Abstract:
In axial flow compressors
and fans, the stable working range is
restricted by the so called stall line where operation may become
unstable and
simultaneously a short fall in compressor performance
appears.
Stall margin improvement has been a major task and experimental
observations over the last two decades have shown that modifying the
shape of the outer annulus wall, above the tips of rotor blades, is
effective in delaying the onset of stall to lower mass flow rates.
A wide variety of wall modifications or casing treatments have been
tried with a range of stall flow improvement which can amount to about
20% .(of the stalled flow)
An alternative technique for stall margin improvement in a single
stage axial flow machine has been studied and results will be
described in the present thesis. The technique for delaying stall
involves a large scale treatment to the outer casing of the compressor
which extends partly over and mainly upstream of the rotor.
The operating principle appears to be that as the flow is reduced
towards the stall point some radially unbalanced cells are transported
from the blade tips where they are collected in the treatment , turned
by the treatment vanes , and re-introduced to the main flow upstream
of the rotor.
A rotor with and without outlet stators has been tested ( and also
with and without casing treatment) over a range of speeds and flow
conditions.
Also the flow condition inside the recessed casing has been simulated
by means of an existing computer code known as PHOENICS.
