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Prediction of Compressor Muffler Frequency Response Function using CFD

The acoustic filters of hermetic reciprocating compressors, also called mufflers, are usually developed through acoustic simulation solving the discretized wave equation to obtain the Frequency Response Function, which translates the acoustic response of the muffler. Nonlinear effects are neglected in this approach, which are attributed to flow patterns, as turbulence phenomena, which occur in the contractions, expansions and changing directions within the geometry. The main aim of this work is to investigate the influence of non-linear effects in the acoustic response of mufflers, solving the flow field by computational fluid dynamics (CFD). A discharge acoustic filter design was simplified for the study purpose and simulated using both CFD and Linear Acoustic techniques; the difference in the two approaches is made by comparing the Frequency Response Function (FRF). The flow effects are analyzed varying the compressor piston displacement and operating conditions. FRF predicted by CFD presents reasonable agreement with acoustics approach for lower frequencies identifying resonances and anti-resonances. It was observed increased disagreement for higher mass flow rates due to the predominance of flow effects over acoustics vibrations modes.