The Tesla turbine is a bladeless centripetal-flow turbine invented by Nikola Tesla in 1913. It functions as nozzles apply a moving fluid to the edges of a set of discs. The engine uses smooth discs rotating in a chamber to generate rotational movement due to the momentum exchange between the fluid and the discs. The discs are arranged in an orientation similar to a stack of CDs on an axle. The Tesla turbine uses the boundary-layer effect, instead of the method employed by more conventional turbines, wherein a fluid acts on blades. The Tesla turbine is also referred to as the bladeless turbine, boundary-layer turbine, cohesion-type turbine, and Prandtl-layer turbine. The latter is named for Ludwig Prandtl. Bioengineering researchers have additionally referred to the Tesla turbine as a multiple-disk centrifugal pump. One of Tesla's intended implementations for this turbine was for the generation of geothermal power, which he described in his work Our Future Motive Power.
Theory In the pump, the radial or static pressure, due to centrifugal force, is added to the tangential or dynamic, thus increasing the effective head and assisting in the expulsion of the fluid. In the motor, on the contrary, the first named pressure, being opposed to that of supply, reduces the effective head and the velocity of radial flow toward the center. Again, in the propelled machine, a great torque is always desirable, calling for an increased number of disks and a smaller distance of separation, while in the propelling machine, for numerous economic reasons, the rotary effort should be the smallest and the speed the greatest practicable. In standard steam turbines, the steam must press on the blades for the rotor to extract energy from the steam; the blades must be carefully oriented to minimize the angle of attack to the blade surface area. In other words, in the optimal regime, the orientation of the blades minimizes the angle (blade pitch) with which the steam is hitting their surface area, to create smooth steam flow and to minimize turbulence. This turbulence reduces the amount of useful energy that can be extracted from the incoming steam flow. In the Tesla turbine, considering that there are no blades to be impacted, the mechanics of the reaction forces are different. The reaction force to the steam head pressure builds relatively quickly, in the form of a steam pressure "belt" along the periphery of the turbine. That belt is most dense and pressurized in the periphery as its pressure, when the rotor is not under load, will not be much less than the (incoming) steam pressure. In a normal operational mode, that peripheral pressure limits the flow of the incoming stream, and in this way, the Tesla turbine can be said to be self-governing. When the rotor is not under load, the relative speeds between the "steam compressed spirals" (SCS, the steam spirally rotating between the disks) and the disks are minimal. When a load is applied to the Tesla turbine, the shaft slows down; that is, the speed of the discs relative to the (moving) fluid increases as the fluid, at least initially, preserves its angular momentum. For example, in a 10 cm (3.9 in) radius, where at 9000 RPM the peripheral disk speeds are 90 m/s (300 ft/s) when there is no load on the rotor, the disks move at approximately the same speed as the fluid, but when the rotor is loaded, the relative velocity differential (between the SCS and the metal disks) increases and, at a rotor speed of 45 m/s (150 ft/s), the rotor has a relative speed of 45 m/s to the SCS. This is a dynamic environment, and these speeds reach these values over a time interval and not instantly. Here, we have to note that fluids start to behave like solid bodies at high relative velocities, and in the case of the Tesla turbine, we also have to take into consideration the additional pressure. With this pressure and relative velocity toward the faces of the discs, the steam should start behaving like a solid body (SCS) dragging on the disks' surfaces. The created "friction" can only lead to the generation of additional heat directly on the disk and in SCS and will be most pronounced in the peripheral layer, where the relative velocity between the metal discs and SCS discs is the highest. This increase in the temperature, due to the friction between the SCS disks and the turbine disks, will be translated to an increase in the SCS temperature, and that will lead to SCS steam expansion and pressure increase perpendicular to the metal discs as well as radially on the axis of rotation, and so this fluid-dynamic model appears to be positive feedback for transmitting a stronger "dragging" on the metal disks and consequently increasing the torque at the axis of rotation.
Design
The guiding principle for developing the Tesla turbine is the idea that, to obtain the highest efficiency, the changes in the velocity and direction of movement of fluid should be as gradual as possible. Therefore, the propelling fluid of the Tesla turbine moves in natural paths, or streamlines, of least resistance. A Tesla turbine consists of a set of smooth disks, with nozzles applying a moving fluid to the edge of the disk. The fluid drags on the disk through viscosity and the adhesion of the surface layer of the fluid. As the fluid slows and adds energy to the disks, it spirals into the center exhaust. Since the rotor is a simple disk, it is more robust and easier to manufacture, compared to a traditional bladed turbine. Tesla wrote:
This turbine is an efficient self-starting prime mover which may be operated as a steam or mixed fluid turbine at will, without changes in construction and is on this account, very convenient. Minor departures from the turbine, as may be dictated by the circumstances in each case, will suggest themselves but if it is carried out on these general lines, it will be found highly profitable to the owners of the steam plant while permitting the use of their old installation. However, the best economic results in the development of power from steam by the Tesla turbine will be obtained in plants especially adapted for the purpose.
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