1. Free Rotating Vaneless Diffuser of
Diffuser Diameter Ratio 1.30 with
Different Speed Ratios and its Effect
on Centrifugal Compressor
Performance Improvement
Seralathan S, Roy Chowdhury D G
Department of Mechanical Engineering
Hindustan Institute of Technology and Science
Hindustan University, Padur 603 103
Tamil Nadu, India

2. Presentation Outline
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Introduction
Literature Review
Objective
Computational Methodology
Results and Discussions
Conclusions
References

3. Introduction
Introduction
The Flow field inside the centrifugal impeller is influenced by
– Inlet geometry
– Bends in the inlet system
– Angles at inlet and exit of the impeller blade
– Curvature & Shape of the impeller blades
– Rotational forces
– Rotational speed of the impeller
– Type of diffuser
– Shape of the volute casing
– Clearance between the rotating impeller and stationary casing
Also, Conditions of flow at impeller exit is complex due to
– Jet Wake Formation
– Secondary flows
– Mixing process

4. Introduction
Introduction
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Fluid from the impeller exit is non- uniform and impeller discharge mixing takes
place in the vaneless space of the diffuser causing a rise in static pressure as well
as significant loss of total pressure
Large losses measured at inlet of the diffuser is due to fault of impeller
With the centrifugal machines, the Mixing losses after the rotor are usually
important source of inefficiency
Centrifugal impeller flows investigated by number of researchers have confirmed
the existence of separated zones which limit the impeller diffusion

5. Introduction
Diffuser
Convert the high kinetic energy fluid which emerges from impeller into a
maximum static pressure rise
Vaneless diffuser sidewalls are stationary --- Dynamic head & logarithmic
path length of the flow causing shear losses are functions of the magnitude and
direction of absolute velocity leaving the impeller
Vaneless diffuser sidewalls are rotating --- Dynamic head & path length of
the flow causing the shear losses are a function of magnitude and direction of
the relative velocity in the diffuser, which is much smaller than absolute velocity
As a result, frictional losses in rotating vaneless diffuser smaller than stationary
vaneless diffuser

6. Introduction
Introduction
Necessary to develop Novel Non-Conventional Diffuser Designs / methods
-- Reducing energy losses associated with diffusion
-- Increasing stable operating ranges of diffusion systems
Rotating Vaneless Diffusers is one among several methods studied and tried out by
the researchers.
1.
Free Rotating Vaneless Diffuser [Free RVD]
Separate entity and rotate at a fraction of the impeller speed by using suitable
arrangement
2.
Forced Rotating Vaneless Diffuser [Forced RVD]
Integral and rotate at same speed as the impeller

7. Free Rotating Vaneless Diffuser
Replacement of the vaneless diffuser section of a typical high-pressure single stage centrifugal
compressor by Free Rotating Vaneless Diffuser
[ C. Rodgers and H. Mnew, April 1975]
Separate entity
Free rotating vaneless
diffuser
Mechanism for free
rotating vaneless diffuser
Diffuser speed becomes a fraction of the impeller speed so that shear
forces between the flow and diffuser are greatly reduced
Boundary layer growth within the rotating diffuser is smaller than
stationary diffuser
Compressor performance improves from both frictional and flow profile
considerations
Fig. 1 and Fig. 2 Free rotating vaneless diffuser (C.Rodgers and H. Mnew, April 1975)

8. Objective
Objective
The objective of this present investigation is to study
numerically the impact of free rotating vaneless
diffuser on the flow diffusion in detail along with the
performance characteristics of a centrifugal
compressor.
•
Impeller with a free rotating vaneless diffuser of diffuser diameter
ratio 1.30 along with stationary vaneless diffuser at downstream for
the remaining radius ratio running at a speed ratio 0.25 times (Free
RVD30
SR0.25) as well as speed ratio 0.75 times (Free RVD30
SR0.75) the impeller rotational speed
with all the other
dimensional details remaining the same.
• Comparisons are done with the basic impeller involving stationary
vaneless diffuser of diffuser diameter ratio 1.40 (SVD).

9. Computational Methodology
Computational Methodology
The numerical investigations are carried out using a commercial CFD code, namely ,
ANSYS CFX 13.0
ICEM CFD
Three dimensional model of centrifugal impeller along with
its fluid domain is created and meshing is done. Unstructured
tetrahedral prism elements are used for grid generation.
CFX-Pre
Boundary conditions, solver parameters, convergence
criteria are defined and a definition file is created.
CFX-Solve
Definition file is solved until the defined convergence
criteria is reached and results file is created.
CFX Post
Results file is opened and the post processing is done.

10. Single Passage of the impeller
Single passage Approach of the
Centrifugal Impeller
Outlet
STATIONARY VANELESS DIFFUSER
Blade
Periodic Boundaries
Shroud
Inlet
Centrifugal Impeller Model
Computational Domain

11. Numerical Validation
Numerical Validation
Comparison of non-dimensional static pressure distribution measured across the width at the exit of
the radial tipped impeller alone with various turbulence models
[ Ф = 0.37 N = 1500 rpm ]
[4] Govardhan, M., Moorthy, B. S. N., Gopalakrishnan, G., 1978. “A preliminary report on the rotating vaneless diffuser for a centrifugal impeller”,
Proceedings of the First International Conference on Centrifugal Compressor, IIT Madras.

12. Boundary Conditions for the
Computational Domain
SVD
Total Pressure - inlet
Mass Flow Rate - outlet.
Rotating Frame of reference to the entire domain.
K-ω Turbulence model
Free RVD

13. Results and Discussions
Performance Characteristics
(a)
(a) Variation of isentropic efficiency
(b)
(b) Variation of energy coefficient

14. Results and Discussions
Diffuser Performance
(a)
(a) Static pressure recovery coefficient
for SVD and Free RVD
(b)
(b) Stagnation pressure loss Coefficient
for SVD and Free RVD

15. Results and Discussions
Flow through centrifugal compressor
R = 1.05
R = 1.28
R = 1.47
Variation
of
tangential velocity
distribution
with
flow coefficient for
SVD, Free
RVD30
SR0.25 and Free
RVD30
SR0.75
measured across the
width of the impelle
r and diffuser at vari
ous radius ratios
R = 1.05,
R= 1.28
and R = 1.47

16. Results and Discussions
Flow through centrifugal compressor
R = 1.05
R = 1.28
R = 1.47
Variation of exit
flow angle with
flow coefficient for
SVD, Free RVD30 SR
0.25 and Free RVD3
0 SR0.75 measured
across the width of
the impeller and diff
user at various radiu
s ratios R = 1.05,
R= 1.28 and R = 1.47

17. Results and Discussions
Flow through centrifugal compressor
R = 1.05
R = 1.28
R = 1.47
Variation
of
stagnation pressure
coefficient with flow
coefficient for SVD,
Free RVD30 SR0.25
and
Free
RVD30
SR0.75
measured
across the width of
the impeller and
diffuser at various
radius ratios R = 1.05,
R= 1.28 and R = 1.47

18. Results and Discussions
Flow through centrifugal compressor
R = 1.05
R = 1.28
R = 1.47
Variation of static
pressure coefficient
with
flow
coefficient for SVD,
Free RVD30 SR0.25
and Free RVD30
SR0.75
measured
across the width of
the impeller and
diffuser at various
radius ratios R =
1.05, R= 1.28 and
R = 1.47

19. Conclusions
The performance characteristics of diffuser configurations involving Free
Rotating Vaneless Diffuser (Free RVD30 SR0.25 and Free RVD30 SR0.75) are
analyzed in terms of efficiency, energy coefficient, stagnation pressure loss
coefficient, static pressure recovery coefficient as well as static pressure rise.
The following conclusions are obtained based on the results.
• A higher static pressure rise with reduced losses is achieved by
Free RVD30 SR0.75 configuration.
• The static pressure recovery coefficient increased by around 23 to 80%
over the entire flow range, by independently rotating the vaneless diffuser
at a speed ratio of 0.75 times the impeller rotational speed.
• The losses in the free rotating vaneless diffuser (Free RVD30 SR0.75) are
lesser due to reduced shear between the through flow and independently
rotating walls of the diffuser. At design flow condition, there is a gain in
energy to the fluid by the freely rotating vaneless diffuser.

20. Conclusions
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The efficiency of the Free RVD30 SR0.75 configuration is marginally lesser by a
round 5.3 to 6.3% with SVD at design and off-design flow coefficients.
The energy coefficient, which is measure of pressure rise in the compressor,
increased by around 17 to 23% for Free RVD30 SR0.75 configuration over the
entire flow range.
This indicates that rate of diffusion is higher in the free rotating vaneless
diffuser configuration. By the comparing the performance characteristics of
free rotating vaneless diffuser configuration with speed ratio 0.25 (Free RVD30
SR0.25) and speed ratio 0.75 (Free RVD30 SR0.75), the performance
improvement for the centrifugal compressor in terms of static pressure rise
with reduced losses is enhanced with speed ratios above 0.25 times the
impeller rotational speed.
Thus, the free rotating vaneless diffuser concept can be put into practice in
low-specific speed centrifugal compressors for achieving a higher static
pressure rise with reduced losses.

21. References
[1] Rodgers, C. (1972) Analytical, Experimental and Mechanical Evaluation of F
ree Rotating Vaneless Diffuser, Final Report, ER 2391, AD 744475.
[2] Rodgers, C. and Mnew, H. (1975) Experiments with a Model Free Rotating
Vaneless Diffuser. ASME Journal of Engineering for Power, pp. 231-244.
[3] Fradin, C. (1975) The Effect of the Rotational Speed of a Vaneless Diffuser
on the Performance of a Centrifugal Compressor, European Space Agency,
Paris, Report No: ESA-TT-202 ONERA-NT-218.
[4] Govardhan, M. Moorthy, B.S.N. and Gopalakrishnan, G. (1978) A Prelimina
ry Report on the Rotating Vaneless Diffuser for a Centrifugal Impeller, Proc
eedings of the First International Conference on Centrifugal Compressor Te
chnology, IIT Madras, Chennai, India.