Performance Assessment of Tesla Expander Using 3D Numerical Simulation

Abstract Bladeless or Tesla turbines consist of several flat parallel disks mounted on a shaft with a narrow gap between them. The analytical solution of Navier-Stokes equations for the flow between disks has been extensively studied in the past. However, there is a significant impact of the stator exit-flow conditions on the performance and flow behavior inside the rotor of the Tesla turbine. There has been limited research on flow characterization and performance evaluation of stator-rotor interaction of the Tesla expander using 3D numerical simulation. The challenge arises due to a very high aspect ratio of the Tesla rotor (diameter to gap ratio > 1000). In order to accurately evaluate the torque on the disks due to shear forces, a very fine resolution mesh is necessary. 3D numerical modeling with a hexahedral mesh of the nozzle/stator with the commercial software is presented. A steady-state solution is obtained using a density-based solver for solution stability. The simulation is performed for a wide range of inlet pressures and rotational speeds. The numerical solution does not consider the effects of ventilation losses, end-disk leakages, exit kinetic energy and bearing losses. This work focuses on the stator performance, the stator-rotor interaction, the rotor entry losses due to disk tip, the rotor tip velocity ratio and the degree of reaction on the performance of the Tesla expander. Challenges in modelling and key fluid dynamic features are extensively discussed. The peak efficiency of 58% is predicted for 3 bar inlet pressure and a rotational speed of 30000 rpm for a 3 kW machine with air as a working fluid. The 3D numerical analysis provides insights on flow characterization, mainly stator-rotor interaction and flow between disks at different mass flows, with the aim to contribute to the fundamental knowledge towards further improvement of the Tesla turbine performance. Numerical results are also compared with experimented test results performed on a 100-W and a 3-kW air expander.