Analysis of Water Evaporation in Twin Screw Compressors using Eulerian Multiphase Approach in CFD

3D CFD modelling is widely employed for the detailed analysis of flow and thermodynamics in complex machines such as twin screw compressors. The advantage of such high resolution simulations is that realistic geometry of the rotors and the ports can be captured. Additionally, the physical effects of fluid thermal interactions and leakage are directly captured by these models. Recent studies have shown that for dry air and oil injected air compressors a good agreement is achieved with measurements, in prediction of performance parameters. In these simulations the Eulerian-Eulerian multiphase modelling has been applied. To implement the same model for water injected compressors presents an additional challenge as the liquid water injected into the compression chamber changes phase and evaporates depending on the local saturation and thermodynamic conditions. Water also forms liquid film on the rotors and housing and thereby influences thermal changes. 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. Evaporation inside compression chamber has two important physical effects, one is that the latent heat of evaporating water lowers the gas temperature and the other is the change of state from water to vapour. 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.