Condensation and its effects on turbomachinery operation are well understood and have been widely investigated. However, only little scientific work on condensation in positive displacement machines has been published. Although, depending on machine type, high expansion rates and, as a consequence, significant supersaturation can be achieved for working fluids with a negative saturation vapour curve. In this paper, the effects of spontaneous steam condensation in screw expanders are discussed. Classical nucleation theory is used for the thermodynamic simulation of operational behaviour. The study shows at which point during the expansion phase spontaneous condensation can be expected and typical nucleation rates are determined. The impact of released latent heat during expansion on chamber states is depicted. Furthermore, a comparison of purely metastable expansion with equilibrium expansion is provided in order to show the full-range discrepancies. Additionally, the influence of internal leakage and throttling effects during the inlet phase on the course of spontaneous condensation and droplet growth is analysed. Typical operating parameters are widely varied in simulation so as to identify the impact of steam parameters and expander parameters on the condensation process.
During the operation of twin screw expanders with slightly superheated vapours or even two-phase fluids, surface condensation on machine parts occurs during the filling period and the expansion phase when the working fluid is in contact with cooler inner surfaces. This heat exchange from the working fluid to adjacent machine parts effects the working cycle and the efficiency of these machines. Short time scales and the periodicity of the process indicate the condensation process is best described by models for dropwise condensation. In this paper the effects of surface condensation on the operation of twin screw expanders are initially discussed in a simulation-based investigation. Chamber model simulation coupled with a thermal analysis is used for the thermodynamic simulation, whereby heat transfer coefficients are systematically varied. It is found that during the inlet phase condensate emerges on the inner surfaces of the machine being substantially cooler than the working fluid. This results in a higher mass being trapped within the working chamber and, thus, an increasing mass flow rate of the machine. An increase in power output is, however, not observed. The results obtained from chamber model simulations are finally compared against experimental data of a screw expander prototype.
The chamber model method is still a powerful tool for the thermodynamic simulation of screw machines and is widely used in the industry and in science. When choosing adequate submodels for the compressible flows through machine clearances and the machine ports high simulation accuracy can be reached with minimal computation resources. However, detailed modelling is required when the complex geometry of twin-screw machines is abstracted to a time-dependent zero-dimensional chamber model. In this paper a two-chamber model simulation tool is used for the thermodynamic simulation of an oil-free twin-screw expander operating with dry air as working fluid. Insights into the modelling process are presented and the importance of precise modelling of gap flows between adjacent capacities is discussed. The simulation results are finally validated against experimental data. Integral values obtained from the experiments, such as expander mass flow rate, indicated power, and effective power, are in good agreement with the numerical calculations. To gain a deeper understanding of the thermodynamics of the expander working cycle, additional investigations are carried out at zero speed. The mass flow rates measured for a variation of stationary rotor positions are compared to values calculated with the two-chamber model method.
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