Bubble coalescence at constant wall temperatures during subcooled nucleate pool boiling (original) (raw)
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Numerical Study of Bubble Coalescence Heat Transfer During Nucleate Pool Boiling
Heat Transfer Engineering, 2018
Bubble growth during nucleate boiling in a large pool of liquid was modeled by numerically solving the unsteady Navier-Stokes laminar flow equations with the energy equation to predict the vapor and liquid flow fields. The analysis assumed two-phase, transient, three-dimensional, laminar flow with the Boussinesq approximation for the buoyancy. The volume of fluid method was used with the level set method to predict the bubble interface motion. The numerical investigations studied the dynamics and heat transfer rates associated with the coalescence of bubbles generated on two microheaters. The results for various wall superheats and liquid subcoolings illustrate the bubble growth and interaction dynamics throughout the coalescence process and the wall heat fluxes associated with the bubble nucleation and coalescence. In some cases, the bubble coalescence traps an evaporating liquid layer between the bubbles that then quickly evaporates resulting in high heat fluxes. In other cases, the bubbles very quickly coalescence while the bubbles are still in the fast inertial controlled growth regime and the liquid layer between the bubbles is pushed out without evaporating, resulting in low heat fluxes as the surfaces are covered with vapor. These results show how similar conditions can lead to very different heat fluxes during coalescence as has been seen experimentally.
Results from numerical simulation and experimental validation of the growth and departure of single and multiple merging bubbles and the associated heat transfer on a horizontal heated surface during pool nucleate boiling under low and earth normal gravity conditions have been reviewed here. A finite difference scheme was used to solve the equations governing mass, momentum and energy in the vapor and liquid phases. The vapor-liquid interface is captured by a level set function while including the influence of phase change at the liquid-vapor interface. Water and PF5060 were used as test liquids.
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.
Study on bubble dynamics for pool nucleate boiling
International Journal of Heat and Mass Transfer, 2000
A characteristic length scale and a time scale are proposed to describe the dynamic growth and departure process of bubbles. A correlation between bubble departure diameter and bubble growth time is established thereby, and a predication formula for bubble departure diameter is suggested by considering the analogue between nucleate boiling and forced convection. #
A bubble dynamics-based model for wall heat flux partitioning during nucleate flow boiling
International Journal of Heat and Mass Transfer, 2017
Many physical mechanisms are responsible for wall heat transfer during nucleate flow boiling, such as evaporation of microlayers, gradual rewetting, transient conduction, and forced convection. The nature of these mechanisms tightly connects with the complex dynamics of nucleating bubbles (e.g., growth, sliding, and merger), leading to considerable challenges of modeling the partitioning of wall heat flux into these mechanisms. In this study, we proposed a mechanistic model for wall heat flux partitioning relying on the coupling of heat transfer mechanisms with relevant bubble dynamics. The heat transfer via evaporation of superheated liquid (including microlayers) and gradual quenching over dry spots during the bubble growth period was determined as the latent heat transported to growing bubbles using bubble energy balance and growth equations. The heat transfer over the areas swept by bubbles while sliding and merger whose thermal effect is counted from after the bubble departure to the instant it changes to forced convection or nucleation was quantified by the conventional transient conduction combining with the bubble growth equation and wall functions. The residual wall heat transfer corresponds to forced convection over the region unoccupied by bubbles and the region it replaces transient conduction during the remaining period of bubbling cycle. These three primary mechanisms mechanistically constitute the present wall heat flux partitioning model that is physically concrete and confirmed to has good predictability against experimental data for nucleate boiling at a variety of flow conditions.
Nuclear Engineering and Design, 2008
This study examines the bubble detachment phenomena under subcooled nucleate boiling conditions, in order to obtain a better understanding of the bubble dynamics on horizontal flat heat exchangers. Refrigerant R134a is chosen as a simulant fluid due to its merits of having smaller surface tension, reduced latent heat, and lower boiling temperature than water. Experiments are run with varying experimental parameters, e.g. pressure, inlet subcooled level, flow rate, etc. Digital images are obtained at frame rates up to 4000 frames/s, showing the characteristics of bubble movements. Bubble departure and bubble liftoff, which are described as bubbles detaching from the original nucleation sites and bubbles detaching from the horizontal heated surface respectively, are both considered and measured. Results are compared against the model proposed by Klausner et al. for the prediction of bubble detachment sizes. While good overall agreement is shown, it is suggested that finite rather than zero bubble contact area should be assumed, which improves the model prediction at the pressure range of 300-500 kPa while playing no significant role at a lower pressure of 150 kPa where the model was originally benchmarked. The impact of heated surface structure is studied whose results provide support to the above assumption.
Experimental Analysis of Bubble Dynamics during Pool Boiling of Water for Heat Transfer Augmentation
2016
ARTICLE INFO The bubble diameter at departure in nucleate pool boiling heat transfer using saturated water affected by wall superheat, size of nucleation site etc. In this paper, effect of wall superheat in nucleate pool boiling heat transfer on single bubble dynamics using saturated water has been studied experimentally. Single bubble is generated using vertical hypodermic needle tip as a nucleation site. The hypodermic needles were used of inner diameters 0.603 mm with a constant depth of 25 mm. Single bubble dynamics was studied using PCO high speed camera operating at 100 frames per second at atmospheric pressure and at a wall superheat of 9 K to 30 K. Bubble growth in saturated water is studied at heat flux 430 kW/m 2 , 900 kW/m 2 , the results show that, bubble departure diameter increases with increase in wall superheat at 430 kW/m 2. The bubble release frequency increases with increase in wall superheat at 900 kW/m 2 .
The bubble departure diameter in nucleate pool boiling heat transfer using saturated water affected by wall superheat, size of nucleation site etc. In this paper, effect of wall superheat in nucleate pool boiling heat transfer on single bubble dynamics using ammonium chloride has been studied experimentally. Single bubble is generated using vertical hypodermic needle tip as a nucleation site. The hypodermic needles were used of inner diameters 0.514 mm with a constant depth of 25 mm. Single bubble dynamics was studied using PCO high speed camera operating at 100 frames per second at atmospheric pressure and at a wall superheat of 3 K to 20 K for heat flux 601 kW/m 2 and 3 K to 30 K for heat flux 950 kW/m 2 . Concentration of ammonium chloride is 2200 ppm at heat flux 601 kW/m 2 and 950 kW/m 2