Phoenix Turbine Builders Club

Turbine Compressors

April 2007

Last month we talked about theoretical turbine efficiencies based upon mathematics models. We also talked about how we might improve overall engine efficiencies by using pre-ignition or pulse detonation events, rather than high cost, inefficient compressors, to generate high velocity gases.

The first step in approaching a turbine design direction is to determine compressor type and configuration.

In Figure 1 we see a compressor efficiency comparison between the two most commonly used turbine compressors. 

Figure 1 - Compressor Efficiency Curves

It is easy to see that axial compressors are higher in efficiency when comparing single stages of each. Single stage pressure ratios are, however, higher with centrifugals. 

Multistage axials are typically used in jet aircraft due to less bulk and cross section -- allowing lighter weights, but they are also more complex and expensive. Most hobby turbines use centrifugal compressors with either a centrifugal or axial hot rotor -- giving the engine the benefit of simplicity and robustness, but at a cost of efficiency. 

Jet engines such as the PT6 and Rolls Royce 250 are used in small aircraft. They use a combination of compressors to gain the advantage of both types. The PT6 (Figure 2) uses three axial stages followed by one centrifugal stage. Rolls engines (Figure 3) use 5-7 axial stages followed by one centrifugal stage.

Since our turbine uses a much lower compression ratio than other designs, we should be able to get by with two axial stages followed by one centrifugal stage.

In past PTBC articles we covered everything from bearing cases to hot rotors and combustion cycles. With this article on compressors, we have completed the basics for designing and building gas turbines using low-cost components. For those of you who are interested in gas turbine experiments, these articles will point you in the right direction -- the rest is up to you.

Next month we'll look at the "other turbine" and how Nikola Tesla applied technology to real world applications. Until then, let's see what you can come up with in gas turbine designs.