Industrial centrifugal compressors in many applications require higher efficiency
and a wider operating range to reduce their life cycle costs. As the cost of energy
increases with the increasing demand for energy, the requirement for a compressor with
good efficiency and a wider operating range is justified. It is very difficult to meet these
two requirements which are almost contradictory. In the present study, an attempt is
made to improve the stable operating range and the efficiency of the centrifugal
compressor, although in separate cases.
There is enough literature available on the vaneless, vaned and low solidity vaned
diffusers and their influence on the performance of a centrifugal compressor stage. But
the investigations on the centrifugal compressor stage with leaned and partial vaned
diffusers are very limited. The thesis has two objectives, to study the effect of leaned
vanes and partial vanes in the diffuser, on the compressor stage performance. The
investigation of the compressor stage with these two diffusers is carried out numerically
and independently.
In the first part, the effect of the leaned vanes instead of the straight curved vanes
in the low solidity vaned diffuser on the stable operating range is examined. The vanes
in the low solidity vaned diffuser were leaned in the direction of impeller rotation by
reducing the diffuser inlet vane angle from hub to shroud uniformly. Five cases of
leaned vaned diffusers were used downstream of the same impeller with different lean
magnitude. The optimum lean angle for the diffuser vanes was found out. The effect of
number vanes in the diffuser is also explored.
In the leaned vaned diffuser, the stall and choke points moved to lower mass flow
rates, on compressor characteristic curve, as the lean angle is increased from the base
case, resulting in an improvement in the stall margin and decrement in the choke
margin. As the improvement in stall margin is more than the decrement in the choke
margin, there is an overall increase in the stable operating range. The diffuser with eight
number of vanes, and 60

leaned vanes gave the best performance amongst the all the
lean configurations. It gave an improvement of 19.3% and 12.2% in the stall marginand operating range respectively over the base case. The optimum leaned vaned diffuser
with 7 vanes gave least performance while, the optimum leaned vaned diffuser with 9
vanes gave the intermediately performance in terms of stall margin and operating range.
The results suggest that the optimum lean angle for the diffuser vanes is about 60
in all

the three cases with diffuser vane numbers 9, 8 and 7.
In the second part, the compressor stage performance with partial vaned diffusers
is analyzed numerically. Two types of partial vaned diffusers are considered for the
study. The hub vaned diffusers with the partial vanes fixed to the hub wall, and shroud
vaned diffusers with the partial vanes fixed to the shroud wall. All the partial vaned
diffusers have the same geometrical dimensions as the vaned diffuser except for the
vane location and vane height. In the study totally 5 cases of hub vaned diffusers and 6
cases of shroud vaned diffusers are studied along with the baseline vaneless diffuse. In
these partial vaned diffusers the vane height is varied from 20-80% of the diffuser
width.
The three sub-objectives of this analysis are (i) to analyze the hub vaned and shroud
vaned diffusers and compare their performance against a base line vaneless diffuser (ii)
to investigate the underlying flow physics behind the diffusers’ performance and (iii)
to find the optimum vane height for the partial vaned diffusers.
The results suggest that, the shroud vaned diffuser performs better than the
corresponding (same vane height) hub vaned diffuser almost over the entire operating
range. The hub and shroud vaned diffusers both gave an improvement in the stage
efficiency and diffuser pressure recovery coefficient (Cp) over the vaneless diffuser.
The improvements in the shroud vaned diffusers are significantly higher than those
observed in the hub vaned diffusers. The improvements in stage efficiency and Cp for
shroud vaned diffusers are around 5% and 15% respectively. While the corresponding
numbers for hub vaned diffusers are only 1% and 6%. Shroud vane diffusers showed
better uniformity of the flow parameters in the spanwise direction compared to vanless
or hub vaned diffuser. The spanwise uniformity in the flow facilitated better diffusion
in the diffuser channel. Considering compressor stage, and the diffuser performance
over the entire operating range, the optimum vane height in the partial vaned diffuser
was found to be 0.3 to 0.4 times the diffuser width.