A QUANTITATIVE STUDY OF FREE AIR RELEASE DURING TRANSIENT FLOW

The phenomenon of air release during hydraulic transients in pipe flow with column separation was investigated experimentally. It is established that when line pressure during a hydraulic transient drops to the vapor pressure of the liquid column separation and cavitation bubbles occur throughout the system, and that air release from saturated water is initiated towards these bubbles by the process of convective diffusion. At the time of cavity collapse, the sudden increase in pressure causes the system to agitate. This agitation significantly increases the rate of air release and with disappearance of all vapor cavities small gas bubbles scattered throughout the system are left behind. These gas bubbles cause a considerable decrease in both the wave speed of the medium and the peak pressures.; From preliminary tests conducted on the model employed in this study and other earlier works, the parameters influencing the mass of released air were identified. These parameters consisted of: the instantaneous line pressure, the time period at low pressure, the maximum mass of releasable air, the initial mass of free air, the agitation, and the temperature of water. These variables in addition to the mass of released air were combined into a set of Pi terms by the method of dimensional analysis.; The experimental model employed in this study was specially designed so that individual column separations could be generated and studied one at a time. Tests were made on air-saturated water and data were collected for three different values of water temperature. The mass of dissolved air was predicted from Henry's Law and the mass of free air was determined by the wave speed equation for gas-liquid mixtures.; With other details of the phenomenon of air released clarified, the main outcome of this study was first the design of a system capable of producing individual column separation and second the derivation of an equation that enables the worker in transient flow to predict the mass of released air from individual column separations. Although this mass depends on another unknown (the peak pressure), it is quite possible in a computer program that an iterative solution starting with an assumption for the mass of released air or the peak pressure may easily provide the answer.