Ahhh, “the damn turbofan”…

That is the name of the latest project we have undertaken, as the underpinning for my continued engineering education. During this process my studies have taken me through numerous books, journals, and websites, and I have learned substantially more than “I don’t think it will take that long to design”. I’m such a moron.

We’re adding more pages to the Engineering Notes section of Design & Motion, with a review of the mathematics and principles of thermodynamics, fluid dynamics, structures, (and more) that I find pertinent during this study. More importantly I hope to create a resource that helps people connect the dots in areas, which for some unknown reason are still so poorly explained. I hope this is helpful for anyone wanting reference materials, those wanting to study along-side, and my friends that just want to laugh when I screw something up.

Study Materials

I just counted 7 books and 41 document and papers on various aspects of turbojet design in our library. I thought it would be good to point out a few noteworthy resources that I will occasionally refer to, those that I and many others have found very helpful (I’ll add more as we progress).

Jet Propulsion, a Simple Guide…

Nicholas Cumpsty’s book is a great text on understanding propulsion from the ground up. He spends a good amount of time in the mathematics with a substantial emphasis on the relationships of each component on the overall design, as well as the relationship of the design goals and specific aircraft applications.

Jet propulsion Excerpt

The book takes the reader through the development of both a civil engine as well as a military application, in both on and off-design goals. Cumpsty includes numerous work problems throughout each chapter as the reader is expected to design the focus engine along the way. Answers for each problem are given, but unfortunately, no worked solutions. “Hey Cambridge, A solutions manual would be a great addition!”

What’s included: (318 pages) Step-by-step identification of needs and design goals, as well as step-by-step exercises of the overall design from start to finish. cooling bleeds, internal bypass mixing, basic blade design, turbojet and turbofan designs, variable inlet ramp calculations, various nozzle types, shock wave propagation, and much more are covered.

What’s Missing: Very specific physical design – the book is a great on and off-design theoretical reference, but lacks nitty-gritty aspects of how to approach compressor blade dynamics, and combustor and afterburner construction, and so on. Cumpsty does however write another book on the detailed aspects of compressor design.

Elements of Propulsion: Gas Turbines and Rockets…

Jack Mattingly’s book is another well regarded reference, and is packed with mathematics and really important information. I used this as a go-to reference when I needed another perspective on very specific, and often odd design goals. Mattingly discusses a very broad scope of propulsion, in a more ‘here’s the math behind the component” type-of-manner, with less discussion, but many worked through examples, and a lot of equations.

mattingly elements of propulsion

Note: I did not read the entire book, so I cannot give a valid review of the book or what’s missing.

What’s in there: (869 pages) Equations, tons or equations. Covers almost every aspect of the overall design theory, with worked through examples.

What’s missing: Explanations. Overall discussions of why certain aspects discussed are important or when alternatives should be found. This book is a good supporting reference to have along side another that explains the design theory.

NASA’s Propulsion Index

This is where I started; no doubt the best first-steps general information regarding jet propulsion. The information is tuned to be useful to both high-school and undergraduate level students. A good place to start is with the Jet Simulator tasks, which walk the reader through the basics of each section in a turbojet engine, and ends with NASA’s online Jet Engine Simulator.

NASA Basic Propulsion

There are numerous pages of basic principles of thermodynamic relationships involved in turbojet engines, along with a great step-by-step discussion on each page detailing the algebra involved in reducing the isentropic principles to the specific final equations.

Note: Beware of the Simulator’s nozzle area relationship calculations. If you read their “Engine Theory” simulator calculations documentation, which I highly recommend, you’ll see a tiny note about their forcing 0.5 Mach at the turbine inlet for choking, and adjust the results accordingly without any external warning. I cannot fathom why NASA didn’t have this paper in big bold “READ THIS” type. It’s seems nearly impossible to find this document.

What’s in there: (turbojet section) The basic thermodynamic principles of each engine section; including intake, compressor, combustor, turbine, nozzle, and isentropic efficiency.

What’s missing: Mid level topics of design and beyond; a great starting point only.

Paper: Design Methodology of a Two Stage Axial Compressor

Believe it or not, this little 3-page paper was one of my favorite reads working through the compressor section. The authors compressed the very detailed subject of a 2D design of a two-stage axial compressor, and packed it into 3 pages, including diagrams, tables, and a brief step through of the calculations.

Favorite paper on basic turbojet axial compressor design

They included only the core 2D features involved in the calculations, and without comment or reason (which I did not find humorous at all). Ultimately, this drove me to a substantially larger study in order to understand, and be able to accurate reproduce the methods used.

There was enough information, and principle steps to allow readers to get a good feel of the methodologies to study, as well as some alternate approaches to apply in their design scenarios. More importantly, it is a great indicator of what aspects that you need more study in.

What’s included: Basic design mathematics worked through the principles of axial compressors; stage calculations, velocity triangles, blade geometry, twist, and spacing, mid-plane design.

What’s missing: Discussions and explanations.

Note: This paper was submitted for INPRESSCO’s International Journal of Current Engineering and Technology. These journals (and many others) contain tons of great discussions on all aspects of engineering, and may be a good reference tool for your studies.

Paper: Design and Optimization of a Multi-Stage Axial-Flow Compressor

This paper was intended as a flow of calculations for an axial compressor optimization software program. It was a tough read and utterly useless until I had a better understanding of compressor mathematics. It does however explain a very solid systematic design, and is quite detailed in calculations, especially in that areas of losses.

Turbojet Axial Compressor Optimization

What’s included: (20 pages) A detailed list of the calculations and variables associated with optimizations of axial compressor blades, work and flow coefficient relationships, inefficiencies and losses, and more.

What’s missing: Discussions beyond those of the optimization software. It’s a great approach to streamlined design after you have studied the principles of incidence, flow/work coefficients, and losses, but not learning what these are.

NPTEL Turbomachinery Aerodynamics

I found this course to be a great value on the core aspects of 2D axial compressor design. Each lecture includes a video of the professors, their discussion and slides. PDF handouts of each lecture are available, which contain the slides, which contain the equations, notes, warnings, etc. 2 tutorial lectures were given which I found quite useful to support the mathematics.

What’s included: Aero-Thermodynamics of flow through an Axial flow Compressor stage; Tip leakage flow and scrubbing; Design of compressor blades, airfoil data, flow analysis and performance estimation; Multi-staging of compressor characteristics; Transonic Compressors and Shock Structure Models; Instability, Losses, and Noise.

What’s missing: The online discussions with the classes were very poorly tended. In some cases I had the same questions and were poorly or not answered.


Works cited:

Cumpsty, Nicholas Jet Propulsion: A Simple Guide to the Aerodynamic and Thermodynamic Design and Performance of Jet Engines. Cambridge, United Kingdom: Cambridge University Press, 2003. Print.

Mattingly, Jack D. Elements of Propulsion, Gas Turbines and Rockets 2nd edition (AIAA Education Series). Reston, Virginia: American Institute of Aeronautics and Astronautics, Inc., 2006. Print

Propulsion Index. National Aeronautics and Space Administration, 2014. NASA website. http://www.grc.nasa.gov/WWW/k-12/airplane/shortp.html. 2015 accessed

Gaddam Srikanth, S.Srinivas Prasad, V.Mahesh Kumar, and B.Mounica Reddy. Design Methodology of a Two Stage Axial Compressor. International Journal of Current Engineering and Technology, 2014, INPRESSCO. http://inpressco.com/special_journal/special-issue-2-feb-2014/. 2014 accessed.

Prof. Dr. Atef M. Alm-Eldien,  Prof. Dr. Ahmed F. Abdel Gawad, Prof. Dr. Gamal Hafaz, Eng. Mohamed G. Abd El Kreim. ICFD11-EG-4103 Design and Optimization of a Multi-Stage Axial-Flow Compressor.  Proceedings of ICFD11: Eleventh International Conference of Fluid Dynamics, 2013, Alexandria, Egypt. http://www.icfd11.org/Eleventh International Conference of Fluid Dynamics.pdf  2014 accessed (host server problems currently) http://paperzz.com/doc/2886896/icfd11-eg-4103—researchgate. 2015 accessed.

Prof. Roy, Bhaskar and Prof. Pradeep, A M. NPTEL Turbomachinery Aerodynamics. IIT Bombay, n.d., http://nptel.ac.in/syllabus/101101058/ March 2015