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Experimental analysis on unsteady characteristics of sheet/cloud cavitating Venturi flow under the effect of dissolved air

Malekshah Emad Hasani, Wróblewski Włodzimierz, Bochon Krzysztof, Majkut Mirosław, Rusin Krzysztof

Archives of Thermodynamics

2022

Vol. 43, no 3

63--84

The highly dynamic and unsteady characteristics of the cavitating flow cause many negative effects such as erosion, noise and vibration. Also, in the real application, it is inevitable to neglect the dissolved air in the water, although it is usually neglected in the previous works to reduce the complexity. The novelty of the present work is analysing the impact of dissolved air on the average/unsteady characteristics of Venturi flow by conducting sets of experimental tests. For this purpose, two different amounts of dissolved air at five pressure levels (i.e. five different sets of cavitation numbers) were considered in the study of cavitating flow inside a Venturi nozzle. The fast Fourier transform analysis of pressure fluctuations proved that the shedding frequency reduces almost by 50% to 66%, depending on the case, with adding the amount of dissolved air. However, the reduction of 14% to 25% is achieved by the vibration transducers. On the other hand, the cavity enlarges as well as bubbly flow is observed in the test chamber at a higher level of dissolved air. Furthermore, it is observed that the re-entrant jet, as the main reason for the cavity detachment, is more effective for the detachment process in cases with a lower level of dissolved air, where the re-entrant jet front penetrates more toward the leading edge.

Influence of dissolved air on bubble attachment to highly oriented pyrolytic graphite

Krasowska M., Sellapperumage P. M. F., Ralston J., Beattie D. A.

Physicochemical Problems of Mineral Processing

2018

Vol. 54, iss. 1

163--173

The effect of air, dissolved in 0.1 M KCl solution, on bubble attachment to the smooth hydrophobic surface of highly oriented pyrolytic graphite was studied. The stability of a wetting film in such a system is governed by surface forces, i.e. electrostatic and van der Waals interactions. At the high ionic strength investigated, the electric double layer forces are both weak and of short range, therefore the stability of the wetting film is dominated by van der Waals interactions. The Hamaker coefficient for the highly oriented pyrolytic graphite-KCl aqueous solution-air system is negative and hence van der Waals interactions are repulsive. A repulsive force should stabilize the wetting film, preventing its rupture and bubble attachment to the highly oriented pyrolytic graphite surface. Many experimental studies have found that wetting films are not stable at graphite or coal surfaces, and air bubbles attach. In the present experiments, the stability of the wetting films decreased with increasing amount of dissolved air. The time required for film drainage, rupture, and air bubble attachment was shortened by two orders of magnitude when the experiments were performed in air saturated 0.1 M KCl solution. This instability was attributed to an increasing number of nano- and submicron- bubbles nucleated at the graphite surface. The Hamaker coefficient across the air-KCl aqueous solution-air system is positive and hence van der Waals interactions are attractive, resulting in wetting film rupture and macroscopic air bubble attachment to a highly oriented pyrolytic graphite surface decorated with resident nano- and submicro-metre bubbles.