The onset of significant void in up-flow boiling of water at low pressure and velocities (original) (raw)
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Nuclear Engineering and Design, 2007
Visualization was performed for the vapor bubbles in water subcooled flow boiling in a vertical heated tube to measure simultaneously the void fraction and the four fundamental bubble parameters: nucleation site density, bubble release frequency, bubble lifetime and bubble size. Using the mass flowrate and liquid subcooling as the experimental parameters, the changes of void fraction and bubble parameters with the wall heat flux were measured. The results of image analysis showed that the vapor void fraction could be approximated by the function of nucleation site density and bubble lift-off diameter; the bubble lift-off diameter was more influential than the nucleation site density. It was hence concluded that the bubble lift-off diameter could be regarded as the key parameter to determine the vapor void fraction under the present experimental conditions. The strong relation of bubble lift-off diameter to superheated liquid layer thickness was indicated for the future model development studies of bubble lift-off diameter.
Bubble behaviour in subcooled flow boiling at low pressures and flow rates
2001
Bubble behaviour in subcooled flow boiling of water at pressures ranging from 1.05 to 3 bar, bulk liquid velocities from 0.08 to 0.8 m/s, heat fluxes from 0.2 to 1 MW/m 2 and subcoolings from 10 to 30 K was investigated experimentally and analytically. Experiments were carried out on a vertical, annular test section with an inner heating surface and upward flow. High-speed photography at rates of 6000-8000 frames/s captured bubbles from inception to collapse, revealing variations of bubble shapes and sizes, as well as bubble sliding and detachment from the wall. Bubble growth and condensation rates, sliding velocities, variation of bubble lifetime and bubble size with flow rate, subcooling, heat flux and pressure were further examined. New correlations were proposed for maximum and detachment bubble diameters, bubble growth rate, bubble growth time, detachment time and condensation time. High-speed photographic results showed changes in typical bubble behaviour with increasing heat ...
International Journal of Heat and Mass Transfer, 2012
Observation of the bubble behavior was made using a high-speed camera to investigate the mechanisms to cause the net vapor generation in subcooled flow boiling. In the experiments, water was used as the test fluid, the flow direction was vertical upward, and the pressure was kept close to the atmospheric pressure. At high liquid subcooling close to the condition of the onset of nucleate boiling, all the bubbles were lifted off the heated surface immediately after the nucleation to disappear quickly in the subcooled bulk liquid due to condensation. It was found that the void fraction did not increase significantly unless the liquid subcooling became low enough for some bubbles to be reattached to the heated surface after the lift-off. When the reattachment took place, the bubble lifetime was substantially elongated since the bubbles slid up the vertical heated surface for a long distance after the reattachment. The reattachment therefore contributed to an increase in the void fraction. It was concluded that in the experimental conditions tested in this work, the bubble reattachment to the heated surface was a key phenomenon to cause the sharp increase of the void fraction at the point of net vapor generation.
Bubble dynamics at boiling incipience in subcooled upward flow boiling
International Journal of Heat and Mass Transfer, 2012
Bubble dynamics in water subcooled flow boiling was investigated through visualization using a highspeed camera. The test section was a vertical rectangular channel, and a copper surface of low contact angle was used as a heated surface. Main experimental parameters were the pressure, mass flux and liquid subcooling. Although all the experiments were conducted under low void fraction conditions close to the onset of nucleate boiling, no bubbles stayed at the nucleation sites at which they were formed. Depending on the experimental conditions, the following two types of bubble behavior were observed after nucleation: (1) lift-off from the heated surface followed by collapsing rapidly in subcooled bulk liquid due to condensation, and (2) sliding along the vertical heated surface for a long distance. Since the bubble lift-off was observed only when the wall superheat was high, the boundary between the lift-off and the sliding could be determined in terms of the Jakob number. Based on the present experimental results, discussion was made for the possible mechanisms governing the bubble dynamics.
Bubble behavior in subcooled flow boiling of water at low pressures and low flow rates
International Journal of Multiphase Flow, 2002
This study presents experimental data for subcooled¯ow boiling of water at pressures from 1.05 to 3 bar, bulk liquid velocities ranging from 0.08 to 0.8 m/s, and subcooling from 10 to 30 K. Experiments were carried out on a vertical, annular test section with inner heating surface and upward water¯ow. High-speed photography at rates of 6000±8000 frames/s captured bubble behavior from inception to collapse, bubble shapes during lifetime, detachment from the wall and typical bubble size. Bubble growth and condensation rates, and variation of bubble lifetime and size with¯ow rate, subcooling, heat¯ux and pressure were, further, examined and new correlations proposed.
Vapor Bubble Dynamics in Upward Subcooled Flow Boiling During Void Evolution
World Academy of Science, Engineering and Technology, International Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering, 2012
Bubble generation was observed using a high-speed camera in subcooled flow boiling at low void fracti on. Constant heat flux was applied on one side of an upward rectangul ar channel to make heated test channel. Water as a working fluid from high subcooling to near saturation temperature was injec ted step by step to investigate bubble behavior during void development . Experiments were performed in two different pressures condition close to 2bar and 4bar. It was observed that in high subcooling when boiling was commenced, bubble after nucleation departed its ori gin and slid beside heated surface. In an observation window mea n r lease frequency of bubble f b,mean, nucleation site Ns and mean bubble volume Vb,mean in each step of experiments were measured to investigate wall vaporization rate. It was found th at in proximity of PNVG vaporization rate was increased significantly in compare with condensation rate which remained in low value. Keywords—Subcooled flow boiling, Bubble dy...
International Journal of Heat and Mass Transfer, 1998
A visual study of vapor bubble growth and departure in vertical up~ow and down~ow forced convection boiling is presented[ A vertical~ow boiling facility was constructed with a transparent\ electrically!heated test section in which the ebullition process could be observed[ High!speed digital images of~ow boiling phenomena were obtained\ which were used to measure bubble growth\ departure diameters\ and lift!o} diameters[ Experiments were conducted for~ow of FC!76 over a commercially!_nished nichrome heating surface\ with mass~ux ranging from 089 to 555 kg m −1 s −0 and heat~ux ranging from 0[2 to 03[5 kW:m 1 [ The~ow was slightly subcooled "DT sub 0[9Ð4[9>C#\ and boiling occurred at isolated nucleation sites[ A major conclusion of this work is that the observed vapor bubble dynamics between up~ow and down~ow are signi_cantly di}erent[ In the up~ow con_guration\ bubbles departing the nucleation site slide along the heater wall\ and typically do not lift o}[ In the down~ow con_guration\ bubbles either lift o} directly from the nucleation site or slide and then lift o}\ depending on~ow and thermal conditions[ The process of vapor bubble sliding appears to be responsible for enhanced energy transfer from the heating surface\ as evidenced by larger heat transfer coe.cients for up~ow than for down~ow under otherwise identical operating conditions[ Þ 0887 Elsevier Science Ltd[ All rights reserved[ Key words] Boiling^Heat transfer^Vapor bubble dynamics Nomenclature C p speci_c heat ðJ kg −0 K −0 Ł d"t# vapor bubble diameter ðm or mmŁ d d vapor bubble departure diameter d L vapor bubble lift!o} diameter G liquid mass~ux ðkg m −1 s −0 Ł h convective heat transfer coe.cient ðW m −1 K −0 Ł h fg latent heat of vaporization ðJ kg −0 Ł Ja Jacob number\ r l C pl DT sat :r v h fg K mean growth constant ðm s −n Ł n mean growth exponent t time ðs or msŁ t w waiting time ðmsŁ
Nuclear Engineering and Design, 2005
Rise characteristics of vapor bubbles after the departure from a nucleation site in forced convective subcooled flow boiling were studied visually using two synchronized high speed video cameras. The test section was a transparent glass tube of 20 mm in inside diameter, filtrated and deionized tap water was used as a working fluid, and the flow direction adopted was vertical upward. The outer surface of test section tube was electrically heated to generate vapor bubbles inside of the tube. In the present experiments, the mass flux and liquid subcooling were varied within 94-1435 kg/m 2 s and 2.2-10 K, respectively. Since the observations were performed at low heat fluxes to avoid the significant increase in the number of active nucleation sites, the obtained bubble images were clear enough to carry out the detailed image analysis for the rise characteristics of individual bubbles. The following three different bubble rise paths were observed after the departure from nucleation sites: some bubbles slid upward the vertical wall for long distance, while other bubbles were detached from the wall after sliding for several millimeters and then migrated toward the bulk liquid; after the migration, some of the detached bubbles were collapsed in subcooled liquid but others remained close to the wall and were reattached to the wall. The results of detailed image analyses suggested that the variation in bubble shape from flattened to more rounded was of primary importance for the occurrence of bubble detachment from the wall.
Measurement of the maximum bubble size distribution in water subcooled flow boiling at low pressure
International Journal of Heat and Mass Transfer, 2017
Bubbles of various sizes are produced at nucleation sites on a heated surface in subcooled flow boiling. However, although many correlations have been developed so far for the mean bubble size, systematic information is scarce for the bubble size distribution. In the present work, bubble nucleation process in water subcooled flow boiling at low pressure was observed in detail using an ITO-deposited transparent glass plate as the heated surface. It was found that bubbles of different sizes are produced at each nucleation site and moreover the mean bubble size differs considerably between different sites. It was hence considered that the reliability of subcooled flow boiling analysis codes can be improved if the effect of the bubble size distribution is included. In view of this, quantitative investigation was done for the bubble size distribution. It was shown that under the experimental conditions tested in this work, the distribution of the maximum size of individual bubbles produced at nucleation sites are fitted well with the gamma distribution. The dependences of the measured maximum bubble size distribution on important dimensionless numbers were explored to develop new correlations for the bubble size distribution.