Tutorial on “Axial Compressor” theory and specification for “Overhaul of Gas Turbine Parts”

The compEDU sessions given as part of the gas turbine technology course are devoted to two chapters:

1. Axial compressor theory: familiarize yourself with compEDU and ease learning of axial compressor theory (secondary reading material).
2. Overhaul of Gas Turbine parts: learn about how gas turbines are operated, maintained and overhauled (primary reading material).

The learning tool has been introduced in the course because last years students felt that a stumbling block in learning about axial compressor theory is the difficulty to visualize the processes described in the book. The hope is that the many images and illustrative explanations will ease this burden, making the learning more efficient.

Additionally, references to some videos supporting the lecture notes are given below.

For more information on the compEDU effort check out http://www.energy.kth.se/compedu/Compedu_home_asp/main/default.asp. A personal license including a DVD for installing the program on your home computer can be purchased for 300 SEK.

Axial compressor theory

The axial compressor theory chapters is used to introduce you to working with compEDU. This session is intended to support the reading of the course book. Hopefully, you will go home with a feeling that you know what axial compressors is all about, after only about one hour of studying.

Start book S2B3 and select Chapter 1, that is S2B3C1. Work through the 2D stage theory. Make sure that you check all links that you find interesting. For example, what new developments regarding axial compressors were introduced in 1926?

Use the next page button to browse through the material.

Make sure you master the five “fundamental concepts” mentioned in the summary.

Overhaul of Gas Turbine Parts

Start S2B14C1. As you go through the chapter try to find answers to the following issues:

• Why is the expected life of a low pressure turbine longer than for a high pressure turbine?
• What materials are used in gas turbines and approximately where are they used. Check some limiting temperatures for some common alloys. What is an approximate upper pressure ratio for an axial compressor considering current limits on nickel based alloys?
• How can the remaining life of a gas turbine part be tested? What is the “surface replication method”? How does this process differ between aero and stationary gas turbines?
• What is “preventive” and “predictive” maintenance?
• What is trend analysis and “fault indices”? What gas path measurements are usually required to perform engine diagnostics.
• Which parameters influence the operating and maintenance cost for a gas turbine engine?
• What is:

• Creep
• Erosion
• Corrosion
  
  

What governs these three deterioration mechanisms? How do they arise and where in the engine?

• Give four “key-service factors”. What is meant by them?
• Define different levels of disassembly inspection.
• What is the typical overhaul interval (major inspection) for a stationary gas turbine?
• What does EOH mean?
• What is a birth history list?
• What aero engine failure conditions are considered safety critical? What do they mean?

Videos supporting lecture notes

It is frequently claimed that the compressors are generally more difficult to design aerodynamically than the turbines. This is mainly attributed to the adverse axial pressure gradients of compressors, that is the flow is in the direction of increasing pressure, causing blade and end wall boundary layers to grow more rapidly than in turbines. In particular, an optimal included angle of divergence of about seven degrees is suggested for radial compressor diffusers. Recall the slide:

An old, but very illustrative video for this flow phenomena is given in compEDU. Select the “BROWSER” tab below in the compEDU main screen. Choose videos and start the S2B1C3 video (flow in a diffuser).

When discussing gas turbine stressing the following slide was used:

High cycle fatigue was attributed to rotor stator interaction, that is the trailing boundary layer of the turbine nozzle impinges on the rotors at high frequency (compared to low cycle fatigue). Start the S5B1C6 (midspan section of a turbine) video for an illustration of this phenomena.