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246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
246 seralathan s
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  • 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 • • • • • • • 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 • • • • 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 • • • • 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.

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