Transient effects in the discharge of compressed air from a cylinder through an orifice
Chang, Yung Fen
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The thesis describes the expansion process in a cylinder and the accompanying transient flow through an orifice when compressed air is discharged from the cylinder. The orifice which is located at one end of the cylinder is small enough to prevent the development of significant wave action. The aim of the investigation is to study the behaviour of flows through circular orifices under transient upstream conditions and to compare such flows with those through similar orifices under steady state conditions. A critical review of previous work is included from which it is evident that further experimental work is necessary both for a better understanding of the nature of transient flow and to obtain results of practical value. The experiments in the present investigation have been designed from-dimensional considerations. The main conclusions that have been made are as follows: The expansion of the air in the cylinder is not reversible. Gradients of temperature in the radial and axial directions are quickly established after the commencement of discharge and heat is transferred from the cylinder wall to the residual air. In practice it is impossible to achieve a truly adiabatic wall. An experimental correlation between the pressure The expansion of the air in the cylinder is not reversible. Gradients of temperature in the radial and axial directions are quickly established after the commencement of discharge and heat is transferred from the cylinder wall to the residual air. In practice it is impossible to achieve a truly adiabatic wall. An experimental correlation between the pressure and weighted average temperature of the air in the cylinder during expansion has been obtained. This relationship can be used to predict the transient average temperature of the air from the instantaneous pressure in the cylinder. The transient mass flow through a given orifice, and the corresponding discharge coefficient, have been found to be functions of the length-diameter ratio of the orifice, the pressure ratio across it and the heat transferred to the residual air. The latter involves a consideration of the temporal sequence of the discharge which makes it impossible to obtain general statements about transient flows. From dimensional considerations, and from experiments, it has been possible to obtain hypothetical transient mass flows without heat transfer. These flows are consistently lower than those under steady state conditions when geometrical and physical similarity is preserved. Heat transfer to the residual air in the cylinder during discharge increases the instantaneous mass flow through the orifice. Its effect on the mass flow may be quite pronounced for relatively slow discharges. The effect on the transient mass flow of the lengthdiameter ratio of an orifice has been found to be similar to that for steady state conditions.