Dynamic Analysis of Scroll Compressors Systems

Abstract

Vibrations of Scroll compressors system are the sources of noise and stresses in the different parts of the assembly. For obvious reasons of security and comfort, the level of vibration and sound of that device must therefore be studied with the utmost care. This master thesis covers the development of a modeling technology that simulates the dynamics of compressor systems mounted on grommets and connected with rails. A vibration model of a multiple compressors system is implemented in a MATLAB code through different numerical approaches. The representation of the compressor system by finite elements associated with an enforcement of the kinematic constraints between the piping and the compressors using a rigid link appears to be the most cost-effective technique. The influence of the fluid dynamics on the vibratory behaviour of the system is studied through the dynamic stiffness method. It emerges that only the local eigenfrequencies of the system are influenced by the fluid. In the range of operating conditions, the only impact of the fluid was an addition of mass to the piping. The numerical results are compared to an experimental modal analysis. The numerical and experimental global modes of the structure are well correlated. However, huge discrepancies are observed for the local modes in terms of frequencies, these differences being related to approximations chosen to represent the boundary conditions at the binding compressor-pipe. Different ways to simulate the rail are explored using a commercial finite element software. An accurate and cost-effective approach is identified based on a shell element representation. A straightforward formulation of a triangular first-order shell element is implemented in MATLAB and is validated compared to the reference numerical model. The super element formulation through Craig-Bampton drastically reduces the number of dof while preserving the modal properties of rail. The rail element is then integrated to the compressor system. Finally, numerical outcomes are compared to an experimental modal analysis performed on a tandem configuration mounted on rails. Some differences related to the global mode shapes between the numerical and experimental results are observed, resulting from an approximate modeling of the bindings compressor-rail.