A cost-effective power cycle shall be developed to supply heat and electricity (co-generation). The aim of this study is to find an appropriate power cycle and working fluid for a friction turbine application. Due to its low production costs, better scalability and low-maintenance requirements, a friction turbine, also known as Tesla turbine, shall be used. In order to find a proper working fluid and estimate the efficiency and power output,a numerical study was performed on five promising working fluids namely Xenon, Argon, Helium, Air and superheated Steam. The simulation model consists of three disks with a diameter of 0,25 m and an outlet diameter of 0,12 mm rotating at constant 10.000 rpm.It is tried to keep the Gap Reynolds number at a constant 73, to ensure a laminar flow regime between the disks. The outcome of the numerical study showed that all working fluids are suitable. The highest efficiency (shaft power / technical power) has been achieved with steam (96%) and a power output of 126 W/gap. The least efficiency with 89% has been realized with air and a Power output of 118 W/gap. The noble gases are in between these fluids.
By comparing the simulation data and taken into account numerical and estimation errors,all five working fluids are at par. Thus, the choice of the working fluid and the power cycle is an economical. While a hot gas air turbine is not as efficient as the steam turbine, the invest and maintenance costs are lower due to less heat exchangers and necessaries like water reconditioningand a phase separator.
Im Folgenden wird die Entwicklung und Evaluation eines 2,2 kW Tesla Turbinen Prototyps beschrieben. Ausgehend von der konstruktiven Verbesserung der Ein- und Auslassströmung – mit dem Ziel der Steigerung des isentropen Wirkungsgrades – wurde ein Turbinenprototyp gefertigt und auf einem Turbinenversuchsstand erprobt. Hierbei wurde untersucht, ob eine Wirkungsgradsteigerung mit einem Profileinsatz möglich ist. Die gewonnenen Ergebnisse lagen im Erwartungsbereich und zeigten, dass der Profileinsatz zu einer Wirkungsgradsteigerung bei beiträgt.
This article deals with the development of a laminar thin film at the outlet area of a disk turbine. The increasing relative velocities at the outlet -caused by the expansion of the working fluid and the reduction of the cross- section area- are reducing the turbine efficiency, resulting in a lower power output and a higher fuel consumption. To overcome this, a certain amount of the working fluid shall condensate at the disk wall propagating a laminar thin film. Within the liquid film higher shear forces will be transported and therefore the power transfer to the disk optimized. Furthermore, and most important, due to the differences in the specific volume between the liquid and the gaseous phase, the volume flow of the gaseous phase is reduced and therefore the relative velocities are effectively reduced, resulting in a higher turbine efficiency.