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Effect of Tip Clearance on a Centrifugal Compressor

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Effect of Tip Clearance on a Centrifugal Compressor
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  International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 9, September (2014), pp. 379-384 © IAEME   379 EFFECT OF TIP CLEARANCE ON A CENTRIFUGAL COMPRESSOR P. Usha Sri*, J. Deepti Krishna** * Professor, Department of Mechanical Engineering, University College of Engineering, Osmania University, Hyderabad, Telangana – 500 007, India * Student, Department of Mechanical Engineering, University College of Engineering, Osmania University, Hyderabad, Telangana – 500 007, India. ABSTRACT Computational analysis of low speed centrifugal compressor is carried out with finite volume method using ANSYS-CFX software. Centrifugal compressor impeller with three values of clearances i.e., 0%, 2% and 5% of blade height at trailing edge are examined at five flow coefficients φ =0.28, 0.34, 0.42 (design value), 0.48 and 0.52. The effect of tip clearance on static pressure from inlet to outlet of the compressor is analyzed. The drop in static pressure with increase in tip clearance is found to be high at the tip of the blade due to high pressure fluid leakage at the tip of the blade. Performance reduction with tip clearance is observed. Total pressure and velocity at outlet are analysed for five flow coefficients. Keywords: Centrifugal Compressor, Flow Coefficient, Tip Clearance. 1.   INTRODUCTION Tip clearance in centrifugal compressor causes the leakage of high pressure fluid from pressure surface to suction surface of the impeller blade, making the flow field highly complex and affecting the performance. The required tip clearance can be obtained by shifting the casing in radial or axial or combined radial and axial directions. Hayami (1997) has found from his experiments that axial movement of the casing has better efficiency over the movement of casing in radial and axial directions. Radial movement of casing increases clearance at inducer, which reduces the operating range. The tip clearance studies are conducted to understand the flow behavior in order to minimise the effect of tip clearance. Swamy and Pandurangadu(2013), Pampreen (1973), Mashimo et al. (1979), Sitaram and Pandey (1990) have conducted experimental studies and suggested that by reducing the tip clearance gap size, the tip clearance effect can be minimised. The effect of tip leakage on flow behavior in rotating impeller passage was computationally carried out by Usha Sri and Sitaram (2004), Hark-Jin Eum et al. (2004), Hathaway et al. (1993).   INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET) ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online) Volume 5, Issue 9, September (2014), pp. 379-384 © IAEME: www.iaeme.com/IJMET.asp Journal Impact Factor (2014): 7.5377 (Calculated by GISI) www.jifactor.com   IJMET   © I A E M E    International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 9, September (2014), pp. 379-384 © IAEME   380 Fig. 1:  Centrifugal compressor 2. COMPUTATIONAL METHODOLOGY The design details of the impeller which is used in the present investigation 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 3mm thickness through out the blade is shown in Fig. 1. A single passage of the impeller with inlet at 50mm ahead of the impeller and outlet at a distance of 35mm downstream of impeller is shown in Fig. 2. Casing is designed with a clearance of 0.7mm throughout the blade height. Three tip clearances of 0%, 2% and 5% of trailing edge blade height are obtained by moving the casing axially. The 0% clearance model, which is not practicable, is generated for reference. 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 compressors of three clearances (0%, 2% and 5%) were analysed at five different flow coefficients (0.28, 0.34, 0.42, 0.48 and 0.52). The design flow coefficient is 0.42. 3. RESULTS AND DISCUSSIONS Centrifugal compressor with three tip clearances 0%, 2% and 5% at five flow coefficients 0.28, 0.34, 0.42, 0.48 and 0.52 were analysed. Static pressure variation from inlet to outlet of the domain and static pressure variation with flow coefficient for three tip clearances were plotted. Total pressure graphs, blade loading charts, pressure contours and velocity vectors are analysed. 3.1 Static pressure distribution from inlet to outlet: Static pressure variation along meridional distance for three tip clearances at five flow coefficients is shown in Fig. 3. Static pressure is constant before the impeller passage. Static pressure drop at impeller leading edge is observed which causes the fluid to accelerate in to the compressor. Pressure is increasing steadily in the impeller passage for all tip clearances due to the energy transfer taking place the impeller. The drop in static outlet blade Periodic Boundaries inlet Fig. 2:  Computational domain of single passage  International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 9, September (2014), pp. 379-384 © IAEME   381 pressure with increase in tip clearance is found to be high at the tip of the blade due to high pressure fluid leakage at the tip of the blade. 3.2 Static pressure variation with flow coefficient:  Static pressure at outlet versus flow coefficient graph for three different tip clearances are shown in fig. 4. The static pressure maximum is found at flow coefficient of 0.34 while reduced pressure rise at flow coefficient 0.28 is observed. At flow coefficient 0.28, for 0% clearance pressure rise is very less as separation of flow is observed. With increase in tip clearance, the static pressure is also reducing for all flow coefficients. Static pressure is reducing with increase in flow coefficient. 3.3 Total pressure variation with flow coefficient:  Total pressure at outlet versus flow coefficient graph for three different tip clearances is shown in fig. 5. Total pressure rise is reduced at flow coefficient 0.28 is observed. At flow coefficient 0.28, for 0% clearance, total pressure rise is very less as separation of flow is observed. With increase in tip clearance, the total pressure is reducing for all flow coefficients. Total pressure is reducing with increase in flow coefficient after design flow coefficient. 3.4 Blade loading chart: Blade loading charts at design flow coefficient 0.42 for three tip clearances were shown in fig. 6 to 8. Low static pressure on suction and high pressure on pressure side of the blade is observed. With increase in tip clearance, static pressure on both pressure side and suction side are reducing. Fig. 3:  Static Pressure from inlet to outlet Fig. 4:  Static Pressure Vs Flow coefficient Fig. 5:  Static Pressure Vs Flow coefficient Fig. 6:  Blade loading for 0% clearance at φ =0.42  International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 9, September (2014), pp. 379-384 © IAEME   382 3.5 Static pressure contours at mid span: Static pressure contours at mid span of the blade for φ =0.42 are shown in fig. 9 to 11. Gradual increase of static pressure from inlet to outlet is clearly observed at all tip clearances. High pressure on pressure side of the blade and low pressure on suction side of the blade are observed at all tip clearances. With increase in tip clearance, reduction in pressure on both pressure side and suction side is found. Fig. 7:  Blade loading for 2% clearance at φ =0.42 Fig. 8:  Blade loading for 5% clearance at φ =0.42 Fig. 9:  pressure contours for 0% clearance with φ =0.42 at mid span Fig. 10:  pressure contours for 2% clearance with φ =0.42 at mid span
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