Influence Of N-butanol And N-nonanol Concentration On Profiles Of The Local Velocities Of Bubbles
INFLUENCE OF N-BUTANOL AND N-NONANOL CONCENTRATION ON PROFILES OF THE LOCAL VELOCITIES OF BUBBLES
M. Krzan, K. Malysa
Institute of Catalysis and Surface Chemistry Polish Academy of Sciences,
ul. Niezapominajek 8, 30-239 Krakow, Poland
e-mail: ncmalysa@cyf-kr.edu.pl
Description of the motion of bubbles is one of the basic problems in fluid mechanics that has bearing on a wide range of applications from the environmental protection to chemical engineering. Despite that bubbles play a significant role in a wide range of industrial processes there still doesn’t exist a general theory describing motion of bubbles of various dimensions in the absence and presence of adsorption coverage over the bubble surface. Rise velocity of the bubble depends mainly on its size and interfacial properties of the bubble surface. It is rather obvious that bubble size must be the important parameter in the motion of the buoyant bubbles. Interfacial properties of the bubble surface affect its velocity because presence of adsorption layer can decrease fluidity of the gas/liquid interface. When the bubble floats the uneven distribution of surfactant molecules over its surface is induced as a result of a viscous drag exerted by fluid on the moving bubble interface. Adsorption coverage is lowered on the upstream part, while an accumulation of adsorbed molecules of surface active substance takes place at the rear part of the bubble. This gradient of the surface concentration can reduce interfacial mobility and consequently lower velocity of the bubble. However, majority of the existing experimental data describes only impact of the high concentration and adsorption coverage of surfactants on terminal velocity of the bubbles.
The paper presents results of studies on local and terminal velocities, sizes and deformations of bubbles in n-butanol and n-nonanol solutions. The local velocities of bubbles in the distilled water and the n-alkanols solutions of different concentration were determined as a function of the distance from the capillary orifice from which the single bubbles were released. Motion of the bubbles was monitored in a glass square tube (4×4cm) and recorded on
videotapes using the CCD camera and professional video recorder. Stroboscopic illumination (100-200 flashes/s) was applied to obtain 4-8 images of the bubble positions on each frame of the videotape. Sequences of the recorded frames were digitized and analyzed using image analysis software.
It was found that profiles of the local velocities of the bubbles were very much dependent on concentration, especially within the range of the lowest concentrations of the n-butanol and n-nonanol solutions studied. In clean water the bubble released from the capillary increased monotonically its velocity till reaching the terminal value of 34.8 cm/s. At the lowest concentrations of n-butanol and n-nonanol the three stages can be distinguished on the Ulocal=f(distance) dependencies. At first stage the bubbles underwent rapid acceleration till reaching a maximum velocity. In the second stage the bubble velocity decreased and in the third stage a constant value of the local velocities (terminal velocity) was observed. Height and width of the maximum were decreasing with increasing n-butanol and n-nonanol concentrations. Finally, at high enough solution concentrations the maximum disappeared. Thus, profiles of the bubble local velocities in solution of high enough concentration were similar as in clean water, i.e. after initial rapid acceleration a value of the constant terminal velocity was reached immediately, but values of the terminal velocities were much smaller in the n-alkanol solutions. In distilled water the bubbles had diameter 1.48±0.02 mm. With increasing solution concentration the bubble diameters decreased slowly (almost linearly). For example at highest concentration of n-nonanol (6·10-4 mol/dm3) the bubbles had diameters 1.28±0.02 mm.
Variations of the bubble velocities and bubble size and deformations with solution concentration were analyzed in a function of the adsorption coverage at surface of the departing bubble. Time available for the adsorption was 1.6s (time of rapid expansion of surface of the growing bubble) for all concentration of the solutions studied. Adsorption kinetics calculations showed that the equilibrium adsorption coverages were established at the surface of the departing bubbles for all n-butanol concentrations. However, in the case of n-nonanol the adsorption coverages were much smaller than the equilibrium ones. Analysis of the bubble terminal velocities in the function of the actual adsorption coverage over the departing bubbles showed that when the coverage was below ca. 10 % then the terminal velocities were larger than predicted from the existing theoretical models. It shows that this relatively small adsorption coverage was enough to immobilize the bubble surface. Bubble deformation (ratio of the horizontal and vertical diameters) was decreasing with increasing the degree of adsorption coverage from about 1.5 in clean water to ca. 1.05 only at adsorption coverages higher than 10 %, when the terminal velocity was reached.
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