The Tesla turbine is an unconventional type of turbine, invented over 100 years ago by Nikola Tesla. Its rotor consists of several parallel circular disks with small gaps in between. At the outer circumference of the disks, nozzles feed a working fluid into the disk gaps, that travels on a spiral path radially inward to the outlet openings in the centre of the disks. The wall shear stress that the fluid exerts on the disks in circumferential direction drives the rotor. So far, Tesla turbines are only known to a small research community and are not used industrially. While their efficiency is generally lower compared to conventional bladed turbines, the rotor design is simpler and more cost-efficient. Along with other unique traits, this possibly puts them at an advantage in certain small-scale expander applications, for example in the utilization of waste energy from industrial processes. To provide a basis for comparing and evaluating Tesla turbines, this work summarizes the state of the art of Tesla turbine research and builds upon it with results from analytical modelling, numerical fluid simulation and experiment on a new application-oriented 5 kW Tesla turbine test facility. Practical design guidelines and strategies are provided. The basis for prediction of Tesla turbine performance is modelling of the spiralling flow inside the flat cylindrical annulus formed by the disk surfaces. A detailed summary of existing analytical solutions, derived from simplified Navier-Stokes equations, is given and two ways of simulating Tesla turbine flow using commercial numerical fluid simulation software are outlined. For choosing the right modelling approach, some knowledge is necessary about whether the flow inside the disk gap is expected to be laminar or turbulent. Three attempts at locating a theoretical regime boundary through linear stability analysis are summarized. Together with previous experimental results, this allows to predict the flow regimes based on a mass flow parameter and a rotational speed parameter. More insight into the flow and additional validation is provided by results from a new direct numerical simulation. A comparison between some laminar analytical models of Tesla turbine flow, numerical simulation with turbulence modelling and previous experimental results shows that the analytical models are accurate in the laminar region of the stability map, and some distance beyond the theoretical stability boundary. The summary and evaluation of modelling approaches is followed by description of a new Tesla turbine test facility, that is used to generate performance maps of a real, application oriented, air-driven Tesla turbine. It is designed to allow modification of some turbine design parameters to evaluate their impact on turbine performance. Different stator designs with one to four nozzles and different nozzle geometries are tried out as well as various rotor configurations with different disk thicknesses, disk spacings, edge geometry and surface roughness. The best turbine performance is achieved for the thinnest disks and the smallest disk spacing, at a power output of over 5 kW and an isentropic efficiency just below 50%. Some test runs are dedicated to measuring turbine losses, which allow to qualitatively rank loss mechanisms. Especially for a small ratio of disk spacing to disk thickness, the space between nozzle and rotor is identified as a major source of turbine losses. The last chapter compiles findings from previous chapters and from literature into advice on some Tesla turbine specific design challenges and chooses and develops a preliminary design strategy. A Tesla turbine model is set up from an analytical rotor model and simplified loss modelling. The outlined design process guides the turbine designer through choosing the major design parameters, for example rotor diameter and disk spacing. Generally, high rotational speed is beneficial for Tesla turbine performance, however the choice of the radial mass flow parameter involves an optimization conflict. Lower mass flow is beneficial for turbine efficiency, but disadvantageous for the ratio of power output to machine size.    «

The Tesla turbine is an unconventional type of turbine, invented over 100 years ago by Nikola Tesla. Its rotor consists of several parallel circular disks with small gaps in between. At the outer circumference of the disks, nozzles feed a working fluid into the disk gaps, that travels on a spiral path radially inward to the outlet openings in the centre of the disks. The wall shear stress that the fluid exerts on the disks in circumferential direction drives the rotor. So far, Tesla turbines are...    »