Measurement of surface dryout near heating surface at high heat fluxes in subcooled pool boiling

Journal of Nuclear Science and Technology

2011

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Experiments were carried out to observe boiling behaviors of water on horizontal and vertical surfaces at pressures from 0.35 to 5 MPa. The growth curves of the primary bubbles are well described by the t 1=2 variation over the whole range of pressures. The growth rates of primary bubbles are proportional to the square root of the Jakob number, and agree with the correlation by Labuntsov with the arbitrary constant ¼ 3. The conventional correlations of bubble growth rates, which are directly proportional to the Jakob number, predict slower growth rates at higher pressures. The coalescence behaviors of the primary bubbles were also measured on the vertical surface at 3.66 MPa. The coalesced bubbles, which were formed by the coalescence of two primary bubbles, grow at rates similar to the rates of the primary bubbles. The nucleation site densities measured on the vertical surface at pressures up to 5 MPa increase in proportion to about the 1.5th power of the pressure under equivalent heat flux conditions. The dependence of the nucleation site densities on the heat flux is very similar to the results obtained near atmospheric pressure where the nucleation site density is proportional to the 1.5th power of the heat flux. The nucleation site densities measured in the range of pressures of 0.35 to 5 MPa and at heat fluxes of 0.05 to 0.35 MW/m 2 agree fairly well with the available correlations.

Section: Results Of Bubble Growth Rates 1 Previous Studies On Pmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 95%

Section: Introductionmentioning

confidence: 99%

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Bubble Growth Rates and Nucleation Site Densities in Saturated Pool Boiling of Water at High Pressures

Journal of Nuclear Science and Technology

2011

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“…No coating for electrical insulation was provided on the surface of the probe tip, but it has been confirmed that the probe is able to measure liquidvapor behaviors with high accuracy. 23) The conductance probe was connected to a moving stage with an accuracy of 0.5 mm in the direction normal to the heating surface. During the measurements, potassium chloride (KCl) was added to the water to increase the electroconductivity of the water, and the AC voltage was imposed between the heating surface and the probe.…”

Section: Measurement System With Conductance Probementioning

confidence: 99%

Critical Heat Flux and Near-Wall Boiling Behaviors in Saturated and Subcooled Pool Boiling on Vertical and Inclined Surfaces

Journal of Nuclear Science and Technology

Ono

NYUI

2009

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The mechanism of the critical heat flux (CHF) where the departure from nucleate boiling (DNB)-type boiling transition takes place has not been fully elucidated. In this paper, we examine the trigger mechanism of the CHF for saturated and subcooled pool boiling on vertical and inclined surfaces based on measurements of the liquid-vapor behaviors near heating surfaces by using a conductance probe. The angle of inclination was varied from 90 (vertical) to 170 (facing almost horizontally downwards). The probe signals and the void fraction distributions showed that a liquid layer remains beneath the vapor masses moving upward along the heating surface at high heat fluxes near the CHF. The thickness of the liquid layer was determined from the location where the probe signals corresponding to the vapor masses disappeared. The thickness of the liquid layer formed on the vertical surface increased with increasing degree of subcooling, which may be the cause of the increases in CHF with increasing degree of subcooling. The measurements of saturated boiling on the inclined surface confirmed that the orientation of the heating surface greatly affects the period it takes for vapor masses to pass, but it negligibly affects the liquid layer thickness. This suggests that the decrease in CHF with increasing angle of inclination is primarily caused by the lengthening of the duration of vapor mass passage.

“…A liquid-rich zone between vapor masses and a heating surface is usually termed a 'macrolayer', and the authors here also used the term 'macrolayer' in previous papers [11,14]. As shown below, however, the liquid-rich zone is able to reach 1mm in thickness for 2-propanol/water mixtures, and it may be inappropriate to call this a macrolayer, which could be associated with a thin liquid-layer of tens to a few hundreds micrometers.…”

Section: Introductionmentioning

confidence: 99%

Measurements of critical heat flux and liquid–vapor structure near the heating surface in pool boiling of 2-propanol/water mixtures

International Journal of Heat and Mass Transfer

Ono

Nakabayashi

2010

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Saturated pool boiling of 2-propanol/water mixtures on a 12mm diameter horizontal disk under atmospheric pressure was investigated. The CHF of the mixtures increased up to 1.7times the CHF of water at 3.0 to 4.7 mol% concentrations of 2-propanol. To examine the mechanism of the CHF enhancement in the mixtures, liquid-vapor structures close to the heating surface were measured using a conductance probe. It was found that in the boiling of the mixtures, liquid-vapor structures show strong non-uniformity in the radial direction of the heating surface. The void fractions at 0.1mm to 1mm above the heating surface are small at the central region and large near the periphery of the heating surface. The liquid layer between the vapor mass and the heating surface is considerably thicker than that of water at the central region and becomes thinner near the periphery of the heating surface. This thicker liquid layer is likely to be the cause of the CHF enhancement in the 2-propanol/water mixtures.