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 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.

Heater Size and Orientation Effect on Pool Boiling of FC-72

2010 ◽  
Author(s):  
Rishi Raj ◽  
Jungho Kim ◽  
John McQuillen ◽  
William Sheredy ◽  
Wendell Booth ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Surface Tension ◽  
Pool Boiling ◽  
Threshold Value ◽  
Orientation Effect ◽  
Relative Importance ◽  
Capillary Length ◽  
Low Gravity ◽  
Size Dependent ◽  
Surface Tension Forces

A recent study on pool boiling for upward facing square heaters reported two pool boiling regimes depending on the relative importance of buoyancy and surface tension forces. At higher gravity levels and/or with larger heaters when the ratio of heater size Lh (length of a side for a square heater) to capillary length Lc was greater than 2.1, boiling was buoyancy dominated and the heat transfer results were heater size independent. Boiling was surface tension dominated and heat transfer results were heater size dependent when Lh/Lc<2.1 (small heaters and/or low gravity conditions). This paper studies the effects of orientation on the balance between buoyancy and surface tension forces. The threshold value of Lh/Lc for transition between pool boiling regimes was found to be 1.8 for heaters oriented at 45°, 90°, and 135°.

Subcooled Pool Boiling Studies on Nano-Textured Surfaces

2007 ◽  
Author(s):  
Vijaykumar Sathyamurthi ◽  
Debjyoti Banerjee
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Pool Boiling ◽  
Type A ◽  
Nucleate Boiling ◽  
Test Fluid ◽  
Type B ◽  
Textured Surfaces ◽  
Transfer Enhancement ◽  
Boiling Regime

Heat transfer in subcooled pool boiling on nano-textured surfaces is reported in this study. Silicon wafers coated with Multiwalled Carbon Nanotubes (MWCNT) forests 9 microns (Type-A), and 25 microns (Type-B) in height and 8–15 nm in diameter with a randomized pitch of 16–30 nm, form the test surfaces. The test fluid is a fluoroinert (PF-5060, Manufacturer: 3M Co.) with a boiling point of 56°C. The test rig is of the constant heat flux type. Heat transfer enhancement of approximately 1.3 to 32% is observed in the nucleate boiling regime for Type-A at subcooling levels of 20°C. Type-B CNT shows an enhancement of about 13–30% in the nucleate boiling regime for 20°C subcooling. Digital images acquired during the tests show increased nucleation occurring on surfaces coated with MWCNT. Potential factors that could explain the observed heat transfer enhancement are: the enhanced surface area (nano-fin effect), disruption of the “microlayer” region in nucleate boiling, an increase in the size of cold-spots and the high thermal conductivity of MWCNT.

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.

Effects of Surface Tension and Binary Diffusion on Pool Boiling of Dilute Solutions: An Experimental Assessment

1999 ◽  
Vol 121 (2) ◽  
pp. 488-493 ◽  
Author(s):  
S. G. Kandlikar ◽  
L. Alves
Keyword(s):  
Heat Transfer ◽  
Surface Tension ◽  
Contact Angle ◽  
Heat Transfer Coefficient ◽  
Boiling Heat Transfer ◽  
Pure Water ◽  
Pool Boiling ◽  
Binary Diffusion ◽  
Water Value ◽  
Diffusion Effects

Pool boiling heat transfer with dilute binary mixtures introduces two additional effects due to binary diffusion, and due to change in the surface tension. The secondary effects due to changes in contact angle and wetting characteristics may also play a role. The present study focuses on identifying these effects for dilute aqueous solutions of ethylene glycol. Pool boiling experiments are conducted to generate data in the range of one to ten percent mass fraction. It is found that in the low concentration region, the binary diffusion effects are insignificant for aqueous solutions of ethylene glycol, and a slight improvement in heat transfer coefficient is observed over the pure water value. The binary diffusion effects are related to a volatility parameter, V1. The heat transfer coefficient does not degrade in the region where V1 < 0.03, and the surface tension does not change appreciably compared to pure water value. This points to the possibility that the changes in contact angle and wetting characteristics play an important role in the pool boiling heat transfer.

The Intertube Falling Film: Part 1—Flow Characteristics, Mode Transitions, and Hysteresis

1996 ◽  
Vol 118 (3) ◽  
pp. 616-625 ◽  
Author(s):  
X. Hu ◽  
A. M. Jacobi
Keyword(s):  
Heat Transfer ◽  
Flow Characteristics ◽  
Horizontal Tube ◽  
Falling Film ◽  
Flow And Heat Transfer ◽  
Transition Criteria ◽  
Continuous Sheet ◽  
Mode Transitions ◽  
Regime Map ◽  
Flow Regime Map

When a liquid film falls from one horizontal tube to another below it, the flow may take the form of discrete droplets, jets, or a continuous sheet; the mode plays an important role in the wetting and heat transfer characteristics of the film. Experiments are reported that explore viscous, surface tension, inertial, and gravitational effects on the falling-film mode transitions. New flow classifications, a novel flow regime map, and unambiguous transition criteria for each of the mode transitions are provided. This research is part of an overall study of horizontal-tube, falling-film flow and heat transfer, and the results may have important implications on the design and operation of falling-film heat exchangers.

Subcooled Pool Boiling in Variable Gravity Environments

2009 ◽  
Vol 131 (9) ◽  
Author(s):  
Rishi Raj ◽  
Jungho Kim ◽  
John McQuillen
Keyword(s):  
Heat Transfer ◽  
Pool Boiling ◽  
Gravity Level ◽  
Dissolved Gas ◽  
Subcooled Liquid ◽  
Size Dependent ◽  
Gravity Effects ◽  
Parametric Effects ◽  
Bubble Growth And Departure ◽  
Continuous Range

Virtually all data to date regarding parametric effects of gravity on pool boiling have been inferred from experiments performed in low-g, 1g, or 1.8g conditions. The current work is based on observations of boiling heat transfer obtained over a continuous range of gravity levels (0g–1.8g) under subcooled liquid conditions (n-perfluorohexane, ΔTsub=26°C, and 1 atm), two gas concentrations (220 ppm and 1216 ppm), and three heater sizes (full heater-7×7 mm2, half heater-7×3.5 mm2, and quarter heater-3.5×3.5 mm2). As the gravity level changed, a sharp transition in the heat transfer mechanism was observed at a threshold gravity level. Below this threshold (low-g regime), a nondeparting primary bubble governed the heat transfer and the effect of residual gravity was small. Above this threshold (high-g regime), bubble growth and departure dominated the heat transfer and gravity effects became more important. An increase in noncondensable dissolved gas concentration shifted the threshold gravity level to lower accelerations. Heat flux was found to be heater size dependent only in the low-g regime.

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