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Research Papers: Evaporation, Boiling, and Condensation

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

[+] Author and Article Information
Rishi Raj

Department of Mechanical Engineering,  University of Maryland, College Park, MD 20742rraj@umd.edu

Jungho Kim1

Department of Mechanical Engineering,  University of Maryland, College Park, MD 20742rraj@umd.edu

John McQuillen

 NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, OH 44135John.B.McQuillen@nasa.gov

The thermophysical properties of FC-72 are very similar to those of n-perfluorohexane. Detailed compositions of both fluids are given in Ref. [15].

The logic behind selecting the above mentioned four test runs as high subcooling and low gas and the quantification of subcooling and gas concentration levels will become clear in Sec. 3.

1

Corresponding author.

J. Heat Transfer 134(1), 011502 (Nov 18, 2011) (13 pages) doi:10.1115/1.4004370 History: Received November 24, 2010; Revised May 17, 2011; Published November 18, 2011; Online November 18, 2011

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.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Plot of the heat flux vs. acceleration for gas concentrations of (a) 220 ppm, and (b) 1216 ppm (ΔTw  = 44 °C, ΔTsub  = 26 °C, and Lh  = 7 mm) [11]

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Figure 2

Heat flux vs. acceleration during transition along with steady state data for 1 g, 1.3 g, and 1.6 g (Lh  = 7 mm)

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Figure 3

Plot of the heat flux vs. acceleration for 2.1 mm and 7 mm heater cases, (a) high subcooling, low gas, and (b) low subcooling, low gas

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Figure 4

Plot of the heat flux vs. acceleration at three superheats and subcoolings (Lh  = 7 mm)

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Figure 5

Plot of the heat flux vs. acceleration at three superheats and subcoolings (Lh  = 2.1 mm)

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Figure 6

Boiling curve at earth gravity for different heater sizes at an elevated pressure with FC-72

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Figure 7

Plot of the power law coefficient m vs. T* in the BDB regime (60 data points, RMSerror  = 13%, 11 °C ≤ ΔTsub ≤27 °C, 123 ppm ≤ cg  ≤ 1216 ppm, and 2.1 mm ≤ Lh  ≤ 7 mm)

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Figure 8

Comparison of measured and predicted heat flux values using the gravity scaling parameter for BDB regime (240 data points)

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Figure 9

A schematic of heat flux vs. acceleration for different subcooling and dissolved gas concentrations

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Figure 10

Comparison between bubble sizes at a gravity level just before transition (left), and the stable bubble in equilibrium (bottom) at a gravity level just after transition for two subcoolings (Lh  = 2.1 mm, Twall  = 90 °C)

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Figure 11

Plot of the power law coefficient m vs. T* in the SDB regime

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Figure 12

Comparison of experimental and predicted heat flux value at 0.01 g for the high subcooling and low gas cases (RMSerror  = 12%)

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Figure 13

(a) Kjump at transition versus subcooling, and the (b) comparison of experimental and predicted heat flux values at 0.01g (RMSerror  = 20%)

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Figure 14

Sensitivity of (a) microgravity acceleration, and (b) low-gravity power law coefficient m on heat flux

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Figure 15

Comparison of experimental and predicted heat flux for 2.1 mm heater based on the scaling law

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Figure 16

(a) Data and (b) schematic illustrating the effect of subcooling and gravity on CHF

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