Simulation of a supersonic stator vane under two-phase inlet conditions

Two-phase expansion has previously been considered as a means to improve the performance of ORC systems for waste-heat recovery applications. However, ORC turbomachinery has almost exclusively been designed for operation with either superheated or saturated vapour, whilst during experimental testing turbine operation close the saturation region is generally avoided. However, for high temperature ORC systems, characterised by high molecular weight fluids, high expansion ratios and supersonic flows, it is postulated that a degree of wetness at the turbine inlet could be accommodated. This raises the question of whether an existing ORC turbine could be operated under two-phase inlet conditions, and whether two-phase expansion could be realised with existing ORC turbomachinery. This work presents an investigation of the performance of a supersonic stator vane, initially designed for superheated expansion, under two-phase inlet conditions. The stator geometry evaluated is installed on the ORC test rig at Lappeenranta University of Technology and was designed for the expansion of MDM from turbine inlet conditions of 265 °C and 8 bar to a Mach number of 2.4. A one-dimensional isentropic analysis is first conducted under the assumption of homogeneous flow to predict the variation in pressure and velocity if the existing area distribution is mapped onto two-phase inlet conditions with the same inlet pressure. The results indicate that for inlet vapour qualities above 0.65 the two-phase region is confined to upstream of the throat. This investigation is followed by a two-dimensional CFD simulation of the stator vane with inlet vapour qualities of 0.85, 0.65 and 0.45. As an initial assumption the two-phase mixture is modelled as a homogeneous binary mixture, and non-equilibrium effects are neglected. The CFD results are consistent with the one-dimensional predictions, indicating that for vapour qualities of 0.85 and 0.65 the two-phase region is confined to upstream of the throat, whilst for a vapour quality of 0.45 the transition to superheated vapour occurs in very close proximity to the throat. In all cases, there is no significant shift in the velocity triangles at stator outlet, indicating similar rotor performance could be expected. Finally, the validity of the equilibrium assumption is discussed.