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 103Tamil Nadu, India

Presentation Outline

• Introduction

• Literature Review

• Objective

• Computational Methodology

• Results and Discussions

• Conclusions

• References

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

Introduction

Introduction

• 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

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

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 bythe researchers.

Introduction

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

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

Replacement of the vaneless diffuser section of a typical high-pressure single stage centrifugalcompressor by Free Rotating Vaneless Diffuser

[ C. Rodgers and H. Mnew, April 1975]

Free Rotating Vaneless Diffuser

Fig. 1 and Fig. 2 Free rotating vaneless diffuser (C.Rodgers and H. Mnew, April 1975)

Separate entity

Free rotating vanelessdiffuser

Mechanism for free rotating vaneless diffuser

Objective

• Impeller with a free rotating vaneless diffuser of diffuser diameterratio 1.30 along with stationary vaneless diffuser at downstream forthe remaining radius ratio running at a speed ratio 0.25 times (FreeRVD30 SR0.25) as well as speed ratio 0.75 times (Free RVD30SR0.75) the impeller rotational speed with all the otherdimensional details remaining the same.

• Comparisons are done with the basic impeller involving stationaryvaneless diffuser of diffuser diameter ratio 1.40 (SVD).

Objective

The objective of this present investigation is to studynumerically the impact of free rotating vanelessdiffuser on the flow diffusion in detail along with theperformance characteristics of a centrifugalcompressor.

Computational Methodology

ICEM CFD

CFX-Pre

CFX-Solve

CFX Post

Boundary conditions, solver parameters, convergence

criteria are defined and a definition file is created.

Definition file is solved until the defined convergence

criteria is reached and results file is created.

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.

Results file is opened and the post processing is done.

The numerical investigations are carried out using a commercial CFD code, namely ,

ANSYS CFX 13.0

Computational Methodology

Centrifugal Impeller Model

Single passage of the impellerOutlet

Inlet

Periodic Boundaries

Blade

Shroud

STATIONARY VANELESS DIFFUSER

Single Passage Approach of the Centrifugal Impeller

Computational Domain

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.

Numerical Validation

Boundary Conditions for the Computational Domain

Total Pressure - inlet

Mass Flow Rate - outlet.

Rotating Frame of reference to the entire domain.

K-ω Turbulence model

SVD Free RVD

Results and DiscussionsPerformance Characteristics

(a) Variation of isentropic efficiency (b) Variation of energy coefficient

(a) (b)

(a) Static pressure recovery coefficient (b) Stagnation pressure loss Coefficient for SVD and Free RVD for SVD and Free RVD

Results and DiscussionsDiffuser Performance

(a) (b)

Variation oftangential velocitydistribution withflow coefficient forSVD, Free RVD30SR0.25 and FreeRVD30 SR0.75measured across thewidth of the impeller and diffuser at various radius ratiosR = 1.05, R= 1.28and R = 1.47

R = 1.05

R = 1.28

R = 1.47

Results and DiscussionsFlow through centrifugal compressor

Variation of exitflow angle withflow coefficient forSVD, Free RVD30 SR0.25 and Free RVD30 SR0.75 measuredacross the width ofthe impeller and diffuser at various radius ratios R = 1.05,R= 1.28 and R = 1.47

R = 1.05

R = 1.28

R = 1.47

Results and DiscussionsFlow through centrifugal compressor

Variation ofstagnation pressurecoefficient with flowcoefficient for SVD,Free RVD30 SR0.25and Free RVD30SR0.75 measuredacross the width ofthe impeller anddiffuser at variousradius ratios R = 1.05,R= 1.28 and R = 1.47

R = 1.05

R = 1.28

R = 1.47

Results and DiscussionsFlow through centrifugal compressor

Variation of staticpressure coefficientwith flowcoefficient for SVD,Free RVD30 SR0.25and Free RVD30SR0.75 measuredacross the width ofthe impeller anddiffuser at variousradius ratios R =1.05, R= 1.28 andR = 1.47

R = 1.05

R = 1.28

R = 1.47

Results and DiscussionsFlow through centrifugal compressor

Conclusions

The performance characteristics of diffuser configurations involving FreeRotating Vaneless Diffuser (Free RVD30 SR0.25 and Free RVD30 SR0.75) areanalyzed in terms of efficiency, energy coefficient, stagnation pressure losscoefficient, 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.

Conclusions

• The efficiency of the Free RVD30 SR0.75 configuration is marginally lesser by around 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 vanelessdiffuser 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.

References

[1] Rodgers, C. (1972) Analytical, Experimental and Mechanical Evaluation of Free 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 Preliminary Report on the Rotating Vaneless Diffuser for a Centrifugal Impeller, Proceedings of the First International Conference on Centrifugal Compressor Technology, IIT Madras, Chennai, India.