Experiments on pool boiling of a dielectric fluid on extended surfaces

1996 ◽  
Vol 23 (4) ◽  
pp. 451-462 ◽  
Author(s):  
G. Guglielmini ◽  
M. Misale ◽  
C. Schenone
Keyword(s):  
Pool Boiling ◽  
Dielectric Fluid ◽  
Extended Surfaces

Self-Propelled Sliding Bubble Motion Induced by Surface Microstructure in Pool Boiling of a Dielectric Fluid Under Microgravity

2015 ◽  
Vol 137 (2) ◽  
Author(s):  
Naveenan Thiagarajan ◽  
Sushil H. Bhavnani ◽  
Vinod Narayanan
Keyword(s):  
Vapor Bubble ◽  
Bubble Dynamics ◽  
Heated Surface ◽  
Pool Boiling ◽  
Bubble Nucleation ◽  
Radius Of Curvature ◽  
Dielectric Fluid ◽  
Bubble Motion ◽  
Reduced Gravity ◽  
Modification Technique

This paper reports bubble dynamics observed during pool boiling over microstructures with an asymmetric saw-tooth cross section, under reduced gravity. The periodic saw-toothed ratchets etched on a silicon surface include fabricated vapor bubble nucleation sites only on the shallow slope. Reduced gravity pool boiling experiments were conducted aboard a Boeing 727 aircraft carrying out parabolic maneuvers. The fluid used was FC-72, a highly wetting dielectric fluid used as a coolant for electronics. Under microgravity, it was observed that the bubble diameters were six times larger than in terrestrial gravity. Also, self-propelled sliding bubble motion along the surface of the saw teeth was observed in reduced gravity. The velocity of the sliding bubbles across the saw teeth, following lateral departure from the cavities, was measured to be as high as 27.4 mm/s. A model for the sliding bubble motion is proposed by attributing it to the force due to pressure differences that arise in the liquid film between the vapor bubble and the saw-toothed heated surface. The pressure difference is due to difference in the radius of curvature of the interface between the crest and trough of the saw teeth. The surface modification technique, which resulted in the sliding bubble motion, has the potential to alleviate dry-out caused due to stagnant vapor bubbles over heat sources under microgravity when the buoyancy forces are negligible compared to the surface tension forces.

Numerical Study of Dynamic Behavior of Dielectric Fluid Bubbles Within Diverging, External Electric Fields

2006 ◽  
Author(s):  
Matthew R. Pearson ◽  
Jamal Seyed-Yagoobi
Keyword(s):  
Electric Field ◽  
Electric Fields ◽  
Numerical Study ◽  
Pool Boiling ◽  
Spherical Shape ◽  
Past Research ◽  
Bubble Surface ◽  
Uniform Electric Field ◽  
Dielectric Fluid ◽  
Uniform Field

Past research in the area of pool boiling within the presence of electric fields has generally focused on the case of uniform field intensity. Any numerical or analytical studies of the effect of non-uniform fields on the motion of bubbles within a dielectric liquid medium have assumed that the bubbles will retain their spherical shape rather than deform. These studies also ignore changes to the electrical field caused by the presence of the bubbles. However, these assumptions are not necessarily accurate as, even in the case of a nominally uniform electric field distribution, bubbles can exhibit considerable physical deformation and the field can become noticeably affected in the vicinity of the bubble. This study explores the effect that a non-uniform electric field can have on vapor bubbles of a dielectric fluid by modeling the physical deformation of the bubble and the alteration of the surrounding field. Numerical results show that the imbalance of electrical stresses at the bubble surface exerts a net dielectrophoretic force on the bubble, propelling the bubble to the vicinity of weakest electric field, thereby enhancing the separation of liquid and vapor phases during pool boiling. However, the proximity of the bubble to one of the electrodes can considerably alter the bubble trajectory due to an attractive force that arises from local distortions of the potential and electric fields. This phenomenon cannot be predicted if bubble deformation and field distortion effects are neglected.

Pool Boiling in Saturated and Subcooled FC-72 Dielectric Fluid From a Porous Graphite Surface

2004 ◽  
Author(s):  
Mohamed S. El-Genk ◽  
Jack L. Parker
Keyword(s):  
Heat Flux ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Porous Coating ◽  
Dielectric Fluid ◽  
Subcooled Boiling ◽  
Porous Graphite ◽  
Rate Of Increase ◽  
Boiling Incipience

Experiments are conducted that investigated pool boiling of FC-72 liquid at saturation and 10, 20, and 30 K subcooling on porous graphite and smooth copper surfaces measuring 10 × 10 mm. The nucleate boiling heat flux, Critical Heat Flux (CHF), and surface superheats at boiling incipience are compared. Theses heat fluxes are also compared with those of other investigators for smooth copper and silicon, etched SiO2, surfaces and micro-porous coating. No temperature excursion at boiling incipience on the porous graphite that occurred at a surface superheats of < 1.0 K. Conversely, the temperature excursions of 24.0 K and 12.4–17.8 K are measured at incipient boiling in saturation and subcooled boiling on copper. Nucleate boiling heat fluxes on porous graphite are significantly higher and corresponding surface superheats are much smaller than on copper. CHF on porous graphite (27.3, 39.6, 49.0, and 57.1 W/cm2 in saturation and 10 K, 20 K, and 30 K subcooled boiling, respectively) are 61.5%–207% higher than those on copper (16.9, 19.5, 23.6, and 28.0 W/cm2, respectively). The surface superheats at CHF on the porous graphite of 11.5 K in saturation and 17–20 K in subcooled boiling are significantly lower that those on copper (25 K and 26–28 K, respectively). In addition, the rate of increase of CHF on porous graphite with increased subcooling is ~ 125% higher than that on copper.

Sliding Bubble-Pumped Motion Induced by Surface Micro-Structure in Pool Boiling of a Dielectric Fluid Under Reduced Gravity

2014 ◽  
Author(s):  
Naveenan Thiagarajan ◽  
Sushil H. Bhavnani ◽  
Vinod Narayanan
Keyword(s):  
Vapor Bubble ◽  
Bubble Dynamics ◽  
Heated Surface ◽  
Pool Boiling ◽  
Bubble Nucleation ◽  
Radius Of Curvature ◽  
Dielectric Fluid ◽  
Bubble Motion ◽  
Reduced Gravity ◽  
Modification Technique

This paper reports bubble dynamics observed during pool boiling over micro-structures with an asymmetric saw-tooth cross-section, under reduced gravity. The periodic saw-toothed ratchets etched on a silicon surface include fabricated vapor bubble nucleation sites only on the shallow slope. Reduced gravity pool boiling experiments were conducted aboard a Boeing 727 aircraft (Zero-g Inc.) carrying out parabolic maneuvers to achieve reduced gravity. The fluid used was FC-72, a highly wetting dielectric fluid used as a coolant for electronics. Under microgravity, it was observed that the bubble diameters were six times larger than in terrestrial gravity. Also, self-propelled sliding bubble motion along the surface of the saw teeth was observed in reduced gravity. The velocity of the sliding bubbles across the saw teeth, following lateral departure from the cavities, was measured to be as high as 27.4 mm/s. A model for the sliding bubble motion is proposed by attributing it to the force due to pressure differences that arise in the liquid film between the vapor bubble and the saw-toothed heated surface. The pressure difference is due to difference in the radius of curvature of the interface between the crest and trough of the saw teeth. The surface modification technique has the potential to alleviate dry out caused due to vapor blanketing of heat sources in microgravity due to negligible buoyancy forces compared to the surface tension forces.

Enhanced Pool Boiling With HFE7000 Over Selectively Sintered Microchannels

2017 ◽  
Author(s):  
Travis S. Emery ◽  
Satish G. Kandlikar
Keyword(s):  
Heat Transfer ◽  
Heat Dissipation ◽  
Pool Boiling ◽  
Electronics Cooling ◽  
Fold Increase ◽  
High Heat ◽  
Heat Fluxes ◽  
Dielectric Fluid ◽  
Maximum Heat ◽  
Maximum Heat Transfer

As the need for efficient thermal management grows, pool boiling’s ability to dissipate high heat fluxes has gained significant interest. The objective of this work was to study the performance of pool boiling at atmospheric pressure using a dielectric fluid, HFE7000. Both plain and enhanced copper surfaces were tested, and these results were then compared to similar testing performed with water and FC-87. The enhanced surfaces utilized microchannels with porous coatings selectively located on different regions of the heat transfer surface. A maximum critical heat flux (CHF) of 41.7 W/cm2 was achieved here, which translated to a 29% CHF increase in comparison to a plain chip. A maximum heat transfer coefficient (HTC) of 104.0 kW/m2°C was also achieved, which translated to a 6-fold increase in HTC when compared to a plain copper chip. More notably, this HTC was achieved at a wall temperature of 38.4 °C. This HTC enhancement was greater than that of water and FC-87 when using the same enhanced surface. The effect of sintering location was found to have a similar effect on CHF with HFE7000 in comparison with water. The effect of microchannel size was shown to have similar effects on CHF when compared with FC-87 and water. From the results found here, it is concluded that the employment of selectively sintered open microchannels with HFE7000 has significant potential for enhanced heat dissipation in electronics cooling applications.

scholarly journals Hydrophilic and Hydrophobic Nanostructured Copper Surfaces for Efficient Pool Boiling Heat Transfer with Water, Water/Butanol Mixtures and Novec 649

Nanomaterials ◽  
2021 ◽  
Vol 11 (12) ◽  
pp. 3216
Author(s):  
Matic Može ◽  
Viktor Vajc ◽  
Matevž Zupančič ◽  
Iztok Golobič
Keyword(s):  
Heat Transfer ◽  
Boiling Heat Transfer ◽  
Heat Dissipation ◽  
Pool Boiling ◽  
Chemical Oxidation ◽  
Dielectric Fluid ◽  
Surface Stability ◽  
Copper Surfaces ◽  
Functionalized Surfaces ◽  
Fluorinated Silane

Increasing heat dissipation requirements of small and miniature devices demands advanced cooling methods, such as application of immersion cooling via boiling heat transfer. In this study, functionalized copper surfaces for enhanced heat transfer are developed and evaluated. Samples are functionalized using a chemical oxidation treatment with subsequent hydrophobization of selected surfaces with a fluorinated silane. Pool boiling tests with water, water/1-butanol mixture with self-rewetting properties and a novel dielectric fluid with low GWP (Novec™ 649) are conducted to evaluate the boiling performance of individual surfaces. The results show that hydrophobized functionalized surfaces covered by microcavities with diameters between 40 nm and 2 µm exhibit increased heat transfer coefficient (HTC; enhancements up to 120%) and critical heat flux (CHF; enhancements up to 64%) values in comparison with the untreated reference surface, complemented by favorable fabrication repeatability. Positive surface stability is observed in contact with water, while both the self-rewetting fluids and Novec™ 649 gradually degrade the boiling performance and in some cases also the surface itself. The use of water/1-butanol mixtures in particular results in surface chemistry and morphology changes, as observed using SEM imaging and Raman spectroscopy. This seems to be neglected in the available literature and should be focused on in further studies.

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