Comparing FEM Transfer Matrix Simulated Compressor Plenum Pressure Pulsations to Measured Pressure Pulsations and to CFD Results

Keith Novak
Jack Sauls


Improving efficiency of positive displacement compressors requires improving current computer compressor simulations to better model compressor behavior. This paper will specifically look at increasing fidelity of the 1D thermodynamic compressor simulation program by incorporating suction and discharge pulsations due to complex suction and discharge three dimensional (3D) plenum geometry. This method allows for quick pressure pulsation analysis and enables the design engineer to make changes early in the compressor development cycle. Finite Element Method (FEM) full harmonic analysis is used to calculate suction or discharge plenum’s impedance transfer functions in the frequency domain. The impedance transfer functions are normalized to the FEM conditions and normalized impedance transfer functions are used to model all compressor operating speeds and operating conditions. The normalized impedance transfer functions are independent of mass flow, compressor speed, valve dynamics, sonic velocity, refrigerant density, refrigerant, bore size and stroke size which allow the 1D compressor simulation program to change any of these variables without have to solve for a new FEM impedance transfer function. We have also worked on a new way to include damping and phase shift between mass flow and pressure pulsations that better agrees with experimental and Computational Fluid Dynamics (CFD) results. Since some of this method is novel or may extend beyond the limits of linear acoustic analysis, it requires good agreement to compressor test results and to CFD results. The main part of the paper will show good agreement between simulated pressure pulsations to suction and discharge test results in a reciprocating compressor. The paper will also show our latest improvements in pulsation modeling to obtain better agreement to CFD resultsthan the original comparison in Bilal et al (2010).