Effect of Electric Fields on Two-Phase Impingement Heat Transfer

2007 ◽  
Author(s):  
Xin Feng ◽  
James E. Bryan
Keyword(s):  
Heat Transfer ◽  
Electric Fields ◽  
Capillary Flow ◽  
Heated Surface ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Geometrical Parameters ◽  
Two Phase ◽  
Transfer Rates ◽  
Impingement Heat Transfer

The effect of electric fields applied to two-phase impingement heat transfer is explored for the first time. The application of an electric field between a capillary and heated surface results in the ability to control the free boundary flow from discreet drops to jets to sprays. Through an experimental study, the impingement heat transfer was evaluated over a range of operating and geometrical parameters using subcooled ethanol as the working fluid. The ability to change the mode of impinging mass did change the surface heat transfer. The characteristics of the impinging mass on heat transfer was dependent on capillary flow rate, applied voltage, capillary to heated surface spacing, capillary geometry, and heat flux. Enhancement occurred primarily at low heat fluxes (below 30 W/cm2) under ramified spray conditions where the droplet momentum promoted thin films on the heated surface. Higher heat fluxes resulted in greater vapor momentum from the surface minimizing the effect of different modes. However, under ramified spray conditions less mass was impacting the heated surface showing that heat transfer rates at higher heat fluxes were achievable with less mass, resulting in greater evaporation efficiency.

Application of Electrohydrodynamic Atomization to Two-Phase Impingement Heat Transfer

2008 ◽  
Vol 130 (7) ◽  
Author(s):  
Xin Feng ◽  
James E. Bryan
Keyword(s):  
Heat Transfer ◽  
Capillary Tube ◽  
Heated Surface ◽  
Heat Fluxes ◽  
Operating Conditions ◽  
Working Fluid ◽  
Heater Surface ◽  
Two Phase ◽  
Impingement Heat Transfer ◽  
Repulsive Forces

The effect of electric fields applied to two-phase impingement heat transfer is explored for the first time. The electric field applied between a capillary tube and heated surface enhances the heat transfer by controlling the free boundary flow modes from discreet drops to jets, to sprays. Through an experimental study, the impingement heat transfer was evaluated over a range of operating conditions and geometrical parameters with subcooled ethanol used as the working fluid. The ability to change the mode of impinging mass did change the surface heat transfer. The characteristics of the impinging mass on heat transfer were dependent on flow rate, applied voltage, capillary tube to heated surface spacing, capillary tube geometry, heat flux, heater surface geometry, and capillary tube array configuration. Enhancement occurred primarily at low heat fluxes (below 30W∕cm2) under ramified spray conditions where the droplet momentum promoted thin films on the heated surface resulting in 1.7 times enhancement under certain conditions. Higher heat fluxes resulted in greater vapor momentum from the surface, minimizing the effect of different impingement modes. The use of capillary tube array allowed for electrohydrodynamics atomization enhancement and higher liquid flow rates, but electrostatic repulsive forces diverted the spray from the heater surface. This reduced the mass flux to the surface, leading to premature dryout under certain conditions.

Effects of Varying Geometrical Parameters on Boiling From Microfabricated Enhanced Structures

2003 ◽  
Vol 125 (1) ◽  
pp. 103-109 ◽  
Author(s):  
C. Ramaswamy ◽  
Y. Joshi ◽  
W. Nakayama ◽  
W. B. Johnson
Keyword(s):  
Heat Dissipation ◽  
Layer Structure ◽  
Single Layer ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Geometrical Parameters ◽  
Small Range ◽  
Two Phase ◽  
Wall Superheat

The current study involves two-phase cooling from enhanced structures whose dimensions have been changed systematically using microfabrication techniques. The aim is to optimize the dimensions to maximize the heat transfer. The enhanced structure used in this study consists of a stacked network of interconnecting channels making it highly porous. The effect of varying the pore size, pitch and height on the boiling performance was studied, with fluorocarbon FC-72 as the working fluid. While most of the previous studies on the mechanism of enhanced nucleate boiling have focused on a small range of wall superheats (0–4 K), the present study covers a wider range (as high as 30 K). A larger pore and smaller pitch resulted in higher heat dissipation at all heat fluxes. The effect of stacking multiple layers showed a proportional increase in heat dissipation (with additional layers) in a certain range of wall superheat values only. In the wall superheat range 8–13 K, no appreciable difference was observed between a single layer structure and a three layer structure. A fin effect combined with change in the boiling phenomenon within the sub-surface layers is proposed to explain this effect.

Heat Transfer Correlations for Liquid Film in the Evaporator of Enclosed, Gravity-Assisted Thermosyphons

1998 ◽  
Vol 120 (2) ◽  
pp. 477-484 ◽  
Author(s):  
M. S. El-Genk ◽  
H. H. Saber
Keyword(s):  
Heat Transfer ◽  
Liquid Film ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Smooth Transition ◽  
Working Fluid ◽  
Two Phase ◽  
Data Set ◽  
Evaporator Section ◽  
Heat Transfer Correlations

Heat transfer correlations were developed for the liquid film region, in the evaporator section of closed, two-phase, gravity-assisted thermosyphons in the following regimes: (a) laminar convection, at low heat fluxes, (b) combined convection, at intermediate heat fluxes, and (c) nucleate boiling, at high heat fluxes. These correlations were based on a data set consisting of a total of 305 points for ethanol, acetone, R-11, and R-113 working fluids, wall heat fluxes of 0.99–52.62 kW/m2, working fluid filling ratios of 0.01–0.62, inner diameters of 6–37 mm, evaporator section lengths of 50–609.6 mm, and vapor temperatures of 261–352 K. The combined convention data were correlated by superimposing the correlations of laminar convention and nucleate boiling using a power law approach, to ensure smooth transition among the three heat transfer regimes. The three heat transfer correlations developed in this work are within ±15 percent of experimental data.

The Influence of External Electric Field on Heat Transfer at Boiling on Non-Uniform Surfaces

2010 ◽  
Author(s):  
Alexey A. Eronin ◽  
Stanislav P. Malyshenko ◽  
Anton I. Zhuravlev
Keyword(s):  
Heat Transfer ◽  
Electric Field ◽  
Electric Fields ◽  
Heated Surface ◽  
Transfer Enhancement ◽  
Two Phase ◽  
Major Mechanism ◽  
External Electric Fields ◽  
Different Types ◽  
Field Traps

Characteristics of heat transfer and hydrodynamics of boiling of liquid nitrogen on the surfaces with different types of non-uniformities at the presence of external electric fields are experimentally investigated. It is shown that the formation of field traps is a major mechanism of heat transfer enhancement. And this effect result in noticeable change of two-phase hydrodynamics in vicinity of heated surface.

scholarly journals Surface Structure Enhanced Microchannel Flow Boiling

2016 ◽  
Vol 138 (9) ◽  
Author(s):  
Yangying Zhu ◽  
Dion S. Antao ◽  
Kuang-Han Chu ◽  
Siyu Chen ◽  
Terry J. Hendricks ◽  
...  
Keyword(s):  
Heat Transfer ◽  
High Performance ◽  
Capillary Flow ◽  
Flow Boiling ◽  
High Heat ◽  
Heat Fluxes ◽  
Heat Sinks ◽  
Two Phase ◽  
Thin Film Evaporation ◽  
Surface Microstructures

We investigated the role of surface microstructures in two-phase microchannels on suppressing flow instabilities and enhancing heat transfer. We designed and fabricated microchannels with well-defined silicon micropillar arrays on the bottom heated microchannel wall to promote capillary flow for thin film evaporation while facilitating nucleation only from the sidewalls. Our experimental results show significantly reduced temperature and pressure drop fluctuation especially at high heat fluxes. A critical heat flux (CHF) of 969 W/cm2 was achieved with a structured surface, a 57% enhancement compared to a smooth surface. We explain the experimental trends for the CHF enhancement with a liquid wicking model. The results suggest that capillary flow can be maximized to enhance heat transfer via optimizing the microstructure geometry for the development of high performance two-phase microchannel heat sinks.

Effect of Jet Arrays and Structured Surfaces on Two-Phase Impingement Heat Transfer

2007 ◽  
Author(s):  
Xin Feng ◽  
James E. Bryan
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Performance ◽  
Jet Impingement ◽  
Heat Fluxes ◽  
Transfer Efficiency ◽  
Surface Wetting ◽  
Two Phase ◽  
Structured Surfaces ◽  
Heat Transfer Efficiency ◽  
Impingement Heat Transfer

When heat fluxes and heat transport exceed 100 W/cm2, heat transfer efficiencies decrease rapidly. Experimental work will be presented exploring how micro jet arrays and structured surfaces can be used to increase heat transfer efficiency. Using water, ethanol, and HFE-7000 as working fluids, the effect of jet momentum, subcooling temperature and surface wetting are experimentally investigated on 1cm2 smooth and structured surfaces. From results obtained so far, heat transfer efficiency increases with increasing surface tension (decreasing surface wetting) with micro-jet arrays. Further, existing correlations for two-phase jet impingement cannot predict the heat transfer performance with acceptable accuracy as they do not account for surface wetting characteristics.

Liquid Crystal Imaging of Flow Boiling in Minichannels

2004 ◽  
Author(s):  
D. Keith Hollingsworth
Keyword(s):  
Heat Transfer ◽  
Liquid Crystal ◽  
Flow Boiling ◽  
Heated Surface ◽  
Working Fluid ◽  
Hysteresis Phenomenon ◽  
Two Phase ◽  
Channel Effects ◽  
Mini Channels ◽  
Boiling Incipience

Quantitative liquid crystal thermography was used to investigate boiling incipience and nucleate flow boiling in rectangular mini-channels with channel heights of 2 mm to 500 μm. Distributions of surface temperature along the heated surface were measured from the liquid crystal images, and streamwise profiles of heat transfer coefficient on the heated surface were calculated. The working fluid was the refrigerant R-11. Observations of the boiling incipience superheat excursion, the hysteresis phenomenon, and saturated flow boiling are presented. Comparisons to established two-phase heat transfer correlations are performed to investigate the existence of “thin channel” effects.

Correlation of Heat Transfer Data to Film Thickness Data of the Thin Film Found in Spray Cooling

2004 ◽  
Author(s):  
Adam G. Pautsch ◽  
Timothy A. Shedd
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Film Thickness ◽  
Heated Surface ◽  
Spray Cooling ◽  
Heat Fluxes ◽  
Experimental Measurements ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Heat Transfer Coefficients

As electronic circuit design and packaging technology progresses, the density and power levels of electronic components is increasing at a nearly exponential rate. The higher heat loads dissipated by these devices are nearing the limits of traditional cooling techniques. One method capable of removing heat fluxes as high as 100 W/cm2 is spray cooling. This process involves the impingement of liquid droplets onto a heated surface, forming a thin two-phase film. In order to create reliable models of the heat transfer during spray cooling, the behavior of the film must be understood. This paper presents an investigation into the behavior of the thin film found in spray cooling. A study was performed to relate experimental measurements of the heat transfer coefficients to experimental measurements of film thickness as they vary spatially over a die surface. Both a single nozzle and a multi-nozzle array were investigated. Measured heat transfer coefficients ranged from 0.2 to 1.2 W/m2K and film thicknesses ranged from 90 to 300 μm.

Choosing Working Fluid for Two-Phase Thermosyphon Systems for Cooling of Electronics

2003 ◽  
Vol 125 (2) ◽  
pp. 276-281 ◽  
Author(s):  
Bjo¨rn Palm ◽  
Rahmatollah Khodabandeh
Keyword(s):  
Heat Transfer ◽  
High Pressure ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Air Cooling ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Heat Transfer Coefficients ◽  
And Performance ◽  
Cooling Of Electronics

The heat fluxes from electronic components are steadily increasing and have now, in some applications, reached levels where air-cooling is no longer sufficient. One alternative solution, which has received much attention during the last decade, is to use heat pipes or thermosyphons for transferring or spreading the dissipated heat. In this paper two-phase thermosyphon loops are discussed. Especially, the choice of fluid and its influence on the design and performance is treated. The discussion is supported by results from simulations concerning heat transfer and pressure drop. In general it is found that high-pressure fluids will give better performance and more compact designs as high-pressure results in higher boiling heat transfer coefficients and smaller necessary tube diameter.

Two-Phase Flow and Heat Transfer in Pin-Fin Enhanced Micro-Gaps With Non-Uniform Heating

2013 ◽  
Author(s):  
Steven A. Isaacs ◽  
Yogendra Joshi ◽  
Yue Zhang ◽  
Muhannad S. Bakir ◽  
Yoon Jo Kim
Keyword(s):  
Heat Transfer ◽  
Two Phase Flow ◽  
Flow Boiling ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Phase Flow ◽  
Two Phase ◽  
Pin Fin ◽  
Flow And Heat Transfer ◽  
Imaging Results

In modern microprocessors, thermal management has become one of the main hurdles in continued performance enhancement. Cooling schemes utilizing single phase microfluidics have been investigated extensively for enhanced heat dissipation from microprocessors. However, two-phase fluidic cooling devices are becoming a promising approach, and are less understood. This study aims to examine two-phase flow and heat transfer within a pin-fin enhanced micro-gap. The pin-fin array covered an area of 1cm × 1cm and had a pin diameter, height and pitch of 150μm, 200μm and 225μm, respectively, (aspect ratio of 1.33). Heating from two upstream heaters was considered. The working fluid used was R245fa. The average heat transfer coefficient was evaluated for a range of heat fluxes and flow rates. Flow regime visualization was performed using high-speed imaging. Results indicate a sharp transition to convective flow boiling mechanism. Unique, conically-shaped two-phase wakes are recorded, demonstrating 2D spreading capability of the device. Surface roughness features are also discussed.

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