Table Of ContentCRANFIELD INTERNATIONAL SYMPOSIUM SERIES
Volume 10
COMBUSTION IN ADVANCED
GAS TURBINE SYSTEMS
Already published in this series:
FLIGHT TEST INSTRUMENTATION, Volume 1 Edited by M. A. Perry, 1961
RECENT DEVELOPMENTS IN NETWORK THEORY Edited by S. R. Deards, 1963
FLIGHT TEST INSTRUMENTATION, Volume 2 Edited by M. A. Perry, 1963
ADVANCES IN AUTOMOBILE ENGINEERING Part I Edited by G. H. Tidbury, 1963
ADVANCES IN AUTOMOBILE ENGINEERING Part II Edited by N. A. Carter, 1963
FLIGHT TEST INSTRUMENTATION, Volume 3 Edited by M. A. Perry, 1965
ADVANCES IN AUTOMOBILE ENGINEERING Part III Edited by G. H. Tidbury, 1965
ADVANCES IN AUTOMOBILE ENGINEERING Part IV Edited by D. Hodgetts, 1966
AEROSPACE INSTRUMENTATION, Volume 4 Edited by M. A. Perry, 1967
COMBUSTION IN ADVANCED
GAS TURBINE SYSTEMS
Proceedings of an International Propulsion Symposium
held at the College of Aeronautics, Cranfield,
April 1967
Edited by
1. E. SMITH
PERGAMON PRESS
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Copyright © 1968
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First edition 1968
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FOREWORD
ALTHOUGH the process of chemical combustion is one that is still far from
being understood, the application of our imperfect knowledge has made a
remarkable contribution to the field of applied combustion. As in any pro
gressive technology the requirements of the customer are always a step or two
ahead of the engineer or designer, and in this respect the science of combustion
engineering is no exception. As a result there is an ever present need for the
rapid communication of ideas and techniques in an attempt to keep this gap as
narrow as possible.
Whereas international organizations exist for the study and dissemination of
information on combustion processes in general it follows, from the very breadth
of the field, that the attention paid to any individual aspect of combustion must
necessarily be limited.
The Cranfield Symposium, organized by the Propulsion Department, College
of Aeronautics, was intended to provide a forum where workers in the field of
Gas Turbine Combustion could meet and discuss their particular problems, free
from the interesting distractions inherently present in a more broadly-based
Symposium.
The papers were selected from as wide a background as possible, approxi
mately an equal number coming from the academic and research establishments
as from industry. The principal criterion for their selection was that they should
contribute to the knowledge of Gas Turbine Combustion, and have application
either to the improvement of current engines or to the design of more advanced
systems for the future.
In order not to trespass on the informahty of the Symposium, and also to
condense the material to a size which could be incorporated in a single volume,
the discussions by authors and delegates have been severely edited. No apology
is made for this, save to delegates who might feel their comments should have
been included, and it is hoped that this restriction in content will be more than
compensated by a wider circulation of this volume.
The Editor, on behalf of the Organizing Committee, would like to express
sincere thanks to Authors, Chairmen and to the Delegates who contributed to
the success of the 1967 Propulsion Symposium, and made the compilation of
this volume possible.
I. E. SMITH
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DESIGN CONSIDERATIONS IN ADVANCED
GAS TURBINE COMBUSTION CHAMBERS
ARTHUR H. LEFEBVRE
College of Aeronautics, Cranfield, England
Introduction
Future trends in the design of aircraft combustion chambers will be governed
to a large extent by the evolution of new engine concepts. At the present time a
wide variety of engine types is being produced, ranging from simple lift engines
to large, multispool, by-pass engines of complex design. For each type of
engine the combustion chamber must be "tailor-made" to meet the stipulated
requirements. Fortunately, as a result of the advances made in our knowledge of
the quantitative relationship between combustor dimensions, pressure-loss
factor, and the operating conditions of temperature, pressure and velocity, this
is now practicable. However, in spite of these advances, the stage is not yet
reached where it is possible to "optimize" a combustor design for any given
application. Attainment of this objective will demand a more detailed knowledge
of the influences of fuel-air mixing and air-flow distribution on combustion
characteristics, and it is with these aspects that this paper is largely concerned.
Basic Considerations
Before considering future trends in combustor design it is of interest to
examine briefly the considerations that dictate the basic geometry of what might
be termed the "conventional" gas turbine chamber. This is an instructive
exercise because it helps to define the essential components needed to carry out
the primary functions of a combustion chamber, and indicates the extent to
which combustor geometry may be modified in order to best meet the various
performance requirements which can difí*er widely between one engine design
and another.
Figure la illustrates the simplest possible form of combustion chamber which
is merely a straight-walled duct connecting the compressor to the turbine.
Unfortunately this simple arrangement is impractical because the pressure loss
incurred would be excessive. The fundamental pressure loss due to combustion
Α. Η. LEFEBVRE
Ι fuel
(a)
fuel
(o) air
fuel local AFR = 15
air
7
FIG. 1. Stages^in the evolution of the "conventionar' aircraft combustion
^ chamber.
is proportional to the square of the air velocity, and for compressor outlet
velocities of the order of 500 ft/sec this loss could amount to about a quarter
of the pressure rise achieved in the compressor. To reduce this pressure loss to
an acceptable level a difTuser is fitted which lowers the air velocity by a factor of
usually about five, as shown in Fig. lb. However, even with a diffuser flow
ADVANCED GAS TURBINE COMBUSTION CHAMBERS 5
velocities are still too high to permit stable combustion, and a flow reversal
must be created in order to provide a low-velocity region in which the flame can
be sheltered and sustained. Figure Ic shows how this may be done using a plain
baffle. The only remaining defect with this arrangement is that, in order to
produce the desired temperature rise, the overall chamber air-fuel ratio is
normally around 50, which lies well outside the Umits of inflammability for
kerosine-air mixtures. Ideally the air-fuel ratio should be close to the stoichio
metric value of 15, since for kerosine this corresponds to maximum flame
speeds. To meet this problem the simple baffle is replaced by a perforated flame-
tube, as illustrated in Fig. Id. The function of the flame-tube is to provide a
region of low velocity in which fuel and air can mix and burn in roughly stoichio
metric proportions. The air not required for combustion is admitted down
stream of the combustion zone to mix with the hot burned products and thereby
reduce their temperature to a value which is acceptable to the turbine. In
practice it is customary to insert between the primary and dilution zones a
so-called "intermediate" zone, whose purpose is to recover losses arising due to
chemical dissociation of the primary combustion products. This it does by the
discreet addition of small amounts of air.
Figure 1 thus illustrates the logical development of the conventional aircraft
combustion chamber in its most widely used form. As would be expected there
are many variations on the basic pattern shown in Fig. Id but, in general,
all chambers incorporate the following key components—air casing, diff*user,
flame-tube and fuel injector.
The choice of a particular type and layout of combustion chamber is deter
mined largely by engine specifications but is also strongly influenced by the
desirabihty of using the available space as effectively as possible. On modern
aircraft engines the chamber is almost invariably of the through-ffow type in
which the air flows in a direction essentially parallel to the axis of the chamber.
All designs fall into one of three main categories—tubular, annular and tubo-
annular, as illustrated in Fig. 2.
fuel
casing flame tube
injector
tubular tubo-annular annular
FIG. 2. Types of aircraft combustion chamber.
6 Α. Η. LEFEBVRE
In most aircraft engines the fuel is injected into the combustion zone in the
form of a finely atomized spray, obtained by forcing the fuel through a fine
orifice under pressure. An alternative method is to vaporize the fuel by leading
it through tubes which are immersed in the hot combustion zone.
Performance Requirements
An aircraft combustion chamber must satisfy a wide range of requirements.
These are well known and may be listed as follows.
1. High combustion efficiency.
2. Stable combustion at all operating conditions.
3. Easy ignition at start-up and when windmilling at high altitude.
4. Uniform distribution of temperature at chamber outlet.
5. No coke deposition or exhaust smoke.
6. Minimum size and weight.
7. Minimum loss of total pressure.
8. Long life between overhauls.
9. Easy to dismantle from engine.
The relative importance of these various requirements varies widely between
one engine type and another. Unfortunately many of the requirements are
mutually incompatible. For example, it is relatively simple to achieve a high
combustion efficiency and a uniform outlet temperature distribution, provided
there is no restriction on chamber length. In the past major difficulties have
arisen through striving to meet all requirements in all combustion chambers.
Under these conditions the final design is inevitably a compromise solution and
its success or failure can only be gauged by the amount of trouble experienced in
test-bed development and in service.
Current Trends
In recent years an important trend in the development of the turbojet engine
has been towards greater cost and complexity in terms of increases in by-pass
ratio, pressure ratio, turbine-inlet temperature, and multiplicity of shafts.
Another trend has been in the opposite direction, i.e. towards lower pressure
ratio and greater simplicity in design, as evidenced by the evolution of the lift
engine. It is of interest to consider the likely effects of these trends on future
combustor designs.
(a) Increase in by-pass ratio. This has led to the development of large, complex
engines in which the combustion chamber occupies such a small proportion of
the total engine volume that there is little advantage to be gained by a reduction
in chamber size. However, a high combustion eflüciency is very important, not
only at cruise conditions, but also during the acceleration phase from idling to
