Research Papers

Rapid Boiling of a Two-Phase Droplet in an Immiscible Liquid at High Superheat

[+] Author and Article Information
Herman D. Haustein

Faculty of Mechanical Engineering, Technion Israel Institute of Technology, Haifa 32000, Israelherman@tx.technion.ac.il

Alon Gany

Faculty of Aerospace Engineering, Technion Israel Institute of Technology, Haifa 32000, Israelgany@tx.technion.ac.il

Ezra Elias

Faculty of Mechanical Engineering, Technion Israel Institute of Technology, Haifa 32000, Israelmerezra@tx.technion.ac.il

J. Heat Transfer 131(12), 121010 (Oct 15, 2009) (7 pages) doi:10.1115/1.3220146 History: Received September 11, 2008; Revised February 25, 2009; Published October 15, 2009

This work studies experimentally the rapid boiling of a droplet rising in a host liquid environment, within a range of superheats (0.2<Ja<0.5) not previously investigated. The direct-contact rapid-boiling process has many advantages in the fields of heat exchange and multiphase flow. By taking into account the superheat, heat transfer, and hydrodynamics of the multiphase-droplet the aim of this study is to create greater insight into the character of this transient-boiling process, for the first time. The sudden depressurization of a water column led to the rapid boiling of liquid propane droplets rising by buoyancy. During this millisecond boiling distinct stages were identified. Appropriate critical times for the transition between stages were defined by a simplified model, among these a novel criterion for the sudden pause in boiling caused by the engulfing liquid-film's collapse. Good agreement was found between these predicted time-points and measured changes in the boiling profile. This form of boiling, though being very rapid and sustaining high heat transfer rates, is still calm in nature, therefore, more predictable and widely applicable. Understanding this form of boiling suggests that the “design” of the boiling curve may be possible by setting the initial parameters.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

Schematic of the water column experiment setup

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Figure 2

Configurations at different levels of boiling (N): (a) initial (observed), (b) rapid (observed and predicted), and (c) liquid-film collapse (predicted)

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Figure 3

The photographic boiling sequence of propane two-phase droplet in water (0.3 mm bubble in a 2.1 mm droplet initially, within an environment at T∞=303 K); boiling completes around 35 ms—last two frames are after completion

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Figure 4

Pressure and bubble diameter of a typical multiphase-droplet boiling

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Figure 5

The calculated boiled mass for a multiphase-droplet: with early collapse (Di=0.8 mm, Do=2.1 mm, at 300 K) and with late collapse (Di=0.3 mm, Do=2.1 mm, at 303 K)

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Figure 6

The velocity measured for two different droplets: with a large and a small initial bubble (t3—liquid-film collapse, S.C.—transition to spherical-cap shape, and P.O.—pressure overshoot)

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Figure 7

Vapor bubble growth experiments versus the two-fluid system theory (due to initial delay, experiments’ time shifted by 6 ms)

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Figure 8

The temperature evolution prediction at the boiling front and outer surfaces of the liquid-film (eq—interface equilibrium and es—energy summation)

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Figure 9

The normalized form of the superheat, heat transfer, and the resulting mass flux

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Figure 10

The comparison of predicted critical times (t1-t2-t3, accordingly) with observed ones: (a) ambient pressure variation 0.2<Ja∗<0.35 and (b) ambient temperature variation 0.3<Ja∗<0.45

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Figure 11

The depressurization rate for different ball taps and a plug on different size ports

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Figure 12

Parameters of observed shapes of multiphase-droplets: sphere, ellipsoid, and truncate sphere (spherical-cap)



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