MODELING OF MULTICYLINDER COMPRESSOR DISCHARGE SYSTEMS
Abstract
The present study deals with the analysis and modeling of the gas oscillations in multicylinder compressor discharge systems. Since only the lower harmonics of the unsteady flows are detrimental to the compressor performance, the emphasis is on the lower frequency predictions. The objective is to develop and apply mathematical models for the multicylinder discharge lines. To account for the fluid dynamic interference mechanisms, a modeling philosophy is developed. It incorporates both kinematic and discharge cavity coupling effects. The solution technique is based on the acoustical impedance concept. The impedances are described in the distributed parameters format and include linear friction terms. The theory is applicable for the acoustic plane wave region. It is compatible with the research goal which calls for a low frequency orientation. The mathematical model is coupled with the equations describing the basic compressor cylinder and valves' dynamic processes, and are solved simultaneously. To supplement the mathematical models with the physical understanding of the dynamic nature of the multicylinder discharge cavities, a lumped parameters analysis is performed. It is advanced from the reported previous attempts by evaluating the forced response of the gas oscillations. The critical design points in the geometric configurations and kinematics have been brought to light. The effect of crank phasing on the dynamic response is determined. The theory is applied to a symmetric and opposed-piston refrigeration compressor. Pressure time histories and frequency spectra in the cylinder, at valve exits, and at anechoic lines are predicted. Excellent correlations between computed and measured results are obtained. Experimentally measured impedances could also be utilized. New methods of measuring acoustical impedances at low frequencies are proposed and established. These employ harmonically, randomly, and transiently oscillating pistons. Both input and transfer impedances are measured directly and efficiently. The methods are applied to a two cylinder discharge system, and excellent correlations between theory and experiment are obtained. The various aspects of the problem are discussed.
Degree
Ph.D.
Subject Area
Mechanical engineering
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