EFFECT OF LEADING EDGE SWEEP ON THE PERFORMANCE OF A CENTRIFUGAL COMPRESSOR IMPELLER. Abstract 1. INTRODUCTION

Transcription

1 EFFECT OF LEADING EDGE SWEEP ON THE PERFORMANCE OF A CENTRIFUGAL COMPRESSOR Abstract IMPELLER Ch. Sivaji Ganesh 1, *Q. H. Nagpurwala 2, C. S. Bhaskar Dixit 3 1 Student, M. Sc. [Engg.], 2 Professor, 3 Professor and Center Manager (RMD), M.S. Ramaiah School of Advanced Studies, Bangalore, *Contact Author nagpurwala@msrsas.org A centrifugal compressor, with backward swept impeller, was designed for industrial applications and its performance was evaluated through numerical simulations. The baseline impeller geometry was then modified at the inducer leading edge to impart different forward and back sweeps. The inducer sweep was also associated with lean in the tangential direction. In all, seven impeller configurations were investigated, with inducer leading edge sweep varying from +25 (back sweep) to -20 (forward sweep). CFD simulations have indicated that the pressure ratio at design mass flow rate remained mostly unaffected by sweep, but showed a tendency to increase beyond an inducer back sweep of 20. Similarly, a forward sweep of 20 gave lowest stall flow rate (highest stall margin). However, with an increase in tip clearance, the impeller with inlet forward sweep of 20 showed an increase in choke and stall mass flow rates. It is concluded that a forward sweep of 20, with baseline tip clearance (0.5 mm), is beneficial in terms of high stall margin and acceptable peak pressure ratio as well as reasonably high efficiency. Keywords: Centrifugal Compressor, Inducer, Inlet Sweep, Tip Clearnce 1. INTRODUCTION Centrifugal compressors find wide application in turbochargers and small turbojet engines because of their compact size, rugged construction, and ability to produce high pressure ratio per stage. However, their efficiencies are lower compared to the axial counterparts. Hence, the designer s goal is to find ways and means to improve efficiency and operating range of centrifugal compressors by modifying certain geometric features, such as imparting blade skew at impeller exit (Arunachalam et al, 2008); trimming the impeller exit width (Raghavendra et al, 2010); and imparting sweep at inducer leading edge (Hazby and Xu, 2007). The effectiveness of sweep in axial compressor blading has been investigated by many researchers, e.g. by Wadia et al [1997], Blaha et al (2000) and Jang et al (2005). It may be noted that the inner working of sweep in axial compressors is somewhat different than in centrifugal compressors. The axial compressor blades are relatively shorter in length along the flow path, and hence the effect of flow redistribution, brought about by sweep, is felt over the entire blade. In contrast, the centrifugal impeller blades are much longer, and the effect of sweep at the inducer leading end may not percolate up to the trailing end. Initial practice was to impart back sweep near the hub and forward sweep near the tip, which resulted in bow shaped blades for axial compressors. The migration of low energy hub boundary layer fluid towards the rotor tip, under centrifugal forces, is shifted downstream of the leading end, thus keeping the tip boundary layer energised, delaying separation and, consequently, delaying stall. In transonic compressors, the forward sweep helps in reducing the meridional component of relative Mach number in the leading end tip region, thus reducing the shock losses. Through careful experimentation and CFD simulations, Wadia et al (1997) showed that a complete forward sweep from hub to tip in axial rotors was more beneficial than a complete back sweep or a mixed sweep, like in bowed rotors. Fischer et al (2003) carried out experimental and numerical analyses of strongly bowed stators of a 4-stage high-speed axial compressor. The bowed stator vanes shifted the flow from near wall regions to mid-span of the vanes in the front portion. This led to a lower loading near the wall, reducing or eliminating the corner stall at the hub. And, when the compressor was highly loaded there was an increase in the overall static pressure rise, total pressure rise and the overall efficiency. Studies by Xu and Chen (2005) on four different rotors, viz, baseline gravity centre stack up blade, forward swept blade, backward swept and root partial forward swept blade, showed that the sweep redistributed the flow, thus reducing the secondary flow loss. It was shown that forward sweep reduced the tip loading in terms of static pressure coefficient. Application of leading edge sweep in a transonic centrifugal impeller is reported by Hazby and Xu (2007). In general, the forward sweep resulted in higher efficiency and wider operating range, whilst efficiency and operating range were reduced for backward swept blades. It is argued that the beneficial effects of forward sweep are due to the reduction of the blade loading in the tip region. It is, thus, recognised that most published information deals with the effect of sweep in axial compressors. Considering the arguments for axial compressors, it is expected that the sweep at inducer leading edge may have beneficial effect in centrifugal compressors also. It is the lack of published information concerning inlet sweep in centrifugal compressors that has provided motivation for the current work. This paper presents the results of numerical simulations on a typical centrifugal compressor with backswept (exit) impeller, designed for industrial application. The baseline impeller geometry was modified to impart forward and back sweeps at inducer leading edge. The overall performance of various impellers without and with inlet sweep is compared, with special reference to SASTECH Journal 55 Volume 9, Issue 2, September 2010

2 peak pressure ratio, peak efficiency and stall margin. The effectiveness of inducer sweep is discussed in terms of flow development through the impeller passages. The tip clearance of the impeller with optimum performance was then increased to find out whether the performance would drop to the level of the baseline impeller. This would, then, ensure that an impeller with inducer leading edge sweep, and having acceptable performance level, could be manufactured with higher tip clearance at relatively lower cost. 2. DEFINITION OF INLET SWEEP The swept blades in axial flow machines are designed, by shifting the blade sections along either the axial or true chord of the unswept blades. However, any shift in the blade sections of a centrifugal impeller, at inducer inlet, results in the variation of the passage area ratio and therefore the relative flow diffusion/acceleration through the blade. The sweep at inducer inlet may be generated by, shifting the blade leading edge along the meridional chord, maintaining the same camber angle distribution (Fig. 1). This may, however, alter the loading distribution within the impeller blade passage, complicating the parametric study on the effects of sweep. 3. CENTRIFUGAL COMPRESSOR 3.1 Baseline Impeller A centrifugal impeller, having 4:1 pressure ratio, 5.5 kg/s mass flow rate and 85% isentropic efficiency, and designed for industrial application (Raghavendra et al, 2010), was used for the current parametric studies on the effect of inducer leading edge sweep. The design rotational speed was rpm. The impeller had 16 blades with exit diameter of mm and an exit back sweep of 30º. The inducer hub and tip diameters were 75.6 mm and mm respectively. A tip clearance of 0.5 mm at the shroud was maintained constant from impeller leading to trailing end. The CAD model of the test impeller, generated using ANSYS Blade Modeler software, is shown in Fig. 3. Fig. 1 Definition of inducer leading edge sweep (Hazby and Xu, 2007) The blade sweep may also result in the variation of the inlet blade angle, with the maximum deviation occurring at the mid blade span. Figure 2 shows the generation of swept blade, using hub and tip sections. Note that the swept blades generated in such a way, have compound leaned profiles if they are viewed in the cross-sectional planes. The compound lean results in a variation in the width of the splitter blocks and a redistribution of the mass flow inside the passages. Fig. 3 CAD model of baseline impeller 3.2 Impellers with Inducer Leading Edge Sweep The baseline impeller geometry was modified by imparting sweep at the inducer leading edge with the help of ANSYS Blade Modeler software. In all, six impeller configurations were generated with inlet forward sweep of 10 and 20 and inlet back sweep of 10, 15, 20 and 25. All other geometric parameters of the baseline impeller were kept unaltered while imparting sweep at inducer leading edge. The definition of inducer forward and back sweep is depicted in Fig.4. It should be noted that the forward sweep was associated with a tangential lean opposite to the direction of impeller rotation, and the back sweep was associated with a tangential lean in the direction of impeller rotation. Fig. 2 Inducer sweep using hub and tip sections (Hazby and Xu, 2007) Fig. 4 Impeller blades with zero, forward and back sweeps at inducer leading edge SASTECH Journal 56 Volume 9, Issue 2, September 2010

3 4. COMPUTATIONAL MODEL AND BOUNDARY CONDITIONS The CFD simulations of all the impeller builds were carried out using FLUENT software. Considering periodic conditions, the computational domain comprised only a single impeller channel. The flow domain was discretised (Fig. 5) with hexahedral elements using ICEM CFD software. Based on grid independence study, a mesh size of ~0.5 million elements was used for all the impeller builds. Steady state solutions were obtained using pressure based, segregated, implicit solver. Turbulent flow was modeled using realisable k-ε model. Air, as ideal gas, was taken as the working fluid. Total pressure and total temperature (standard atmospheric conditions) were specified at impeller inlet and static pressure was specified as the variable boundary condition at impeller exit (Fig. 6). Constancy of mass flow rate between inlet and outlet of the computational domain was monitored and the error was kept less than with a residual reduction of the order of 1.0e RESULTS AND DISCUSSIONS 5.1 Overall Performance The overall performance of the baseline centrifugal compressor impeller and of those with sweep at the inducer leading edge was obtained through numerical simulations at design rotational speed. The performance characteristics are plotted in Fig. 7. The impeller-shroud tip clearance in all the cases was kept at 0.5 mm. in Fig. 7(b) it is observed that the peak isentropic efficiencies of the impellers with 10, 15 and 20 inlet back sweep are lower than those of the baseline impeller except the case of inlet back sweep 25, whereas the impellers with forward inlet sweep of 10 and 20 show a marginal increase peak in isentropic efficiency. The highest stall margin is found in case of the forward inlet sweep of 20, but the choke mass flow rate is higher than the baseline impeller. The choke mass flow rate is found to be maximum for an inlet back sweep of 25. A forward inlet sweep of 20, with 0.5 mm tip clearance, is beneficial in terms of high stall margin and acceptable peak pressure ratio as well as reasonably high efficiency. (%) (a) Fig. 5 Discretised flow domain Fig. 6 Boundary conditions From the compressor characteristics of pressure ratio vs massflow rate in Fig. 7(a), it is observed that the total pressure ratios for impellers with 10, 15, and 20 inlet back sweep are lower than that of the baseline impeller, whereas the impellers with forward inlet sweep of 10 and 20 exhibit higher pressure ratios than the baseline impeller. Further, from the compressor characteristics of isentropic efficiency vs massflow rate Fig. 7 Comparison of overall performance of the impellers with different inlet sweep angles (tip clearance = 0.5 mm) The variations of overall performance parameters, viz. choke and stall mass flow rates, efficiency and peak pressure ratio with inducer sweep are shown in Fig. 8. The ve sign indicates forward sweep and the +ve sign indicates back sweep. The pressure ratio at design mass flow rate remains almost constant with sweep, but shows a tendency to increase beyond back sweep of 20, which is shown in Fig. 8(a). The highest value is found to occur at an inducer back sweep of 25. The stall flow rate is not found to be affected by sweep, except for forward sweep of 20, which gives the lowest stall flow rate of 4.19 kg/s, as shown in Fig. 8(d). The efficiency at design mass flow rate is relatively lower for forward sweep than for back sweep, which is shown in Fig. 8(b). The highest value (89.77%) is found to occur at an inducer back sweep of 25. From Fig. 8(c), it is observed that the choke mass flow rate for different impeller configurations varies between 6 kg/s and 6.5 kg/s only. (b) SASTECH Journal 57 Volume 9, Issue 2, September 2010

4 (a) 5.2 Flow Behaviour through Centrifugal Compressor Impeller - Meridional and Spanwise Planes The flow field of the baseline impeller, and of those with inlet sweep, are examined through relative Mach number distribution in spanwise and meridional planes at choke, maximum efficiency, and stall conditions. The purpose is to find possible explanations for the effect of inducer leading edge sweep on the performance of the compressor. Five meridional planes (Fig. 9) and three span-wise planes (Fig. 10) are considered for the analyses. Two impeller channels are mapped for both the planes. Referring to Fig. 9, the meridional Planes-I and II are at the impeller inlet and at the end of the inducer respectively; Plane-III is taken at a location where the flow turns from axial to radial direction; Plane-IV is further downstream; and the Plane-V is taken at the impeller exit. The three spanwise planes (Fig 10) are: 1) near the hub (5% from hub), 2) at the mean surface between hub and shroud (50% from hub), and 3) near the blade tip (95% from hub). (b) Fig. 9 Location of meridional planes along the impeller flow path (c) Fig. 8 Variation of performance parameters with inlet sweep (d) Fig. 10 Location of spanwise planes across the impeller height 5.3 Flow Development in Impeller (Baseline Case) The flow development in the baseline impeller (zero sweep), from inlet to exit, is presented in Fig. 11 at three operating points. Under maximum flow condition (choke point), the inducer tip Mach number is high and reduces gradually towards hub. At the end of the inducer (Plane-II) a low velocity region in the form of a vortex develops at the shroud, covering almost the complete blade pitch. As the flow SASTECH Journal 58 Volume 9, Issue 2, September 2010

5 proceeds downstream (Plane-III), the vortex at the shroud weakens, showing sign of flow acceleration. However, at impeller exit, the relative flow again decelerates at the shroud, giving some non-uniformity to the impeller exit flow. (a) Choke mass flow rate (b) Max. efficiency flow rate impeller is similar to that at the maximum efficiency operating point. However, at impeller exit (Plane-V), the retarded flow region is stronger and larger, and occupies a position towards the blade suction surface. In all the three cases, the flow at the impeller exit is nonuniform. It is very important to have flow uniformity at the impeller exit for efficient operation of the downstream diffuser. The flow quality at the impeller exit can be improved by incorporating novel design features. 5.3 Flow Development in Impeller (Effect of Sweep) The effect of inlet sweep on the flow through the impeller blade passages is discussed with the help of relative Mach number distribution on meridional planes. Only two inlet sweep cases, viz. forward sweep 20 and back sweep 20, are considered. Figure 12 and 13 show the Mach number distribution on meridional planes at choke and stall flow rates respectively for the two swept impeller configurations. These are to be studied in conjunction with the results of the baseline impeller, as presented in Fig. 11. For the baseline impeller, the Mach numbers are high at the inducer tip and gradually reduce towards the hub owing to reduction in tangential speed. The same trend is observed for the inducer with 20 back sweep (Fig.12a). However, the high Mach number zone near the shroud is concentrated at the mid-pitch. At the end of the inducer (Plane-II), there is separation on the shroud surface owing to convex curvature and high blade loading. The separated region grows at Plane-III, but the flow tends to accelerate near the shroud at Plane- IV and there is indication of flow separation once again at the impeller exit. (a) Impeller with 20 inlet back sweep (c) Stall flow rate Fig. 11 Relative Mach number contours on meridional planes (zero inlet sweep, 0.5 mm tip clearance) At the operating point corresponding to maximum efficiency (Fig. 11(b)), the mass flow rate is lower with consequent lower Mach numbers at impeller inlet. As the flow proceeds downstream through the inducer and the main blade, a pocket of low velocity fluid appears near the shroud-pressure surface corner and on the pressure surface. The later one disappears at and beyond Plane-IV, but the former one grows and occupies position in the middle of the two successive blades. The flow at the impeller exit acquires non-uniformity. At the operating point corresponding to stall mass flow rate (Fig.11 ), the flow behaviour through the (b) Impeller with 20 inlet forward sweep Fig. 12 Relative Mach number contours on meridional planes at choke mass flow rate SASTECH Journal 59 Volume 9, Issue 2, September 2010

6 For the inducer with 20 forward sweep (Fig. 12(b)), the flow field at the inlet Plane-I is markedly different. The tip Mach numbers are relatively lower than for baseline and 20 back swept inducer, and the local high Mach number zone is located near the suction surface. The relative Mach number reduces towards the hub and almost comes to the level of the other two impeller configurations. At Plane-II, there is low Mach number region on the shroud but there is no indication of flow separation. However, there is flow separation zone on the pressure surface, extending form hub to almost about 60% of the blade height. As the flow proceeds further, it accelerates, both at the shroud and on the pressure surface (Plane-III and IV). At Plane-V, corresponding to the impeller exit, the flow is more uniform than for the other two impeller configurations. (a) Impeller with 20 inlet back sweep (b) Impeller with 20 inlet forward sweep Fig. 13 Relative Mach number contours on meridional planes at stall mass flow rate It must be noted that for backswept inducer, the tip leading end is downstream of the hub leading end and for forward swept inducer the tip leading end is upstream of the hub leading end. Hence, the flow approaching the impeller travels through different distances on the blade till it reaches Plane-I, which is slightly behind the tip leading end for the backswept inducer and slightly behind the hub leading end for the forward swept inducer. This mismatch of the approaching flow entering the inducer at different radii causes redistribution of the axial and relative Mach numbers with consequent changes in the flow pattern at Plane-I and further downstream. Moreover, the different sense of associated lean at the inducer leading edge also adds to this flow redistribution. The relative Mach number contours on the meridional planes under stall condition for the two impellers with 20 inlet back sweep and 20 inlet forward sweep are shown in Fig. 13. It may be noted that these contours correspond to the respective stall points of the two impellers. Since the stall margin for impeller with 20 forward swept inducer is larger than that with 20 back swept inducer, the stall mass flow rate for the former is lower than for the later impeller configuration. Owing to low flow rates, the relative Mach numbers are also low throughout the impeller passage. Pocket of retarded flow develops right from Plane- I and it is larger in case of impeller with forward swept inducer. Retarded flow can be observed on the pressure surface also, but this region disappears at the impeller exit. It may also be noted that the retarded flow pocket, which covers almost the entire blade pitch at Plane-I near the shroud, shifts towards pressure surface at Plane-II (end of inducer) and then migrates towards suction surface at the impeller exit. The flow blockage at the exit for the two impeller configurations is more or less same, but the mass flow rate for the forward swept inducer case is comparatively lower. Mach number distribution in the inducer region on the spanwise plane at 95% span from impeller hub is shown in Fig. 14 for the three inlet sweeps of 0, +20, and -20. For impellers with zero and 20 inlet back sweep, there is large flow acceleration on the suction surface followed by sudden deceleration, indicative of the presence of shock near the inducer tip. The separated flow region tends to adversely affect the impeller performance. However, the initial flow acceleration in case of 20 forward inlet sweep is mild, followed by gradual deceleration without a shock, thus giving better overall performance and lower stall margin. (a) zero sweep (b) +20 sweep (c) -20 sweep Fig.14 Relative Mach number distribution on spanwise plane at 95% span from hub SASTECH Journal 60 Volume 9, Issue 2, September 2010

7 performance parameters equal to that of the baseline impeller with 0.5 mm tip clearance. (a) 0.5 mm tip clearance Fig. 15 Effect of tip clearance on compressor performance 6. EFFECT OF TIP CLEARANCE The impeller with inlet forward sweep of 20 and with a baseline tip clearance of 0.5 mm seems to be beneficial in terms of high stall margin and acceptable peak pressure ratio as well as reasonably good efficiency. Hence, this impeller configuration was selected to numerically investigate the effect of increased tip clearance on the performance of the centrifugal compressor. The tip clearance was increased from 0.5 mm to 0.75 mm and numerical simulations were performed at the design rotational speed as has been explained earlier. The motivation behind this parametric study was to find out whether at an increased tip clearance the impeller configuration (20 inlet forward sweep) would exhibit performance equivalent to the baseline impeller (zero inlet sweep). A compressor with higher rotor tip clearance will have less stringent manufacturing tolerance and will undoubtedly bring down the manufacturing cost. Figure 15 shows the comparison of overall performances of the impeller with inlet forward sweep of 20 for 0.5 mm and 0.75 mm tip clearances. The performance of the baseline impeller (0.5 mm tip clearance) is also plotted for reference. It is observed that the peak pressure ratio and the efficiency of the impeller with larger clearance are decreased compared to the impeller with smaller clearance and the values are even slightly lower than those for the baseline impeller. Also, the stall margin of the compressor with increased clearance is reduced slightly below that of the baseline compressor. It can be concluded that an impeller with slightly lower tip clearance than 0.75 mm will have (b) 0.75 mm tip clearance Fig. 16 Relative Mach number distribution on meridional planes at choke mass flow rate (inlet forward sweep 20 ) The flow development through the impeller with 0.75 mm tip clearance and 20 forward swept inducer is presented in Fig. 16. The relative Mach number distributions are plotted on the planes as in the previous Section. It is observed that the change in tip clearance has not brought about any visibly large effect on the overall flow behaviour in the meridional planes. 7. CONCLUSIONS Numerical investigations have been performed to determine the effect of leading edge sweep on the performance and stall margin of a centrifugal compressor impeller. The major conclusions drawn are as follows: The mass flow rate is found to be maximum at inlet back sweep of 25. The pressure ratio at design mass flow rate remains almost constant with inlet sweep, but shows a tendency to increase beyond back sweep of 20. The highest value is found to occur at inlet back sweep of 25. The efficiency at design mass flow rate is relatively lower for forward sweep than for back sweep. The highest value (89.77%) is found to occur at inlet back sweep of 25. The stall flow rate is not appreciably affected by sweep, except for forward sweep of 20, which gives a lowest stall flow rate of 4.19 kg/s, and consequently highest stall margin. SASTECH Journal 61 Volume 9, Issue 2, September 2010

8 There is an increase in choke and stall mass flow rates with increase in tip clearance from 0.5mm to 0.75mm for impeller with inlet forward sweep of 20. The stall margin reduces to the level of base flow impeller with slight reduction in total pressure ratio. A forward sweep of 20, with smaller tip clearance, seems to be beneficial in terms of high stall margin and acceptable peak pressure ratio as well as reasonably good efficiency. 8. ACKNOWLEDGMENTS The authors wish to thank Mr. Abdul Nassar and Mr. Ananthesha of MSRSAS, Bangalore for their help in sorting out problems related to grid generation and FLUENT solver. The authors are also grateful to the Director, MSRSAS, Bangalore for granting permission to publish this work. 9. REFERENCES [1] Arunachalam, V. C., Nagpurwala, Q. H., Deshpande, M. D. and Shankapal, S. R., Numerical Studies on the Effect of Impeller Blade Skew on Centrifugal Compressor Performance, Proceedings of the 4 th BSME-ASME International Conference on Thermal Engineering, IUT, Dhaka [2] Blaha C., Kablitz S., Hennecke D. K., Eisenlohr U. S., Pirker K. and Haselhoff S., Numerical Investigation of the Flow in an Aft-Swept Transonic Compressor Rotor. Report published by Darmstadt University, Munich, Germany [3] Fischer A., Roes W. and Seume J. R., Performance of Strongly Bowed Stators in a 4-Stage High Speed Compressor, ASME Paper GT [4] Hazby H. R. and Xu L., Effect of Leading Edge Sweep on Performance of a Transonic Centrifugal Impeller, Proceedings of the International Symposium on Air Breathing Engines (ISABE), ISABE Paper [5] Jang C. M., Li P. and Kim K.Y., Optimisation of Blade Sweep in a Transonic Axial Compressor Rotor, Journal of Japanese Society of Mechanical Engineers, Series B, Vol. 4, Issue 48, pp [6] Raghavendra M., Nagpurwala Q. H., and Abdul Nassar,. Numerical Investigation of the Eeffect of Exit Width Trim on the Performance of a Centrifugal Compressor Impeller, Proceedings of the 8 th Asian Computational Fluid Dynamics Conference, Hong Kong, January [7] Wadia A. R., Szucs, P. N. and Crall D. W., Inner Workings of Aerodynamic Sweep, ASME Paper No. 97-GT [8] Xu C. and Chen W. J., Computational Analysis on a Compressor Blade, International Conference on Jets, Wakes and Separated Flows, Toba-shi, Mie, Japan. ICJWSF SASTECH Journal 62 Volume 9, Issue 2, September 2010