In the pump, the radial or static pressure, due to centrifugal force, is added to the tangential or dynamic (pressure), 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 the supply, reduces the effective head and the velocity of radial flow towards the center. Again, the propelled machine a great torque is always desirable, this calling for an increased number of disks and 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.— Nikola Tesla
In standard steam turbine, the steam has to press on the blades in order for the rotor to extract energy from the speed of the steam, due to the difference between the relative speed of the steam and the blades. In the bladed steam turbine, the blades must be carefully orientated, in the optimal speed regime of the turbine’s work, in such a way as to minimize the angle of the steam attack to the blade surface area. In their words, in the optimal regime the orientation of the blades are trying to minimize the angle (blade pitch) with which the steam is hitting their surface area, as to create smooth steam flow, to try to minimize the turbulence. These eddies are created in reaction to the steam impacting (although the minimized angle in the optimal turbine speed) the surface of the blades. In this dynamic, the first eddies are a loss to the useful energy that can be extracted from the system and second, as they are in opposite direction, they subtract from the energy of 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, actually builds, relatively quickly, as a steam pressure “belt” along the periphery of the turbine. That belt is most dense, pressurized, in the periphery as its pressure, when the rotor is not under load, will be a not much less than the (incoming) steam pressure. In a normal operational mode, that peripheral pressure, as Tesla noted, plays a role of BEMF (Back Electro Motive Force), limiting 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 is minimal.
When a load is applied on the Tesla turbine shaft slows down, i.e. the relative speed of the discs to the (moving) fluid increases as the fluid, at least initially, preserves its own angular momentum. For example, we can take a 10 cm (3.9 in) radius where at 9000 PM 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 with the fluid, but when the rotor is loaded, the relative velocity differential (between the SCS and the metal disks) increases and 45 m/s (150 ft/s) rotor speed has a relative speed of 45 m/s to the SCS. This is a dynamic environment and these speeds reach these values over time delta and not instantly. Here we have to note that fluids start to behave like solid bodies at high relative velocities, and in TT case, we also have to take in consideration the additional pressure. According to the old literature on steam boilers it is said, that steam at high speed, resulting from high pressure source, cuts steel as a “knife cuts butter”. According to the logic, this pressure and relative velocity towards the faces of the discs, the steam should start behaving like a solid body (SCS) dragging on disk metal surfaces. The created “friction” can only lead to the generation of an 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 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 (SCS trying to expand, in order to absorb additional heat energy), and so this fluid dynamic model appears to be a positive feedback for transmitting a stronger “dragging” on the metal disks and consequently increasing the torque at the axis of rotation.