Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

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 100kPa. 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 (550kPa 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 compressors
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
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