Institute of Fundamental Technological Research Polish Academy of Sciences (original) (raw)
Related papers
Experimental analysis of vapour bubble growing on a heated surface
2002
Using high speed video camera and numerical processing of the digital images transient description of the geometry and the interface velocity for vapour bubble growing at the heated surface is achieved. Particle Image Velocimetry and Thermometry are applied to obtain details about velocity and temperature in the surrounding flow field.
Visualization of vapour bubble growth
Journal of Visualization, 2001
The growth of a single microscopic water vapor bubble under low-pressure condition at the heated surface is investigated experimentally. Details about the shape development of the bubble and velocity of the interface are studied using an image-processing scheme specifically developed for this purpose. The interface velocity is evaluated using an optical flow based PIV technique. The flow seeded with thermochromic liquid crystals is used to evaluate velocity and temperature fields surrounding the bubble. It is found that under the investigated conditions observed bubbles usually exhibit non-spherical shape. The bubble detachment precedes development of a neck-like structure. In many details it recalls images of large air bubbles moving through a very viscous fluid or the break-up of a liquid jet.
Automated high-speed video analysis of the bubble dynamics in subcooled flow boiling
International Journal of Heat and Fluid Flow, 2004
Subcooled flow boiling is a commonly applied technique for achieving efficient heat transfer. In the study, an experimental investigation in the nucleate boiling regime was performed for water circulating in a closed loop at atmospheric pressure. The testsection consists of a rectangular channel with a one side heated copper strip and a very good optical access. For the optical observation of the bubble behaviour the high-speed cinematography is used. Automated image processing and analysis algorithms developed by the authors were applied for a wide range of mass flow rates and heat fluxes in order to extract characteristic length and time scales of the bubbly layer during the boiling process. Using this methodology, a huge number of bubble cycles could be analysed. The structure of the developed algorithms for the detection of the bubble diameter, the bubble lifetime, the lifetime after the detachment process and the waiting time between two bubble cycles is described. Subsequently, the results from using these automated procedures are presented. A remarkable novelty is the presentation of all results as distribution functions. This is of physical importance because the commonly applied spatial and temporal averaging leads to a loss of information and, moreover, to an unjustified deterministic view of the boiling process, which exhibits in reality a very wide spread of bubble sizes and characteristic times. The results show that the mass flux dominates the temporal bubble behaviour. An increase of the liquid mass flux reveals a strong decrease of the bubble life-and waiting time. In contrast, the variation of the heat flux has a much smaller impact. It is shown in addition that the investigation of the bubble history using automated algorithms delivers novel information with respect to the bubble lift-off probability.
Flow field around growing and rising vapour bubble by PIV measurement
Journal of Visualization, 2005
A study on flow field measurement around growing and rising vapour bubbles by use of PIV technique is presented. Bubbles were generated from single artificial cavities. Experiments have been conducted with saturated boiling of distilled water at atmospheric pressure. In the experiment fluid velocity field surrounding the bubbles was visualized by use of polyamide tracer particles and a sheet of a YAG pulse laser beam. The images were recorded with a cross-correlation CCD-camera. It has been shown that for lower heat flux density bubble growths in an almost quiescent bulk of liquid. For higher heat flux density the train of bubbles creates a vapour column with strong wake effect. Maximum liquid velocity recorded is approximately equal to the terminal velocity of bubble rising in a stagnant liquid.
2012
Experiments have been carried out to study bubble behavior in subcooled flow boiling of water in a horizontal annulus at mass fluxes from 400 to 1200 kg/m 2-s, heat fluxes from 0.1 to 1 MW/m 2 , and pressures varying from 1 to 4 bar using high-speed visualization methods. National Instruments Labview IMAQ Vision Builder automated image-processing software was used to analyze the images obtained by high-speed visualization to obtain bubble size and bubble density. The parametric effects of pressure, mass flux, and heat flux on bubble behavior have also been brought out. Experimental results were validated by comparing with the predicted bubble sizes by using the Zeitoun and Shoukri (1996) correlation and were found to be in good agreement. It was found that bubble behavior is significantly affected by mass flux of working fluid and applied heat flux, whereas pressure of working fluid influences the bubble formation process indirectly.
Experimental analysis of a single vapor bubble condensing in subcooled liquid
Chemical Engineering Journal, 2002
An experimental investigation of the dynamics of, and the heat transfer to, the vapor bubbles condensing in a miscible or an immiscible liquid is presented in this paper. Unpublished experiments of Freon-113, pentane and hexane bubbles condensing in water and Freon-113 bubbles condensing in subcooled Freon-113 are analyzed and compared to previously published experiments of pentane/water, isopentane/water and pentane/glycerol systems. The experimental results of both the mechanical and thermal behaviors are compared to existing models. Throughout the comparison, we examine the effect of the shape and rigidity of condensing bubbles as well as the effects of the contaminants and noncondensibles on the velocity of, and the heat transfer to, the bubbles. Empirical correlations for the drag coefficient and the Nusselt number for a wide range of experimental parameters are developed. These correlations are simple to use (especially in contrast to existing complicated models requiring numerical solutions) and agree well with the experimental results.
International Journal of Heat and Mass Transfer, 2018
This paper presents results of the experimental study of the subcooled boiling flow on the surface of the vertical cylindrical steel heater. The initial stages of formation of the fully developed nucleate boiling were investigated over the unsteady stepwise heat flux range from 0.56 to 1.76 MW/m 2. The field temperature of heated liquid layers at the moment of the bubble departure was described by numerical modeling under nonstationary conditions. A high-speed digital video camera was used to capture the bubble dynamics. The effects of heat release conditions and inlet water temperature on the bubble maximum diameter and nucleation frequency were studied. We found that the increased heat flux results in reduction of the maximum diameter at non-stationary heat release. Calculations on the basis of prediction models showed that the non-stationary nucleate boiling data cannot be generalized using the models developed by Prodanovic et al. (2002) and Song (2016) with the coefficients obtained for the stationary boiling. However, the heat balance approach to calculation of the bubble maximum diameter proved to be promising, and the correlation between the bubble diameter and the overheated layer thickness was verified.
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...
Dynamics of a Vapor Bubble in Film Boiling and the Superheat Effect
WSEAS TRANSACTIONS ON HEAT AND MASS TRANSFER
This study aims at developing an improved numerical simulation of the film boiling regime phenomenon to understand and visualize the growth of vapor bubble at a heated surface during low and high superheats. The simulation of the bubble dynamics including the bubble growth, departure, coalescence, rising, and frequency of detachment under different wall superheats is numerically investigated. The continuity, momentum, and energy equations are solved for the two immiscible fluids phases using the finite volume method. The phase change model and the results exhibited a good agreement with the theoretical models. The obtained results show that the velocity of bubble growth and its frequency of emission promotes heat exchange. It is found that the shape of a bubble has been influenced by the wall superheat. It is also found that the high superheat generates a large amount of steam in which the steam bubble takes the shape of a fungus. So, a clear correlation exists between heat transfer...
Experiments have been carried out in order to study bubble spacing in subcooled flow boiling of water in horizontal annulus at mass fluxes from 400 to 1200 kg/m2-s, heat fluxes from 0.1 to 1 MW/m2 and pressures varying from 1 to 4 bar using high speed visualization methods. Appropriate automated image processing technique was used to analyze the images obtained by high speed visualization to study bubble spacing. The parametric effect of pressure, mass flux and heat flux on bubble spacing has also been brought out. It was found that bubble spacing is significantly affected by pressure, mass flux and heat flux.