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Investigation of compressor map width enhancement and the inducer flow field

using a shroud bleed slot

S Sivagnanasundaram, S Spence and J Early School of Mechanical and Aerospace Eng, Queen's University Belfast, UK

B Nikpour

Cummins Turbo Technologies, Huddersfield, UK

Abstract Modern engine developments to satisfy stringent exhaust gas emission targets require increased compressor map width, higher pressure ratio and higher efficiency from the turbocharger compressor. The range of the compressor mass flow is limited by surge at lower flow rates and by choke at higher flow rates. To help extend the flow range, the shroud bleed slot is the most common Map Width Enhancement (MWE) technique used in turbocharger compressors. This paper describes the investigation of several widths of shroud bleed slot using CFD with manual multi-block structured grid generation. The numerical model has been carefully developed and validated with experimental measurements for the baseline case. The influence of operating conditions near surge and choke on the bleed slot flow field has been analysed, along with the impact of the returning bleed flow on the incidence angle at the impeller blade leading edge as it mixes with the main compressor inlet flow. One of the slot variations modelled showed an improvement in the compressor map width over the baseline geometry, which is explained through consideration of the incidence angle distribution at the inlet to the impeller. Nomenclature

Absolute flow velocity (m/s)

Swirl velocity (m/s) Tip speed (m/s) Relative flow velocity (m/s)

Incidence angle (deg) CFD Computational Fluid Dynamics CTT Cummins Turbo Technologies GGI General Grid Interface MWE Map Width Enhancement N.D. Non-Dimensional SST Shear Stress Transport

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1. Introduction The development of modern internal combustion engines requires the turbocharger centrifugal compressor to provide a broader map width and higher pressure ratio while simultaneously increasing the efficiency characteristics. For a compressor the term “Map Width” refers to the range of flow rates over which the operation of the compressor is stable between surge and choke flow rates. The compressor map tends to become narrower and the efficiency level tends to reduce with increasing pressure ratio. This adds difficulties to the design of a compressor to provide a broad operating range, when high pressure ratio and high efficiency are also required as part of the design specification. Surge occurs at the lower extreme of the compressor flow range where the gradient of a compressor’s pressure characteristic becomes positive; therefore any reduction in mass flow rate will result in a reduction in the outlet pressure from the compressor accompanied by a reversal of the high pressure flow downstream of the compressor. This reversed flow then ‘fills’ the compressor inlet and momentarily the compressor impeller operates at a higher flow rate (i.e. the overall flow rate + the reversed flow rate) and therefore becomes stable until the cycle is repeated when the inducer ‘empties’ again. In other words, the correct direction of flow through the compressor will not be restored until flow reversal has caused the downstream pressure to drop sufficiently to return the compressor to a stable part of its operating range (i.e. a negative gradient for the pressure characteristic).

Figure 1 Inducer inlet velocity triangle indicating the incidence angle

Surge is a cyclic unsteady event which leads to large pressure and mass flow fluctuations, and ultimately compressor failure as a result of excessive mechanical vibrations. A common cause of surge is the occurrence of stalled flow due to excessive incidence at the leading edge of the impeller blades, although it can also result from stall in the diffuser.

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At the target operating condition the inducer inlet is usually designed in such a way that the flow enters the inducer with zero or close to zero incidence. However, at off-design conditions the flow approaches the blade leading edge at an incidence angle which can be positive or negative. The incidence angles depicted in Figure 1, which are associated with the low flow rate extremes, are termed as positive incidence angles. If the incidence is large enough the flow separates near the blade wall resulting in localised flow recirculation and reduced pressure ratio, leading the system to surge. As illustrated in Figure 1, introducing a positive swirl component to the inlet flow tends to reduce excessive positive incidence. Consequently it reduces the possibility of flow separations on the suction side of the blade and moves the onset of surge to a lower mass flow rate. In an attempt to increase the compressor map width and overcome the problems with surge, a number of approaches have been adopted. Casing treatments and bleed slots, also known as a ring groove arrangement, have been among the most widely investigated [1-4]. Fisher's investigation [1] showed that there was not only a significant improvement in surge flow, but the inducer flow was also stabilised considerably with the addition of a shroud bleed slot. Work on casing treatments was carried out by Jensen et al [5] and MacDougal and Elder [6] in the early 80s. Reference [7] reports a state of the art high pressure ratio compressor for a helicopter engine using a series of fixed vanes at the inducer tip to facilitate inducer flow recirculation. As technology has improved, the ability to provide cheap and durable variable geometry inlet guide vanes and diffusers has also been exploited [8, 9, 10]. Passive and active surge control systems have been implemented in industrial compressors to help provide surge stabilization, but do not seem practical to employ in turbocharger applications. In the present study, CFD simulations were performed for a turbocharger compressor with various shroud bleed slot geometries intended to enhance the stable operating range. The validation of the baseline compressor model has been undertaken using a full stage CFD analysis. However, numerical investigations of the map width enhancement using different slot widths were performed using single passage models (asymmetrical compressor housing not included) and compared with the single passage prediction for the baseline case. The effect of the flow incidence at the inducer inlet due to the recirculating flow has been studied and the influence of this incidence on the compressor map width has been considered. 2. Description of baseline compressor

Figure 2 Schematic of baseline numerical model

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A centrifugal compressor designed by Cummins Turbo Technologies (CTT) has been used as the baseline geometry for this investigation. The baseline compressor stage consists of an inlet duct with bleed slot and noise baffle, impeller, vaneless diffuser and asymmetrical scroll. Although a series of single passage models were used to explore variations in bleed slot width, validation of the computational model with experimental measurements was undertaken using a model of the full compressor stage, including the scroll. Figure 2 shows the schematic of the impeller model with the original bleed slot. The standard bleed slot, which has a width of 3 mm, is placed just after the inducer throat of the impeller at an angle of 45°. 3. Grid generation and computational method Single passages of the impeller, diffuser, slot and inlet duct were modelled and manual multi-block techniques using ANSYS ICEM CFD 12.0 were applied to develop a fully hexahedral mesh for a single passage comprising main blade and splitter. Figure 3 shows an example of a single blade passage of the impeller modelled, with a separate surface for the slot opening, for use in the mesh generator. The single passage model, which included corresponding segments of the inlet duct and vaneless diffuser, comprised one million elements, with the impeller blade passage containing about 700k elements. The tip gap between the impeller blades and the shroud included 8 layers of hex elements. An O-grid was applied around the impeller blade walls and the curved area of the slot passage. The mesh was compressed next to the wall so as to achieve y+ values of 2 for the nodes next to the wall, as required for use of the SST turbulence model. A grid independence study confirmed that this mesh was sufficient to provide a mesh-independent solution.

Figure 3 Single blade passage of impeller

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To perform a full stage CFD analysis, the single passage mesh was duplicated to fill 360°, forming a complete impeller, diffuser, slot and inlet duct. The asymmetrical scroll passage was meshed separately and consisted of approximately 600k tetrahedral and prism elements. The CFX 12.0 software was used to perform steady state CFD analyses. The diffuser, slot, inlet duct and scroll were specified as four different stationary domains, while the impeller was specified as a rotating domain. The frozen rotor interface approach was used to connect the rotating and stationary frames of reference and the GGI connection was used between the two stationary frames of reference. Total atmospheric pressure and temperature values were applied as inlet boundary conditions, and the average static pressure over the whole outlet area was used as the outlet boundary condition. 4. Numerical analysis 4.1. Validation of baseline model and effects of slot on compressor performance In the preliminary stage of the numerical modelling, the full stage model of the compressor with the baseline bleed slot was validated with existing experimental data. Only the overall compressor performance measurements were available for validation of the model, and these are compared with the predictions obtained from the full stage model in Figure 4. The data presented relates to 87% of the maximum speed for this compressor stage. The efficiency and the flow rate were non-dimensionalised by considering the maximum experimental efficiency and the corresponding mass flow rate at this speed as 100%. The model can be seen to predict both the efficiency and pressure ratio characteristics accurately at this speed. Having established confidence in the full stage model, the compressor was then simulated using a single passage only, with no scroll. The full stage model comprised 10M elements and required considerable computational time to predict enough points for a speed line, therefore the single passage modelling approach was chosen as the basis for further models with different slot widths. Figure 4 depicts the performance predicted by the single passage model with an exception of choke flow condition. There is a good agreement between the experimental data and the single passage simulations. The discrepancy at this speed is highlighted by the absence of scroll, resulting in a slightly higher choking mass flow rate. This was not considered as a problem since the focus of the work was the recirculating slot flow and its mixing with the impeller inlet flow near the surge condition. The exit pressure used to calculate the total-total pressure ratio for the single passage model was the mean of the mass averaged total and static pressures at exit from the vaneless diffuser. This was a simplified representation of the fact that approximately 50% of the kinetic energy would be recovered in the scroll with the remainder being lost in the diffusion process. While it is acknowledged that volute losses are not constant throughput the compressor map, this approach was considered adequate for studying and comparing the bleed slot flow and its interaction with the incoming flow at the impeller leading edge. Figures 5 (a) and (b) represent the flow pattern within the compressor stage at near choke and stall conditions respectively, with the inclusion of the shroud bleed slot and noise baffle. As expected, at the near choke condition a portion of the through-flow is diverted into the impeller through the slot, by-passing the inducer throat. This means that, on the choke side of the compressor map, the maximum mass flow rate is not limited by the inducer throat choking; instead it is supplemented by additional in-flow through the bleed slot. On the other hand, at or

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near the stall condition a portion of the inducer flow is re-circulated out through the slot and then mixed with the main incoming flow. The recirculated flow has a swirl component since the compressor blades have already done some work on this flow before it enters the bleed slot. This swirl, which is positive, reduces the large incidence near the tip of the blades (as illustrated in Figure 1) and so reduces the possibility of flow separations on the suction side of the blade.

Figure 4 Baseline compressor pressure ratio and efficiency (@ 87% of the

maximum speed)

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Slot recirculation

Mixing of the slot flow with main through flow

Figure 5a Schematic of velocity distribution and streamline plot representing near choke condition

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Impeller Stalled flow

The suction of the inducer flow and returning flow by

the slot

The recirculation of the returning flow inside the slot

Figure 5b Schematic of velocity distribution and streamline plot representing near stall flow condition

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Flow incidence at the inducer inlet is a significant factor in the onset of inducer stall and surge. Figure 6 illustrates the incidence angle distribution, predicted by the CFD model, just upstream of the leading edge of the main blade of the baseline compressor with and without a bleed slot. At choke, in the case of no slot, the positive incidence angle was present from the hub up to 50% of the span as well as near the tip of the blade between about 95% and 100% of the span and negative incidence was predicted between 50% and 95% of the span. The region of positive incidence near the shroud is a consequence of reduced axial velocity due to the influence of the boundary layer in the inlet. For the choke condition, addition of the bleed slot increases the incidence relatively uniformly across the entire span, indicating that the mass flow (and axial velocity component) through the inducer is reduced when the bleed slot is added and some of the flow can enter through the slot and bypass the inducer throat. On the other hand, at the stall condition, a large positive incidence predicted near the blade tip in the absence of the bleed slot was reduced considerably by the addition of the bleed slot. This reduction of incidence at the tip occurred due to the mixing of the re-circulated flow with its positive swirl component with the main in-flow at the inducer inlet.

Figure 6 Incidence angle distributions at inducer inlet of the baseline

compressor with and without slot

4.2. Effect of various slot widths An investigation of map width has been performed using the same compressor impeller with the addition of various slot widths. The centre of the baseline slot has been maintained in the same position while the width of the slot has been varied. The baseline geometry employed a 3 mm slot; two further slot widths of 2 mm and 4 mm were also simulated. In the following graphs and discussions, the different configurations have been denoted MWE2, MWE3 and MWE4 for slot widths of 2, 3 and 4 mm respectively.

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Figure 7 shows the comparison of the compressor pressure ratio and efficiency predicted for the three different slot widths using the single passage model. From Figure 7 it is evident that the increasing slot width enhances the map width of the compressor considerably by reducing the surge mass flow rate. For this analysis the predicted inception of surge was taken as being the highest static pressure that was achieved with a converged numerical solution. Note that the plots show pressure ratio based on total pressure at stage outlet (calculated according to the method described in Section 4.1), so in the case of MWE4, the left most point in Figure 7 actually represents the highest static pressure, even though the total pressure appears slightly reduced. The wider slot width has improved the map width by reducing the surge mass flow rate by 15% compared to the baseline case. The pressure difference between the main inlet duct and the slot opening along the shroud is the driving force for the slot flow. Increasing the slot width allows the same pressure difference to pass a larger volume of flow in order to enhance the map width at either side of the compressor map. However, the prediction results show that the widest slot has actually yielded a reduction of around 2% in the choking mass flow rate rather than the increase that was expected. Moreover, the efficiency of the compressor also dropped by 2-3% as the slot width is increased to 4mm.

Figure 7 Comparison of compressor pressure ratio and efficiency obtained with modified slot geometries (@ 87% of the maximum speed)

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This numerical study shows that, at or near stall conditions, increasing slot width has a significant influence on the enhancement of map width due to the mixing of the recirculating flow with the main inflow. The wider slot (MWE4) recirculates about 45% of the main incoming flow, as shown in Figure 8, and stabilises the flow at even lower stage mass flow rates compared to the other two cases. As the width of the slot increases the mixing of the recirculating flow occupies a larger portion of the inducer inlet span (from tip to hub). Figure 9 shows the inducer incidence angle distributions at the leading edge of the main blade, clearly showing how the largest slot makes the greatest impact towards reducing incidence. Figure 10 depicts the swirl velocity component in the flow just upstream of the inducer leading edge due to the mixing of the recirculated slot flow. As the slot width is increased to 4mm the outer 60% of the inducer span is influenced by the swirl in the recirculating flow. This reduces the incidence significantly, therefore reducing the possibility of flow separation and stabilising the inducer flow. Modifying the mixing regime between the recirculating flow and the inlet flow to control the distribution of swirl and therefore the incidence at the leading edge of the impeller would probably yield further improvements in the surge mass flow rate. It was noted that the expected increase in choke flow rate was not observed as the width of the slot was increased. However, there is still some part of the inflow that passes through the slot, as illustrated in Figure 8, thereby reducing the velocities at the inducer throat. Further analysis is being conducted to understand the reason for this.

Figure 8 Slot flow at choke and surge conditions due to various slot

geometries

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Figure 9 Incidence angle distributions at inducer inlet due to various slot geometries

Figure 10 Swirl components at inducer inlet flow due to recirculating flow

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5. Conclusions Three-dimensional steady-state CFD simulations were performed for a single passage of a turbocharger centrifugal compressor stage to investigate the enhancement of the compressor map width using a shroud bleed slot with various slot geometries. Three different slot geometries were obtained by modifying the original slot width. The results showed that increasing the slot width has a considerable influence on the compressor performance in terms of moving the surge point to lower flow rates. In this analysis the wider slot width (4 mm) reduced the surge flow by about 15% compared to the baseline slot (3 mm). In addition, the predicted flow field at the inducer inlet and the interaction of the recirculating flow with main inflow has been analysed and its effect on the inlet incidence angle has been clearly demonstrated. It was shown that reduction of incidence due to improved mixing of the recirculation flow could be a factor for stabilising the flow within the passage and so improving the available flow range for compressor operation. On the other hand this analysis showed that the increased slot width had an adverse effect on the stage efficiency and pressure ratio. Further studies are underway to determine the causes of these effects. 6. Acknowledgement The authors are grateful to ANSYS Inc for access to and technical support with the CFD and grid generation software used for this research study. © Authors 2010 7. References 1. Fisher, F.B., (1988), “Application of Map Width Enhancement Devices to

Turbocharger Compressor Stage”, Society of Automotive Engineers, Inc. 2. Nikpour, B., (2004) “Turbocharger compressor flow range improvement for

future heavy duty diesel engines”, THIESEL 2004 Conference on Thermo- and Fluid Dynamic Processes in Diesel Engines.

3. Hunziker, R., Dickmann, H P., and Emmrich, R., (2001), “Numerical and experimental investigation of a centrifugal compressor with an inducer casing bleed system”, IMechE Pro Instn Mech Engrs, Vol 215 Part A.

4. Ishida, M., Sakaguchi, D., and Ueki, H., (2006) “Effect of pre-whirl on unstable flow suppression in a centrifugal impeller with ring groove arrangement”, ASME Turbo Expo 2006, GT2006-90400.

5. Jansen, W., Carter, A. F., and Swarden, M. C., (1980), "Improvements in surge margin for centrifugal compressors", AGARD Conference Proceedings 282.

6. Macdougal, I., and Elder, R. L., (1982), "The improvement of operating range in a small, high speed, centrifugal compressor using casing treatment", IMechE Conference, paper C32/82.

7. Barton, M. T., Mansour, M. L., Liu J. S., and Palmer D.L, (2006), "Numerical Optimization of a Vaned Shroud Design for Increased Operability Margin in Modern Centrifugal Compressors", Journal of Turbomachinery, ASME, Vol 128, pp. 627-631.

8. Kim, Y., Engeda, A., Aungier, R., and Amineni, N., (2002), "A compressor stage with wide flow range vaned diffusers and different inlet configurations", IMechE, Vol 216, Part A.

9. Engeda, A., (2001), "The unsteady performance of a centrifugal compressor with different diffusers", Proceedings of the Institution of Mechanical Engineers, Journal of Power and Energy, Part A, v 215, n 5, 2001, p 585-599.

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10. Coppinger, M., and Swain, E., (2000), "Performance prediction of an industrial centrifugal compressor inlet guide vane system", IMechE Paper, Vol 214 Part A.