Performance optimization of Screw Compressors based on numerical investigation of the flow behaviour in the discharge chamber

In the field of screw compressors, although the basic operation of such flow systems is well known and the analytical methods for their performance prediction are well established, only few attempts of investigating the flow in screw compressors by means of CFD can be identified in the available literature. The present paper aims at contributing to this field by establishing CFD as permanent tool for compressor design and optimization during the new product development process at a screw compressor manufacturer.The major challenges during this process related to the implementation of a valid meshing technique, as this is a far more unconventional process in the case of screw compressors than any other meshing techniques available within the commercial solvers, due to the very tight interlobe clearances. The definition of an optimum meshing procedure and, subsequently, of appropriate boundary conditions, allowed the successful setup of flow simulations in an oilfree screw compressor and the prediction of the compressor performance, with a good degree of confidence. Within the scope of the present paper, the CFD analysis focused on two main objectives. Firstly, a baseline model for the optimization exercise was determined: the discharge chamber (discharge port, pipe and flanges) of the modular casing of the single-wall/ rolling-element bearing/ compressor, characterized by a rotor diameter of 127 mm. Secondly, the performance of the baseline was assessed against that of several new designs which aimed at improving the gas flow in the discharge chamber.At the comparison stage, the correct definition of accurate performance indicators was very important, as these indicators had to capture the overall flow improvements achieved by the new designs. The flow analysis included both qualitative and quantitative evaluations of parameters like the variation of the pressure and temperature in the discharge chamber, torque, power, and entropy variation. This detailed analysis allowed for design improvements of the discharge chamber to be implemented and “virtually” tested in order to determine an optimized compressor geometry, which will be subsequently incorporated in the actual modular casing of the compressor.