Spray Cooling of High Aspect Ratio Open Microchannels

2007 ◽  
Vol 129 (8) ◽  
pp. 1052-1059 ◽  
Author(s):  
Johnathan S. Coursey ◽  
Jungho Kim ◽  
Kenneth T. Kiger
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Thermal Performance ◽  
High Aspect Ratio ◽  
Flat Surface ◽  
Spray Cooling ◽  
Heat Sinks ◽  
Projected Area ◽  
Two Phase ◽  
Flux Flow

Direct spraying of dielectric liquids has been shown to be an effective method of cooling high-power electronics. Recent studies have illustrated that even higher heat transfer can be obtained by adding extended structures, particularly straight fins, to the heated surface. In the current work, spray cooling of high-aspect-ratio open microchannels was explored, which substantially increases the total surface area and allows more residence time for the incoming liquid to be heated by the wall. Five such heat sinks were constructed, and their thermal performance was investigated. These heat sinks featured a projected area of 1.41×1.41cm2, channel width of 360μm, a fin width of 500μm, and fin lengths of 0.25mm, 0.50mm, 1.0mm, 3.0mm, and 5.0mm. The five enhanced surfaces and a flat surface with the same projected area were sprayed with a full cone nozzle using PF-5060 at 30°C and nozzle pressure differences from 1.36–4.08atm(69–121ml∕min). In all cases, the enhanced surfaces improved thermal performance compared to the flat surface. Longer fins were found to outperform shorter ones in the single-phase regime. Adding fins also resulted in the onset of two-phase effects (and higher-heat transfer) at lower wall temperatures than the flat surface. The two-phase regime was characterized by a balance between added area, changing flow flux, flow channeling, and added conduction resistance. Spray efficiency calculations indicated that a much larger percentage of the liquid sprayed onto the enhanced surface evaporated than with the flat surface. Fin lengths between 1mm and 3mm appeared to be optimum for heat fluxes as high as 124W∕cm2 (based on projected area) and the range of conditions studied.

Spray Cooling of Small-Pitched, Straight-Finned, Copper Heat Sinks

2005 ◽  
Author(s):  
Johnathan S. Coursey ◽  
Jungho Kim ◽  
Kenneth T. Kiger
Keyword(s):  
Single Phase ◽  
Flat Surface ◽  
Heated Surface ◽  
Spray Cooling ◽  
Heat Fluxes ◽  
Heat Sinks ◽  
Lower Wall ◽  
Projected Area ◽  
Two Phase ◽  
Flow Flux

Spraying a dielectric liquid such as PF-5060 (95% pure FC-72) has been shown to be an effective method of cooling high power electronics. Recent studies have illustrated the potential enhancement of spray cooling by the addition of extended structures, particularly straight fins, to the heated surface. In the current work, these studies are extended to finer fin widths and pitches and longer fin lengths. Four such heat sinks were EDM wire machined. These 1.41 × 1.41 cm2 heat sinks featured a fin pitch of 0.86 mm; a fin width of 0.5 mm; and fin lengths of 0.5 mm, 1 mm, 3 mm, and 5 mm, which substantially increase the total area, allowing more residence time for the incoming liquid to be heated by the wall. The four enhanced surfaces and a flat surface with the same projected area were sprayed with a full cone nozzle using PF-5060 at 96 mL/min, 24°C, and 3.65 atm (38.5 psig). In all cases, the enhanced surfaces improved thermal performance. Longer fins were found to outperform shorter ones in the single-phase regime. Adding fins also resulted in two-phase effects and higher heat transfer at lower wall temperatures than the flat surface. Finally, the two-phase regime appeared to be marked by a balance between added area, changing flow flux, channeling, and added conduction resistance. Although critical heat flux (CHF) was not reached for the finned surfaces, fin lengths between 1–3 mm appeared to be optimum for heat fluxes as high as 131 W/cm2 and the range of conditions studied.

Effect of Microgrooved Surface and Surface Orientation on Spray Cooling Heat Transfer

2009 ◽  
Author(s):  
Dong-Fang Chen ◽  
Da-Wei Tang ◽  
Xue-Gong Hu
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Thermal Performance ◽  
High Speed ◽  
Heat Transfer Performance ◽  
Flat Surface ◽  
Spray Cooling ◽  
Surface Orientation ◽  
Microgrooved Surface ◽  
Water Spray Cooling

Experiments were performed to investigate the flow structure and boiling heat transfer characteristics of water spray cooling on flat and microgrooved surfaces using a high-speed camera and a microscope. The heaters were made of cooper, with surface size of 2.0cm×7.4cm. Three orientations of the heater surfaces were selected: horizontal upward-facing, vertical, and horizontal downward-facing. A full-cone spray nozzle was placed normal to these heated surfaces. The heat transfer was directly measured using thermocouples within the heater. The experimental results show the bubble’s growth, coalescence along/between microgrooves, and break-up as wall heat flux reaches some higher values. It was found that the heat transfer for microgrooved surface is generally higher than that of flat surface at a given flow rate with the same surface orientation. The thermal performance of vertical microgrooved surface was highest at low temperatures; the thermal performance of the horizontal upward-facing was highest at higher wall temperature. The heat transfer performance for the horizontal downward-facing microgrooved surface had the highest critical heat flux (CHF).

Numerical Study of Gas-Liquid Two-Phase Flow in Ultra-High-Aspect-Ratio Microchannel With Capillary-Structured Wall

2019 ◽  
Author(s):  
Xiang Mei ◽  
Zhenyu Liu ◽  
Huiying Wu
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
High Aspect Ratio ◽  
Two Phase Flow ◽  
Flow Behavior ◽  
Phase Flow ◽  
Two Phase ◽  
Chip Cooling ◽  
Evaporation Heat ◽  
Evaporation Heat Transfer

Abstract The understanding of the liquid-gas flow and heat transfer in the high-aspect-ratio microchannel is very important to realize the high-efficiency phase change chip cooling. In this work, a novel ultra-high-aspect-ratio microchannel with capillary-structured wall was developed to enhance the evaporation heat transfer in microchannel, in which the capillary grooves on the side walls (capillary-structured wall) were designed to avoid the dryout phenomenon. A three-dimensional VOF model was established to predict the immiscible gas-liquid flow in microchannel. The influences of wettability of capillary grooves on the gas-liquid two-phase flow behavior in microchannel were investigated based on the numerical predictions. The slug bubble can be observed for different inlet flow conditions. Variation of pressure loss between inlet and outlet of microchannel with time were studied for different flow rates and gas-liquid ratios. The results show that the existence of capillary structured wall has a significant influence on the liquid-gas two-phase flow behavior in the microchannel. The liquid flow in microgrooves is driven by the capillary force, which can supply more liquid to the side wall to promote the evaporation heat transfer process. The design of capillary-structured wall for ultra-high-aspect-ratio microchannel in this work provides a new approach to improve the performance of the chip cooling technique with microchannels.

Two-Phase Flow and Heat Transfer in Rectangular Micro-Channels

2003 ◽  
Author(s):  
Weilin Qu ◽  
Seok-Mann Yoon ◽  
Issam Mudawar
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Flow Pattern ◽  
Two Phase Flow ◽  
Flow Patterns ◽  
Heat Sinks ◽  
Phase Flow ◽  
Micro Channel ◽  
Two Phase ◽  
Micro Channels

Knowledge of flow pattern and flow pattern transitions is essential to the development of reliable predictive tools for pressure drop and heat transfer in two-phase micro-channel heat sinks. In the present study, experiments were conducted with adiabatic nitrogen-water two-phase flow in a rectangular micro-channel having a 0.406 × 2.032 mm cross-section. Superficial velocities of nitrogen and water ranged from 0.08 to 81.92 m/s and 0.04 to 10.24 m/s, respectively. Flow patterns were first identified using high-speed video imaging, and still photos were then taken for representative patterns. Results reveal that the dominant flow patterns are slug and annular, with bubbly flow occurring only occasionally; stratified and churn flow were never observed. A flow pattern map was constructed and compared with previous maps and predictions of flow pattern transition models. Annual flow is identified as the dominant flow pattern for conditions relevant to two-phase micro-channel heat sinks, and forms the basis for development of a theoretical model for both pressure drop and heat transfer in micro-channels. Features unique to two-phase micro-channel flow, such as laminar liquid and gas flows, smooth liquid-gas interface, and strong entrainment and deposition effects are incorporated into the model. The model shows good agreement with experimental data for water-cooled heat sinks.

Heat Transfer in a Rotating, Two-Pass, Variable Aspect Ratio Cooling Channel With Profiled V-Shaped Ribs

2020 ◽  
Author(s):  
I-Lun Chen ◽  
Izzet Sahin ◽  
Lesley M. Wright ◽  
Je-Chin Han ◽  
Robert Krewinkel
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Heat Transfer Enhancement ◽  
Thermal Performance ◽  
Channel Modeling ◽  
Cooling Channel ◽  
Transfer Enhancement ◽  
Rotation Numbers ◽  
First Pass ◽  
Rib Turbulators

Abstract The thermal performance of two V-type rib configurations is measured in a rotating, two-pass cooling channel. Modeling modern, high pressure, turbine blades, the cross-section of the cooling channel varies from the first pass to the second pass. The coolant travels radially outward in the rectangular first pass with an aspect ratio of 4:1. Near the tip region, the coolant turns 180°, and travels radially inward in a 2:1 rectangular channel. The serpentine passage is positioned such that both the first and second passes are oriented 90° to the direction of rotation. The leading and trailing surfaces of both the first and second pass of the channel are roughened with V-type rib turbulators. The thermal performance of two V-type configurations is measured in this two-pass channel. The first V-shaped configuration is similar to a traditional V-shaped turbulator with a narrow gap at the apex of the V. The configuration is modified by off-setting one leg of the V to create a staggered discrete, V-shaped configuration. The ribs are oriented 45° relative to the streamwise coolant direction. In both passes, the rib spacing is P/e = 10 and the rib height – to – channel height is e/H = 0.16. The heat transfer enhancement and frictional losses are measured for both rib configurations with varying Reynolds and rotation numbers. The Reynolds number varies from 10,000 to 45,000 in the AR = 4:1 first pass; this corresponds to 16,000 to 73,500 in the AR = 2:1 second pass. Considering the effect of rotation, the rotational speed of the channel varies from 0–400 rpm with maximum rotation numbers of 0.39 and 0.16 in the first and second passes, respectively. The heat transfer enhancement on both the leading and trailing surfaces of the first pass of the 45° V-shaped channel is slightly reduced with rotation. In the second pass, the heat transfer increases on the leading surface while it decreases on the trailing surface. The 45° staggered, discrete V-shaped ribs provide increased heat transfer and thermal performance compared to the traditional V-shaped and standard, 45° angled rib turbulators.

Experimental Investigation of Heat Transfer in Impingement Air Cooled Plate Fin Heat Sinks

2005 ◽  
Vol 128 (4) ◽  
pp. 412-418 ◽  
Author(s):  
Zhipeng Duan ◽  
Y. S. Muzychka
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Thermal Resistance ◽  
Thermal Performance ◽  
Heat Transfer Model ◽  
Heat Sinks ◽  
Transfer Model ◽  
Transfer Coefficients ◽  
Developing Flow ◽  
Rectangular Channels

Impingement cooling of plate fin heat sinks is examined. Experimental measurements of thermal performance were performed with four heat sinks of various impingement inlet widths, fin spacings, fin heights, and airflow velocities. The percent uncertainty in the measured thermal resistance was a maximum of 2.6% in the validation tests. Using a simple thermal resistance model based on developing laminar flow in rectangular channels, the actual mean heat transfer coefficients are obtained in order to develop a simple heat transfer model for the impingement plate fin heat sink system. The experimental results are combined into a dimensionless correlation for channel average Nusselt number Nu∼f(L*,Pr). We use a dimensionless thermal developing flow length, L*=(L∕2)∕(DhRePr), as the independent parameter. Results show that Nu∼1∕L*, similar to developing flow in parallel channels. The heat transfer model covers the practical operating range of most heat sinks, 0.01<L*<0.18. The accuracy of the heat transfer model was found to be within 11% of the experimental data taken on four heat sinks and other experimental data from the published literature at channel Reynolds numbers less than 1200. The proposed heat transfer model may be used to predict the thermal performance of impingement air cooled plate fin heat sinks for design purposes.

Two-Phase Heat Sinks With Microporous Coating

2011 ◽  
Author(s):  
Tadej Semenic ◽  
Seung M. You
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Heat Sink ◽  
Heat Sinks ◽  
Porous Coating ◽  
Two Phase ◽  
Small Channel ◽  
Rectangular Channels ◽  
Microporous Coating ◽  
Channel Aspect Ratio

To minimize flow boiling instabilities in two-phase heat sinks, two different types of microporous coatings were developed and applied on mini- and small-channel heat sinks and tested using degassed R245fa refrigerant. The first coating was epoxy-based and was sprayed on heat sink channels while the second coating was formed by sintering copper particles on heat sink channels. Mini-channel heat sinks had overall dimensions 25.4 mm × 25.4 mm × 6.4 mm and twelve rectangular channels with a hydraulic diameter 1.7 mm and a channel aspect ratio of 2.7. Small-channel heat sinks had the same overall dimensions, but only three rectangular channels with hydraulic diameter 4.1 mm and channel aspect ratio 0.6. The microporous coatings were found to minimize parallel channel instabilities for mini-channel heat sinks and to reduce the amplitude of heat sink base temperature oscillations from 6 °C to slightly more than 1 °C. No increase in pressure drop or pumping power due to the microporous coating was measured. The mini-channel heat sinks with porous coating had in average 1.5-times higher heat transfer coefficient than uncoated heat sinks. Also, the small-channel heat sinks with the “best” porous coating had in average 2.5-times higher heat transfer coefficient and the critical heat flux was 1.5 to 2-times higher compared with the uncoated heat sinks.

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