Page 156 - Fluid Mechanics and Thermodynamics of Turbomachinery

CHAPTER 5

Axial-flow Compressors and

Fans

A solemn, strange and mingled air, ’t was sad by fits, by starts was wild.
(W. COLLINS, The Passions.)

Introduction
The idea of using a form of reversed turbine as an axial compressor is as old as
the reaction turbine itself. It is recorded by Stoney (1937) that Sir Charles Parsons
obtained a patent for such an arrangement as early as 1884. However, simply
reversing a turbine for use as a compressor gives efficiencies which are, according to
Howell (1945), less than 40% for machines of high pressure ratio. Parsons actually
built a number of these machines (circa 1900), with blading based upon improved
propeller sections. The machines were used for blast furnace work, operating with
delivery pressures between 10 and 100 kPa. The efficiency attained by these early,
low pressure compressors was about 55%; the reason for this low efficiency is now
attributed to blade stall. A high pressure ratio compressor (550 kPa delivery pressure)
was also built by Parsons but is reported by Stoney to have “run into difficulties”.
The design, comprising two axial compressors in series, was abandoned after many
trials, the flow having proved to be unstable (presumably due to compressor surge).
As a result of low efficiency, axial compressors were generally abandoned in favour
of multistage centrifugal compressors with their higher efficiency of 70 80%.
It was not until 1926 that any further development on axial compressors was
undertaken when A. A. Griffith outlined the basic principles of his aerofoil theory
of compressor and turbine design. The subsequent history of the axial compressor
is closely linked with that of the aircraft gas turbine and has been recorded by
Cox (1946) and Constant (1945). The work of the team under Griffith at the Royal
Aircraft Establishment, Farnborough, led to the conclusion (confirmed later by rig
tests) that efficiencies of at least 90% could be achieved for ‘small’ stages, i.e. low
pressure ratio stages.
The early difficulties associated with the development of axial-flow compres-
sors stemmed mainly from the fundamentally different nature of the flow process
compared with that in axial-flow turbines. Whereas in the axial turbine the flow
relative to each blade row is accelerated, in axial compressors it is decelerated.
It is now widely known that although a fluid can be rapidly accelerated through a
passage and sustain a small or moderate loss in total pressure the same is not true for
a rapid deceleration. In the latter case large losses would arise as a result of severe
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