Advanced Gas Turbine Cycles A Brief Review of Power Generation Thermodynamics

Many people have described the genius of von Ohain in Germany and Whittle in the United Kingdom, in their parallel inventions of gas turbine jet propulsion; each developed an engine through to first flight. The best account of Whittle’s work is his Clayton lecture of 1946 [l]; von Ohain described his work later in [2]. Their major invention was the turbojet engine, rather than the gas turbine, which they both adopted for their new propulsion engines.

Feilden and Hawthorne [3] describe Whittle’s early thinking in their excellent biographical memoir on Whittle for the Royal Society. “‘I‘he idea for the turbojet did not come to Whittle suddenly, but over a period of some years: initially while he was a final year flight cadet at RAF Cranwell about 1928; subsequently as a pilot officer in a fighter squadron; and then finally while he was a pupil on a flying instructor’s course.. . . While involved in these duties Whittle continued to think about his ideas for high-speed high altitude flight. One scheme he considered was using a piston engine to drive a blower to produce a jet. He included the possibility of burning extra fuel in the jet pipe but finally had the idea of a gas turbine producing a propelling jet instead of driving a propeller”.

But the idea of gas turbine itself can be traced back to a 1791 patent by Barber, who wrote of the basic concept of a heat engine for power generation. Air and gas were to be compressed and burned to produce combustion products; these were to be used to drive a turbine producing a work output. The compressor could be driven independently (along the lines of Whittle’s early thoughts) or by the turbine itself if it was producing enough work.

Here lies the crux of the major problem in the early development of the gas turbine. The compressor must be highly efficient-it must use the minimum power to compress the gas; the turbine must also be highly efficient-it must deliver the maximum power if it is to drive the compressor and have power over. With low compressor and turbine efficiency, the plant can only just be self-sustaining-the turbine can drive the compressor but do no more than that.

Stodola in his great book of 1925 [4] describes several gas turbines for power generation, and Whittle spent much time studying this work carefully. Stodola tells how in 1904, two French engineers, Armengaud and Lemae, built one of the first gas turbines, but it did little more than turn itself over. It appears they used some steam injection and the small work output produced extra compressed air-but not much. The overall efficiency has been estimated at 2-3% and the effective work output at 6- 10 kW.

Much later, after several years of development (see Eckardt and Rufli [5]), Brown Boveri produced the first industrial gas turbine in 1939, with an electrical power output of 4MW. Here the objective of the engineering designer was to develop as much power as possible in the turbine, discharging the final gas at low temperature and velocity; as opposed to the objective in the Whittle patent of 1930, in which any excess energy in the gases at exhaust from the gas generator-the turbine driving the compressor-would be

used to produce a high-speed jet capable of propelling an aircraft.

It was the wartime work on the turbojet which provided a new stimulus to the further development of the gas turbine for electric power generation, when many of the aircraft engineers involved in the turbojet work moved over to heavy gas turbine design. But surprisingly it was to be the late twentieth century before the gas turbine became a major force in electrical generation through the big CCGTs (combined cycle gas turbines, using bottoming steam cycles).

This book describes the thermodynamics of gas turbine cycles (although it does touch briefly on the economics of electrical power generation). The strictures of classical thermodynamics require that “cycle” is used only for a heat engine operating in closed form, but the word has come to cover “open circuit” gas turbine plants, receiving “heat” supplied through burning fuel, and eventually discharging the products to the atmosphere (including crucially the carbon dioxide produced in combustion). The search for high gas

turbine efficiency has produced many suggestions for variations on the simple “open circuit” plant suggested by Barber, but more recently work has been directed towards gas turbines which produce less COz, or at least plants from which the carbon dioxide can be disposed of, subsequent to sequestration.

There are many books on gas turbine theory and performance, notably by Hodge [6], Cohen, Rogers and Saravanamuttoo [7], Kerrebrock [8], and more recently by Walsh and Fletcher [9]; I myself have added two books on combined heat and power and on combined power plants respectively [10,11]. They all range more widely than the basic thermodynamics of gas turbine cycles, and the recent flurry of activity in this field has encouraged me to devote this volume to cycles alone. But the remaining breadth of gas turbine cycles proposed for power generation has led me to exclude from this volume the coupling of the gas turbine with propulsion. I was also influenced in this decision by the existence of several good books on aircraft propulsion, notably by Zucrow [12], Hill and Peterson [13]; and more recently my friend Dr Nicholas Cumpsty, Chief Technologist of Rolls Royce, plc, has written an excellent book on “Jet Propulsion” [ 141.

I first became interested in the subject of cycles when I went on sabbatical leave to MIT, from Cambridge England to Cambridge Mass. There I was asked by the Director of the Gas Turbine Laboratory, Professor E.S.Taylor, to take over his class on gas turbine cycles for the year. The established text for this course consisted of a beautiful set of notes on cycles by Professor (Sir) William Hawthorne, who had been a member of Whittle’s team. Hawthorne’s notes remain the best starting point for the subject and I have called upon them here, particularly in the early part of Chapter 3. Hawthorne taught me the power of temperature-entropy diagram in the study of cycles, particularly in his discussion of “air standard” cycles-assuming the working fluid to be a perfect gas, with constant specific heats. It is interesting that Whittle wrote in his later book [15] that he himself “never found the (T,s diagram) to be useful”, although he had a profound understanding of the basic thermodynamics of gas turbine cycles. For he also wrote “When in jet engine design, greater accuracy was necessary for detail design, I worked

in pressure ratios, used y = 1.4 for compression and y = 1.3 for expansion and assumed specific heats for combustion and expansion corresponding to the temperature range concerned. I also allowed for the increase in mass flow in expansion due to fuel addition (in the range 1.5-2%). The results, despite guesswork involved in many of the assumptions, amply justified these methods to the point where I was once rash enough to declare that jet engine design has become an exact science”. Whittle’s modifications of air standard cycle analysis are developed further in the later parts of Chapter 3.

Hawthorne eventually wrote up his MIT notes for a paper with his research student, Graham de Vahl Davis [ 161, but it is really Will Hawthorne who should have written this book. So I dedicate it to him, one of several great engineering teachers, including Keenan,  Taylor and Shapiro, who graced the mechanical engineering department at MIT when I was there as a young assistant professor.

My subsequent interest in gas turbines has come mainly from a happy consulting arrangement with Rolls Royce, plc and the many excellent engineers I have worked with there, including particularly Messrs.Wilde, Scrivener, Miller, Hill and Ruffles. The Company remains at the forefront of gas turbine engineering. I must express my appreciation to many colleagues in the Whittle Laboratory of the Engineering Department at Cambridge University. In particular I am grateful to Professor John Young who readily made available to me his computer code for “real gas” cycle calculations; and to Professors Cumpsty and Denton for their kindness in extending to me the hospitality of the Whittle Laboratory after I retired as Vice-Chancellor of the Open University. It is a stimulating academic environment.

I am also indebted to many friends who have read chapters in this book including John Young, Roger Wilcock, Eric Curtis, Alex White (all of the Cambridge Engineeering Department), Abhijit Guha (of Bristol University), Pericles Pilidis (of Cranfield University) and Giampaolo Manfrida (of Florence University). They have made many suggestions and pointed out several errors, but the responsibility for any remaining mistakes must be mine.

Mrs Lorraine Baker has helped me greatly with accurate typing of several of the chapters, and my friend John Stafford, of Compu-Doc (silsoe-solutions) has provided invaluable help in keeping my computer operational and giving me many tips on preparing the material. My publishing editor, Keith Lambert has been both helpful and encouraging. Finally I must thank my wife Sheila, for putting up with my enforced isolation once again to write yet another book.

J. H. Horlock

Cambridge, June 2002

 

 

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