Design of a Centrifugal Compressor Impeller for Micro Gas Turbine Application by Bosman Botha van der Merwe Thesis presented in fulfilment of the requirements for the degree of Master of Science in Engineering in the Faculty of Mechanical and Mechatronic Engineering at Stellenbosch University Supervisor: Dr S.J. van der Spuy Co-supervisor: Prof T.W. von Backström December 2012 Stellenbosch University http://scholar.sun.ac.za DECLARATION By submitting this thesis electronically, I declare that the entirety of the work contained therein is my own, original work, that I am the sole author thereof (save to the extent explicitly otherwise stated), that reproduction and publication thereof by Stellenbosch University will not infringe any third party rights and that I have not previously in its entirety or in part submitted it for obtaining any qualification. Date:………………….. Copyright © 2012 Stellenbosch University All rights reserved. i Stellenbosch University http://scholar.sun.ac.za ABSTRACT Design of a Centrifugal Compressor Impeller for Micro Gas Turbine Application B.B. van der Merwe Department of Mechanical and Mechatronic Engineering, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa Thesis: MSc. Eng. (Mech) December 2012 The use of micro gas turbines (MGTs) for the propulsion of unmanned aerial vehicles (UAVs) has become an industry standard. MGTs offer better performance vs. weight than similar sized, internal combustion engines. The front component of an MGT serves the purpose of compressing air, which is subsequently mixed with a fuel and ignited to both power the turbine which drives the compressor, and to produce thrust. Centrifugal compressors are typically used because of the high pressure ratios they deliver per stage. The purpose of this project was to design a centrifugal compressor impeller, and to devise a methodology and the tools with which to perform the aforementioned. A compressor impeller adhering to specific performance and dimensional requirements was designed. The new compressor was designed using a mean-line performance calculation code. The use of the code was vindicated through comparison with the results from a benchmark study. This comparison included mean-line, Computational Fluid Dynamic (CFD), and experimental results: the new design mean-line results were compared to the results of CFD simulations performed on the same design. The new design was optimised using an Artificial Neural Network (ANN) and Genetic Algorithm. Prior to and during optimisation, the ANN was trained using a database of sample CFD calculations. A Finite Element Analysis (FEA) was done on the optimised impeller geometry to ensure that failure would not occur during operation. According to CFD results, the final design delivered good performance at the design speed with regards to pressure ratio, efficiency, and stall margin. The mechanical stresses experienced during operation were also within limits. Experimental results showed good agreement with CFD results of the optimised impeller. Keywords: micro gas turbine, centrifugal compressor, impeller, CFD, experimental, optimisation, FEA. ii Stellenbosch University http://scholar.sun.ac.za UITTREKSEL Ontwerp van „n Mikrogasturbine Sentrifigaalkompressor Rotor (“Design of a Centrifugal Compressor Impeller for Micro Gas Turbine Application”) B.B. van der Merwe Departement van Meganiese and Megatroniese Ingenieurswese, Universiteit van Stellenbosch, Privaatsak X1, Matieland 7602, Suid-Afrika Tesis: MSc. Ing. (Meg) Desember 2012 Die gebruik van mikrogasturbines vir die aandrywing van onbemande vliegtuie het ‟n standaard geword in die industrie. Mikrogasturbines bied beter werkverrigting teen gewig as binnebrandenjins van soortgelyke grote. Hierdie eienskap verseker dat mikrogasturbines as aandryfmotors vir onbemande vliegtuie uiters voordelig is. Die voorste komponent van ‟n mikrogasturbine dien om lug saam te pers, wat dan met brandstof gemeng en daarna aan die brand gesteek word om krag aan die kompressor en stukrag te voorsien. Sentrifugaalkompressors word tipies gebruik as gevolg van die hoë drukverhoudings wat hierdie komponente per stadium kan lewer. Die doel van hierdie projek was om ‟n sentrifugaalkompressor te ontwerp, en ‟n metode en die hulpmiddels te ontwikkel om laasgenoemde uit te voer. ‟n Kompressor rotor wat voldoen het aan sekere werkverrigtings en dimensionele vereistes is ontwerp. Die nuwe kompressor rotor is met behulp van 1-dimensionele werkverrigting-berekeningskode ontwerp. Die berekeningsakkuraatheid van die kode en díé van ‟n kommersiële Berekenings Vloeidinamika pakket is bevestig deur die berekende resultate te vergelyk met die van eksperimente. Die nuwe rotor is gevolglik deur middel van ‟n Kunsmatige Neurale Netwerk en Genetiese Algoritme geoptimeer. Die Kunsmatige Neurale Netwerk is voor en gedurende optimering deur Berekenings Vloeidinamika simulasies opgelei. Die meganiese sterkte van die geoptimeerde rotor is nagegaan met behulp van ‟n Eindige Element Analise. Dit is gedoen om te verseker dat die rotor nie sal faal by die bedryfspunt nie. Berekenings Vloeidinamika resultate het getoon dat die finale rotor ontwerp ‟n goeie werkverrigting lewer by die ontwerpspoed, met betrekking tot drukverhouding, bennutingsgraad, en stakingsmarge. Eksperimentele resultate het goeie ooreenstemming met die Berekenings Vloeidinamika resultate van die geoptimeerde rotor getoon. Sleutelwoorde: mikrogasturbine, sentrifigaalkompressor, rotor, Berekenings Vloeidinamika, eksperimenteel, optimering, Eindige Element Analise. iii Stellenbosch University http://scholar.sun.ac.za ACKNOWLEDGEMENTS I would not have been able to finish this journey if not for certain individuals and institutions. I am sincerely grateful to all of them. My father, Hendrik, who first introduced me to the world of engineering and who‟s memory lives on in our hearts. My mother, Malinda. You are my guiding light. All my friends and family. There are too many of you to single anyone out. Thanks for bearing with me. My supervisors for all their insights and motivation. Dr S.J. van der Spuy for his unwavering optimism and Prof. T.W. von Backström for bringing me back to earth. Ballast and my supervisors for the financial support of this project. The guys in “Die Lasraam”. Thanks for the coffee brakes, pizza specials, and all- around good vibes! iv Stellenbosch University http://scholar.sun.ac.za DEDICATION To my dad… v Stellenbosch University http://scholar.sun.ac.za TABLE OF CONTENTS Declaration ................................................................................................................ i Abstract .................................................................................................................... ii Uittreksel ................................................................................................................. iii Acknowledgements ................................................................................................. iv Dedication ................................................................................................................ v Table of Contents .................................................................................................... vi List of Figures ......................................................................................................... xi List of Tables ........................................................................................................ xiv Nomenclature ........................................................................................................ xvi 1 Introduction .................................................................................................... 1 1.1 Project Objective ................................................................................ 2 1.2 Operation of Centrifugal Compressors ............................................... 3 1.2.1 Efficiency and pressure ratio .............................................. 6 1.2.2 Surge and choke ................................................................. 7 1.3 Thesis Outline ................................................................................... 11 2 Literature Survey .......................................................................................... 12 2.1 Introduction ...................................................................................... 12 2.2 Micro Gas Turbines .......................................................................... 12 2.3 Analysis and Design of Centrifugal Compressors ............................ 13 2.3.1 Mean-line analysis ........................................................... 13 2.3.2 Computational Fluid Dynamics ....................................... 14 2.3.3 Design optimisation ......................................................... 16 vi Stellenbosch University http://scholar.sun.ac.za Table of Contents vii 2.3.4 Finite Element Analysis ................................................... 17 3 Benchmark Analyses .................................................................................... 18 3.1 Introduction ...................................................................................... 18 3.2 Impeller CAD ................................................................................... 18 3.2.1 GeomTurbo file format .................................................... 19 3.2.2 Geometric scans ............................................................... 21 3.2.3 CAD and geomTurbo generation