Numerical assessment of a two-phase Tesla turbine: Parametric analysis

 The scenarios on the future energy systems invariably point to heat pumps as an emerging technology to reach efficiency goals alongside energy and CO2 reduction goals and to a progressively increasing use of chillers and refrigeration units. In this study, a technology that could enhance the efficiency of systems based on inverse cycles is studied. Particularly, the two-phase flow behaviour of a Tesla turbine is numerically investigated. The main objectives were to clarify the role of the second phase, the actual operating range, and examine the flow mixing. Two computational approaches are developed, including the CFD analysis by commercial software and a customised home built mathematical model. The calculations are performed for twophase R404a fluid over a range of rotational speed, plate gap size, and plate roughness. All the critical liquid-vapour interactions are discussed and are determined utilising the CFD solution of the Eulerian-Eulerian approach with a frame motion technique. The other model is a homogeneous finite-difference solution, and the mass transfer is directly determined based on the phase diagram of the fluid neglecting other phase interaction parameters. Two approaches are compared to some available experimental results from the literature, revealing an excellent agreement. The results show the average power output of 0.8 Watts with a delivered torque of 3.6 mN-m at a rotational speed of about 2000 RPM. The numerical analysis explains the different effects of the two-phase conditions on the turbine efficiency, paving the way to an accurate design of boundary layer turboexpanders operating under these conditions.