Experimental analysis of vapour bubble growing on a heated surface (original) (raw)
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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.
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.
Vapour bubble growth and detachment at the wall of shear flow
Heat and Mass Transfer, 2009
N-pentane micro-bubbles are created on a small heated film flushed-mounted at the lower wall of a horizontal channel. The bubble growth and detachment in the shear flow are filmed with a high-speed video camera. The time evolutions of the bubble radius and bubble centre position are measured from image processing. The growth rate is determined and compared to models of the literature. The experimental results are also used to estimate the different forces acting on the bubble during its growth and after its detachment.
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.
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.
Growth of a dry spot under a vapor bubble at high heat flux and high pressure
International Journal of Heat and Mass Transfer, 2001
We report a 2D modeling of the thermal diffusion-controlled growth of a vapor bubble attached to a heating surface during saturated boiling. The heat conduction problem is solved in a liquid that surrounds a bubble with a free boundary and in a semi-infinite solid heater by the Boundary Element Method. At high system pressure the bubble is assumed to grow slowly, its shape being defined by the surface tension and the vapor recoil force, a force coming from the liquid evaporating into the bubble. It is shown that at some typical time the dry spot under the bubble begins to grow rapidly under the action of the vapor recoil. Such a bubble can eventually spread into a vapor film that can separate the liquid from the heater thus triggering the boiling crisis (Critical Heat Flux).
ICONE19-44083 Whole-Field Velocity Measurements of Isothermal Bubble Plume Using PTV
The Proceedings of the International Conference on Nuclear Engineering (ICONE), 2011
The current work focuses on obtaining bubble dynamics properties in a plume by the shadowgraphy technique. Bubble parameters such as the bubbles diameter and velocities were estimated using an in-house developed algorithm. Two gases were independently used in this study, namely Sulfur-Hexafluoride (SF6) and Air. Three separate cases were observed: small (0.01 L/min), medium (0.15 L/min), and large (0.30 L/min) flow rates. The bubble plume was created by injecting the gas from the bottom of a glass tank. The tank was illuminated with two halogen lamps. High speed cameras were situated in the front of the tank. The average axial velocity of Air bubbles was calculated to be 13.25 cm/s, 13.92 cm/s, 28.17 cm/s for small, medium, and large flow rates, respectively. The average axial velocity of SF6 bubbles was calculated to be 8.05 cm/s, 8.02 cm/s, and 24.64 cm/s for small, medium, and large flow rates, respectively. It was found that as the bubbles position increased vertically, they were more buoyant-driven than flow-driven. The present study constitutes part of a larger project devoted to the analysis of thermal-hydraulic characteristics of bubble plumes. Results from these series of experiments are intended to provide new and accurate information for the development and validation of correlations used in two phase flow models.
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...