Numerical Simulations of the Dynamics and Heat Transfer Associated With a Single Bubble in Subcooled Pool Boiling (original) (raw)
Related papers
2007
Under subcooled boiling conditions, the liquid may contain dissolved noncondensabe gases. During phase change at the bubble-liquid interface, noncondensable gases will be injected into the bubble along with vapor. Due to heat transfer into subcooled liquid, vapor will condense in the upper regions of the bubble and the bubble interface is impermeable to noncondensables. As a result, noncondensabe gases will accumulate at the top of bubbles. This existing gradient of noncondensable concentration inside bubble determines the saturation temperature gradient around the bubble surface. The nonuniform saturation temperature may cause a difference in surface tension which would give rise to thermocapillary convection in the vicinity of the interface. So far, this description is merely a hypothesis. It is felt that much inspection is in vital demand to clarify the uncertainty as to the role of noncondensables throughout this process. In this study, air is taken as noncondensable gas, and the aim is to investigate the effects of noncondensable air on heat transfer and bubble dynamics. The results from a numerical procedure coupling level set function with moving mesh method show the evidence of effects of noncondensable air imposed on heat transfer and the induced flow pattern is presented as well.
Dynamics of vapor bubbles and associated heat transfer in various regimes of boiling
2018
The dynamics of bubble formation during boiling is highly significant considering its influence on the heat transfer rate associated with various applications. Depending on the heat flux, the mode of boiling transforms from the nucleate boiling regime to the film boiling regime. The present thesis is focused on the study of the varying characteristics of boiling regimes through direct numerical simulations. The liquidvapor interface-capturing is performed using the CLSVOF (Coupled Level-Set and Volume of Fluid) approach. In the film boiling regime, the phenomenon of bubble formation is governed by the instabilities at the liquid-vapor interface instigated by the combined influence of surface tension, buoyancy, heat flux, vapor thrust or any other applied external field (electric field in the present study). The dynamical disturbances destabilize the interface which results in bubble formation with the passage of time. The bubble release during film boiling is found to be more of a d...
Nuclear Engineering and Design, 2010
To enhance the multi-dimensional analysis capability for a subcooled boiling two-phase flow, the onegroup interfacial area transport equation was improved with a source term for the bubble lift-off. It included the bubble lift-off diameter model and the lift-off frequency reduction factor model. The bubble lift-off diameter model took into account the bubble's sliding on a heated wall after its departure from a nucleate site, and the lift-off frequency reduction factor was derived by considering the coalescences of the sliding bubbles. To implement the model, EAGLE (elaborated analysis of gas-liquid evolution) code was developed for a multi-dimensional analysis of two-phase flow. The developed model and EAGLE code were validated with the experimental data of SUBO (subcooled boiling) and SNU (Seoul National University) test, where the subcooled boiling phenomena in a vertical annulus channel were observed. Locally measured two-phase flow parameters included a void fraction, interfacial area concentration, and bubble velocity. The results of the computational analysis revealed that the interfacial area transport equation with the bubble lift-off model showed a good agreement with the experimental results of SUBO and SNU. It demonstrates that the source term for the wall nucleation by considering a bubble sliding and lift-off mechanism enhanced the prediction capability for the multi-dimensional behavior of void fraction or interfacial area concentration in the subcooled boiling flow. From the point of view of the bubble velocity, the modeling of an increased turbulence induced by boiling bubbles at the heated wall enhanced the prediction capability of the code.
Heat and Mass Transfer Around a Bubble on a Horizontal Surface in a Subcooled Flow
2016
Early state heat and mass transfer processes around a nucleated bubble in a subcooled flow boiling is studied numerically. The model uses both boiling and condensation processes, in which microlayer evaporation, thermal boundary layer conduction and kinetic theory evaporation and condensation heat and mass transfer mechanisms, are included. In addition, the model includes a microlayer thickness prediction. The model is applied on a two-dimensional and a three-dimensional computational domain at different subcooling temperatures and flow velocities in presence of gravity. The heated surface superheat and the system pressure are kept constant in all simulations. The two phase model, Volume of Fluid (VOF) in ANSYS/FLUENT is used. The results of the study show the importance of each of the heat transfer mechanisms in various stages of the bubble growth. At the early stages of the bubble growth, the microlayer heat transfer is the dominant transfer mechanism, however, as the bubble grows, the evaporation through bubble upper surface becomes significant. Furthermore, results indicate that the isothermal bubble assumption, which is used in prior models, is not valid for the whole life of bubble growth.
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.
International Journal of Chemical Engineering, 2012
Bubble dynamics of a single condensing vapor bubble in a subcooled pool boiling system with a centrally heated cylindrical tank has been studied in the Rayleigh number range 7.9 × 10 12 < Ra < 1.88 × 10 13. The heat source in the system is steam condensing inside a vertical tube. The tube was placed in the center of the tank (300 mm i.d., 300 mm height) which is well filled with water. Experimental investigation has been carried out with High Speed Camera while Computational Fluid Dynamics (CFD) investigation has been performed using Volume of Fluid (VOF) method. The heat source has been modeled using simple heat balance. The rise behavior of condensing bubbles (change in size during rise and path tracking) was studied and the CFD model was validated both quantitatively and qualitatively.
Applied Thermal Engineering, 2019
In this work, a confined space bioreactor with bacterial suspension was developed to experimentally investigate the adsorption of microorganisms at the gas-liquid interface and the effects of inner diameter, inclined angle, and gas flow rate on bubble dynamic behavior. The results indicated that the adsorption of microorganisms at the gas-liquid interface was affected by the bacterial chemotaxis and the gas-liquid interface expansion, and the confined space was conducive to accelerate the detachment of smaller bubble and exacerbate the fluctuation of gas-liquid interface. Moreover, the larger inner diameter induced that the bubble was squeezed by the 2 side walls of the confined space bioreactor, restricting the bubble shocking and slowing down the bubble rising velocity for increasing the bubble residence time. Furthermore, the smallest bubble departure size and the shortest inrush time were observed at horizontal capillary orifice, but the largest inrush bubble number was obtained at down-45°capillary orifice. In addition, the inrush bubble number increased with increasing gas flow rate, but the bubble departure sizes were not much different. The results can be used to optimize the design of aeration system for improving the performance of the bioreactor.
International Journal of Multiphase Flow, 2016
This second of two companion papers presents an analysis of sliding bubble and wall heat transfer parameters measured during subcooled boiling in a square, vertical, upward flow channel. Bubbles were generated only from a single nucleation site for better observation of both the sliding bubble characteristics and their impact on wall heat transfer through optical measurement techniques. Specific interests include: (i) bubbles departure and subsequent growth while sliding, (ii) bubbles release frequency, (iii) coalescence of sliding bubbles, (iv) sliding bubbles velocity, (v) bubbles size distribution and (vi) wall heat transfer influenced by sliding bubbles. The results showed that sliding bubbles involve two distinct growth behaviors: (i) at low mass fluxes, sliding bubbles grew fast near the nucleation site, subsequently shrank, and then grew again, (ii) as mass flux increased, however, sliding bubbles grew more steadily. The bubbles originating from the single nucleation site coalesced frequently while sliding, which showed close relation with bubbles release frequency. The sliding bubble velocity near the nucleation site consistently decreased by increasing mass flux, while the observation often became reversed as the bubbles slid downstream due to the effect of interfacial drag. The sliding bubbles moved faster than the local liquid (i.e., u r <0) at low mass flux conditions, but it became reversed as the mass flux increased. The size distribution of sliding bubbles followed Gaussian distribution well both near and far from the nucleation site. The standard deviation of bubble size varied insignificantly through sliding compared to the changes in mean bubble size.
Symmetry, 2020
In this study, we examined the condensing behavior of single and multiple bubbles of pure steam in a subcooled liquid phase using a fully compressible two-phase homogeneous mixture method that is solved by an implicit dual-time preconditioned technique. The interface between the liquid and vapor phases was determined by the advection equations using a compressive high-resolution interfacing capturing method. The spurious current reduced near the interface, a smoothing filter is applied to the progress curvature calculation. The sensitivity study carried out to predict the empirical constant by using Lee’s mass transfer model. A comparison of the numerical and experimental results highlighted that the proposed model accurately predicted the behavior of the definite condensing bubble. Furthermore, the single and multiple bubble condensation behaviors were investigated for different initial subcooled temperatures, and bubble diameters under various gradient flow, such as velocity gradi...