The Impulse Tribune

Referring to Figure 1 we see a typical bladed section of an impulse turbine.

The key to understanding how any turbine operates is to understand the aerodynamic forces.

In the case of the impulse turbine, high velocity gases operate on the concave surfaces of the blades almost exclusively. In other words, this is a "bucket effect" means of extracting energy.

Gas directed into the concave surface of the blades and at an angle of about 45 to 85 degrees, relative to the shaft, will transfer power to the shaft through impulse.

The unique characteristic of impulse engines is that the velocity of the gas decreases upon exiting the blades, whereas the pressure remains constant. Energy is transferred by changing the velocity of the gas -- not its pressure.

The Reaction Turbine

In Figure 2 we see a typical reaction turbine rotor.

Notice the difference between the blade cross section of the reaction turbine compared to the impulse blade.

The reaction blade acts like a wing section of a plane, whereas the impulse blade acts like the piston of an engine.

In the reaction turbine, kinetic gas energy is converted to shaft power by decreasing the velocity of the gas and lowering gas pressure -- just like on an airplane wing. As gas enters from the left of the blade section and travels across the blade surface, there is a decrease in pressure on the upper surface, and an increase in pressure on the lower surface. As the gas leaves the trailing edge there is a decrease in gas velocity, pressure, and a downward angle -- resulting in a lifting or reaction force.

The Tesla or Disk Turbine

In Figure 3 we see a representative Tesla or disk turbine section.

Notice that there are no blades -- simply narrowly spaced disks with round washers in the mid and outer periphery, and a star washer in the center.

High velocity gas enters the disk turbine edge-on, or 90 degrees perpendicular to the plates. The gas is directed on a tangent to the plates -- in other words, the gas is vectored across the upper edge of the plates.

As the gas enters the disk pack, it first encounters the outer washers. Spherical aerodynamics plays a key role in "spooling up" this type of turbine. The washers act as impulse elements on their leading edge, and as drag elements on their trailing edge (relative to the gas vector). This impulse-drag effect is essential in starting up the turbine.

This interaction of adhesion and spiral gas movement pulls the disk in the direction of the gas.

Because there is a drop in velocity and pressure, as well as a vector change in the gas path, this energy transfer must be classified as a reaction force transform. While the washers in this design operate as impulse elements, the plates act as reaction elements.

One last, but very important, characteristic of this turbine is the gating or shuttering effect.

When the turbine is spooling up to about 50 percent of its rated speed, kinetic gases pass through the plates with minimal back pressure. From 50% to 100% of rated speed (determined by gas velocity and disk diameter), centrifugal forces operating on the gas between the plates create a back pressure to incoming gas.

As the turbine peripheral speed approaches the speed of the incoming gas, centrifugal gas back pressure closes the gate between the plates -- shutters off incoming gas. That's also why the turbine peripheral speed never attains parity with the incoming gas velocity -- the gate or shutter never fully closes. If it did, the turbine action would stop.