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Hydrodynamika bubliny narážející na stěnu. Experiment a modelování
Zedníková, Mária ; Vejražka, Jiří ; Růžička, Marek ; Drahoš, Jiří
The contribution deals with both experimental and theoretical investigation of collision of a bubble with a horizontal hydrophobic (glass) surface. The terminal bubble velocity, impact velocity, contact time of bubble with the wall and restitution coefficient (ratio of restitution and terminal velocity) are evaluated (by means of a high-speed camera) as a function of bubble size, which is ranging from 0.34 to 2.07 mm. The experimental results are compared with the models based on a balance of forces acting on a bubble during the interaction process. A good agreement is showed between the experimental data and theoretical predictions.
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Optické sondy a jejich přesnost měření
Vejražka, Jiří ; Večeř, Marek ; Orvalho, Sandra ; Sechet, P. ; Cartellier, A.
The optical probes are commonly used for experimental measurements in gas-liquid multiphase systems. During the measurement, it is evaluated the intensity of the light, which reflects from the tip of the optical fiber placed in the measuring point. From the obtained signals, it is possible to evaluate the bubble residence time, local gas hold-up and it is possible to estimate the size and the velocity of the bubbles. In this study, the signal of the optical probe is compared with the data obtained by direct observation by high-speed camera. It was found that optical probe works well in the case of large and fast moving bubbles. In the case of small and slow bubbles the data measured by optical probe are not usable. The dimensionless criterion for minimal Weber number characterizing the reliability of the probe was derived on the base of motion equation for the bubbles.
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Hydrodynamika bubliny narážející na stěnu, experiment a modelování
Zedníková, Mária ; Vejražka, Jiří ; Růžička, Marek ; Drahoš, Jiří
The contribution deals with both experimental and theoretical investigation of collision of a bubble with a horizontal hydrophobic (glass) surface. The terminal bubble velocity, impact velocity, contact time of bubble with the wall and restitution coefficient (ratio of restitution and terminal velocity) are evaluated (by means of a high-speed camera) as a function of bubble size, which is ranging from 0.4 to 1.1 mm. The experimental results are compared with the models based on a balance of forces acting on a bubble during the interaction process. A good agreement is showed between the experimental data and theoretical predictions.
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Bubble Production Controlled by Needle Movement
Vejražka, Jiří ; Stanovský, Petr ; Zedníková, Mária
For the research of multiphase flows, it is often needed to produce bubbles of well-defined size. Examples of such a research are studies of bubble acoustic emission, bubble interactions with solid particles (e.g. in flotation process) or interactions between bubbles. To produce a well-defined bubble is, however, rather difficult, and it is even more difficult to vary the bubble size between different experimental runs. For this reason, we have produces an instrument ("bubble generator", which produces bubbles in a controlled manner, enableng to set indepedently the bubble size, bubbling frequency and total number of bubbles. The bubbling control is achieved by moving the needle, on which the bubbles are produced.
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Hydrodynamické interakce mezi bublinami stoupajícími v řadě
Stanovský, Petr ; Vejražka, Jiří ; Růžička, Marek ; Drahoš, Jiří
Knowledge about hydrodynamic interaction between bubbles are essential to uderstanding of bubbly flows behavior. The aim of this work is to check the existence of stable separation distance between bubbles experimentally. Air bubbles were produced in ultra-filtered water using a special device - the bubble generator. Pictures were taken with a digital SLR kamera. Bubbles were illuminated by the back light, using a light diffuser. Pictures were processed in MATLAB using the Image Processing Toolbox. Equivalent bubble radius, nonsphericity of bubbles, horizontal spacing and vertical displacement. was evaluated from processed pictures. Behavior of bubbles was compared with theoretical assumptions and models.
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Experimental Study of Bubble Interactions with Wall.
Zedníková, Mária ; Vejražka, Jiří ; Růžička, Marek
The work deals with the experimental study of hydrodynamic behaviour of bubbles with various sizes bouncing on a wall in liquid. The aim of the work is to obtain bubble trajectory and bubble velocity. The experiments are made using a high speed camera Kodak Ektapro (max 10 000 fps). From the images, the bubble diameter, its centre, steady-state bubble rise velocity and also local velocity especially during the bouncing. The rebounding conditions are also evaluated. The results are compared with the models available in the literature.
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Numerické modelování impaktního proudění tekutiny na stěnu a experimentální ověření výsledků pomocí teplotní anemomerie
Cvetinović, D. ; Tihon, Jaroslav ; Vejražka, Jiří ; Drahoš, Jiří
The configuration of a submerged, circular, unconfined impinging air jet, which issues from a bell-shaped converging nozzle, is characterized numerically and experimentally. A commercial CFD package is repeatedly employed to simulate three different configurations to validate the different turbulence models, namely k–? Re-Normalized Group (RNG), k–? Shear Stress Transport (SST) and the Reynolds Stress Model (RSM). A hot-wire anemometry technique is applied to investigate extensively the flow field of the impinging jet. The comparison of results suggests that the k–? SST turbulence model is the most reliable for the flow configurations under investigation.
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Spontaneous Bubble Production on the Needle
Vejražka, Jiří ; Zedníková, Mária ; Stanovský, Petr
The study of spontaneous bubble production on the needle is necessary to understand all possibilities to control the bubble production on the needle with desired volume and desired production frequency. For this reason the system, where the gas flows into the chamber and continues through the needle, is studied. This system is modeled using modified model of Oguze and Prosperetti (1993). The model results enable to understand how the chamber volume, the diameter and the length of the needle influence the forming process and size of the bubbles. The experimental results are also presented.
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