Air entrainment by fluid jets and streams

Baxter, Allan M (1955) Air entrainment by fluid jets and streams. PhD thesis, University of Glasgow.

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Model experiments on the preliminary design of a section of pipeline for the Glen Shira hydroelectric scheme showed that under certain conditions air was entrained by a stream of water and carried into the main pipeline. Although the design was altered to overcome this, these experiments emphasised the need for further knowledge of the behaviour of such systems. Information was lacking regarding the proper scaling laws to use, the possibility of predicting behaviour of a prototype from model tests in such cases and the mechanism whereby air is entrained at a free surface. An investigation into the more fundamental aspects of the problem was therefore made and forms the subject of this thesis. The experiments were also designed to supply data which would be useful in the design of hydraulic compressors, devices which depend for their functioning on air entrainment. A technique had to be developed whereby air was entrained by a jet of water striking a free surface and subsequently collected and measured, and the experimental difficulties encountered are described. In the method finally evolved, jets of water from nozzles of various sizes (.0353 inch to .1497 inch diameter) were directed vertically into water in a receiver into which was also led a secondary flow of sufficient magnitude to carry the entrained air into a separator. After separation from the water, the air was measured by a gas flow-meter. A series of experiments was also made to investigate the possibility of increasing the quantity of air entrained, this being of importance for the design of hydraulic compressors. The results of the experiments are analysed and discussed. A comparison is made with the results of Shirley who, in a thesis to the State University of Iowa, has given the only known comparable data. Allowing for the experimental difficulties, the difference in method and range of jet sizes used, reasonably good agreement is recorded. The results show that the Froude Number alone is not the correct similarity criterion if the start of air entrainment is of importance. For the smaller jets, the entrainment starts at a critical value of Weber Number. The quantity of air entrained depends most closely on Reynolds Number. When plotted on a base of d2V2 (where d = jet diameter and V = jet velocity) both the results of Shirley and those of the writer lie scattered about a mean straight line. This curve and others included in the thesis enable estimates to be made of the quantity of air likely to be entrained toy jets working under given conditions. The supplementary experiments showed that simple circular jets are "best for a hydraulic compressor. The data collected would therefore be applicable to the design of such a machine. For model experiments, such as those mentioned at the beginning, the experiments indicated that different similarity criteria are required depending on whether conditions at the start of entrainment or the quantity of air entrained is the major concern. Both cannot be simulated simultaneously in one model.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: A ST Thomson
Keywords: Fluid mechanics, Hydraulic engineering
Date of Award: 1955
Depositing User: Enlighten Team
Unique ID: glathesis:1955-73655
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jun 2019 08:56
Last Modified: 14 Jun 2019 08:56

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