On the Scaling of Pool Boiling Heat Flux With Gravity and Heater Size

2011 ◽  
Author(s):  
Rishi Raj ◽  
Jungho Kim ◽  
John McQuillen
Keyword(s):  
Surface Tension ◽  
Heat Flux ◽  
Pool Boiling ◽  
Threshold Value ◽  
Prediction Errors ◽  
Gravity Level ◽  
Experimental Conditions ◽  
Scaling Parameter ◽  
Boiling Regime ◽  
Using Data

A framework for scaling pool boiling heat flux is developed using data from various heater sizes over a range of gravity levels. Boiling is buoyancy dominated for large heaters and/or high gravity conditions and the heat flux is heater size independent. The power law coefficient for gravity is a function of wall temperature. As the heater size or gravity level is reduced, a sharp transition in the heat flux is observed at a threshold value of Lh/Lc = 2.1. Below this threshold value, boiling is surface tension dominated and the dependence on gravity is smaller. The gravity scaling parameter for the heat flux in the buoyancy dominated boiling regime developed in the previous work is updated to account for subcooling effect. Based on this scaling parameter and the transition criteria, a methodology for predicting heat flux in the surface tension dominated boiling regime, typically observed under low-gravity conditions, is developed. Given the heat flux at a reference gravity level and heater size, the current framework allows the prediction of heat flux at any other gravity level and/or heater size under similar experimental conditions. The prediction is validated using data at over a range of subcoolings (7°C ≤ ΔTsub ≤ 32.6°C), heater sizes (2.1 mm ≤ Lh ≤ 7 mm), and dissolved gas concentrations (3 ppm ≤ cg ≤ 3500 ppm). The prediction errors are significantly smaller than those from correlations currently available in the literature.

On the Scaling of Pool Boiling Heat Flux With Gravity and Heater Size

2011 ◽  
Vol 134 (1) ◽  
Author(s):  
Rishi Raj ◽  
Jungho Kim ◽  
John McQuillen
Keyword(s):  
Surface Tension ◽  
Heat Flux ◽  
Pool Boiling ◽  
Threshold Value ◽  
Prediction Errors ◽  
Gravity Level ◽  
Experimental Conditions ◽  
Scaling Parameter ◽  
Boiling Regime ◽  
Using Data

A framework for scaling pool boiling heat flux is developed using data from various heater sizes over a range of gravity levels. Boiling is buoyancy dominated for large heaters and/or high gravity conditions and the heat flux is heater size independent. The power law coefficient for gravity is a function of wall temperature. As the heater size or gravity level is reduced, a sharp transition in the heat flux is observed at a threshold value of Lh/Lc = 2.1. Below this threshold value, boiling is surface tension dominated and the dependence on gravity is smaller. The gravity scaling parameter for the heat flux in the buoyancy dominated boiling regime developed in the previous work is updated to account for subcooling effect. Based on this scaling parameter and the transition criteria, a methodology for predicting heat flux in the surface tension dominated boiling regime, typically observed under low-gravity conditions, is developed. Given the heat flux at a reference gravity level and heater size, the current framework allows the prediction of heat flux at any other gravity level and/or heater size under similar experimental conditions. The prediction is validated using data at over a range of subcoolings (11 °C ≤ ΔTsub ≤ 32.6 °C), heater sizes (2.1 mm ≤ Lh ≤ 7 mm), and dissolved gas concentrations (3 ppm ≤ cg ≤ 3500 ppm). The prediction errors are significantly smaller than those from correlations currently available in the literature.

Gravity Scaling Parameter for Pool Boiling Heat Transfer

2010 ◽  
Vol 132 (9) ◽  
Author(s):  
Rishi Raj ◽  
Jungho Kim ◽  
John McQuillen
Keyword(s):  
Heat Flux ◽  
Pool Boiling ◽  
Gravity Level ◽  
Dissolved Gas ◽  
Experimental Conditions ◽  
Scaling Parameter ◽  
Flux Data ◽  
Earth Gravity ◽  
Variable Gravity ◽  
Continuous Range

Although the effects of microgravity, earth gravity, and hypergravity (>1.5 g) on pool boiling heat flux have been studied previously, pool boiling heat flux data over a continuous range of gravity levels (0–1.7 g) was unavailable until recently. The current work uses the results of a variable gravity, subcooled pool boiling experiment to develop a gravity scaling parameter for n-perfluorohexane/FC-72 in the buoyancy-dominated boiling regime (Lh/Lc>2.1). The heat flux prediction was then validated using heat flux data at different subcoolings and dissolved gas concentrations. The scaling parameter can be used as a tool to predict boiling heat flux at any gravity level in the buoyancy dominated regime if the data under similar experimental conditions are available at any other gravity level.

Gravity Scaling Parameter for Pool Boiling Heat Transfer

2009 ◽  
Author(s):  
Rishi Raj ◽  
Jungho Kim ◽  
John McQuillen
Keyword(s):  
Heat Flux ◽  
Pool Boiling ◽  
Gravity Level ◽  
Low Gravity ◽  
Experimental Conditions ◽  
Scaling Parameter ◽  
Flux Data ◽  
Variable Gravity ◽  
Continuous Range ◽  
Good Agreement

The effect of low gravity on pool boiling heat flux has been studied by researchers, but pool boiling heat flux data over a continuous range of gravity levels (0g–1.8g) was unavailable until recently. The current work uses the results of a variable gravity subcooled pool boiling experiment to develop a gravity scaling parameter for prediction purposes. The heat flux prediction at various gravity levels was found to be in good agreement with the measured heat flux data. The scaling parameter can be used as a tool to predict boiling heat flux at any gravity level if the data under similar experimental conditions are available at any other gravity level. The scaling parameter has been demonstrated to be valid for pool boiling of n-perfluorohexane in heater size independent boiling regime (Lh/Lc>2.8).

Heater Size and Gravity Based Pool Boiling Regime Map: Transition Criteria Between Buoyancy and Surface Tension Dominated Boiling

2010 ◽  
Vol 132 (9) ◽  
Author(s):  
Rishi Raj ◽  
Jungho Kim
Keyword(s):  
Heat Transfer ◽  
Surface Tension ◽  
Pool Boiling ◽  
Threshold Value ◽  
Gravity Level ◽  
Capillary Length ◽  
Boiling Regime ◽  
Transition Criteria ◽  
Regime Map ◽  
Transition Criterion

A pool boiling regime map demarcating the boundary between the surface tension and buoyancy dominated boiling regimes is developed based on heater size and gravity. For large heaters and/or high gravity conditions, boiling is dominated by buoyancy, and the ebullition cycle dominates the contribution to heat transfer. As the gravity level and/or heater size is decreased, surface tension forces become increasingly dominant, and a decrease in heat transfer is observed. The ratio of the heater size Lh (length of a side for a square heater) to the capillary length Lc is found to be a suitable parameter to define the transition criterion between these regimes. Based on the data obtained using FC-72 and pentane, the threshold value of Lh/Lc above which pool boiling is buoyancy dominated was found to be about 2.1. This transition criterion was found to be the same for gravity levels between ∼0g–1.7g and liquid subcoolings between 6.6°C and 26.6°C.

On the Role of Marangoni Effects on the Critical Heat Flux for Pool Boiling of Binary Mixtures

1996 ◽  
Vol 118 (1) ◽  
pp. 103-109 ◽  
Author(s):  
W. R. McGillis ◽  
V. P. Carey
Keyword(s):  
Surface Tension ◽  
Heat Flux ◽  
Binary Mixtures ◽  
Critical Heat Flux ◽  
Full Range ◽  
Pool Boiling ◽  
Marangoni Effect ◽  
Restoring Force ◽  
Marangoni Effects

The Marangoni effect on the critical heat flux (CHF) condition in pool boiling of binary mixtures has been identified and its effect has been quantitatively estimated with a modified model derived from hydrodynamics. The physical process of CHF in binary mixtures, and models used to describe it, are examined in the light of recent experimental evidence, accurate mixture properties, and phase equilibrium revealing a correlation to surface tension gradients and volatility. A correlation is developed from a heuristic model including the additional liquid restoring force caused by surface tension gradients. The CHF condition was determined experimentally for saturated methanol/water, 2-propanol/water, and ethylene glycol/water mixtures, over the full range of concentrations, and compared to the model. The evidence in this study demonstrates that in a mixture with large differences in surface tension, there is an additional hydrodynamic restoring force affecting the CHF condition.

Heat Transfer Behavior of Silica Nanoparticles in Pool Boiling Experiment

2008 ◽  
Vol 130 (4) ◽  
Author(s):  
Denitsa Milanova ◽  
Ranganathan Kumar
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Chemical Composition ◽  
Particle Deposition ◽  
Ph Value ◽  
Pool Boiling ◽  
High Heat ◽  
Boiling Regime ◽  
High Heat Transfer ◽  
The Wire

The heat transfer characteristics of silica (SiO2) nanofluids at 0.5vol% concentration and particle sizes of 10nm and 20nm in pool boiling with a suspended heating Nichrome wire have been analyzed. The influence of acidity on heat transfer has been studied. The pH value of the nanosuspensions is important from the point of view that it determines the stability of the particles and their mutual interactions toward the suspended heated wire. When there is no particle deposition on the wire, the nanofluid increases critical heat flux (CHF) by about 50% within the uncertainty limits regardless of pH of the base fluid or particle size. The extent of oxidation on the wire impacts CHF, and is influenced by the chemical composition of nanofluids in buffer solutions. The boiling regime is further extended to higher heat flux when there is agglomeration on the wire. This agglomeration allows high heat transfer through interagglomerate pores, resulting in a nearly threefold increase in burnout heat flux. This deposition occurs for the charged 10nm silica particle. The chemical composition, oxidation, and packing of the particles within the deposition on the wire are shown to be the reasons for the extension of the boiling regime and the net enhancement of the burnout heat flux.

Investigation of Pool Boiling Critical Heat Flux Enhancement on a Modified Surface Through the Dynamic Wetting of Water Droplets

2012 ◽  
Vol 134 (7) ◽  
Author(s):  
Ho Seon Ahn ◽  
Joonwon Kim ◽  
Moo Hwan Kim
Keyword(s):  
Surface Tension ◽  
Heat Flux ◽  
Contact Angle ◽  
Critical Heat Flux ◽  
Pool Boiling ◽  
Water Layer ◽  
Contact Angles ◽  
Water Droplets ◽  
Dynamic Wetting ◽  
Modified Surface

Dynamic wetting behaviors of water droplet on the modified surface were investigated experimentally. Dynamic contact angles were measured as a characterization method to explain the extraordinary pool boiling critical heat flux (CHF) enhancement on the zirconium surface by anodic oxidation modification. The sample surface is rectangular zirconium alloy plates (20 × 25 × 0.7 mm), and 12 μl of deionized water droplets were fallen from 40 mm of height over the surface. Dynamic wetting movement of water on the surface showed different characteristics depending on static contact angle (49.3 deg–0 deg) and surface temperature (120 °C–280 °C). Compared with bare surface, wettable and spreading surface had no-receding contact angle jump and seemed stable evaporating meniscus of liquid droplet in dynamic wetting condition on hot surface. This phenomenon could be explained by the interaction between the evaporation recoil and the surface tension forces. The surface tension force increased by micro/nanostructure of the modified zirconium surface suppresses the vapor recoil force by evaporation which makes the water layer unstable on the heated surface. Thus, such increased surface force could sustain the water layer stable in pool boiling CHF condition so that the extraordinary CHF enhancement could be possible.

Pool Boiling Experiments on Multiwalled Carbon Nanotube (MWCNT) Forests

2006 ◽  
Vol 128 (12) ◽  
pp. 1335-1342 ◽  
Author(s):  
Hee Seok Ahn ◽  
Nipun Sinha ◽  
Mei Zhang ◽  
Debjyoti Banerjee ◽  
Shaoli Fang ◽  
...  
Keyword(s):  
Heat Flux ◽  
Silicon Wafer ◽  
Pool Boiling ◽  
Chemical Vapor ◽  
Film Boiling ◽  
Copper Block ◽  
Multiwalled Carbon ◽  
Boiling Regime ◽  
Vertically Aligned ◽  
Working Liquid

In this study, two silicon wafer substrates were coated with vertically aligned multiwalled carbon nanotubes (MWCNT) “forests” and were used for pool boiling studies. The MWCNT forests (9 and 25μm in height) were synthesized on the silicon wafer substrates using chemical vapor deposition (CVD) process. The substrates were clamped on a cylindrical copper block with embedded cartridge heaters. The heat flux was measured using sheathed K-type thermocouples, which were placed inside the cylindrical copper block. Pool boiling experiments using refrigerant PF-5060 as the working liquid were conducted to obtain the pool “boiling curve.” The experiments were conducted in nucleate and film boiling regimes to investigate the effect of MWCNT height on pool boiling performance. Reference (control) experiments were also performed with an atomically smooth bare silicon wafer (without MWCNT coating). The results show that the MWCNT forests enhanced critical heat flux (CHF) by 25-28% compared to control experiments. For the film boiling regime, Type-B MWCNT (25μm in height) yields 57% higher heat flux at Leidenfrost point (film boiling regime) compared to control experiments. However, for the Type-A MWCNT (9μm in height) the film boiling heat flux values are nearly identical to the values obtained for the control experiments performed on bare silicon.

Effect of Fouling on Nucleate Pool Boiling in Microgravity and Earth Gravity

1999 ◽  
Author(s):  
Daiju Motoya ◽  
Ikuya Haze ◽  
Masahiro Osakabe
Keyword(s):  
Heat Flux ◽  
High Speed ◽  
Pool Boiling ◽  
Film Boiling ◽  
Gravity Level ◽  
Nucleate Pool Boiling ◽  
Bubble Volume ◽  
High Speed Video ◽  
Earth Gravity ◽  
Bare Surface

Abstract Nucleate pool boiling of water on clean and fouling surfaces was conducted in microgravity and earth gravity. The microgravity experiments were conducted in 8 s JAMIC drop shaft in Hokkaido of Japan. Platinum wires of 0.2 mm in diameter with or without fouling scale were used to provide uniform heat flux and measurement of the mean temperature of wires. The generated bubble volume was measured with high-speed video or CCD images. The more vigorous bubbling was observed on the fouling wire compared to that on the clean wire at a same heat flux both in earth gravity and microgravity. The enhancement of the bubbling was associated with the fact that the hydrophilic porous structure in the fouling scale provided the sufficient number of active sites for bubbling nucleation. The wettability of the surface with the fouling scale was much higher than that of the clean bare surface. The bubble departure diameter on the fouling wire was smaller due to the high wettability than that on the clean wire. The latent heat transportation ratio to the total heat flux was calculated with the generated bubble volume measured with high-speed video or CCD images. The ratio was approximately the same at the clean and fouling wires in spite of the apparent difference in bubbling behavior, but it was significantly affected with the gravity level. The ratio increased with an increase of the heat flux in the earth gravity but it remained at the smaller value in the microgravity. The nucleate heat transfer coefficient on the bare surface did not depend on the gravity levels although the bubbling behavior strongly affected with the gravity level. As the wire radius is small compared to the capillary length scale in microgravity, a growing and coalescing bubble sometimes completely covered the clean wire, evaporating all liquid in contact with the surface and inducing a transition to film boiling. However, on the fouling wire, many small bubbles were generated and sprang from the surface in various directions in microgravity. The spring out action of bubbles suppressed the transition to the film boiling on the fouling wire in the present experimental range.

Pool Boiling Critical Heat Flux in Reduced Gravity

1999 ◽  
Vol 121 (4) ◽  
pp. 865-873 ◽  
Author(s):  
D. P. Shatto ◽  
G. P. Peterson
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Qualitative Data ◽  
Pool Boiling ◽  
Heat Fluxes ◽  
Reduced Gravity ◽  
Empirical Correlations ◽  
Using Data ◽  
Semi Empirical ◽  
Reduced Pressures

An experimental investigation has been conducted to measure pool boiling critical heat fluxes in reduced gravity. A horizontal cylindrical cartridge heater immersed in water at reduced pressures during parabolic flights on NASA’s KC-135 resulted in boiling on the heater surface. Visual observations and qualitative data trends indicate that the conventional Taylor-Helmholtz. instability model still governs the critical heat flux mechanism over the range of gravitational accelerations of the current study, which range from 0.0005 < g/go < 0.044. Using data from more than 40 individual tests, two semi-empirical correlations have been developed to account for the effect of thermocapillary flow, which tends to decrease the critical heat flux below the predictions of previous correlations.

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