Liquid-Vapor Structure near Heating Surface at High Heat Flux in Subcooled Pool Boiling

International Journal of Heat and Mass Transfer

2012

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This study investigated the saturated pool boiling of water on a 12 mm diameter horizontal heating surface coated with titanium oxide (TiO2) nanoparticles under atmospheric pressure. The TiO2 coated surface was formed during nucleate boiling of TiO2 (25 nm diameter)-water-surfactant nanofluid on the plain heating surface at high heat flux conditions. It was found that the CHF of water boiling on the TiO2 coated surface increased up to about 1.8 times the CHF for the uncoated (plain) surface. To examine the mechanism of the CHF enhancement by TiO2 nanoparticle coating, liquid-vapor behaviors close to the heating surface were measured using a conductance probe with a tip diameter smaller than 5 μm. The probe signals and the void fraction distributions showed that there is little difference in the liquid-vapor structure in the boiling on the uncoated surface and on the TiO2 coated surface, that a liquid rich layer (a so called macrolayer) remains on the heating surface, and that in boiling on the TiO2 coated surface it does not dry out even at heat fluxes far higher than the CHF of the uncoated surface. The thickness of the macrolayer formed beneath large vapor masses was determined from the location where the probe signals corresponding to the large vapor masses disappear. It was found that the macrolayers formed on the TiO2 coated surface are thicker than those on the uncoated surface, and it is considered that this is the most likely to be a cause of the CHF enhancement with the TiO2 coated heating surface.

Section: Experimental Apparatusmentioning

confidence: 99%

Section: Macrolayer Thickness Formed Beneath Vapor Massesmentioning

confidence: 99%

CHF and near-wall boiling behaviors in pool boiling of water on a heating surface coated with nanoparticles

International Journal of Heat and Mass Transfer

2012

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“…22). An AC voltage of 24 kHz, the resonance frequency of the measurement circuit, was applied between the conductance probe and the heating surface, and the inverse of the resonance frequency of the electrical circuit, 42 ms, was the time resolution of the measurements.…”

Section: Measurement System With Conductance Probementioning

confidence: 99%

“…Full details of the method used to determine the liquid layer thickness are indicated in the previous paper, 22) and an outline of the method is shown next. The time-series data of the pulse signals were measured at various locations away from the heating surface by moving the probe in the direction normal to the heating surface (total number of pulses: 4,096 or 8,192).…”

Section: (5) Liquid Layer Thickness Beneath Vapor Massesmentioning

confidence: 99%

“…21) Through those measurements, the existence of the liquid layer beneath the vapor mass was confirmed and the thickness of the liquid layer was determined by several researchers. Ono and Sakashita 22) measured the liquid-vapor behaviors in saturated and subcooled pool boiling on upward surfaces, and established that the thickness of the liquid layer beneath the vapor masses increases with increasing degree of subcooling. Furthermore, Ono and Sakashita 23) measured the 2-dimensional dryout behaviors of the liquid layer and concluded that the macrolayer dryout model proposed by Katto and Yokoya 4) is the most appropriate model of the CHF for pool boiling on upward heating surfaces, and that the increase in CHF with the degree of subcooling is due to the formation of a thicker liquid layer.…”

Section: Introductionmentioning

confidence: 99%

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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.

Temperature measurements near a heating surface at high heat fluxes in subcooled pool boiling

Ono

2009

Heat Trans. Asian Res.

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In previous papers (Int J Heat Mass Transfer, 2008;50:3481-3489, 2009;52: 814-821), the authors conducted measurements of liquid-vapor structures in the vicinity of a heating surface for subcooled pool boiling on an upward-facing copper surface by using a conducting probe method. We reported that the macrolayer dryout model is the most appropriate model of the CHF and that the reason why the CHF increases with increasing subcooling is most likely that a thick macrolayer is able to form beneath large vapor masses and the lowest heat flux of the vapor mass region shifts towards the higher heat flux. To develop a mechanistic model of the CHF for subcooled boiling, therefore, it is necessary to elucidate the effects of local subcooling on boiling behaviors in the vicinity of a heating surface. This paper measured local temperatures close to a heating surface using a micro-thermocouple at high heat fluxes for water boiling on an upward-facing surface in the 0 to 40 K range of subcooling. A value for the effective subcooling, defined as the local subcooling during the period while vapor masses are being formed was estimated from the detected bottom peaks of the temperature fluctuations. It was established that the effective subcooling adjacent to the surface remains at considerably lower values than the bulk liquid subcooling. This suggests that, from nucleation to coalescence, the subcooling of a bulk liquid has a smaller effect on the behavior of primary bubbles than the extent of the subcooling would appear to suggest. An empirical correlation of the effective subcooling is proposed to provide a step towards quantitative modeling of the CHF for subcooled boiling.