Modelling Screw Compressors and Expanders for HVACR using SCORG

Rotary positive displacement machines such as screw compressors and expanders, rotary vane, gear, g-rotor and other similar machines are common in refrigeration, air-conditioning and utilisation of waste heat. They consume large amount of energy which in turn affects environment and increases carbon footprint. It is essential to reduce their impact on environment which can be achieved by their careful design and optimisation.

Software SCORG is the design platform for screw machines which allows use both high fidelity and lower order prediction models to evaluate performance and optimise compressor or expander for the refrigeration or other system in which it is used. Use of high fidelity Computational Fluid Dynamics (CFD) and Computational Continuum Mechanics (CCM) for analysis of rotary positive displacement machines is exponentially increasing every year. This is mostly due to the development of methods for generating numerical grids of complex deforming domains of such machines.

However, there are several challenges with the commercial CFD/CCM codes which are preventing generic use of these methods for modelling of rotating positive displacement machines. These are

i) conservativeness of the spatial discretisation caused by an inappropriate grid generation,
ii) numerical instabilities and dissipation caused by misalignments of the flow direction and mesh,
iii) application of inappropriate discretisation of equations and
iv) the speed and accuracy of multiphase models used in CFD of positive displacement machines.
In this paper, the authors review methods used for modelling of rotary positive displacement machines and challenges which developers and users of these methods face. The talk offers solutions and directions for resolving these issues which is implemented in the software suite SCORG that allows comprehensive and ubiquitous calculation of rotary positive displacement machines for Refrigeration, Air-conditioning, utilisation of waste heat through ORC systems etc.