Numerical analysis of a novel two-phase turbine using thermal non-equilibrium, homogeneous nucleation phase change

Abstract

A total flow geothermal system with objective to co-generate fresh water from the expelled flash is being investigated for well heads located in the east African rift countries. Numerical analysis using two-phase models has been used to design the two-phase turbine of the power plant. A base turbine configuration of a curved rotating nozzle was evaluated against available test data at 400 kPa feedwater pressure, temperature of 117 °C and a back-pressure condition of 6 kPa in a water-cooled condenser operated at near vacuum. Along with the sensitivity of modelling factors on turbine performance, analysis indicated that the channel cross section, curvature, torque characteristics and pumping effect in the nozzle were required to be improved for obtaining a higher power output and efficiency. A novel two-phase turbine configuration has been proposed in this article that consists of a rotor–stator arrangement which facilitates reduction in the pumping effect at the inlet and transports the feedwater to a throat located at a larger radius of the disk. The new nozzle channel curvature is designed such that the suction surface produces negligible opposing torque, and the pressure surface produces a uniformly increasing torque. The new turbine is estimated to produce a 2.3 kW power output at 4623 rpm and although the feedwater flow increased by 34%, a 40% higher specific power output is obtained at an isentropic efficiency of 24%. Fresh water recovery was estimated to be around 12% for these working conditions. Two-phase flow evolution in the turbine has been presented as local variation of pressure, thermal non-equilibrium, and vapour distribution with varying operating speed. The proposed turbine is also capable of producing work output at high back pressure, close to atmospheric conditions so that it can be used in combined power and freshwater generation thermal plants.