|
|
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.
|
|
| |
|
| |
|
|
Shape of So-Called Ellipsoidal Bubbles Rising in a Liquid
Wichterle, K. ; Smutná, K. ; Večeř, Marek
Small bubbles have a spherical shape. Big bubbles are of an unstable shape. The shape of a big bubble resembles that of pulsating hemisphere with a flat or concave base. Relatively stable medium-sized bubbles are interesting in view of technical applications. The medium-sized bubbles have approximately a shape of an oblate ellipsoid. Shape of smaller bubbles in low viscosity liquids is determined by gravitational force and surface tension force. Eötvös number comprises of the gravitational and surface tension forces. Ellipsoidal shape is expected for 0,2 < Eo < 30. This corresponds to bubbles in water with bubble size 1 - 2000 mm3. Longstanding records of bubble motion made possible to evaluate bubble shape and shape oscillations. Volume of recorded bubbles was 10 to 800 mm3, water and glycerol solutions were used as liquids. A ratio of horizontal and vertical bubble size, given in literature, is correct in principle.
|
|
| |
|
| |
|
|
Koncové efekty v rotační viskometrii za přítomnosti zdánlivého skluzu
Wein, Ondřej ; Večeř, Marek ; Havlica, Jaromír
Common methods of treating primary data from rotational viscometry are tested using numerical simulations of primary circular flow of shear-thinning liquids in an actual sensor Z40 DIN according to ISO 3219 and a new KK (Morse-taper) sensor, suggested for rotational viscometry under slip effect. Theory of the sensors is presented, including the conversion of primary data into fluidity and slip viscometric material functions, as well as a statistical analysis of their relevance. The data treatment, based on the concept of local filtration, is illustrated on several aqueous solutions of Hercules CMC and Welan gum.
|
|
| |
|
|
Movement Periodicity of Wobbling Ellipsoidal Bubbles
Machačová, K. ; Raška, P. ; Wichterle, K. ; Večeř, Marek ; Růžička, Marek
Experiments with bubbles observed in countercurrent flow allow us to study movement of bubbles for long time period and diagnose their speciality. The oscillations of bubble velocity and shape deformation are statistically treated in this work for ellipsoidal bubbles. Prevailing behavior of these bubbles is movement from side to side (zig-zag) with small inclination to helical one. Simple model describing the hydrodynamic causation of horizontal oscillations and their magnitude was suggested. Typical values for bubbles with equivalent diameter from 4 to 10 mm are: rising velocity 200-300 mm/s, maximum lateral velocity 150 mm/s and oscillation frequency 6 Hz. Good agreement between our results and recent publications is enunciate.
|
|
| |
guest ::
