Phoenix Turbine Builders Club

Gas Turbines: It's All a Matter of Momentum

February 2007

As I mentioned in our January 2007 article , 21st Century Prime Mover: the Gas Turbine Challenge, our main focus for this year is on the gas turbine. We'll begin by studying historic designs that have proven to be somewhat successful in the past, and are in fact still in use today, with one exception -- Tesla's gas turbine.

There are basically two types of turbines due to fluid flow characteristics:

1. Impulse turbines
2. Reaction turbines

Both of these types were used in power production beginning in the late 1800's to early 1900's. In the U.S., the two main competitors in power production were Westinghouse and General Electric. They utilized Curtis and Parsons blades designs, one being an impulse bladed rotor, the other being a reaction rotor. 

Impulse Turbines

In an impulse bladed turbine there is no pressure drop across the blades, whereas with the reaction blade there is a pressure drop between the leading edge and the trailing edge of the blade.

Another type of impulse turbine is the Pelton wheel. Its mechanics are very similar to the piston engine, but without reciprocating movement. This design is being utilized in a pulse detonation engine at www.turbinetruckengines.com and the Floyd engine at http://Floyddesign.com.  The design works, but at a cost of lower power-per-pound ratios.

Still another impulse type of turbine is the typical turbo booster found on cars and trucks. In this design they use a centrifugal bladed hot rotor rather than a paddle wheel, but again, the power-per-pound ratio remains rather low. 

Reaction Turbines

The early Whittle design (figure 1) used a centrifugal compressor and an inflow centrifugal impulse hot rotor for compressor power. He later changed the design to use an axial reaction hot rotor for greater efficiencies (figure 2). 

Chrysler used the revised classic Whittle design in its 1960's turbine-powered car. (figure 3)

OHain's German Jumo design used axial compressors and axial hot rotors (figure 4). And while the Whittle design was much more robust, it suffered from much lower efficiencies due to the characteristics of the centrifugal compressor. The German design used reaction blades that delivered much higher power per pound of engine. The problems associated with this design included a tendency towards overheating and over revving the engine -- problems continuing to plague the design even today.

The last design we are going to take a look at is the mixed flow or Pratt & Whitney PT6 (figure 5). The PT6 is the most prolific gas turbine used in small turboprop aircraft today. It is a unique design that utilizes a reverse airflow -- from the back of the engine, through a couple of axial compression stages, then through a single centrifugal compressor, then into combustion chambers, axial hot rotors, and finally the front of the engine.

It's All a Matter of Momentum

In this introductory session we are going to focus on the basic principle that makes all turbines operate and deliver power: momentum. It is the momentum of matter moving through the engine that delivers useful power. 

In the case of the gas turbine, it is the momentum of the air and gases of combustion moving through the engine that is converted to shaft horsepower and thrust. In the case of the pure thrust engine, the momentum of the exiting gases produce forward reaction thrust. In the case of the fanjet, turboprop, or turboshaft engine, most of the gas momentum is converted to shaft horsepower - allowing greater overall efficiencies.

In all cases, it is the mass of air and fuel multiplied by their velocity that delivers and power. And since air and combustion gases weight so little, large amounts must be moved through the engine to deliver horsepower.

Well, we will leave it there for now. Next time we'll look at a few efficiency curves, take a look at the "other turbine" -- the disk turbine -- and how we might approach a workable design.