Multi-microchannel Heat Sink Research Articles

Flow Distribution in Parallel Rectangular Multi Microchannels in Single Phase

This study proposed newly designed inlet manifolds to manage non-uniformity in parallel multi microchannel heat sink by introducing edges with a curved shape were introduced in order to reduce flow-recirculation at the sharp edges. This resulted in a better flow distribution in the parallel channels. A comprehensible numerical study has been performed using ANSYS-Fluent and a three-dimensional computational domain, incorporating the effect of conjugated heat transfer, was employed in this study. R134a was used as the working fluid and copper was selected as the heat sink material. The dimensionless channel flow ratio and flow maldistribution factor were introduced to quantify the flow distribution inside individual channels and the uniformity of this flow distribution. A uniform flow distribution is achieved when the maldistribution factor value approaches 0.

Flow boiling pressure drop characteristics in a multi-microchannel heat sink

We make an effort in this study to experimentally investigate the flow boiling pressure drop characteristics in a multi-microchannel heat sink. For the multi-microchannel heat sink, 27 parallel microfluidic channels are considered. Microchannels are fabricated on the copper block and have a hydraulic diameter of 421 µm and length of 40 mm. We perform experiments considering the refrigerant, considered the working fluid in this study. A high mass flux (G ∼ 400 kg/m2 s–1200 kg/m2 s) for a range of wall heat flux (q′′ ∼ 10 kW/m2–170 kW/m2) and varying degrees of saturation temperature (Tsat ∼ 13 °C, 18 °C, and 23 °C) are considered for the experiments. We demonstrate the correlative effect of the total pressure drop on the two-phase flow pattern in the channel. By depicting the interplay between the total pressure drop (∆p) and the frictional pressure gradient, we show that the frictional pressure gradient increases with the increasing mass flux while it decreases with the increasing degree of saturation temperature. We show the significant impact of the heat flux and saturation temperature on the frictional pressure gradient. In addition, we develop a new correlation for the pressure drop characteristics taking the interference effect of the frictional pressure drop in the multi-microchannel heat sink. The inferences of this experimental study will have far-ranging consequences for the design of heat exchangers, leading to the optimization of microscale thermal management equipment.

Effect of inlet subcooling on flow boiling in microchannels

The paper describes experimental work to assess the effect of inlet subcooling on the flow boiling characteristics of HFE-7200 in a multi-microchannel heat sink. The experiments were performed at a mass flux of 200 kg/m2 s and system pressure of 1 bar for inlet subcooling conditions of 5 K, 10 K and 20 K and wall heat flux between 25.9 and 180.7 kW/m2. The equivalent base heat flux was between 99.1 and 605.3 kW/m2. The microchannel evaporator had 44 channels which were 0.36 mm in width and 0.7 mm in depth. The thickness of the fin between the channels was 0.1 mm and the base area of the microchannels was 20 mm × 20 mm. Increasing inlet subcooling generally decreased two-phase pressure drop in the channels and delayed flow regime transitions in the heat sink. Flow instability was also found to be dependent on the degree of inlet subcooling. The extended subcooled region of the channels at higher degrees of subcooling resulted in lower average heat transfer coefficients in the heat sink, especially at low heat fluxes. The effect of inlet subcooling on local heat transfer coefficients was less pronounced in the saturated boiling region. The study verified the importance of the degree of subcooling when comparing heat transfer and pressure drop characteristics in microchannel heat sinks and when optimising the design of integrated thermal management systems for high heat flux electronic devices.

Experimental Study of Backward Conduction in Multi-Microchannel Heat Sink

In this paper, the effect of conjugate phenomenon on the heat transfer in microchannel heat sinks is experimentally investigated. Circular channels, 18 in number, each having length of (40 ± 0.4) mm and diameter of (400 ± 09) μm were fabricated in rectangular metallic plates. The plates of three different materials, i.e., copper, brass and stainless steel, were used to prepare the heat sinks. Water and ethanol were used as coolants. Heat input during the experimentation ranged from 3 to 25 W. Maranzana number, M, the measure of conjugate phenomenon, was observed to vary from 0.015 to 2.60 in the range of flow rates considered for the coolants. Rise in temperature of the fluid at inlet location was observed under every experimental condition. Under constant heat flux conditions, the temperature of the fluid at the entrance increased up to 20 °C for the conditions indicating extreme values of the parameter ‘M.’ The maximum value of Nusselt number determined from the experimental data was observed to be smaller (2.78) than those from the expected (4.36) for circular channels. This difference is attributed to the phenomenon of longitudinal conduction in microchannels. A correlation for heat transfer in microchannels is suggested taking into account the effect of longitudinal conduction. Regression analysis for the correlation indicated good agreement between the dependent and independent variables.

Experimental investigation on the flow boiling of R134a in a multi-microchannel heat sink

The present study reports on an experimental evaluation of flow boiling of R134a inside a multi-microchannel heat sink. The copper test section consisted of 27 parallel rectangular channels with 0.470mm depth, 0.382mm width, 0.416mm fin thickness and 40mm length. The experiments were performed at saturation temperatures of 18, 23 and 28°C, mass fluxes of 800, 1000 and 1200kgm−2s−1 and constant vapor quality at the heat sink inlet at 0.05. The wall heat flux was increased from 50kWm−2 until reaching critical heat flux, and maximum value of 460kWm−2 was reached. With the collected data from the experiments, the effects of mass flux, heat flux, saturation temperature and steam quality on the heat transfer coefficient are emphasized and the reasons are discussed. At low vapor quality, heat flux plays a major role in increasing the heat transfer coefficient, while the effect of mass flux is negligible. With rising heat flux, convective boiling begins to control the heat transfer mechanism, and the heat transfer coefficient increases with rising vapor quality and mass flux, until dry-out point. In all conditions, high heat transfer coefficients are obtained for high saturation temperatures. In open literature, the correlation generated by Mortada et al. for heat transfer coefficient is the one that makes the most accurate prediction for current study. Based on the experimental results, a correlation is proposed to calculate the heat transfer coefficients for R134a flow boiling inside the multi-microchannel at high mass and heat flux predicting the measured ones better than those in the literature.

Experimental investigation of single-phase turbulent flow of R-134a in a multiport microchannel heat sink

This experimental study aims to investigate the heat transfer characteristics of single-phase turbulent flow of R-134a refrigerant in a rectangular multi-micro channel heat sink having 27 channels where each channel has a hydraulic diameter of 421μm. Experimental results were obtained for inlet temperatures ranging from 24 to 33°C, mass fluxes from 1485 to 2784kgm−2s−1 and wall heat fluxes from 3 to 24kWm−2. The results indicate that the heat transfer coefficients are found to be higher at lower inlet temperatures than those at higher ones. In addition, when equal amount of heat supplied to the heat sink, the heat transfer coefficients increase with increasing the mass flux of refrigerant. They were also compared with 12 well-known correlations and it was seen that 4 of 12 were in good agreement with each other with the average deviation <10%. The findings demonstrate that well-known correlations in fundamental sources can be used to predict the heat transfer coefficient of R-134a during its single phase flow in a multiport microchannel heat sink under turbulent regime.

Experimental evaluation of the thermal performances of a thermosyphon cooling system rejecting heat by natural and forced convection

In the present paper, a closed-loop thermosyphon cooling system for power electronics has been developed and tested. The evaporator is a multi-microchannel heat sink designed for high heat fluxes. The power electronics module is emulated by four independent electrical heaters, which can generate a non-uniform heat flux on the evaporator. The condenser is air-cooled and is tested in both natural convection and forced convection modes. R1234ze has been selected as the working fluid. Although in natural convection mode the cooling system experiences a maximum thermal resistance of approximately 0.6K/W, the complete passiveness makes such a system very attractive. With values of heat fluxes of 107W/cm2 and 25W/cm2 equally distributed on the four heaters, the maximum heater temperature measured is 53°C at a saturation temperature of 45°C. In forced convection mode, two different values of air flow have been tested. A reduction of 50% on the overall thermal resistance has been measured with an imposed air flow rate of 87.8m3/h, consuming only 5.29W to drive the fan for dissipating 70.2W. At this condition, the maximum heater temperature is reduced to 36°C. In summary, it has been demonstrated that a thermosyphon coupled with air cooling can dissipate high heat fluxes of power electronics with no or little energy consumption.

Flow boiling heat transfer of R134a in multi microchannels

Experiments were conducted to investigate flow boiling heat transfer of R134a in a multi microchannel heat sink. The heat sink consisted of 25 rectangular microchannels with nominal dimensions of 300µm wide, 700µm deep (Dh=420µm) and 200µm separating wall thickness. The heat sink was made of oxygen free copper by CNC machining and was 20mm long and 15mm wide. The experiments were conducted at 6.5bar system pressure and covered a footprint area-based heat flux range 11.46–403.1kW/m2 and mass flux range 50–300kg/m2s. A high speed camera was used to capture the flow patterns simultaneously with heat transfer measurements. Three flow patterns were observed namely bubbly, slug and wavy-annular flow when the heat flux increased gradually. The heat transfer coefficient increased with heat flux and there was no mass flux effect. Assessing existing correlations indicated that the correlations of Mahmoud and Karayiannis (2013) and Cooper (1984) predict the data very well with a mean absolute error less than 20% compared to the other correlations.

Experimental CHF for low-GWP fluids and R134a. Effect of the Lh/D ratio at low and high mass velocities

New saturated critical heat flux (CHF) experiments for refrigerants flowing into a multi-microchannel heat sink are presented in this paper. These tests integrate an existing database (Mastrullo et al., 2016) by adding new fluids and operating conditions. The test section consists of an aluminum heat sink heated from the bottom and made up of seven rectangular microchannels, each of them 35mm long and 2mm wide. The possible heights are 1.0 and 0.5mm, whereas the heated lengths are 25 and 35mm, thus obtaining three different Lh/D ratio of 19, 27 and 44, respectively. R134a and the low-GWP refrigerants R1234ze, R1234yf and R32 are used as working fluids. The mass flux was varied from 146 to 1504kg/(m2s) and tests were performed at saturation temperatures from 24.7 to 75.5°C. With a new operative definition of critical heat flux, the experimental results show that CHF is strongly enhanced with higher mass fluxes, while it is only slightly influenced by the saturation temperature. The effect of Lh/D is a CHF reduction at low mass velocities (G<500kg/(m2s)), while the trend is reversed for high mass velocities. Finally, the agreement of the present database with some of the most quoted CHF correlations is also tested. The comparison results show that the correlation of Callizo (2010) best fits the whole database and also the low-G data, whereas the predictive methods of Ong and Thome (2011) and Wojtan et al. (2006) work better at higher mass fluxes.

An experimental study on two-phase flow patterns and heat transfer characteristics during boiling of R134a flowing through a multi-microchannel heat sink

The present paper presents an experimental study of flow patterns and heat transfer characteristics of flow boiling of R134a in a multi-microchannel heat sink. The copper test section has 27 parallel rectangular channels with a depth of 470μm, a width of 382μm, a length of 40mm, and a fin thickness of 416μm. The experimental results are presented for saturation temperatures of 13, 18, and 23°C, and mass fluxes of 150, 400, and 600kg/m2s. The wall heat flux and inlet vapor quality values were between 3 and 127kW/m2, and 0.05 and 0.92, respectively. The effects of pertinent parameters on the heat and fluid flow characteristics such as saturation temperature, mass flux, heat flux, and inlet vapor quality are studied and discussed. The heat transfer coefficient at high saturation temperatures (i.e. 23°C) is higher than low saturation temperatures (i.e. 13°C) in the heat flux range of 40–120kW/m2. For high heat flux ranges, the heat transfer coefficient increases with increasing mass flux. The convective boiling heat transfer mechanism will play a major role in wavy and annular flow patterns. For wall heat fluxes higher than 80kW/m2, the existence of a partial dry-out phenomenon in the multi-microchannel leads to a decrease in heat transfer coefficient. The results unveil the significant effect of flow patterns on heat transfer characteristics. Based on the experimental data, a correlation is proposed to calculate the heat transfer coefficient for R134a flow in the multi-microchannel heat sink that is useful in electronic cooling applications.

Design, fabrication and test of an integrated multi-microchannel heat sink for electronics cooling

In this paper, MEMS fabrication techniques are used to assemble a micro device which integrates a simulated heat source, a multi-microchannel heat sink and flow sensors.The front side of the silicon chip comprises one 1000nm-thick Aluminum heater and six 400Å-thick Ruthenium thermal sensors microfabricated to achieve precise and interface-free heat flux imposition and temperature measurements, respectively. The heat sink on the backside is made of 27 parallel rectangular channels with hydraulic diameter Dh=96μm, length to diameter ratio L/Dh=104 and relative roughness of 2.4%. The channels are closed with glass to allow visualization of the flow, and sealed by anodic bonding, instead of using adhesives, thermal fusion or mechanical sealing, in order to guarantee resistant sealing over the entire range of operating pressures and temperatures while still preserving the depth of the microchannels.Primary fabrication tests are performed to evaluate the influence of geometrical parameters (thin film width and length) in thermal response and joule heating performance for surface temperatures in the range from 25°C to 106°C obtained with currents from 0mA to 250mA. The temperature coefficient of electrical resistivity, TCR, of the Ru thermal sensors is found to be (1.72±0.45)×10−3K−1. The thermo-fluid-dynamic performance of the device is assessed with hydrofluoroether coolant (HFE-7000) with Reynolds numbers up to 640 (mass flux up to 2727kgm−2s−1) and constant wall heat fluxes q″ up to 10.8Wcm−2. Preliminary tests have shown that, without boiling, the temperature of the chip can be kept below 85°C (Tw,avg=65.4°C) with a maximum heat flux of 10.8Wcm−2 and a flow rate of 3050mLh−1. Boiling of the coolant fluid allows to increase the heat flux up to 26.6Wcm−2 with a flow rate of 1300mLh−1 (Re=277) with perfect robustness and reliability of the system.

Flow Boiling of R134a in a Multi-Microchannel Heat Sink With Hotspot Heaters for Energy-Efficient Microelectronic CPU Cooling Applications

This paper focuses on two-phase flow boiling of refrigerant R134a inside a copper multi-microchannel heat sink for microelectronic central processing unit cooling applications. The heat sink is composed of 100 parallel microchannels, 100 μm wide, 680 μm high, and 15 mm long, with 72-μm-thick fins separating the channels. The base heat flux was varied from 2.57 to 189 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and the mass flux from 205 to 1000 kg/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> s, at a nominal saturation temperature of 63°C. Over 40 000 local heat transfer coefficients were measured at 35 locations using local heaters and temperature sensors, for which different heat transfer trends were identified. The main ones were that the heat transfer coefficient increased with heat flux and was independent of mass flow rate. Heat transfer coefficients as high as 270 000 W/m K (relative to the base area) were reached, keeping the chip under 85°C with a maximum of 94 kPa of pressure drop, for no inlet subcooling and a coolant flow rate of 1000 kg/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> s.

Saturated critical heat flux in a multi-microchannel heat sink fed by a split flow system

An extensive experimental campaign has been carried out for the measurement of saturated critical heat flux in a multi-microchannel copper heat sink. The heat sink was formed by 29 parallel channels that were 199 μm wide and 756 μm deep. In order to increase the critical heat flux and reduce the two-phase pressure drop, a split flow system was implemented with one central inlet at the middle of the channels and two outlets at either end. The base critical heat flux was measured using three HFC Refrigerants (R134a, R236fa and R245fa) for mass fluxes ranging from 250 to 1500 kg/m 2 s, inlet subcoolings from −25 to −5 K and saturation temperatures from 20 to 50 °C. The parametric effects of mass velocity, saturation temperature and inlet subcooling were investigated. The analysis showed that significantly higher CHF was obtainable with the split flow system (one inlet–two outlets) compared to the single inlet–single outlet system, providing also a much lower pressure drop. Notably several existing predictive methods matched the experimental data quite well and quantitatively predicted the benefit of higher CHF of the split flow.

High Heat Flux Two-Phase Cooling in Silicon Multimicrochannels

This paper presents performances of two-phase cooling of a chip at very high heat flux with refrigerant R236fa in a silicon multimicrochannel heat sink. This heat sink was composed of 134 parallel channels, 67 mum wide, 680 mum high, and 20 mm long, with 92- mum -thick fins separating the channels. The base heat flux was varied from 3 to 255 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , the volume flow rate from 0.18 to 0.67 I/min, and the exit vapor quality from 0 to 80%. The working pressure and saturation temperature were set at 273 kPa and 25 degC, respectively. The present database includes 1040 local heat transfer coefficients. The base temperature of the chip could be maintained below 52 degC while dissipating 255 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with 10 degC of inlet subcooling and 90 kPa of pressure drop. A comparison of the respective performances with an extrapolation of the present results shows that two-phase cooling should be able to cool the chip 13 K lower than liquid cooling for the same pumping power at a base heat flux of 350 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

High heat flux flow boiling in silicon multi-microchannels – Part III: Saturated critical heat flux of R236fa and two-phase pressure drops

New experimental critical heat flux results for saturated boiling conditions have been obtained for R236fa flowing in a silicon multi-microchannel heat sink composed of 67 parallel channels, 223 μm wide, 680 μm high and with 80 μm thick fins separating the channels. The microchannel length was 20 mm. The footprint critical heat fluxes measured varied from 112 to 250 W/cm 2 and the wall critical heat fluxes from 21.9 to 52.2 W/cm 2 for mass velocities from 276 to 992 kg/m 2s. When increasing the mass velocity, the wall critical heat flux was observed to increase. The inlet saturation temperatures (20.31 ⩽ T sat,in ⩽ 34.27 °C) and the inlet subcoolings (0.4 ⩽ Δ T sub ⩽ 15.3 K) were found to have a negligible influence on the saturated CHF. The best methods for predicting the data were those of Wojtan et al. [L. Wojtan, R. Revellin, J. R. Thome, Investigation of critical heat flux in single, uniformly heated microchannels, Exp. Therm. Fluid Sci. 30 (2006) 765–774] and Revellin and Thome [R. Revellin, J. R. Thome, A theoretical model for the prediction of the critical heat flux in heated microchannels, Int. J. Heat Mass Transfer 50 (in press)]. They both predict the experimental CHF results with a mean absolute error of around 9%. Using the critical vapour quality, an annular-to-dryout transition is also proposed as a limit in a diabatic microscale flow pattern map. Pressure drop measurements were measured and analysed, showing that the homogeneous model could correctly predict the observed trends.

High heat flux flow boiling in silicon multi-microchannels – Part I: Heat transfer characteristics of refrigerant R236fa

This article is the first in a three part study on flow boiling of refrigerants R236fa and R245fa in a silicon multi-microchannel heat sink. The heat sink was composed of 67 parallel channels, which are 223 μm wide, 680 μm high and 20 mm long with 80 μm thick fins separating the channels. The base heat flux was varied from 3.6 to 221 W/cm 2, the mass velocity from 281 to 1501 kg/m 2 s and the exit vapour quality from 2% to 75%. The working pressure and saturation temperature were set nominally at 273 kPa and 25 °C, respectively. The present database includes 1217 local heat transfer coefficient measurements, for which three different heat transfer trends were identified, but in most cases the heat transfer coefficient increased with heat flux and was almost independent of vapour quality and mass velocity. Importantly, it was found for apparently the first time that the heat transfer coefficient as a function of vapour quality reaches a maximum at very high heat fluxes and then decreases with further increase of heat flux.

High heat flux flow boiling in silicon multi-microchannels – Part II: Heat transfer characteristics of refrigerant R245fa

This article is the second in a three-part study. This second part focuses on flow boiling heat transfer of refrigerant R245fa in a silicon multi-microchannel heat sink and their comparison with the results presented in part I for refrigerant R236fa. This heat sink was the same as utilized in part I. The test conditions covered base heat fluxes from 3.6 to 190 W/cm 2, mass velocities from 281 to 1501 kg/m 2 s and the exit vapour qualities from 0% to 78%. The effect of saturation pressure on heat transfer was tested from 141 to 273 kPa for R245fa and the effect of sub-cooling from 0 to 19 K. The R245fa database includes 693 local heat transfer coefficient measurements, for which four different heat transfer trends were identified, although in most cases the heat transfer coefficient increased with heat flux, was almost independent of vapour quality and increased with mass velocity. The entire database, including both R245fa and R236fa measurements, was compared with four prediction methods for flow boiling heat transfer in microchannels. The three-zone model of Thome et al. (J.R. Thome, V. Dupont, A.M. Jacobi, Heat transfer model for evaporation in microchannels. Part I: presentation of the model, International J. Heat Mass Transfer 47 (2004) 3375–3385) was found to give the best predictions, capturing 90% of the data within ±30% in the slug and annular flow regimes ( x > 5%).