Abstract:
It is well
recognised that the volute plays a important role on the stage performance
of a turbo machine in terms of
pressure recovery, losses, efficiency and flow range, in
addition to
influencing stability and radial loads. Although there is a demand for
further
improvements in efficiency and operating range of compressor components,
less attention has been
paid to the effect of volute design.
The
goal of the present study was to experimentally measure the flow through a high
pressure ratio 5:1 centrifugal compressor incorporating a vaned diffuser and a volute
as
part of a fully representative, production version turbocharger test facility. By
running the unit as in a gas turbine cycle' (that is with the compressor flow passed
through the combustors and then through the hot axial turbine of the turbocharger) it
was
possible to generate the 1.2MW of power that the test compressor required at its
design point. This approach allowed the compressor to be tested at a duty that was
relevant to
todays industrial needs.
The
present study shows that the source of the circumferential pressure distortion at
the
impeller tip was the non-axisymmetric volute. The vaned diffuser did not fully
attenuate this distortion, and consequently some of this distortion was measured at the
impeller tip. The impeller tip pressure distortion varied as the operating point moved
on the
compressor characteristic and it was seen to be greatest at the surge point.
The
performance duties of the compressor components were examined in detail. The
radial vaned diffuser had a
major influence on both the stage and volute performance,
and
imposed a very narrow operating range on the test volute. The volute had a nearly
constant loss coefficient and the
pressure recovery therefore mainly depended on the
dynamic head at the volute inlet at the duty points encountered on the tests. The test
volute was modified to have a cutback°
tongue which gave a better compressor
matching at high flows.
The flow
pattern at the impeller exit was seen to comprise two distinct regions, firstly
a wake° or accumulation of low relative
energy fluid which occupied most of the
shroud surface and extended towards the suction side, and secondly a lower loss flow
which was
present at the hub pressure comer.
The
survey also investigated the structure of the flow in the test volute. The volute did
not
operate as a constant pressure collector° and there was a swirling flow within it.
This Vortex distribution was seen to
depend on the radial velocity distribution at the
volute inlet.
Larger radial velocities contributed to stronger swirling flows. The lowest
total
pressure showed itself in the core of the Vortex over the volute cross sections.
There was a radial decrease of the
tangential velocities from the inner to the outer
radius of the volute cross sections. The flow into the test volute is not uniform.
Additional
insight into the nature of the compressor flows was derived from a
complimentary CFD analysis which is also described.