Journal of Mechanical Science and Technology 32 (12) (2018) 5711~5721 www.springerlink.com/content/1738-494x(Print)/1976-3824(Online) DOI 10.1007/s12206-018-1118-4 The evaluation of numerical methods for determining the efficiency of Tesla turbine operation † Krzysztof Rusin * , Włodzimierz Wróblewski and Sebastian Rulik Silesian University of Technology, 44-100 Gliwice, Poland (Manuscript Received May 8, 2018; Revised July 9, 2018; Accepted July 30, 2018) ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Abstract The Tesla turbine operation is based on the use of tangential stresses arising from the fluid viscosity and turbulence and from the phe- nomenon of the fluid adhesion to the surface it flows past. The paper presents a description and testing of the Tesla turbine model, point- ing to the impact of the applied turbulence models on the prediction of the Tesla turbine operating conditions. Non-stationary simulations are performed using the Ansys CFX 18 commercial code. The following turbulence models are analysed: the RNG k-ε, the k-ω SST and the SST-SAS in two variants of time and space discretization. The flow field structures and the flow unsteadiness occurring in the gaps between the rotor discs are described. The distribution of power unit arising on the discs is determined and the predictions as to the power generated by the turbine coming from numerical analysis and preliminary experimental investigations are compared. A comparison of efficiency estimation is made using different methods. Keywords: Bladeless turbine; CFD simulation; Tesla turbine; Turbine efficiency evaluation; Turbulence models ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction The Tesla turbine, also known as the bladeless turbine or the boundary layer turbine, was built by Nikola Tesla in 1906. It was patented in 1915 [1]. Though successful at the begin- ning, Tesla soon encountered obstacles that at that time were insurmountable. As no appropriate materials capable of with- standing high stresses arising in discs (estimated by Tesla at the level of 350 MPA [2]) were available, the works on the turbine development were discontinued. At present, due to the progress in design materials, the Tesla turbine has again be- come the subject of research. Although the turbine structure is relatively simple, the flow in it is rather complex. The fluid is fed to the rotor at an ap- propriate angle by a supply system made of nozzles. It is in the nozzles where the fluid is expanded, which involves a rise in its velocity. The optimal geometry of the nozzles is ex- tremely important in terms of the turbine performance because the losses occurring there are the main causes of the drop in efficiency [3, 4]. The medium velocity at the nozzle outlet often exceeds the speed of sound, which may generate shock waves [5]. Being strongly non-isentropic phenomena, shock waves also involve a drop in the turbine efficiency. Moreover, propagating in the rotor, they favour flow disturbance and incidents of the fluid jet separation [6]. Another essential fac- tor that affects the turbine efficiency is the ratio between pres- sure values at the inlet and at the outlet of the nozzle. If the fluid in the nozzle expands to a pressure level exceeding the value prevailing beyond the supply system, expansion and secondary compression may occur [7]. These are entropy- generating processes and as such – they cause a decrease in efficiency. Having expanded in the nozzle, the fluid is directed to the region of the rotor composed of flat discs mounted on a shaft. Due to adhesion forces between the particles of the discs and the fluid, the latter adheres to the disc walls. The transfer of energy from the working medium to the disc occurs owing to the moment diffusion, which is an effect of the fluid viscos- ity. The layers of the jet moving more slowly absorb particles from the faster layers of the fluid and thus increase their mo- mentum. Being strongly bound by intermolecular forces with the fluid first layer, the disc also increases its momentum, which makes the rotor rotate. The fluid elements move spi- rally in relation to the outlets located close to the rotor axis. The outlet system geometry and the method in which the working medium is extracted from the turbine also have an effect on the turbine total efficiency [4, 8]. The main problems that may result in a drop in efficiency is an abrupt change in the fluid outflow direction and its high outlet velocity increas- ing the kinetic loss. The Tesla turbine has a number of advantages [9, 10]. The most important is the unsophisticated structure, which means * Corresponding author. Tel.: +48 322371425 E-mail address: krzysztof.rusin@polsl.pl † Recommended by Associate Editor Donghyun You © KSME & Springer 2018