scholarly journals Orientation Effects in Two-Phase Microgap Flow

2019 ◽  
Vol 141 (3) ◽  
Author(s):  
Franklin L. Robinson ◽  
Avram Bar-Cohen
Keyword(s):  
Technology Development ◽  
Flow Boiling ◽  
Heat Fluxes ◽  
Forced Flow ◽  
Management Technique ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Aerospace Applications ◽  
Dielectric Fluids ◽  
Mass Fluxes

The high power density of emerging electronic devices is driving the transition from remote cooling, which relies on conduction and spreading, to embedded cooling, which extracts dissipated heat on-site. Two-phase microgap coolers employ the forced flow of dielectric fluids undergoing phase change in a heated channel within or between devices. Such coolers must work reliably in all orientations for a variety of applications (e.g., vehicle-based equipment), as well as in microgravity and high-g for aerospace applications, but the lack of acceptable models and correlations for orientation- and gravity-independent operation has limited their use. Reliable criteria for achieving orientation- and gravity-independent flow boiling would enable emerging systems to exploit this thermal management technique and streamline the technology development process. As a first step toward understanding the effect of gravity in two-phase microgap flow and transport, in the present effort the authors have studied the effect of evaporator orientation, mass flux, and heat flux on flow boiling of HFE7100 in a 1.01 mm tall × 13.0 mm wide × 12.7 mm long microgap channel. Orientation-independence, defined as achieving similar critical heat fluxes (CHFs), heat transfer coefficients (HTCs), and flow regimes across orientations, was achieved for mass fluxes of 400 kg/m2 s and greater (corresponding to a Froude number of about 0.8). The present results are compared to published criteria for achieving orientation- and gravity-independence.

scholarly journals Orientation Effects in Two-Phase Microgap Flow

2018 ◽  
Author(s):  
Franklin L. Robinson ◽  
Avram Bar-Cohen
Keyword(s):  
Technology Development ◽  
Flow Boiling ◽  
Flow Regimes ◽  
Heat Fluxes ◽  
Forced Flow ◽  
Management Technique ◽  
Gravitational Acceleration ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Dielectric Fluids

The high power density of emerging electronic devices is driving the transition from remote cooling, which relies on conduction and spreading, to embedded cooling, which extracts dissipated heat on-site. Two-phase microgap coolers employ the forced flow of dielectric fluids undergoing phase change in a heated channel within or between devices. Such coolers must work reliably in all orientations for a variety of applications (e.g., vehicle-based equipment), as well as in microgravity and high-g for other applications (e.g., spacecraft and aircraft). The lack of acceptable models and correlations for orientation- and gravity-independent operation has limited the use of two-phase coolers in such applications. Previous research has revealed that gravitational acceleration plays a diminishing role in establishing flow regimes and transport rates as the channel size shrinks, but there is considerable variation among the proposed microscale criteria and limited research on two-phase flows in low aspect ratio microgap channels. Reliable criteria for achieving orientation- and gravity-independent flow boiling would enable emerging systems to exploit this thermal management technique and streamline the technology development process. As a first step toward understanding the effect of gravity on two-phase microgap flow and transport, in the present effort the authors have studied the effect of evaporator orientation and mass flux on near-saturated flow boiling of HFE7100 in a 1.01 mm tall by 13.0 mm wide by 12.7 mm long microgap channel. Orientation-independence, defined as achieving similar critical heat fluxes, heat transfer coefficients, and flow regimes across evaporator orientations, was achieved for mass fluxes of 400 kg/m2-s and greater. The present results are compared to published criteria for achieving gravity-independence.

Flow Boiling of R134a in Circular Microtubes—Part I: Study of Heat Transfer Characteristics

2011 ◽  
Vol 133 (5) ◽  
Author(s):  
Saptarshi Basu ◽  
Sidy Ndao ◽  
Gregory J. Michna ◽  
Yoav Peles ◽  
Michael K. Jensen
Keyword(s):  
Heat Transfer ◽  
Flow Boiling ◽  
Saturation Pressure ◽  
Heat Fluxes ◽  
Average Error ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Heat Transfer Coefficients ◽  
Mass Fluxes ◽  
Two Phase Heat Transfer

An experimental study of two-phase heat transfer coefficients was carried out using R134a in uniformly heated horizontal circular microtubes with diameters from 0.50 mm to 1.60 mm over a range of mass fluxes, heat fluxes, saturation pressures, and vapor qualities. Heat transfer coefficients increased with increasing heat flux and saturation pressure but were independent of mass flux. The effects of vapor quality on heat transfer coefficients were less pronounced and varied depending on the quality. The data were compared with seven flow boiling correlations. None of the correlations predicted the experimental data very well, although they generally predicted the correct trends within limits of experimental error. A correlation was developed, which predicted the heat transfer coefficients with a mean average error of 29%. 80% of the data points were within the ±30% error limit.

Reduced Pressure Boiling Heat Transfer in Rectangular Microchannels With Interconnected Reentrant Cavities

2005 ◽  
Vol 127 (10) ◽  
pp. 1106-1114 ◽  
Author(s):  
Ali Koşar ◽  
Chih-Jung Kuo ◽  
Yoav Peles
Keyword(s):  
Heat Transfer ◽  
Heat Fluxes ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Convective Boiling ◽  
Reduced Pressure ◽  
Exit Pressure ◽  
Mass Fluxes ◽  
Mechanism Transition ◽  
Two Phase Heat Transfer

Boiling flow of deionized water through 227μm hydraulic diameter microchannels with 7.5μm wide interconnected reentrant cavities at 47 kPa exit pressure has been investigated. Average two-phase heat transfer coefficients have been obtained over effective heat fluxes ranging from 28 to 445W∕cm2 and mass fluxes from 41 to 302kg∕m2s. A map is developed that divides the data into two regions where the heat transfer mechanisms are nucleation or convective boiling dominant. The map is compared to similar atmospheric exit pressure data developed in a previous study. A boiling mechanism transition criterion based on the Reynolds number and the Kandlikar k1 number is proposed.

Flow Boiling in Microchannels

2005 ◽  
Author(s):  
Kathleen H. Peters ◽  
Francis A. Kulacki
Keyword(s):  
Flow Boiling ◽  
Heat Fluxes ◽  
Nickel Plate ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Convective Boiling ◽  
Data Set ◽  
Heat Transfer Coefficients ◽  
Micro Channels ◽  
Constant Wall Heat Flux

Experiments are reported on convective boiling of water in a system of parallel micro-channels with a constant wall heat flux and highly sub-cooled inlet flow. The test section comprises a nickel plate containing ∼388 micro-channels along the center-plane with a mean hydraulic diameter of 145 μm with and a mean separation of 35 μm. The data set spans wall heat fluxes from 94 to 152 kW/m2, 8 < G < 24 kg/m2-s, and 2 < Re < 40. These parameters produce Weber, capillary and boiling numbers one to two orders of magnitude below those of the current published database. Overall heat transfer coefficients in flow boiling are estimated in the range 65 to 325 kW/m2-K. The present experiments more nearly emulate conditions for practical micro-channel heat exchangers compared to those of reported studies using artificially induced two-phase flows and either one or several micro-channels.

Evaporation of Ammonia in a Smooth Horizontal Tube: Heat Transfer Measurements and Predictions

1999 ◽  
Vol 121 (1) ◽  
pp. 89-101 ◽  
Author(s):  
O. Zu¨rcher ◽  
J. R. Thome ◽  
D. Favrat
Keyword(s):  
Heat Transfer ◽  
Flow Boiling ◽  
Saturation Temperature ◽  
Local Heat Transfer ◽  
Horizontal Tube ◽  
Steel Tube ◽  
Heat Fluxes ◽  
Stainless Steel Tube ◽  
Transfer Coefficients ◽  
Two Phase

Experimental test results for flow boiling of pure ammonia inside horizontal tubes were obtained for a plain stainless steel tube. Tests were run at a nominal saturation temperature of 4°C, nine mass velocities from 20–140 kg/m2 s, vapor qualities from 1–99 percent and heat fluxes from 5–58 kW/m2. Two-phase flow observations showed that the current test data covered the following regimes: fully stratified, stratified-wavy, intermittent, annular, and annular with partial dryout. The Kattan-Thome-Favrat flow boiling model accurately predicted the local heat transfer coefficients measured in all these flow regimes with only two small modifications to their flow map (to extend its application to G < 100 kg/m2 s). Their flow boiling model was also successfully compared to the earlier ammonia flow boiling data of Chaddock and Buzzard (1986). The Gungor-Winterton (1987) correlation instead gave very poor accuracy for ammonia.

Flow Boiling of Coolant (HFE-7000) Inside Structured and Plain Wall Microchannels

2009 ◽  
Vol 131 (12) ◽  
Author(s):  
C.-J. Kuo ◽  
Y. Peles
Keyword(s):  
Heat Transfer ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Mass Fluxes ◽  
Side Walls ◽  
Parallel Microchannels ◽  
The Heat Transfer Coefficient

Flow boiling was experimentally studied using coolant HFE-7000 for two types of parallel microchannels: a plain-wall microchannel and a microchannel with structured reentrant cavities on the side walls. Flow morphologies, boiling inceptions, heat transfer coefficients, and critical heat fluxes were obtained and studied for mass fluxes ranging from G=164 kg/m2 s to G=3025 kg/m2 s and mass qualities (energy definition) ranging from x=−0.25 to x=1. Comparisons of the performance of the enhanced and plain-wall microchannels were carried out. It was found that reentrant cavities were effective in reducing the superheat at the onset of nucleate boiling and increasing the heat transfer coefficient. However, they did not seem to increase the critical heat flux.

Fabrication of Silicon Microchannel With Integrated Heater and Temperature Sensors for Flow Boiling Studies

2008 ◽  
Author(s):  
S. Gedupudi ◽  
G. P. Cummins ◽  
H. Lin ◽  
A. J. Walton ◽  
K. Sefiane ◽  
...  
Keyword(s):  
Cross Sections ◽  
Heat Dissipation ◽  
Flow Boiling ◽  
Heat Fluxes ◽  
Heat Sinks ◽  
Single Channels ◽  
Two Phase ◽  
Mass Fluxes ◽  
Wall Superheat ◽  
Uniform Wall

Two-phase microchannel heat sinks are a promising solution to meet the requirements for cooling electronic components with high-density heat dissipation. However, their design requires a thorough understanding of flow boiling and pressure drop in microchannels. The channels described in this paper have been fabricated in silicon, with rectangular cross-sections ranging in hydraulic diameter between 0.62 and 0.1 mm, for studies of boiling in single channels. To facilitate visualisation, the top of each channel is covered with Pyrex 7740, predrilled for fluid inlet and outlet connections. Integrated tantalum resistors are located uniformly along the bottom of the channel for temperature sensing. Tantalum pentoxide and PECVD silicon dioxide (which also conformally coats the channel walls) are used to electrically insulate the sensor from any liquid in the channel. The heater is an integrated aluminium serpentine track on the back of the bottom wafer. The channel is etched down to the sensors on the bonded bottom silicon wafer using the Bosch process. The objective related to the development of these silicon microchannels is to achieve heat fluxes of 2 MW m−2 with low, near-uniform wall superheat (by means of bubble triggering and artificial nucleation sites). Experiments will be carried out with mass fluxes varying from 100 to 500 kg m−2 s−1, using de-ionized water and an organic fluid as the working fluids.

Flow Boiling Heat Transfer Enhancement from Carbon Nanotube-Enhanced Surfaces

2014 ◽  
Vol 348 ◽  
pp. 20-26
Author(s):  
I. Pranoto ◽  
C. Yang ◽  
L.X. Zheng ◽  
K.C. Leong ◽  
P.K. Chan
Keyword(s):  
Heat Transfer ◽  
Carbon Nanotube ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Aluminum Surface ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Heat Transfer Coefficients ◽  
Mass Fluxes ◽  
Flow Boiling Heat Transfer

This paper presents an experimental study of flow boiling heat transfer from carbon nanotube (CNT) structures in a two-phase cooling facility. Multi-walled CNT (MWCNT) structures of dimensions 80 mm × 60 mm were applied to a horizontal flow boiling channel. Two CNT structures with different properties viz. NC-3100 and MERCSD were tested with a dielectric liquid FC-72. The height of the CNT structures was fixed at 37.5 μm and tests were conducted at coolant mass fluxes of 35, 50, and 65 kg/m2·s under saturated flow boiling conditions. The experimental results show that the CNT structures enhance the boiling heat transfer coefficients by up to 1.6 times compared to the smooth aluminum surface. The results also show that the CNT structures increase significantly the Critical Heat Flux (CHF) of the smooth aluminum surface from 66.7 W/cm2 to 100 W/cm2.

Hotspot Thermal Management With Flow Boiling of Refrigerant in Ultrasmall Microgaps

2017 ◽  
Vol 139 (1) ◽  
Author(s):  
Mohamed H. Nasr ◽  
Craig E. Green ◽  
Peter A. Kottke ◽  
Xuchen Zhang ◽  
Thomas E. Sarvey ◽  
...  
Keyword(s):  
Thermal Management ◽  
Flow Boiling ◽  
Silicon Layer ◽  
Flow Regimes ◽  
Visual Observation ◽  
Heat Fluxes ◽  
Two Phase ◽  
Pin Fin ◽  
Mass Fluxes ◽  
Gap Height

As integration levels increase in next generation electronics, high power density devices become more susceptible to hotspot formation, which often imposes a thermal limitation on performance. Flow boiling of R134a in two microgap heat sink configurations was investigated as a solution for hotspot thermal management: a bare microgap and inline micro-pin fin populated microgap, both with 10 μm gap height, were tested in terms of their ability to dissipate heat fluxes approaching 5 kW/cm2 at the heat source. Additional parameters investigated include mass fluxes up to 3000 kg/m2 s at inlet pressures up to 1.5 MPa and exit qualities approaching unity. The microgap testbeds investigated consist of a silicon layer which is heated from the bottom using resistive heaters and capped with glass to enable visual observation of two-phase flow regimes. Wall temperature, device thermal resistance, and pressure drop results are presented and mapped to the dominant flow regimes that were observed in the microgap.

Heat Transfer Mechanism and Flow Pattern During Flow Boiling of Water in a Vertical Narrow Channel: Experimental Results

2006 ◽  
Author(s):  
Ewelina Sobierska ◽  
Rudi Kulenovic ◽  
Rainer Mertz
Keyword(s):  
Heat Transfer ◽  
Two Phase Flow ◽  
Flow Boiling ◽  
Heat Fluxes ◽  
Transfer Mechanism ◽  
Phase Flow ◽  
Fluid Temperature ◽  
Heat Transfer Mechanism ◽  
Two Phase ◽  
Mass Fluxes

Experimental investigations on flow boiling phenomena in a vertical narrow rectangular microchannel with the hydraulic diameter dh = 0.48 mm were carried out. The experiments were performed under fluid-inlet subcooling conditions with deionised and degassed water for different mass fluxes. Investigations on pressure drop and heat transfer during single-and two-phase flow have been carried out. Moreover, flow visualisation of the two-phase flow patterns along the channel was performed using a digital high-speed video camera. The present work outlines local heat transfer coefficients for three mass fluxes (200, 700 and 1500 kg/m2s) and heat fluxes (30–110, 35–150 and 65–200 kW/m2, respectively) during two-phase flow. The fluid temperature at the inlet was about 50 °C what corresponds to inlet subcooling, depending on flow pressure conditions, from 34 °C to 57 °C. The visual observations were used to obtain a better insight about the heat transfer mechanism.

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