Frequency response of pressure pulsations and source identification in a suction manifold

A complicated suction manifold geometry is modelled as a simplified cylindrical annular cavity to study gas pulsations in a multi-cylinder compressor. Linear acoustic plane wave theory and a four pole parameter formulation are used to derive and solve the governing inhomogeneous equation for the forced pressure response in the manifold. By matching the lowest natural frequency of the idealized annular cavity model to that of the actual system using finite element modelling, the best possible match between the free response characteristics of the complicated and idealized manifolds is obtained. A simulation procedure for estimating gas pressure pulsations in the annular cavity connected to an anechoic inlet pipe is then described. Complicated interactions between multiple cylinder valve ports are shown to produce interesting changes in the frequency response for changes in the operating speed, and hence, the flow rate characteristics through the valves. By including a delay time for opening the valve in the mass flow rate profiles and considering the difference between the experimental and simulation results, the variable stroke of the piston and the delay times for opening the valve are estimated without solving the valve dynamic and thermodynamic equations. It is shown that the estimate of the gas pulsation in the suction manifold, which was identified using an iterative procedure, is in good agreement with the experimental result in the case of lower mass flow rate. By applying mass flow rate sinks at each valve as identified, the correlation between analytical and experimental results is shown to be much better than if the idealized, kinematically obtained source functions are used instead.