Numerical Simulation and Experimental Validation of the Dynamics of Multiple Bubble Merger During Pool Boiling Under Microgravity Conditions (original) (raw)
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Single-Bubble Dynamics During Pool Boiling Under Low Gravity Conditions
Journal of Thermophysics and Heat Transfer, 2002
Results of an experimental study on growth and detachment mechanisms of a single bubble on a heated surface conducted during the parabola ights of the KC-135 aircraft are described. An arti cial cylindrical cavity 10 ¹m in diameter was etched in the center of a silicon wafer. The wafer was heated on the back side, and the wall superheat was controlled. Degassed distilled water was used as the test liquid. Bubble growth time, bubble size and shape from nucleation to liftoff were measured under subcooled and saturation conditions at system pressures varying from 0.101 to 0.115 MPa. The wall superheats were varied from 2.5 to 8.0 ± C. Signi cantly larger bubble diameters and longer bubble growth periods than those at Earth normal gravity were measured. Bubble diameters as large as 20 mm at liftoff were observed as opposed to about 2.5 mm at Earth normal gravity. Consistent with results of numerical simulations, it is found that for the same wall superheat and liquid subcooling the bubble liftoff diameter can be approximately related to the gravity level through the relation D d / g ¡ 0:5 and the growth period as t g / g ¡ 1:05 . The effect of wall superheat and liquid subcooling on bubble liftoff diameter is found to be small. However, the growth periods are found to be very sensitive to liquid subcooling at a given wall superheat. Small accelerations along the heater surface can lead to sliding motion of the bubble prior to liftoff. At the same gravitational acceleration the liftoff diameter of sliding bubbles is smaller than that of nonsliding bubbles.
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.
Convective Boiling Between 2D Plates: Microgravity Influence on Bubble Growth and Detachment
Microgravity Science and Technology, 2010
The experiment detailed in this paper presents results obtained on the nucleation, growth and detachment of HFE-7100 confined vapour bubbles. Bubbles are created on an artificial nucleation site between two-dimensional plates under terrestrial and microgravity conditions. The experiments are performed by varying the shear flow by changing the convective mass flow rate, and varying the bubble nucleation rate by changing the heat flux supplied. The experiments are performed under normal (1 g) and reduced gravity (μg). The distance between the plates is equal to 1 mm. The results of these experiments are related to the detachment diameters of bubbles on the single artificial nucleation site and to the associated effects on the heat transfer by the confinement influence. The experimental device allows the observation of the flow using both visible video camera and infrared video camera. Here, we present the results obtained concerning the influence of gravity on the bubble detachment diameter and the images of 2D bubbles obtained in microgravity by means of an infrared camera. The following parameters: nucleation site surface temperature, bubble detachment diameter and bubble nucleation frequency evidence modifications due to microgravity.
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...
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.
Bubble spreading during the boiling crisis: modelling and experimenting in microgravity
Microgravity - Science and Technology
Boiling is a very efficient way to transfer heat from a heater to the liquid carrier. We discuss the boiling crisis, a transition between two regimes of boiling: nucleate and film boiling. The boiling crisis results in a sharp decrease in the heat transfer rate, which can cause a major accident in industrial heat exchangers. In this communication, we present a physical model of the boiling crisis based on the vapor recoil effect. Under the action of the vapor recoil the gas bubbles begin to spread over the heater thus forming a germ for the vapor film. The vapor recoil force not only causes its spreading, it also creates a strong adhesion to the heater that prevents the bubble departure, thus favoring the further spreading. Near the liquid-gas critical point, the bubble growth is very slow and allows the kinetics of the bubble spreading to be observed. Since the surface tension is very small in this regime, only microgravity conditions can preserve a convex bubble shape. In the experiments both in the Mir space station and in the magnetic levitation facility, we directly observed an increase of the apparent contact angle and spreading of the dry spot under the bubble. Numerical simulations of the thermally controlled bubble growth show this vapor recoil effect too thus confirming our model of the boiling crisis.
Investigating the Bubble Behavior in Pool Boiling in Microgravity Conditions
2008
time to carry out the experiment. Several parabolas were done to get the required microgravity periods to conduct the research. Boiling was done in two cylindrical tanks with fixed boiling pads in the bottom of the tank. Thermo couples were used to determine the temperature while running the experiment and temperature data was stored in the computer. While the experiment was running, video cameras were used to capture the boiling behavior which includes the bubble behavior. After conducting 40 parabolas of microgravity levels, most of the successful microgravity level parabolas were examined by reviewing the video clips. Frame by frame analysis of half a second time-frame captures of the clip were used to determine the bubble position. Center of the bubble, shape of the bubble (maximum radius and minimum radius in vertical and horizontal axis), rate of change of radius, vertical and horizontal displacement of the center, rate of change of the position of the bubble coordinates and the vertical component of the bubble velocity were determined by manually analyzing the data. Bubble displacement and the coordinates were measured in pixels. A couple of softwares such as Power DVD and Photoshop image processing softwares were used to examine the characteristics of the bubble frames. In one particular parabolic path (a parabolic path with a most clear and detectable bubble visibility) one single bubble was selected in microgravity boiling period from the bottom of the tank and until it reached the top of the tank to determine the data. After producing the Excel spread sheets of data, results were gathered. As an outcome of the results, bubble radius in vertical and horizontal direction was calculated with the average radius. Under the microgravity conditions bubble's vertical displacement was in a spiral path. It could be expressed graphically in 2-D coordinate system.
International Journal of Heat and Mass Transfer, 1975
Recently a new mechanistic model for pool and nucleate flow boiling was developed in our group. This model is based on the balance of forces acting on a bubble and considers the evaporation of the microlayer underneath the bubble, thermal diffusion around the cap of bubble due to the super-heated liquid and condensation due to the sub-cooled liquid. Compared to other models we particularly consider the temporal evolution of the microlayer underneath the bubble during the bubble growth by consideration of the dynamic contact angle and the dynamic bubble base expansion. This enhances, in our opinion, the model accuracy and generality. In this paper we further evaluate this model with experiments and direct numerical simulation (DNS) in order to prove the importance of dynamic contact angle and bubble base expansion.
International Journal of Heat and Mass Transfer, 2017
A numerical simulation method to model nucleate pool boiling from multiple nucleation sites has been developed and applied to different boiling-water regimes, ranging from discrete bubbles to the vapor mushroom region. The method is based on an interface tracking method in which the liquid-vapor interface is resolved by a color function within the framework of Computational Fluid Dynamics (CFD). Conjugate heat transfer between the wall and the fluid is included in order to capture the temperature field appropriately, since this has a significant influence on the bubble growing process. The microlayer, which is the thin liquid film existing beneath a growing bubble, is taken into account using a specialized model specifically developed by the authors. A validation case is chosen to test the model, based on an experiment by Gaertner, featuring the boiling of water from a heated, horizontal plate under atmospheric pressure. Estimation of the nucleation site density and the local activation temperatures are taken from experimental measurement, and introduced into the simulation through an in-built, nucleation-site model. The applied heat flux ranges from 50 to 300 kW/m 2 , the heat-transfer surface being of dimensions 20 mm  20 mm. The computed heat transfer coefficient agrees well with the measured value, demonstrating the capability of the described CFD model to predict boiling heat transfer in a mechanistic sense for the flow regimes examined. Comparison of bubble shapes between experiment and computation also shows good agreement. In addition, a variety of statistical data, such as the heat flux partitioning and the ratio of vapor-to-liquid area over the heat transfer surface, which cannot be measured in the experiments, but can be derived from the results of the simulations.
Computations of explosive boiling in microgravity
2003
Dynamics of the explosive growth of a vapor bubble in microgravity is investigated by direct numerical simulation. A front tracking/finite difference technique is used to solve for the velocity and the temperature field in both phases and to account for inertia, viscosity, and surface deformation. The method is validated by comparison of the numerical results with the available analytical formulations such as the evaporation of a one-dimensional liquid/vapor interface, frequency of oscillations of capillary waves, and other numerical results. Evolution of a three-dimensional vapor nucleus during explosive boiling is followed and a fine scale structure similar to experimental results is observed. Two-dimensional simulations yield a similar qualitative instability growth.