You might wonder how the energy of a fluid can cause a metal disk to spin. After all, if a disk is perfectly smooth and has no blades, vanes or buckets to "catch" the fluid, logic suggests that the fluid will simply flow over the disk, leaving the disk motionless. This, of course, is not what happens. Not only does the rotor of a Tesla turbine spin -- it spins rapidly.
The reason why can be found in two fundamental properties of all fluids: adhesion and viscosity. Adhesion is the tendency of dissimilar molecules to cling together due to attractive forces. Viscosity is the resistance of a substance to flow. These two properties work together in the Tesla turbine to transfer energy from the fluid to the rotor or vice versa. Here's how:
- As the fluid moves past each disk, adhesive forces cause the fluid molecules just above the metal surface to slow down and stick.
- The molecules just above those at the surface slow down when they collide with the molecules sticking to the surface.
- These molecules in turn slow down the flow just above them.
- The farther one moves away from the surface, the fewer the collisions affected by the object surface.
- At the same time, viscous forces cause the molecules of the fluid to resist separation.
- This generates a pulling force that is transmitted to the disk, causing the disk to move in the direction of the fluid.
The thin layer of fluid that interacts with the disk surface in this way is called the boundary layer, and the interaction of the fluid with the solid surface is called the boundary layer effect. As a result of this effect, the propelling fluid follows a rapidly accelerated spiral path along the disk faces until it reaches a suitable exit. Because the fluid moves in natural paths of least resistance, free from the constraints and disruptive forces caused by vanes or blades, it experiences gradual changes in velocity and direction. This means more energy is delivered to the turbine. Indeed, Tesla claimed a turbine efficiency of 95 percent, far higher than other turbines of the time.
But as we'll see in the next section, the theoretical efficiency of the Tesla turbine has not been so easily realized in production models.