COMPUTATIONAL ANALYSIS OF CENTRIFUGAL COMPRESSOR WITH GROOVES ON CASING

Transcription

1 INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET) International Journal of Mechanical Engineering and Technology (IJMET), ISSN (Print), ISSN ISSN (Print) ISSN (Online) Volume 6, Issue 2, February (2015), pp IAEME: Journal Impact Factor (2015): (Calculated by GISI) IJMET I A E M E COMPUTATIONAL ANALYSIS OF CENTRIFUGAL COMPRESSOR WITH GROOVES ON CASING P. Usha Sri*, J. Deepthi Krishna** *Professor, Department of Mechanical Engineering, University College of Engineering, Osmania University, Hyderabad, Telangana , India. **Student, Department of Mechanical Engineering, University College of Engineering, Osmania University, Hyderabad, Telangana , India. ABSTRACT Computational results of flow field in a centrifugal impeller with grooves on casing are presented in the paper. A low speed centrifugal compressor with 2% tip clearance is considered. Analysis is carried out for three different cases, one without grooves on the casing, second with two grooves on casing and third with three grooves on the casing. The three cases are studied at five different flow coefficients φ=0.28, 0.34, 0.42 (design value), 0.48 and 0.52 using Ansys-CFX. Pressure ratio improvement with two grooves on casing is observed. The leakage of flow over tip of the blade through the grooves from pressure side to suction side of the blade is interacting with passage wake near casing and reduction in passage wake region area is observed with two grooves on casing. Increase in the velocity of the fluid in passage wake is also observed with two grooves on casing. Increase in pressure ratio is observed for the casing with two grooves at all flow coefficients. For three grooves on casing also, increase of velocity in passage wake is observed but reduction in pressure ratio is observed due to more leakage flow. Reduction in pressure ratio for all flow coefficients is observed with three grooves on casing. Key Words: Centrifugal Compressor, Flow coefficient, Grooves on casing, passage wake. 1. INTRODUCTION In turbomachines, to desensitize tip clearance effects, squealer tips / partial shrouds, tip geometry modifications, casing treatment etc. are suggested in the literature. P. Usha Sri and N. Sitaram observed that the impeller with the chamfer on suction surface of the blade tip shows small improvement in 1

2 performance. With an increase in the chamfer dimension on suction surface of blade tip, performance improvement is observed. Through experiments, N. Sitaram and S.M. Swamy have observed performance improvement on centrifugal compressor with pressure side partial shrouds. S. Senthil and N. Sitaram studied the performance of a centrifugal compressor by means of squealer tips. They observed increase in energy coefficient and efficiency with squealer tips on pressure surface. S. Senthil and N. Krishna Mohan found that sloping type squeeler tip has beneficial effects. Chi-Young Park et. al have observed improvement of performance and surge margin with ring groove system on centrifugal compressor. Fayez M. Wassef et. al have conducted experiments on centrifugal compressor and observed improvement in limit of stability with addition of a ring and a groove in the casing in diffuser region. 2. COMPUTATIONAL METHODOLOGY The design details of the impeller which is used in the investigations are given below: Inducer hub diameter, d 1h = 160 mm Inducer tip diameter, d 1t = 300 mm Impeller tip diameter, d 2 = 500 mm Blade height at the exit, b 2 = 34.7 mm No. of blades of impeller, N b = 16 Blade angle at inducer hub, β 1h = 53 o Blade angle at inducer tip, β 1t = 35 o Blade angle at exit, β 2 = 90 o Thickness of the blade, t = 3 mm Rotor speed, N = 2000 rpm All angles are with respect to the tangential direction. Centrifugal impeller with above specifications with 3 mm thickness throughout the blade, 2% tip clearance is shown in Fig. 1. Assuming periodicity, single passage of centrifugal impeller is analysed. A single passage of the impeller with inlet at 50 mm ahead of the impeller and outlet at a distance of 35 mm downstream of impeller is shown in Fig. 2. Casing is designed with a clearance of 0.7 mm throughout the blade height. Total pressure is used for inlet boundary condition and mass flow rate at outlet. Rotating frame of reference is given to the domain. ANSYS-CFX software is used for obtaining the solution and standard k-ε turbulence model is used for the closure. The centrifugal compressor is analysed at five different flow coefficients (0.28, 0.34, 0.42, 0.48 and 0.52), the design flow coefficient being Fig. 1 Centrifugal compressor 2

3 Fig. 2 Computational domain of single passage On casing of the impeller, circular grooves of width 3 mm and depth 4 mm are made. Three cases of centrifugal compressor, one without grooves, second with two grooves on casing at streamwise locations of 0.515, and third with three grooves on casing at streamwise locations of 0.515, 0.555, are modeled as shown in figure 3 to 5. Grid with boundary conditions ions for three different cases is shown in figures 6-8. Fig. 3 Compressor without Fig. 4 Compressor with two Fig. 5 Compressor with three grooves on casing grooves on casing grooves on casing Fig. 6 Compressor without Fig. 7 Compressor with two Fig. 8 Compressor with three grooves on casing grooves on casing grooves on casing 3

4 3. RESULTS AND DISCUSSIONS A low speed centrifugal compressor of radial blades with grooves on casing is analysed. The pressure contours, velocity contours on the casing of the compressor are plotted for flow coefficient of Velocity vectors, total pressure contours and static pressure contours are plotted on a meridional plane at a streamwise location of Pressure variation along streamwise direction, pressure ratio at all flow coefficients and percentage deviation in velocity along streamwise direction with grooved casing are presented. 3.1 Static Pressure Contours on Casing: Pressure contours on casing for three cases, without grooves on casing, with two grooves on casing and with three grooves on casing is shown in figures The contours show gradual pressure rise from inlet to outlet of the compressor due to dynamic action of the rotating impeller. Pressure gradient above the blade is observed due to the high pressure on pressure side and low pressure on suction side of the blade. For the casing with two grooves, no significant change in pressure is observed. For the casing with three grooves, low pressures on both pressure and suction side of the blade is observed due to more leakage of flow from the grooves. Fig. 9 Pressure contours on casing without grooves Fig. 10 Pressure contours on Fig. 11 Pressure contours on casing with two grooves casing with three grooves 3.2 Velocity Contours on Casing Velocity contours on casing for three different cases s are shown in figures The contours show gradual increase of velocity from inlet to outlet of the impeller. Fluid flow with low velocity from pressure side of the blade to suction side is observed through grooves. Fig. 12 Velocity contours on casing without grooves Fig. 13 Velocity contours on Fig. 14 Velocity contours on casing with two grooves casing with three grooves 4

5 3.3 Static pressure Contours at Meridional Plane Pressure contours at a meridional plane just after the grooves for three cases is shown in figures The contours show high pressure on pressure side (PS) of the blade, low pressure on suction side (SS) of the blade. With two grooves on casing, a slight pressure drop is observed on suction side. With three grooves on casing, significant reduction in pressure on suction side is observed. Fig. 15 Pressure contours on meridional plane for without grooves on casing Fig. 16 Pressure contours on with two grooves on casing Fig. 17 Pressure contours on with three grooves on casing 3.4 Total Pressure Contours at Meridional Plane Total pressure contours on meridional plane just after the grooves for three cases are shown in figures On suction side near casing, low total pressure area caused due to passage wake is observed. With two grooves on casing, low total pressure area of passage wake is reduced. The leakage flow from the grooves is interacting with passage wake. With three grooves on casing, the passage wake area is further reduced, but total pressure in this area is much lower due to more leakage of flow from three grooves. Fig. 18 Total pressure contours on meridional plane for without grooves on casing Fig. 19 Total pressure contours on meridional plane for impeller with two grooves on casing Fig. 20 Total pressure contours on impeller with three grooves on casing 3.5 Velocity Contours at Meridional Plane Velocity contours on meridional plane for three cases are shown in figure The contours show improved velocities on suction side with grooves. The low velocity passage wake area on suction side of the blade is reducing with two grooves and three grooves on casing. However, the velocity in passage wake region is much lower with three grooves on casing as more leakage flow is interacting with the main flow. 5

6 Fig. 21 Velocity contours on meridional plane for without grooves on casing Fig. 22 Velocity contours on with two grooves on casing Fig. 23 Velocity contours on with three grooves on casing 3.6 Velocity Vectors at Meridional ional Plane Velocity vectors on meridional plane for three cases are shown in figures Passage wake with low velocity is observed on suction side near casing of the blade. With grooves on casing, reduction in passage wake area is observed. Fig. 24 Velocity vectors on meridional plane for without grooves on casing Fig. 25 Velocity vectors on with two grooves on casing Fig. 26 Velocity vectors on with three grooves on casing 3.7 Static Pressure Contours at Span 0.7 Pressure contours in blade to blade view, at span 0.7 is shown in figures The contours show gradual pressure rise from inlet to outlet of the compressor due to dynamic action of the rotating impeller. With two grooves on casing, no pressure change is observed. But with three groves on casing, the pressure at outlet is reduced. Fig. 27 Pressure contours for without grooves on casing Fig. 28 Pressure contours for impeller with two grooves on casing Fig. 29 Pressure contours for impeller with three grooves on casing 6

7 3.8 Velocity Contours at Span 0.7 Velocity contours in blade to blade view, at span 0.7 is shown in figures The contours show low velocity region on suction side of the blade. With grooves on casing, the low velocity region is reducing and also velocity improvement is observed. Fig. 30 Velocity contours for without grooves on casing Fig. 31 Velocity contours for impeller with two grooves on casing Fig. 32 Velocity contours for impeller with three grooves on casing 3.9 Static Pressure Distribution Static pressure distribution along streamwise direction from inlet to outlet is shown in figure 33. This plot elucidate that the pressure from inlet to the outlet of the compressor is increasing gradually along the stream wise direction due to the dynamic head developed by the rotating impeller. A drop in static pressure near streamwise direction of 0.2 is observed for all cases due to the acceleration of the flow in to the eye of the impeller. Static pressure reduction is observed with the grooves on casing near the location of grooves. But after the grooves location, static pressure is almost equal to the casing without grooves. With two grooves on casing, static pressure at outlet is more than the static pressure of casing without grooves Total Pressure Distribution Total pressure distribution along streamwise direction around the grooves is shown in figure 34. Gradual increase of pressure along streamwise direction because of dynamic action of the impeller is observed. Total pressure improvement with grooves is observed. With grooves on casing, the fluid flow through grooves from pressure side to suction side of the blade is interacting with passage wake on the suction side of the blade. Though pressure change is not significant, substantial velocity improvement is the cause for total pressure rise around the grooves location Static Pressure no groove Total Pressure no groove 2 grooves 3 grooves Stream wise location Fig. 33 Static Pressure from inlet to outlet Stream wise location Fig. 34 Total Pressure distribution 7

8 3.11 Percentage Deviation of Velocity with Grooves The velocity from inlet to outlet of the compressor passage of grooved casing is compared with the casing without grooves. Percentage deviation in velocity with two grooves is shown in figure 35. Near the streamwise location where grooves are present, a significant velocity improvement of 35% is observed. Percentage Deviation in Velocity Sreamwise Location Fig. 35 Percentage deviation in velocity from inlet to outlet 3.12 Pressure Ratio at Off Design Conditions Outlet to inlet pressure ratio at different flow coefficients is shown in fig. 36. Increase in pressure ratio with two grooves on casing is observed at all flow coefficients. But with three grooves on casing, reduction in pressure ratio is observed at all flow coefficients because of more fluid leakage through the three grooves Static Pressure Ratio No Groove 2 Grooves 3 Grooves Flow Coefficient Fig. 36 Pressure ratio for different flow coefficients 8

9 4. CONCLUSIONS A low speed centrifugal compressor with 2% tip clearance with two grooves on casing and three grooves on casing are analysed at five different flow coefficients. The results are compared with the casing without grooves. With three grooves on casing, though velocity at the meridional section around the grooves is increased, the pressure at outlet is reduced because of more leakage of flow over the blades from grooves. With two grooves on casing, due to the interaction of the leakage flow from grooves with passage wake, the velocity and total pressure improvement is observed. Also with two grooves on casing, static pressure rise at outlet is observed for all coefficients. 5. ACKNOWLEDGEMENTS The authors acknowledges All India Council for Technical Education (AICTE) for the financial assistance provided for the project under R&D scheme. REFERENCES 1. P. Usha Sri and N. Sitaram, Computational Investigation of Flow in a Centrifugal Impeller with Chamfered Blade Tips: Effect of Tip Clearance, National Conference on CFD Applications in Power &b Industry Sectors, organised by BHEL R&D, Hyderabad during Nov 17-18, S. Senthil and N. Sitaram, 2002, Performance Improvement of a Centrifugal Compressor by means of Squealer Tips, Proc. of the 4 th ICPF Beijing, China, S. Senthil and N. Krishna Mohan, Experimental Investigation on a centrifugal compressor by means of rectangular, elliptical and sloping squeeler tips, Indian Journal of Science and Technology, Vol.2, No. 7, July 2009, pp Chi-Young Park, Young-Seok Choi, Kyoung-Yong Lee and Joon-Yong Yoon, Numerical Study on the Range Enhancement of a centrifugal compressor with a ring groove system, Springer, Journal of Mechanical Science and Technology 26 (5)(2012) Fayez M. Wassef, Ahmed S. Hassan, Hany A. Mohamed and Mohamed A. Zaki, Stability and Performance of a Low Speed Centrifugal Compressor with Modified Casing, Journal of Engineering Sciences, JES, Assiut University, Vol. 32, No. 5, pp , N. Sitaram and S.M. Swamy, Performance Improvement of a Centrifugal Compressor by Passive Means, International Journal of Rotating Machinery, Volume 2012, Article ID , 9 pages. 7. P. Usha Sri and J. Deepti Krishna, Effect of Tip Clearance on A Centrifugal Compressor International Journal of Mechanical Engineering & Technology (IJMET), Volume 5, Issue 9, 2014, pp , ISSN Print: , ISSN Online: Jyothi P.N, A. Shailesh Rao, M.C. Jagath, and K. Channakeshavalu, Understanding The Melt Flow Behaviour of Za Alloys Processed Through Centrifugal Casting International Journal of Mechanical Engineering & Technology (IJMET), Volume 4, Issue 1, 2013, pp , ISSN Print: , ISSN Online: Shalin Marathe and Rishi Saxena, Numerical Analysis on Effect of Exit Blade Angle on Cavitation In Centrifugal Pump International Journal of Mechanical Engineering & Technology (IJMET), Volume 4, Issue 3, 2013, pp , ISSN Print: , ISSN Online: