Preliminary Design of Two-Phase Turbo-Expanders for Partial Evaporation Organic Rankine Cycle Power Systems

The partial evaporation organic Rankine cycle has been shown to be able to improve significantly the net power output of organic Rankine cycle power systems by expanding a fluid in two-phase conditions instead of a saturated or superheated vapor. However, the design of turbo-expanders for this application has not been undertaken yet, due to the complex fluid-dynamic modelling and the structural challenges posed by two-phase flows. This paper aims to provide a first insight into the preliminary design of two-phase axial turbines for partial evaporation organic Rankine cycle power systems by implementing two-phase flows features into a validated mean-line model for axial turbine design. The most important phenomena caused by the presence of a two-phase flow in the cascades are modelled based on the results from previous work conducted on wet steam turbines and on particulate flows in turbomachinery. The modelled phenomena include the deposition of droplets on the blades with the subsequent generation of a liquid film and the mixing between the bulk flow and the generated liquid film downstream the blade trailing edge. The effect of the presence on the two-phase flow on various losses is discussed in the work and the influence of the liquid film is investigated. The results suggest that the impact of the mixing loss on the designed geometries and their performance is significant. However, the decrease in efficiency is significantly mitigated by the reduction of droplets deposition. Therefore, the results from this paper will serve as a basis for future more fundamental research work on the deposition phenomenon and liquid film motion.