Nucleate Boiling Heat Transfer on Plain and Microporous Surfaces in Subcooled Water

2017 ◽  
Vol 139 (8) ◽  
Author(s):  
Seongchul Juna ◽  
Jinsub Kima ◽  
Hwan Yeol Kimb ◽  
Seung M. Youa
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
Average Particle Size ◽  
Nucleate Boiling ◽  
Average Particle ◽  
Transfer Coefficients ◽  
Sem Image ◽  
Subcooled Water ◽  
Plain Surface ◽  
Bubble Sizes

The growth of hovering bubbles on Copper, High-Temperature Thermally-Conductive Microporous Coating (Cu-HTCMC) and plain surface were compared at 1,000 kW/m2 in nucleate boiling with different subcoolings. Images obtained by a high speed camera operating at 2,000 frames per second were used. The Cu-HTCMC was created by sintering copper powders with the average particle size of 67 μm and ∼300 μm thickness, which showed the optimized nucleate boiling and critical heat flux enhancement. The hovering bubble size became smaller as subcooling increased for both Cu-HTCMC and plain surface due to condensation by surrounding subcooled water. At 30 K subcooling, big hovering bubbles disappeared on both surfaces. Small bubbles were shown on plain surface and mists were shown on Cu-HTCMC surface. The hovering bubble sizes were close and the growth times were comparable for both surfaces in saturated and 10 K subcooling cases. However, the bubbles on Cu-HTCMC surface were smaller than those of plain surface at 20 K and 30 K subcoolings. This is believed to be due to the microporous structures shown in the SEM image (top left figure). The heat transfer coefficients of Cu-HTCMC were ∼300 kW/m2K for various subcoolings, about 6 times higher than those of plain surface (top right figure). The figure indicates slightly increasing trend of the heat transfer coefficient with subcooling. This is believed to be the result of the disappearance of relatively big size bubbles in Cu-HTCMC case.

scholarly journals Effect of Subcooling on Pool Boiling of Water from Sintered Copper Microporous Coating at Different Orientations

2018 ◽  
Vol 2018 ◽  
pp. 1-9 ◽  
Author(s):  
Seongchul Jun ◽  
Jinsub Kim ◽  
Seung M. You ◽  
Hwan Yeol Kim
Keyword(s):  
Heat Transfer ◽  
Boiling Heat Transfer ◽  
Pool Boiling ◽  
Average Particle Size ◽  
Nucleate Boiling ◽  
Copper Surface ◽  
Average Particle ◽  
Pool Boiling Heat Transfer ◽  
Microporous Coating ◽  
Plain Surface

The subcooling effect on pool boiling heat transfer using a copper microporous coating was experimentally studied in water for subcoolings of 10 K, 20 K, and 30 K at atmospheric pressure and compared to that of a plain copper surface. A high-temperature thermally conductive microporous coating (HTCMC) was made by sintering copper powder with an average particle size of 67 μm onto a 1 cm × 1 cm plain copper surface with a coating thickness of ~300 μm. The HTCMC surface showed a two times higher critical heat flux (CHF), ~2,000 kW/m2, and up to seven times higher nucleate boiling heat transfer (NBHT) coefficient, ~350 kW/m2K, when compared with a plain copper surface at saturation. The results of the subcooling effect on pool boiling showed that the NBHT of both the HTCMC and the plain copper surface did not change much with subcooling. On the other hand, the CHF increased linearly with the degree of subcooling for both the HTCMC and the plain copper surface. The increase in the CHF was measured to be ~60 kW/m2for every degree of subcooling for both the HTCMC and the plain surface, so that the difference of the CHF between the HTCMC and the plain copper surface was maintained at ~1,000 kW/m2throughout the tested subcooling range. The CHFs for the HTCMC and the plain copper surface at 30 K subcooling were 3,820 kW/m2and 2,820 kW/m2, respectively. The experimental results were compared with existing CHF correlations and appeared to match well with Zuber’s formula for the plain surface. The combined effect of subcooling and orientation of the HTCMC on pool boiling heat transfer was studied as well.

Nucleate Boiling Comparison between Teflon-Coated Plain Copper and Cu-HTCMC in Water

2018 ◽  
Vol 140 (8) ◽  
Author(s):  
Seongchul Jun ◽  
Jin Sub Kim ◽  
Jungho Lee ◽  
Seung M. You
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Boiling Heat Transfer ◽  
Average Particle Size ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Average Particle ◽  
Thermally Conductive ◽  
Bubble Sizes ◽  
Coated Surfaces

The hydrophobic, Teflon-coated surfaces on plain copper and Cu-HTCMC (High temperature Thermally Conductive Microporous Coating) compared on pool boiling heat transfer of water. The HTCMC was created by sintering of copper powders with the average particle size of 67 µm and about 300 µm coating thickness that showed a good boiling heat transfer and the CHF enhancement from the previous study at saturation of water [1]. The Teflon-coated surfaces were created by coating of Amorphous Fluoroplastic (AF) 2400 resin on both plain copper and Cu-HTCMC. The static angles of both surfaces showed hydrophobic as about 120-130°. The departure bubble sizes created by merged bubbles of both surfaces are comparable as about 7 mm at 5 kW/m2 and the sizes are increased as heat flux increases. However, unlike to the plain surface, the smaller bubbles on Cu-HTCMC are not observed at the heat flux of 5 kW/m2 because the number of nucleation sites created in the porous structure are huge smaller bubbles are merged as soon as they grow from pores. As heat flux reaches the surfaces are covered by vapor film and reached the critical heat flux (CHF) at much lower heat fluxes compared to hydrophilic surfaces but the CHF values of Teflon-coated Cu-HTCMC is 640 kW/m2 and the value is more than tenfold higher than that of Teflon-coated plain copper.

Pool Boiling Heat Transfer Enhancement of Water Using Brazed Copper Microporous Coatings

2015 ◽  
Author(s):  
Seongchul Jun ◽  
Hyoseong Wi ◽  
Ajay Gurung ◽  
Miguel Amaya ◽  
Seung M. You
Keyword(s):  
Heat Transfer ◽  
Coating Thickness ◽  
Boiling Heat Transfer ◽  
Average Particle Size ◽  
Nucleate Boiling ◽  
Copper Surface ◽  
Heat Fluxes ◽  
Porous Coating ◽  
Average Particle ◽  
Nucleate Boiling Heat Transfer

A novel, high-temperature, thermally-conductive, microporous coating (HTCMC) is developed by brazing copper particles onto a copper surface. This coating is more durable than many previous microporous coatings and also effectively creates reentrant cavities by optimizing brazing conditions. A parametric study of coating thicknesses of 49–283 μm with an average particle size of ∼25 μm was conducted using the HTCMC coating to understand nucleate boiling heat transfer (NBHT) enhancement on porous surfaces. It was found that there are three porous coating regimes according to their thicknesses. The first regime is “microporous” in which both NBHT and critical heat flux (CHF) enhancements gradually grow as the coating thickness increases. The second regime is “microporous-to-porous transition” where NBHT is further enhanced at lower heat fluxes but decreases at higher heat fluxes for increasing thickness. CHF in this regime continues to increase as the coating thickness increases. The last regime is named as “porous”, and both NBHT and CHF decrease as the coating thickness increases further than that of the other two regimes. The maximum nucleate boiling heat transfer coefficient observed was ∼350,000 W/m2K at 96 μm thickness (“microporous” regime) and the maximum CHF observed was ∼2.1 MW/m2 at ∼225 μm thickness (“porous” regime).

Pool Boiling Heat Transfer Enhancement of Water Using Brazed Copper Microporous Coatings

2016 ◽  
Vol 138 (7) ◽  
Author(s):  
Seongchul Jun ◽  
Hyoseong Wi ◽  
Ajay Gurung ◽  
Miguel Amaya ◽  
Seung M. You
Keyword(s):  
Heat Transfer ◽  
Coating Thickness ◽  
Boiling Heat Transfer ◽  
Average Particle Size ◽  
Nucleate Boiling ◽  
Copper Surface ◽  
Heat Fluxes ◽  
Porous Coating ◽  
Average Particle ◽  
Thermally Conductive

A novel, high-temperature, thermally conductive, microporous coating (HTCMC) is developed by brazing copper particles onto a copper surface. This coating is more durable than many previous microporous coatings and also effectively creates re-entrant cavities by varying brazing conditions. A parametric study of coating thicknesses of 49–283 μm with an average particle size of ∼25 μm was conducted using the HTCMC coating to understand nucleate boiling heat transfer (NBHT) enhancement on porous surfaces. It was found that there are three porous coating regimes according to their thicknesses. The first regime is “microporous” in which both NBHT and critical heat flux (CHF) enhancements gradually grow as the coating thickness increases. The second regime is “microporous-to-porous transition” where NBHT is further enhanced at lower heat fluxes but decreases at higher heat fluxes for increasing thickness. CHF in this regime continues to increase as the coating thickness increases. The last regime is named “porous,” and both NBHT and CHF decrease as the coating thickness increases beyond that of the other two regimes. The maximum NBHT coefficient observed was ∼350,000 W/m2K at 96 μm thickness (microporous regime) and the maximum CHF observed was ∼2.1 MW/m2 at ∼225 μm thickness (porous regime).

Microporous Coatings to Maximize Pool Boiling Heat Transfer of Saturated R-123 and Water

2015 ◽  
Vol 137 (8) ◽  
Author(s):  
Joo Han Kim ◽  
Ajay Gurung ◽  
Miguel Amaya ◽  
Sang Muk Kwark ◽  
Seung M. You
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Boiling Heat Transfer ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Transfer Coefficients ◽  
Maximum Heat ◽  
Microporous Coating ◽  
Plain Surface ◽  
Improved Performance

The present research is an experimental study for the enhancement of boiling heat transfer using microporous coatings. Two types of coatings are investigated: one that is bonded using epoxy and the other by soldering. Effects on pool boiling performance were investigated, of different metal particle sizes of the epoxy-based coating, on R-123 refrigerants, and on water. All boiling tests were performed with 1 cm × 1 cm test heaters in the horizontal, upward-facing orientation in saturated conditions at atmospheric pressure and under increasing heat flux. The surface enhanced by the epoxy-based microporous coatings significantly augmented both nucleate boiling heat transfer coefficients and critical heat flux (CHF) of R-123 relative to those of a plain surface. However, for water, with the same microporous coating, boiling performance did not improve as much, and thermal resistance of the epoxy component limited the maximum heat flux that could be applied. Therefore, for water, to seek improved performance, the solder-based microporous coating was applied. This thermally conductive microporous coating, TCMC, greatly enhanced the boiling performance of water relative to the plain surface, increasing the heat transfer coefficient up to ∼5.6 times, and doubling the CHF.

Bubble Growth During Nucleate Boiling in Microchannels

2010 ◽  
Author(s):  
David M. Christopher ◽  
Xipeng Lin
Keyword(s):  
Heat Transfer ◽  
Bubble Growth ◽  
High Speed ◽  
Stokes Equations ◽  
Electronics Cooling ◽  
Nucleate Boiling ◽  
Nucleation Site ◽  
High Heat Flux ◽  
Transfer Coefficients ◽  
Vof Model

The flow and heat transfer in microchannels has been of great interest for some years now due to the significantly higher heat transfer coefficients useful for enhancing the heat transfer in very small but high heat flux applications such as electronics cooling. Nucleate boiling heat transfer in microchannels is also of great interest for generating even higher heat transfer rates; however, numerous studies have shown that the bubble formation immediately fills the entire microchannel with vapor significantly reducing the heat transfer since the bubble size is normally of the same size as the microchannel. The bubble growth process is very fast and difficult to study experimentally, even with high speed cameras. This study numerically analyzes the flow and bubble growth in a microchannel for various conditions by solving the Navier-Stokes equations with the VOF model with an analytical microlayer model to provide the large amount of vapor produced by the curved region of the microlayer. As each bubble forms, the large pressure drop around the bubble causes the bubble to quickly break away from the nucleation site and move quickly downstream. The bubbles are quite small with the size depending on the bulk flow velocity, subcooling and the heating rate. The numerical results compare quite well with preliminary experimental observations of bubble growth on a microheater embedded in the channel wall for FC-72 flowing in a microchannel.

An Experimental Study on Flow Boiling Characteristics of pHEMA Nano-Coated Surfaces in a Microchannel

2016 ◽  
Author(s):  
Abdolali Khalili Sadaghiani ◽  
Yağmur Şişman ◽  
Gözde Özaydın İnce ◽  
Ali Koşar
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Transfer Coefficients ◽  
Rectangular Microchannel ◽  
Camera System ◽  
Coated Surfaces

In this study, the effect of pHEMA (Polyhydroxyethylmethacrylate) nanostructure coated surfaces on flow boiling was investigated in a rectangular microchannel. Experiments were conducted using deionized water as the working fluid to investigate flow boiling in a microchannel with dimensions of 14 cm length, 1.5 cm width, and 500 μm depth. The effect of pHEMA coatings (coated on 1.5 × 1.5 cm2 silicon plates) on heat transfer coefficients and flow patterns was assessed and supported using a high speed camera system. Although the contact angle decreases on nano-coated surfaces, due to surface porosity, boiling heat transfer coefficient increases. Furthermore, visualization results indicated that uncoated surfaces experienced a smaller nucleate boiling region. It was also observed that dryout occurs at higher heat fluxes for coated surfaces.

Flow Boiling Heat Transfer of Subcooled Water on Sintered Microporous Surfaces

2017 ◽  
Vol 139 (8) ◽  
Author(s):  
Yeonghwan Kim ◽  
Junsub Kim ◽  
Seung M. You ◽  
Jungho Lee ◽  
Jeong Lak Sohn ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Average Particle ◽  
Temperature Uniformity ◽  
Subcooled Water ◽  
Wall Superheat ◽  
Flow Boiling Heat Transfer ◽  
Plain Surface

Electronic devices such as battery packs in electric vehicles and LED lights require advanced control in temperature uniformity for their optimum performance and prolonged lifetime. Flow boiling heat transfer of subcooled water (∆Tsub = 20K) in a 300 mm long minichannel with the cross section of 20×10 mm2 was investigated to improve the temperature uniformity over the entire minichannel. The minichannel was uniformity heated from the bottom copper surface. A 10 mm thick Pyrex glass was used for the top plate of the channel to visualize two-phase flow during the experiment. Microporous coating was fabricated by sintering copper particles on the top surface of the copper block. The average particle size was 50 μm, the average coating thickness was 300 μm, and the porosity was 41%, respectively. At the heat flux of 100 kW/m2, more bubbles are shown on the microporous surface compared with plain surface, resulting in better boiling heat transfer performance. These bubbles were large and stationary as liquid is evaporated and condensed to transport the heat as if heat pumps. As heat flux increases, bubble nucleation becomes more intensive, however, the larger stationary bubbles observed at 100 kW/m2 started to decrease. Most of the generated bubbles flowed through the downstream and they shrank quickly upon departure from the wall due to the 20K subcooling. High speed video showed some streaks of these small bubbles, and more streaks were observed as the heat flux increased. As shown in the left graph above, at 50 kW/m2 in subcooled flow boiling, both plain and microporous surfaces show similar local wall temperature because both are placed in the single-phase regime. In contrast, the difference of wall superheat between plain and porous surface is relatively large at higher heat flux of 500 kW/m2. Sintered microporous surface showed smaller increase in wall superheat compared with plain surface at higher wall superheat. [This study was supported by National Research Council of Science and Technology (NST) grant, Korea (Grant No. KIMM-NK203B)].

scholarly journals Operating temperatures of oil-lubricated medium-speed gears: Numerical models and experimental results

2003 ◽  
Vol 217 (2) ◽  
pp. 87-106 ◽  
Author(s):  
H Long ◽  
A A Lord ◽  
D T Gethin ◽  
B J Roylance
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Surface Temperature ◽  
Rotational Speed ◽  
High Speed ◽  
Applied Load ◽  
Frictional Heat ◽  
Transfer Coefficients ◽  
Gear Teeth ◽  
Heat Transfer Coefficients

This paper investigates the effects of gear geometry, rotational speed and applied load, as well as lubrication conditions on surface temperature of high-speed gear teeth. The analytical approach and procedure for estimating frictional heat flux and heat transfer coefficients of gear teeth in high-speed operational conditions was developed and accounts for the effect of oil mist as a cooling medium. Numerical simulations of tooth temperature based on finite element analysis were established to investigate temperature distributions and variations over a range of applied load and rotational speed, which compared well with experimental measurements. A sensitivity analysis of surface temperature to gear configuration, frictional heat flux, heat transfer coefficients, and oil and ambient temperatures was conducted and the major parameters influencing surface temperature were evaluated.

Numerical and Experimental Investigation Into the Effects of Nanoparticle Mass Fraction and Bubble Size on Boiling Heat Transfer of TiO2–Water Nanofluid

2016 ◽  
Vol 138 (8) ◽  
Author(s):  
Cong Qi ◽  
Yongliang Wan ◽  
Lin Liang ◽  
Zhonghao Rao ◽  
Yimin Li
Keyword(s):  
Heat Transfer ◽  
Bubble Size ◽  
Boiling Heat Transfer ◽  
Mass Fraction ◽  
Experimental Methods ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Mass Fractions ◽  
Boiling Heat Transfer Coefficient ◽  
Bubble Sizes

Considering mass transfer and energy transfer between liquid phase and vapor phase, a mixture model for boiling heat transfer of nanofluid is established. In addition, an experimental installation of boiling heat transfer is built. The boiling heat transfer of TiO2–water nanofluid is investigated by numerical and experimental methods, respectively. Thermal conductivity, viscosity, and boiling bubble size of TiO2–water nanofluid are experimentally investigated, and the effects of different nanoparticle mass fractions, bubble sizes and superheat on boiling heat transfer are also discussed. It is found that the boiling bubble size in TiO2–water nanofluid is only one-third of that in de-ionized water. It is also found that there is a critical nanoparticle mass fraction (wt.% = 2%) between enhancement and degradation for TiO2–water nanofluid. Compared with water, nanofluid enhances the boiling heat transfer coefficient by 77.7% when the nanoparticle mass fraction is lower than 2%, while it reduces the boiling heat transfer by 30.3% when the nanoparticle mass fraction is higher than 2%. The boiling heat transfer coefficients increase with the superheat for water and nanofluid. A mathematic correlation between heat flux and superheat is obtained in this paper.

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