Understanding Turbine Basics

September 2003

Summer is over, fall colors are beginning to show up on the trees, and we are preparing for another winter season here in the Upper Peninsula of Michigan. We expect to get our usual fair share of snow in this "snow belt" of the nation. We can average anywhere from 10-15 feet of snow per season in the Munising area, to 30 feet just west of us in the Houghton-Calumet area.

Since we have a cold season here that lasts about six months, we have much higher heating bills compared to people living farther south. With the deregulated utility companies again threatening heating fuel hikes as much as 30 percent, we have to invent innovative ways of keeping the costs down -- like burning wood for heat, or figuring out ways to use waste products like recycled oils.

As we mentioned some months back, by the end of the year we intend to publish a waste oil-to-electricity generator/co-generator plan for those of you who are looking for a way to cut your energy bills down to size, or pull the plug completely. Before we get to the complete system, it is necessary to revisit some of the basics of system components to understand what makes turbines work, and what makes them work well.

Since future production of electricity (worldwide) depends on a solar-steam cycle as well as utilizing waste oils and coal, we are focusing our turbine builders club efforts on steam turbines.

This month we are going to take a look at three distinct types of turbines:

After comparing the characteristics of the three, you may come to the same conclusions we have -- the Tesla type turbine fits the bill for the 21st century.

The Impulse Turbine

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.

As the gas blows past the periphery washers, it spirals through the narrow disk spacing towards the center outlet. The spiraling gas tends to adhere to the disk surface just like air molecules adhere to aircraft skin, or as water adheres to boat skin.

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.

This also explains:

1. why the disk turbine achieves its highest efficiencies when disk peripheral speed comes closest to gas velocity,
2. why torque increases as the rotor decreases in speed down to about 60 percent of rated speed, and
3. why the turbine stalls below 50 percent of its rated speed.

Next month we are going to revisit the importance of the inlet nozzle, and how it is the key factor in improving turbine efficiency. Until then, keep on building and experimenting.

Ken Rieli