gradient. Eq. (15) enables the determination of the experimental value of the convective heat transfer coefficient a:
hl ¼
Q_ el Q_ loss ðTW Tin ÞAw
(15)
From comparison with predicted values of heat transfer, it is found that jet impingement heat transfer is dominant in 90% of total channel length. Fig. 11 shows cooling performance of cooling device compared to commercially available air cooling system, i.e. traditional heat sink with forced air convection. The comparison of different type of heat exchanger performances was performed in plot of the heat exchanger data in terms of transferred heat flux density q_ c versus wall (heater) temperature Tw. Because hybrid module is compared with radiator with forced air convection, transferred heat flux density is described as heat transferred through heater surface Ah ¼ 2 2 cm.
q_ c ¼
Q_ c Q_ el Q_ loss ¼ Ah Ah
(16)
Both were mounted with aid of thermal grease on the contact surface. During measurements without thermal grease an increase in the surface temperature up to 15 K was observed for that of a
Table 2 Comparison of selected properties of selected working fluids with new generation of constituent substances (HFE 7100).
Fig. 7. Comparison of the effect of heat transfer intensification and the value of the fluid flow resistance.
Properties
HFE 7100
Water
Ethanol
Flash point Toxicity ODP GWP Freeze point, C at normal pressure Boiling point, C at normal pressure Critical temperature, C Critical pressure, MPa
None Low 0 320 135
No flammability No 0 0 0
Flammability Low 0 <25 114.5
61
99.98
78.5
195.3 2.23
373.94 22.06
241 6.3
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T. Muszynski, R. Andrzejczyk / Applied Thermal Engineering xxx (2015) 1e7
Fig. 8. Fluid flow regimes for a jet impingement in minichannel.
properly installed. Much better performance of the microjetemicrochannel module is manifested by lower temperatures of cooled surface. Unfortunately, it appears that the heat fluxes above 40 W/cm2 for hybrid module approaches the temperature of above 90 C, thus making it impossible to implement in system with such requirements. Due to high thermal capacity of water, cooling system is capable of removing large heat quantities, with low volumetric flow rate. Economy of such system is increased if its capacity is fully used. Fig. 12 shows effectiveness of heat removal in hybrid cooling module. The effectiveness of heat exchanger performance was described as theoretical pumping power Pp, divided by transferred heat Q_ c in terms of transferred heat flux density q_ c. Pumping power was expressed as total measured pressure drop in system times volumetric flow rate of water:
_ Pp ¼ VDP
(17)
Ultimately, the system described in the paper is designed to work with fluids such as HFE 7100 in a two-phase flow conditions. Optimal in this respect seems to use HFE 7100 at a normal boiling point of 60 C, which gives hope to obtain the greatest heat flux removal at this temperature.
Fig. 9. Hydraulic Characteristics of tested hybrid microjet e microchannel module.
Fig. 10. Total heat transfer coefficient of hybrid microjet e microchannel module.
6. Conclusions Preliminary experimental data of hybrid microjet e microchannel cooling device were presented. The regime of heat transfer of single-phase convection was explored. The results of this study are of technological importance for the efficient design of cooling systems of personal computers or electronic devices to enhance cooling performance. Obtained data are compared to available “on the market” solutions. During measurements a significant difference in the surface temperature was observed for that of proposed cooling device. This channel-type confinement enabled the circular jets to be used more effectively by increasing the flow rate over the rectangular heated
Fig. 11. Comparison of heat removal of hybrid microjet e microchannel module (þ) with traditional heat sink ().
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T. Muszynski, R. Andrzejczyk / Applied Thermal Engineering xxx (2015) 1e7
7
Subscripts air air b brass c cooling l liquid p pump w wall parameter in inlet to nozzle
References
Fig. 12. Effectiveness of heat removal of hybrid microjet e microchannel module.
surface. For the proposed hybrid cooling module the comparison with predicted values of heat transfer, it is found jet impingement heat transfer is dominant in 90% of total channel length which was 20 mm. Unfortunately, it appears that the heat fluxes above 400 kW/m2 for hybrid module approaches the temperature of above 90 C, thus making it impossible to implement in system with such requirements. Ultimately, the system described in the paper is designed to work with fluids such as HFE 7100 and ethanol in a two-phase flow conditions at a normal boiling point of 60 C. Nomenclature A cp D Eu G h L m_ Nu P Pr Re Q_ T V g v,u
r l
surface area, [m2] specific heat, [J/kg K] jet diameter, [m] Eulers number, [-] mass flow rate, [kg/m2 s] heat transfer coefficient, [W/m2 K] channel length, [m] mass flux, [kg/s] Nusselt number, [-] Power, [W] Prandtl number, [-] Reynolds number, [-] Heat flux, [W] temperature, [K] velocity, [m/s] gravity, [m/s2] velocity components, [m/s] density, [kg/m3] thermal conductivity, [W/mK]
[1] Y. Utaka, S. Okuda, Y. Tasaki, Configuration of the micro-layer and characteristics of heat transfer in a narrow gap mini/micro-channel boiling system, Int. J. Heat Mass Transf. 52 (2009) 2205e2214. [2] T. Alam, P.S. Lee, C.R. Yap, L. Jin, A comparative study of flow boiling heat transfer and pressure drop characteristics in microgap and microchannel heat sink and an evaluation of microgap heat sink for hotspot mitigation, Int. J. Heat Mass Transf. 58 (2013) 335e347. [3] S.V. Garimella, R.A. Rice, Confined and submerged liquid jet impingement heat-transfer, J. Heat Transf. 117 (1995) 871e877. [4] K.I. Choi, R.F. Barron, R.O. Warrington, Fluid Flow and Heat Transfer in Micro Tubes, in: Micromechanical Sensors, Actuators and Systems, ASME, 1991. DSC-Vol. 32. [5] D. Yu, R. Warrington, R. Barron, T. Ameel, An experimental and theoretical investigation of fluid flow and heat transfer in microtubes, in: ASME/JSME Thermal Engineering Conference, vol. 1, ASME, 1995. [6] T.M. Adams, S.I. Abdel-Khalik, S.M. Jeter, Z.H. Qureshi, An experimental investigation of single-phase forced convection in microchannels, Int. J. Heat. Mass Transf. 41 (1998) 851e857. [7] T.M. Adams, S.M. Ghiaasiaan, S.I. Abdel-Khalik, Enhancement of liquid forced convection heat transfer in microchannels due to the release of dissolved noncondensables, Int. J. Heat. Mass Transf. 42 (1999) 3563e3573. [8] G.M. Mala, D.Q. Li, Flow characteristics of water in microtubes, Int. J. Heat. Fluid Flow. 20 (1999) 142e148. [9] G.P. Celata, M. Cumo, M. Guglielmi, G. Zummo, Experimental investigation of hydraulic and single phase heat transfer in 0.130 mm capillary tube, Microscale Thermophys. Eng. 6 (2002) 85e97. [10] D. Mikielewicz, T. Muszynski, Experimental study of heat transfer intensification using microjets, in: Int. Symp. On Convective Heat and Mass Transfer in Sustainable Energy, Hammamet, Tunisia, 2009. [11] S.V. Garimella, Heat transfer and flow fields in confined jet impingement, Annu. Rev. Heat. Transf. 11 (2000) 413e494. [12] Z. Wu, R. Du, Design and experimental study of a miniature vapor compression refrigeration system for electronics cooling, Appl. Therm. Eng. 31 (2011) 385e390. [13] H.Y. Li, S.M. Chao, G.L. Tsai, Thermal performance measurement of heat sinks with confined impinging jet by infrared thermography, Int. J. Heat Mass Transf. 48 (2005) 5386e5394. [14] S. Geedipalli, A.K. Datta, V. Rakesh, Heat transfer in a combination microwaveejet impingement oven, Food Bioprod. Process 86 (2008) 53e63. [15] M.K. Sung, I. Mudawar, Single-phase and two-phase heat transfer characteristics of low temperature hybrid micro-channel/micro-jet impingement cooling module, Int. J. Heat Mass Transf. 51 (2008) 3882e3895. [16] P. Naphon, S. Wongwises, Investigation on the jet liquid impingement heat transfer for the central processing unit of personal computers, Int. Commun. Heat Mass Transf. 37 (2010) 822e826. [17] F.P. Incropera, D.P. DeWitt, Fundamentals of Heat and Mass Transfer, New York, Wiley, 2007. [18] B.N. Taylor, C.E. Kuyatt, Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, National Institute of Standards and Technology Technical Note 1297, 1994. [19] W. Owhaib, Experimental Heat Transfer, Pressure Drop, and Flow Visualization of R134a in Vertical Mini/micro Tube, Royal Institute of Technology, KTH Stockholm, Sweden, 2007 [Phd Thesis]. [20] G.F. Hewitt, G.L. Shires, T.R. Bott, Process Heat Transfer, CRC Press, Begell House, Boca Raton, 1994. [21] D. Chisholm, Two-phase Flow in Pipelines and Heat Exchangers, Godwin, New York, 1983.
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Heat transfer characteristics of hybrid microjet e Microchannel cooling module Tomasz Muszynski*, Rafal Andrzejczyk Gdansk University of Technology, Faculty of Mechanical Engineering, Department of Energy and Industrial Apparatus, Narutowicza 11/12, 80-233 Gdansk, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2015 Received in revised form 10 July 2015 Accepted 28 August 2015 Available online xxx
The paper presents experimental investigation of heat transfer intensification in a microjet emicrochannel cooling module. Applied technology takes benefits from two very attractive heat removal techniques. When jets are impinging on the surface, they have a very high kinetic energy at the stagnation point, also in microchannels boundary layer is very thin allowing to obtain very high heat fluxes. Main objective of this paper was to experimentally investigate the performance of a microjet emicrochannel cooling module. Intense heat transfer in the test section has been examined and described with precise measurements of thermal and flow conditions. Reported tests were conducted under steady state conditions for single phase liquid cooling. Obtained database of experimental data were compared to standard cooling techniques, and compared with superposed semi-empirical models for minichannels and microjet cooling. Gathered data with analytical solutions and numerical computer simulation allows the rational design and calculation of hybrid modules and optimum performance of these modules for various industrial applications. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Microjets Heat transfer intensification Microchannels
1. Introduction The paper presents experimental investigation of heat transfer intensification in a microjetemicrochannel cooling module. Applied technology takes benefits from two very attractive heat removal techniques. When jets are impinging on the surface, they have a very high kinetic energy at the stagnation point, also in microchannels boundary layer is very thin allowing to obtain very high heat fluxes. Accurate control of cooling parameters is required in ever wider range of technical applications. Achieving of high heat fluxes can be obtained by means of numerous technologies. Using liquids such as water, boiling is likely to occur when the surface temperature exceeds the coolant saturation temperature. The high convection of jet impingement commonly prolongs the heat flux range where partial boiling is available. Partial boiling of a water jet may cover a heat flux range of 30e300 W/cm2, which corresponds to the heat generation densities of many industrial applications where jet impingement boiling can be applied as an effective cooling method. * Corresponding author. E-mail addresses: [email protected] [email protected] (R. Andrzejczyk).
(T.
Muszynski),
Rafal.
Boiling is associated with large rates of heat transfer because of the latent heat of evaporation and because of the enhancement of the level of turbulence between the liquid and the solid surface. This enhancement is due to the mixing action associated with the cyclic nucleation, growth, and departure or collapse of vapor bubbles on the surface. Utaka et al. [1] measured the thickness of the micro-layer that forms on a heating surface by vapor growth using the laser extinction method for mini/micro-channel gap sizes of 0.5, 0.3 and 0.15 mm, in order to clarify the heat transfer characteristics of boiling in a mini/micro-channel vapor generator. Furthermore, the heat transfer characteristics in narrow gap mini/micro-channel boiling were investigated for the micro-layer dominant region. It was shown that the heat transfer is enhanced due to the microlayer evaporation. Alam et al. [2] carried out a comparative study to investigate the heat transfer and pressure drop characteristics of deionized water (DI) in microgap heat sink and compare results with similar data obtained for microchannel heat sink. These studies were carried out with the inlet DI water temperatures 86 C at different mass fluxes ranging from 400 to 1000 kg/m2 s, for effective heat flux 0e85 W/cm2. In the case of heat transfer enhancement techniques, such as impinging microjets and microchannels, significant rates of heat
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T. Muszynski, R. Andrzejczyk / Applied Thermal Engineering xxx (2015) 1e7
transfer can be achieved [3]. In the past years, many reports concerning the heat transfer and pressure drop behavior inside circular and noncircular microchannels have been published for example: Choi et al. [4], Yu et al. [5], Adams et al. [6,7], Mala and Li [8], Celata et al. [9]. But there are only a few papers on the subject undertaken. It is known that reducing the dimensions of the size of nozzle leads to an increase in the economy of cooling and improves its quality [10]. Space restrictions in many applications of impinging jets, due to installation limitations, results in inevitable confinement. Jet confinement in the stagnation or wall jet regions may affect the flow field and heat transfer to be different from those of the unconfined jet as reviewed by Garimella [11]. This is also the case when heat transfer intensification techniques are fabricated on the target surface, thus necessitating a thermal analysis that considers the degree of confinement. Also, jet impingement boiling can be seriously deteriorated by confinement due to considerable volumetric expansion by phase change, which increases the size of the pressure drop and can decrease the maximum heat flux. Although there exists great potential for applying partial boiling to cooling applications, few investigations of the matter have been reported in the open literatures. In study of Wu and Du [12] a miniature Vapor Compression Refrigeration system is designed, built and tested. The system is incorporating microchannel technology for electronics cooling. In case of jet impingement, high fluid velocities substantially extends the heat flux range corresponding to the partial boiling regime, thus covering the diversity of the heat removal rates required in industrial applications. For single-phase convection, local measurements of heat transfer are commonly employed as module-averaged ones, but the installation of as many sensors as required to obtain fine resolution data sets can be difficult when high-heat flux is involved. Numerous research papers are contributing to the jet impingement heat transfer enhancement. Li et al. [13] determined the thermal performance of heat sinks with various geometries, and with confined impingement cooling. The effects of the impinging Reynolds number, the width and the height of the fins, the distance between the nozzle and the tip of the fins on the thermal resistance were investigated. Geedipalli et al. [14] simulated the combination heating of food using microwave and jet impingement by coupling Maxwell's equations. Sung and Mudawar [15] studied the jet impingement single-phase and two-phase heat transfer characteristics. Naphon and Wongwises [16] studied the jet impingement single-phase cooling in personal computers central processing units. As mentioned earlier, numerous papers presented the study on heat and fluid flow in mini- and micro-channel or jet impingement heat transfer on flat plate. Almost all the works reported on the study of air jet impingement heat transfer. Heat transfer capability is limited by the working fluid transport properties. Present study describes results of research related to the design and construction of the hybrid microjet e microchannel, which may be applied as a cooling module in electronics.
Fig. 1. The schematic diagram of test section.
valve. In order to reduce pressure drop necessary to create a steady laminar jet, nozzles were created as a slot jet in a 1 mm thick aluminum plate. Detailed view of test module microchannel plate is presented on Fig. 2. The water jets impinged on the bottom of 0.7 mm high, 0.5 mm wide and 20 mm long microchannels. Every channel vas supplied by single water slot microjet, placed at the entrance of the microchannel. Slot microjet forms a rectangular microjet of 500 mm width. The cooled surface was made from brass with 20 20 mm
2. Experimental setup Present study shows results of steady state heat transfer experiments, conducted for single phase cooling in order to obtain wall temperature and heat fluxes. Fig. 1 shows the view of the test section diagram with data acquisition apparatus. It consisted of the module, fluid supplying system, the measuring devices and DC power supply. Working fluid was fed by a pulsation free gear pump from a supply tank. Desired fluid flow rate was obtained by means of power inverter and flow control
Fig. 2. Hybrid microjet e microchannel module construction.
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T. Muszynski, R. Andrzejczyk / Applied Thermal Engineering xxx (2015) 1e7
bottom surface. Temperature was measured at the bottom surface of hybrid module by single T-type thermocouple. All thermocouples are pre-calibrated with a dry box temperature calibrator. Heat is supplied by a kanthal heater mounted at the bottom of the module. They are connected to the National Instruments data acquisition set. The signal from thermocouples was processed with the aid of the LabVIEW application. Heater is powered by a DC power supply and the total power input is determined by measuring current intensity and voltage. During tests heater was capable of dissipating up to 240 W. The whole set was thermally insulated. Heater operating power values are precisely controlled and measured. The applied power losses through conduction into the insulation and radiation to the surroundings are accurately calculated and accounted for in all tests according to the following procedure. Air side heat transfer coefficient was determined for case of natural convection, [17].
aair
l ¼ 1; 18 air $ d
1 3 Tair $g$d n2
!1 8
ðTw Tair ÞPr
(1)
The characteristic dimension d was taken as the casing insulation width. The thermodynamic properties of air were determined for ambient temperature. The influences of radiation heat transfer was omitted in calculations. The total thermal resistance was calculated as:
Ri ¼
1 d þ i aair li
3
To allow continuous operation of the cooling system, heat is removed by efficient microchannel heat sink. Fig. 4 presents view of full test facility, close-up on test module is presented on Fig. 5. The test pressure was kept constant by a diaphragm tank. Heat flux in all experiments covered range of the single-phase convection. The wall temperature was measured from the thermocouples welded to the backside of the heater, temperature drop along microchannel plate thickness was calculated assuming a onedimensional conduction through the plate. When a solid body is being heated by the hotter fluid surrounding it, heat is first convected to the body and subsequently conducted within the body. The Biot number is the ratio of the internal resistance of a body to heat conduction to its external resistance to heat convection. Therefore, a small Biot number represents small resistance to heat conduction, and thus small temperature gradients within the body. In this article it was assumed that experimental module behaves like lumped system, in order to simplify measuring system by use of single thermocouple to measure wall temperature. Heat transfer analysis that utilizes this idealization of heat transfer problems without much sacrifice from accuracy, in this case:
Bi ¼
a$Lc < 0:01 lb
(4)
(2)
where the thermal conductivity of insulation is li ¼ 0.03 W/m-K. Finally the total loss was calculated as:
1 Q_ loss ¼ $A$ðTw Tair Þ Ri
(3)
In all cases the total heat loses were smaller than 5% of heater power. Data are taken from a steady state measuring points in order to exclude heat capacity of the installation. Due to low volumetric flow rate of coolant it was measured by means of a coriolis flow meter. Test module construction allows to modify both channel and jets dimensions, Fig. 3 presents one of created microchannel test plates. The surface roughness of the mirrochannel plate was determined in the Ship Design and Research Center in Gdansk. For the measurements Mitutoyo Surftest 211 gauge was used. Before measurements the instrument was calibrated using a sample roughness standard Mitutoyo 178e601. The measurements, determined roughness parameters as Ra ¼ 0.57 mm, Rz ¼ 3.46 mm.
Fig. 3. Microchannel plate view.
Fig. 4. Test section view.
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T. Muszynski, R. Andrzejczyk / Applied Thermal Engineering xxx (2015) 1e7 Table 1 Measurement uncertainties. Variable
Uncertainty (%)
Accuracy
m_ (TwTo) Vjet Pp q_
0.5% 2.1% 1.41% 1% 1.0% 2.5% 2.5%
±0.5 g/s ±1.44 K ±0.02 m/s ±0.1 mW ±4 kW/m2 ±0.05 kW/m2K ±18
a Re
Fig. 5. Test module view.
or jet impingement heat transfer on the flat plate. Almost all the works reported on the study of jet air impingement heat transfer. However, heat transfer capability is limited by the working fluid transport properties. At present there is limited data available for jet liquid impingement heat transfer especially for the central processing unit (CPU) of personal computers (PC). Based on the literature review selected were the correlations dedicated to determining the heat transfer coefficient during laminar single-phase flow:
Nu ¼ 0:023$Re0:8 $Pr0:4 where, a is heat transfer coefficient inside channel, Lc is brass plate thickness as can be seen on Fig. 6, and lb is brass heat conductivity. Lumped system analysis is exact when Bi ¼ 0. It is generally accepted that lumped system analysis is applicable if Bi 0:1. If so the variation of temperature with location within the body is slight and can reasonably be approximated as being uniform. In order to determine the reliability of the experimental results, an uncertainty analysis was conducted on all measured quantities as well as the quantities calculated from the measurement results. Uncertainties were estimated according to the standard procedures described by NIST [18]. The combined standard uncertainty is used to express the uncertainty of measurement results. The uncertainty that defines an interval about the measurement result y within which the value of the measurand Y is confidently believed to lie. The measure of uncertainty intended to meet this requirement is termed expanded uncertainty U, and is obtained by multiplying uc(y) by a coverage factor k. Thus U ¼ k$uc(y) and it is confidently believed that y U Y y þ U, which is commonly written as Y ¼ y ± U. Overall, the uncertainty in the calculated Nusselt number is lower than 30%. Uncertainties of the other calculated variables, are shown in Table 1. 3. Working fluid selection As mentioned earlier, the numerous papers presented the study on heat transfer and pressure drop in the mini- and micro-channel
(5)
and authors own correlation [10]for heat transfer coefficient in microjet impingement:
1:897 D Nu ¼ 0:9871$Re0:8843 $Pr1:776 $ d
(6)
where D[m] is jet orifice diameter and d[m] is surface diameter on which jet impinges. Then, using the method of superposition it is assumed that the total value of the transfer coefficient is equal to the sum coefficients resulting from the contribution of individual phenomena. Also considered is the impact of flow resistance, represented as the value of the dimensionless Euler's number described as:
Eu ¼
DP r$w2
(7)
In Eq. (7) predicted pressure drop DP is the sum of the frictional pressure drop in the microchannel, pressure loss originating from the contraction at the minichannel inlet and expansion at its outlet. Hydrostatics pressure drop is neglected in the study as the error incurred can be neglected. Such assumption is acceptable and stems from authors experience as well as literature review. In open literature there are many papers, for example Owhaib [19]. The measured pressure drop reads:
DP ¼ DPF þ DPexpþcontr
(8)
The pressure drop associated with the contraction at the entrance to the minichannel has been calculated in line with the formula by Hewitt et al. [20]:
DPcontr ¼
G2 $ 2$rl
1 1 þ 1 g2 Cc
(9)
In Eq. (7) g represents the ratio of the test tube cross-section and the inlet plenum cross-section (Asect./Ainlet). The Cc coefficient is calculated from the Chisholm equation [21]:
Cc ¼
Fig. 6. Microchannel plate schematic.
1 0:639$ð1 gÞ0:5 þ 1
(10)
For the pressure loss as associated with the expansion at the outlet from the section the Hewitt et al. formula [20] is applied:
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T. Muszynski, R. Andrzejczyk / Applied Thermal Engineering xxx (2015) 1e7
DPexp ¼
G2 $g$ð1 gÞ rl
(11)
Individual substances were then compared in terms of Nusselt number ratio as a function of the heat transfer coefficient, and the number of Euler, as a function of resistance. Fig. 7 presents this comparison in function of Reynolds number. Certainly it can be said that both the thermodynamic properties, cost-effectiveness and safety related to the implementation of environmental policy, the best agents dedicated to the use with the hybrid module is HFE 7100 and water. In case of this study ethanol was discarded due to its flammability, water was chosen as a coolant because of its good characteristics and availability, as described in Table 2. In course of future investigations HFE 7100 is believed to be a best working fluid due to its compatibility with electronic equipment.
4. Correlation of heat transfer data In the analysis of experimental results the simple superpositioning technique is used to correlate single-phase heat transfer data. The schematic of microjet impingement in a channel is presented in Fig. 8. The channel portion on which the microjet heat transfer mechanisms are dominant can be determined from the energy balance on the element of the cooled channel. The water jets were examined with the average coolant temperature at nozzle exit, Tin ¼ 20 C. Both the bulk velocity at orifice exit (V) and volumetric flow rate (Q) conditions were considered to be the same in all channels. The superimposing technique consists of applying different correlations of general form to the different surface regions. The surface consists of an impingement region and channel flow region. Assuming surface temperature is uniform along entire microchannel length, overall heat transfer coefficient can be written as:
hL L ¼ hjet Ljet þ hmch L Ljet
(12)
Heat transfer in microchannel hmch can be found from
5
DittuseBoelter correlation for laminar flow Eq. (5), impingement heat transfer is taken from Eq. (6). The Reynolds numbers used to characterize heat transfer are defined as:
Rejet ¼
Gjet Djet m
(13)
and
Remch ¼
Gmch Dmch m
(14)
5. Experimental results In the course of experiments data of transferred heat amount along with surface temperatures have been recorded. Data were taken for water as a test fluid. Hydraulic characteristics of evaluated geometry of the microjet e microchannel module, is presented on Fig. 9. In order to allow economic operation of presented module, data were taken for laminar region. Which is obtained by setting flow rate so that Rejet < 1000 and Remch < 1000. Fig. 10 shows the heat transfer coefficient (HTC) obtained with water, for varying applied heat flux. In every case increased fluid inlet velocity resulted in higher heat fluxes removal. HTC is obtained by total heat removed by coolant, divided by wetted surface area (Aw), and temperature gradient. Eq. (15) enables the determination of the experimental value of the convective heat transfer coefficient a:
hl ¼
Q_ el Q_ loss ðTW Tin ÞAw
(15)
From comparison with predicted values of heat transfer, it is found that jet impingement heat transfer is dominant in 90% of total channel length. Fig. 11 shows cooling performance of cooling device compared to commercially available air cooling system, i.e. traditional heat sink with forced air convection. The comparison of different type of heat exchanger performances was performed in plot of the heat exchanger data in terms of transferred heat flux density q_ c versus wall (heater) temperature Tw. Because hybrid module is compared with radiator with forced air convection, transferred heat flux density is described as heat transferred through heater surface Ah ¼ 2 2 cm.
q_ c ¼
Q_ c Q_ el Q_ loss ¼ Ah Ah
(16)
Both were mounted with aid of thermal grease on the contact surface. During measurements without thermal grease an increase in the surface temperature up to 15 K was observed for that of a
Table 2 Comparison of selected properties of selected working fluids with new generation of constituent substances (HFE 7100).
Fig. 7. Comparison of the effect of heat transfer intensification and the value of the fluid flow resistance.
Properties
HFE 7100
Water
Ethanol
Flash point Toxicity ODP GWP Freeze point, C at normal pressure Boiling point, C at normal pressure Critical temperature, C Critical pressure, MPa
None Low 0 320 135
No flammability No 0 0 0
Flammability Low 0 <25 114.5
61
99.98
78.5
195.3 2.23
373.94 22.06
241 6.3
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T. Muszynski, R. Andrzejczyk / Applied Thermal Engineering xxx (2015) 1e7
Fig. 8. Fluid flow regimes for a jet impingement in minichannel.
properly installed. Much better performance of the microjetemicrochannel module is manifested by lower temperatures of cooled surface. Unfortunately, it appears that the heat fluxes above 40 W/cm2 for hybrid module approaches the temperature of above 90 C, thus making it impossible to implement in system with such requirements. Due to high thermal capacity of water, cooling system is capable of removing large heat quantities, with low volumetric flow rate. Economy of such system is increased if its capacity is fully used. Fig. 12 shows effectiveness of heat removal in hybrid cooling module. The effectiveness of heat exchanger performance was described as theoretical pumping power Pp, divided by transferred heat Q_ c in terms of transferred heat flux density q_ c. Pumping power was expressed as total measured pressure drop in system times volumetric flow rate of water:
_ Pp ¼ VDP
(17)
Ultimately, the system described in the paper is designed to work with fluids such as HFE 7100 in a two-phase flow conditions. Optimal in this respect seems to use HFE 7100 at a normal boiling point of 60 C, which gives hope to obtain the greatest heat flux removal at this temperature.
Fig. 9. Hydraulic Characteristics of tested hybrid microjet e microchannel module.
Fig. 10. Total heat transfer coefficient of hybrid microjet e microchannel module.
6. Conclusions Preliminary experimental data of hybrid microjet e microchannel cooling device were presented. The regime of heat transfer of single-phase convection was explored. The results of this study are of technological importance for the efficient design of cooling systems of personal computers or electronic devices to enhance cooling performance. Obtained data are compared to available “on the market” solutions. During measurements a significant difference in the surface temperature was observed for that of proposed cooling device. This channel-type confinement enabled the circular jets to be used more effectively by increasing the flow rate over the rectangular heated
Fig. 11. Comparison of heat removal of hybrid microjet e microchannel module (þ) with traditional heat sink ().
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T. Muszynski, R. Andrzejczyk / Applied Thermal Engineering xxx (2015) 1e7
7
Subscripts air air b brass c cooling l liquid p pump w wall parameter in inlet to nozzle
References
Fig. 12. Effectiveness of heat removal of hybrid microjet e microchannel module.
surface. For the proposed hybrid cooling module the comparison with predicted values of heat transfer, it is found jet impingement heat transfer is dominant in 90% of total channel length which was 20 mm. Unfortunately, it appears that the heat fluxes above 400 kW/m2 for hybrid module approaches the temperature of above 90 C, thus making it impossible to implement in system with such requirements. Ultimately, the system described in the paper is designed to work with fluids such as HFE 7100 and ethanol in a two-phase flow conditions at a normal boiling point of 60 C. Nomenclature A cp D Eu G h L m_ Nu P Pr Re Q_ T V g v,u
r l
surface area, [m2] specific heat, [J/kg K] jet diameter, [m] Eulers number, [-] mass flow rate, [kg/m2 s] heat transfer coefficient, [W/m2 K] channel length, [m] mass flux, [kg/s] Nusselt number, [-] Power, [W] Prandtl number, [-] Reynolds number, [-] Heat flux, [W] temperature, [K] velocity, [m/s] gravity, [m/s2] velocity components, [m/s] density, [kg/m3] thermal conductivity, [W/mK]
[1] Y. Utaka, S. Okuda, Y. Tasaki, Configuration of the micro-layer and characteristics of heat transfer in a narrow gap mini/micro-channel boiling system, Int. J. Heat Mass Transf. 52 (2009) 2205e2214. [2] T. Alam, P.S. Lee, C.R. Yap, L. Jin, A comparative study of flow boiling heat transfer and pressure drop characteristics in microgap and microchannel heat sink and an evaluation of microgap heat sink for hotspot mitigation, Int. J. Heat Mass Transf. 58 (2013) 335e347. [3] S.V. Garimella, R.A. Rice, Confined and submerged liquid jet impingement heat-transfer, J. Heat Transf. 117 (1995) 871e877. [4] K.I. Choi, R.F. Barron, R.O. Warrington, Fluid Flow and Heat Transfer in Micro Tubes, in: Micromechanical Sensors, Actuators and Systems, ASME, 1991. DSC-Vol. 32. [5] D. Yu, R. Warrington, R. Barron, T. Ameel, An experimental and theoretical investigation of fluid flow and heat transfer in microtubes, in: ASME/JSME Thermal Engineering Conference, vol. 1, ASME, 1995. [6] T.M. Adams, S.I. Abdel-Khalik, S.M. Jeter, Z.H. Qureshi, An experimental investigation of single-phase forced convection in microchannels, Int. J. Heat. Mass Transf. 41 (1998) 851e857. [7] T.M. Adams, S.M. Ghiaasiaan, S.I. Abdel-Khalik, Enhancement of liquid forced convection heat transfer in microchannels due to the release of dissolved noncondensables, Int. J. Heat. Mass Transf. 42 (1999) 3563e3573. [8] G.M. Mala, D.Q. Li, Flow characteristics of water in microtubes, Int. J. Heat. Fluid Flow. 20 (1999) 142e148. [9] G.P. Celata, M. Cumo, M. Guglielmi, G. Zummo, Experimental investigation of hydraulic and single phase heat transfer in 0.130 mm capillary tube, Microscale Thermophys. Eng. 6 (2002) 85e97. [10] D. Mikielewicz, T. Muszynski, Experimental study of heat transfer intensification using microjets, in: Int. Symp. On Convective Heat and Mass Transfer in Sustainable Energy, Hammamet, Tunisia, 2009. [11] S.V. Garimella, Heat transfer and flow fields in confined jet impingement, Annu. Rev. Heat. Transf. 11 (2000) 413e494. [12] Z. Wu, R. Du, Design and experimental study of a miniature vapor compression refrigeration system for electronics cooling, Appl. Therm. Eng. 31 (2011) 385e390. [13] H.Y. Li, S.M. Chao, G.L. Tsai, Thermal performance measurement of heat sinks with confined impinging jet by infrared thermography, Int. J. Heat Mass Transf. 48 (2005) 5386e5394. [14] S. Geedipalli, A.K. Datta, V. Rakesh, Heat transfer in a combination microwaveejet impingement oven, Food Bioprod. Process 86 (2008) 53e63. [15] M.K. Sung, I. Mudawar, Single-phase and two-phase heat transfer characteristics of low temperature hybrid micro-channel/micro-jet impingement cooling module, Int. J. Heat Mass Transf. 51 (2008) 3882e3895. [16] P. Naphon, S. Wongwises, Investigation on the jet liquid impingement heat transfer for the central processing unit of personal computers, Int. Commun. Heat Mass Transf. 37 (2010) 822e826. [17] F.P. Incropera, D.P. DeWitt, Fundamentals of Heat and Mass Transfer, New York, Wiley, 2007. [18] B.N. Taylor, C.E. Kuyatt, Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, National Institute of Standards and Technology Technical Note 1297, 1994. [19] W. Owhaib, Experimental Heat Transfer, Pressure Drop, and Flow Visualization of R134a in Vertical Mini/micro Tube, Royal Institute of Technology, KTH Stockholm, Sweden, 2007 [Phd Thesis]. [20] G.F. Hewitt, G.L. Shires, T.R. Bott, Process Heat Transfer, CRC Press, Begell House, Boca Raton, 1994. [21] D. Chisholm, Two-phase Flow in Pipelines and Heat Exchangers, Godwin, New York, 1983.
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