Vapor bubble growth in heterogeneous boiling—I. Formulation (original) (raw)
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International Journal of Heat and Mass Transfer, 1975
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A mechanistic model for predicting the maximum diameter of vapor bubbles in a subcooled boiling flow
International Journal of Heat and Mass Transfer, 2016
Vapor bubbles attached to the heated surface in a subcooled boiling flow usually reach their maximum size during the latter phase of the bubble growth period when the liquid microlayer trapped under them is almost depleted. The heat transfer at the bubble during this phase involves only the transient heat conduction through a so-called relaxation microlayer surrounding the lower bubble surface and the condensation at the bubble dome. On this physical base, a new mechanistic model for predicting the maximum diameter of attached vapor bubbles in a subcooled boiling flow is proposed in this study. The new model is derived from the lumped energy balance for the bubbles. It is then validated using published experimental databases on the maximum bubble diameter measured for subcooled boiling flows of water under a wide range of flow conditions. A good agreement between the predicted maximum bubble diameter and the experimental one is obtained. The average relative error is less than about 35.5%. This model is expectedly worthy of being used in the analysis of subcooled boiling flows.
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International Journal of Multiphase Flow, 2011
Experiments were conducted to analyze flow boiling characteristics of water in a single brass microchannel of 25 mm length, 201 lm width, and 266 lm depth. Different heat flux conditions were tested for each of two different mass flow rates over three different values of inlet fluid temperature. Temporal and spatial surface temperature profiles were analyzed to show the relative effect of axial heat conduction on temperature rise along the channel length and the effect of flow regime transition on local surface temperature oscillation. Vapor bubble growth rate increased with increasing wall superheat. The slower a bubble grew, the further it was carried downstream by the moving liquid. Bubble growth was suppressed for increased mass flux while the vapor bubble was less than the channel diameter. The pressure spike of an elongating vapor bubble was shown to suppress the growth of a neighboring bubble by more than 50% of its volume. An upstream progression of the Onset of Bubble Elongation (OBE) was observed that began at the channel exit and progressed upstream. The effects of conjugate heat transfer were observed when different flow regime transitions produced different rates of progression for the elongation sequence. Instability was observed at lower heat fluxes for this single channel experiment than for similar studies with multiple channels.
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Bubble nucleation and growth to confinement during flow boiling in microchannels lead to high heat transfer coefficients. They may also create pressure fluctuations that change the superheat driving evaporation and cause flow reversals that promote transient dry-out and uneven distribution of flow between parallel channels. The work described in this paper is part of a programme to develop models for these processes that will aid the design of evaporative cooling systems for devices operating at high heat fluxes. Video observations of water boiling in a single copper channel of rectangular cross-section, 0.38 x 1.6 mm and a heated length 40 mm, were performed. The top side of the channel was a glass window. Results are presented for a heat flux, averaged over the area of the three metal sides, of 210 and 173 W/m 2 K for incompressible and compressible inlet flow conditions. The inlet pressure was about 1.12 bar and the mass flux was 747.5 kg/m 2 s for both conditions examined. The results demonstrated the strong influence of compressibility on the mode of bubble detachment and growth and therefore on flow patterns, pressure fluctuations and heat transfer rates. The fluid mechanics of boiling in this size channel were also successfully investigated by 3-D numerical simulation for bubbles growing at a defined rate with a fixed inlet flow rate using the 3-D CFD code FLUENT 6 (no upstream compressibility). The study examined the fluid mechanics of bubble motion with heat transfer, but the mass transfer across the bubble-liquid interface was not simulated in the present work. A small vapour bubble was injected at the wall to ensure the bubble generation is under a quasi nucleation condition. Its growth was driven by an internal source of vapour, at a rate derived by analysis of the experimental measurements of growth. The simulation reproduced well the observed motion and shape of the bubble. The simulation was then extended to model bubbles generated and growing randomly in a 2-D channel.
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 Dynamics and Boiling Heat Transfer in Microsystems
ASME 2008 6th International Conference on Nanochannels, Microchannels, and Minichannels, 2008
Bubble dynamics and boiling heat transfer are received great attention in recent years, due to the wide applications in micro devices or systems. The literature is becoming rich in conventional microchannels. One should be careful to apply these results in silicon microchannels, which have much smoother wall surface and smaller channel size. The fundamental issues related to non-dimensional parameters, bubble nucleation, flow patterns, flow instabilities and heat transfer mechanisms are reviewed in this paper. Besides, future studies on the bubble dynamics and boiling heat transfer are suggested.
A numerical study of bubble growing during saturated and sub- cooled flow boiling in micro channels
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A CFD study of bubbles growing in a mini-channel with a diameter of 0.64 mm has been done. Coupled level set and volume of fluid (CLSVOF) method is applied to capture the two phase interface. Georeconstruct method is used to re-construct the two-phase interface. A constant velocity inlet boundary with mass flux 335 ���� /�� 2 �� and a heated boundary wall with constant heat flux (10���� /�� 2 ) is applied. Both saturated and sub-cooled inlet condition are studied. The growth of bubbles and the transition of flow regime differs each other under these two conditions. Sub-cooling significantly lowers the bubble growth rate. However, it does not affect the heat transfer coefficient at the same level due to its complicated heat transfer mechanism.
Dependence of bubble behavior in subcooled boiling on surface wettability
Nuclear Engineering and Design, 2010
This paper presents the results of visualization experiments that were carried out to investigate the dynamics of vapor bubbles generated in water pool boiling. In the experiments, vapor bubbles were generated on a vertical circular surface of a copper block containing nine cartridge heaters, and the contact angle of the heated surface was used as a main experimental parameter. The experiments were performed under subcooled as well as nearly saturated conditions. To enable clear observation of individual bubbles with a high speed camera, the heat flux was kept low enough to eliminate significant overlapping of bubbles. When the contact angle was small, the bubbles were lifted-off the vertical heated surface within a short period of time after the nucleation. On the other hand, when the contact angle was large, they slid up the vertical surface for a long distance. When bubbles were lifted-off the heated surface in subcooled liquid, bubble lifetime was significantly shortened since bubbles collapsed rapidly due to condensation. It was shown that this distinct difference in bubble dynamics could be attributed to the effects of surface tension force.