Analysis of Water Evaporation in Twin Screw Compressors Using CFD
Journal Title: Mašinstvo - Year 2018, Vol 15, Issue 2
Abstract
Twin screw compressors are extensively used in applications which require pressures higher then allowable pressure in oil free compressors but would benefit from oil free operation such as food processing, pharmaceutical industry etc. The major limitation for achieving high pressures is due to prohibitively high temperature increase in oil free operation modes. One of the solutions is to inject water in the working chamber which will reduce temperatures and allow for sealing. However, excess water requires a system for water separation and a special treatment to prevent development of diseases caused by bacterial growth. On the other hand, if a limited amount of water is injected in the working chamber which will eventually evaporate and reduce temperature of the system without residuals of liquid water in the system. 3D CFD modelling of such processes allows use of realistic geometry of the rotors and the ports to be captured and the physical effects of fluid thermal interactions and leakage to be analysed. In this paper a CFD model for water injected twin screw compressor that accounts for evaporation effects has been presented. Empirical form of the Lee evaporation-condensation model for phase change has been applied in the compression chamber using the phase specific mass and energy sources. Calculation of the amount of water required to just saturate the compressed air at delivery pressure is used to set the mass flow rate of water at two operating speeds. The effect of the suction air temperature and relative humidity is studied. Including vapour as a third phase in the CFD model adds a complexity to already challenging deforming grids required for twin screw domains. Hence a mass and energy source formulation is proposed in the presented study to account for the vapour phase and evaporation effects, thus limiting the number of phases to be modelled. Local drop in gas temperature, distribution of water and regions of evaporation were identified by the simulations. Thermal hot spots on the rotor were located. Reduction in the leakage of gas and its exit temperature was well predicted by the model. Such simplified evaporation model can be further used in the design of water injected screw compressors and extended to predict thermal deformation of the rotors and the housing.
Authors and Affiliations
Sham Rane, Ahmed Kovačević, Nikola Stošić, Graham Stupple
Keywords
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