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Research Papers

Design and Analysis of a Novel Split Sliding Variable Nozzle for Turbocharger Turbine OPEN ACCESS

[+] Author and Article Information
Liangjun Hu

Research and Innovation Center,
Ford Motor Company,
Dearborn, MI 48124
e-mail: lhu4@ford.com

Harold Sun

Research and Innovation Center,
Ford Motor Company,
Dearborn, MI 48124
e-mail: hsun3@ford.com

James Yi

Research and Innovation Center,
Ford Motor Company,
Dearborn, MI 48124
e-mail: jyi1@ford.com

Eric Curtis

Research and Innovation Center,
Ford Motor Company,
Dearborn, MI 48124
e-mail: ecurtis@ford.com

Jizhong Zhang

Diesel Engine Turbocharging Laboratory,
China North Engine Research Institute,
Tianjin 300400, China
e-mail: dtzjz@163.com

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received November 3, 2017; final manuscript received November 25, 2017; published online April 6, 2018. Editor: Kenneth Hall.

J. Turbomach 140(5), 051006 (Apr 06, 2018) (10 pages) Paper No: TURBO-17-1205; doi: 10.1115/1.4038878 History: Received November 03, 2017; Revised November 25, 2017

Variable geometry turbine (VGT) has been widely applied in internal combustion engines to improve engine transient response and torque at light load. One of the most popular VGTs is the variable nozzle turbine (VNT) in which the nozzle vanes can be rotated along the pivoting axis and thus the flow passage through the nozzle can be adjusted to match with different engine operating conditions. One disadvantage of the VNT is the turbine efficiency degradation due to the leakage flow in the nozzle endwall clearance, especially at small nozzle open condition. With the purpose to reduce the nozzle leakage flow and to improve turbine stage efficiency, a novel split sliding variable nozzle turbine (SSVNT) has been proposed. In the SSVNT design, the nozzle is divided into two parts: one part is fixed and the other part can move along the partition surface. When sliding the moving vane to large radius position, the nozzle flow passage opens up and the turbine has high flow capacity. When sliding the moving vane to small radius position, the nozzle flow passage closes down and the turbine has low flow capacity. As the fixed vane does not need endwall clearance, the leakage flow through the nozzle can be reduced. Based on calibrated numerical simulation, there is up to 12% turbine stage efficiency improvement with the SSVNT design at small nozzle open condition while maintaining the same performance at large nozzle open condition. The mechanism of efficiency improvement in the SSVNT design has been discussed.

Turbocharger has been widely applied in diesel engines and has significant impact on engine power, fuel economy, and exhaust emissions. In the last two decades, variable geometry turbines (VGTs) have become a prevailing technology in diesel applications. VGT could improve diesel engines' fuel economy and emissions [1,2]. Many different kinds of variable geometry turbines [36] have been developed, such as pivoting vane, moving wall, and variable flow turbine design. One of the most popular VGT technologies is pivoting vane or variable nozzle turbine (VNT). VNT uses swing vanes to control the flow passage inside nozzle to change the nozzle flow capacity and to match engines' various operating conditions. At low engine speed, the nozzle vanes in VNT close down to restrict the exhaust gas flow through the turbine, thereby increasing turbine power and boost pressure. At higher engine speed, the nozzle vanes open up to maximize the exhaust gas flow, thereby avoiding turbo over-speed and maintaining the boost pressure required by the engine.

One disadvantage of the VNT is the turbine efficiency degradation due to the leakage flow in the nozzle endwall clearance, which is needed to allow the nozzle vanes to rotate without sticking. Many researches have been performed to investigate the nozzle endwall clearance's effect on turbine performance. Hayami et al. [7] investigated the influence of nozzle clearance on turbine performance by experimental method, and the results indicated that the clearance leakage flow can seriously distort the flow field of the rotor, thus greatly deteriorates turbine efficiency. Tamaki et al. [8] studied the effect of nozzle clearance on the flow field of a VNT at different nozzle openings by both experimental and computational fluid dynamics (CFD) methods. The results showed that at small nozzle opening, the flow losses caused by nozzle clearance were the main cause for the low turbine efficiency; while at large nozzle opening, the effect of the leakage flow was negligible and the losses were mainly caused by nozzle wake flow. Hu et al. [9] conducted a research on a variable nozzle vane with various clearance heights. The results revealed that at small nozzle open condition, the turbine stage efficiency dropped up to 20%. Walkingshaw et al. [10] studied the effects of nozzle vane hub side and shroud side clearance on turbine performance by both numerical and experimental methods. They found that the hub side clearance could improve turbine stage efficiency up to 4.5 points and 3 points at the minimum and 25% mass flow rate (MFR) position, respectively. They suggested the use of asymmetric clearances to improve turbine stage efficiency. Tomoki et al. [11] studied the rotor-stator interaction of a radial turbine with variable nozzle vanes and found that the shock wave and nozzle endwall clearance flow were the principal excitation source for the turbine wheel vibration. Hu et al. [12] studied nozzle endwall clearance's effect on turbine efficiency and forced response. They found that nozzle endwall clearance could significantly intensify the unsteadiness of the pressure loading on the turbine wheel as well as turbine forced response.

Nozzle endwall clearance leads to significant efficiency degradation at small nozzle open condition. Moreover, turbocharger turbine efficiency at those conditions is critical to customer driving cycles. As shown in Fig. 1, the driving cycle of a vehicle was plotted on the matched turbine map. The majority of the operating conditions of the turbine are at small nozzle open positions where the nozzle endwall clearance flow greatly penalizes turbine efficiency. It is important to improve turbine efficiency at small nozzle open condition for better fuel economy at customer driving cycle.

Given the negative impact of nozzle endwall clearance on the turbine performance, a novel split sliding variable nozzle turbine (SSVNT) has been proposed [13]. Illustrated in Fig. 2, a split surface divides the nozzle vane into two parts: one part is fixed and the other part can slide along the partition surface on the fixed vane. The mechanism to adjust nozzle flow passage in the SSVNT design is different from that of a traditional pivoting VNT. The sliding vane and the fixed vane together form an integrated vane. Moving of the sliding vane will change the thickness of the integrated vane, as well as the distance between the adjacent vanes. Then, the throat area of the nozzle could be adjusted to match different engine operating conditions. When moving the sliding vane to large radius position, the nozzle flow passage opens up and the turbine has high flow capacity. When moving the sliding vane toward small radius position, the nozzle flow passage closes down and the turbine has low flow capacity. As there is no endwall clearance for the fixed vane, the total clearance of the integrated vane can be reduced, and so does the nozzle endwall leakage flow. As a result, turbine efficiency could be potentially improved.

In current research, a turbocharger turbine with conventional VNT was used as the base turbine. The base VNT turbocharger was from a production diesel engine in 2014. Split sliding variable nozzle was designed to replace the base variable nozzle, while the turbine rotor was unchanged. The present paper consists of three main parts. The first part is the introduction of numerical method used in current research, including CFD method validation. The second part is the design of the SSVNT, introducing the main design features and parameters of the SSVNT. Finally, turbine aero performance with the split sliding nozzle was analyzed and compared with the base VNT turbine, followed by the summary.

CFD Method.

The commercial code EURANUS, integrated in Fine/Turbo interface, was used in the numerical simulation. It solves the time-dependent Reynolds averaged N-S equations. The equations were discretized using central difference scheme and a steady-state flow solution was achieved upon the convergence of a 4-stage explicit Runge-Kutta integration scheme. In order to speed up the convergence, a full multi-grid technique was applied. The one equation turbulence model, Spalart-Allmaras turbulence model, was chosen in the current study as its prediction of the turbine performance agreed well with the test data at the nozzle design and off-design conditions.

Multiblock structure mesh of the rotor and nozzle was generated by igg/autogrid software. To well capture the geometry of the impeller and to improve mesh quality, O mesh was used around the blade surface block and H mesh was applied in the other blocks. Grid clustering was applied around all the wall surfaces to calculate the boundary layer and the averaged y+ was about 3. For the split sliding nozzle vane, the mesh without endwall clearance was first generated by Autogrid and then the mesh of the clearance in the sliding nozzle was manually generated in IGG. The main mesh and the clearance mesh were connected using FNMB technique [14]. Figure 3 shows the mesh of the VNT nozzle as well as the leakage flow through the nozzle endwall clearance. Figure 4 shows the single passage CFD model of a split sliding variable nozzle with the turbine blade. The fixed vanes do not have endwall clearance and the tip clearance mesh only exists on the top of sliding vane. The clearance between the fixed vane and the sliding vane has also been included. The clearance between the fixed vane and the sliding vane was 1.3c, where c is the single side nozzle endwall clearance shown in Fig. 3.

In the CFD validation of the base turbine with or without nozzle endwall clearance, turbine stage models including turbine housing, full nozzle vane passage, and single turbine blade passage were built. Mixing plane was used between the rotor and the stator interface. Steady CFD simulations were performed to predict the turbine performance.

In the SSVNT design process, to reduce the required CPU resources and computation time, volute was not included in the turbine performance analysis, and single passage of the nozzle and rotor was simulated by steady CFD simulation. Nozzle inlet flow direction was calculated according to the volute geometry [15]. Single passage of the base turbine nozzle and rotor was also simulated and the predicted turbine performance was set as a reference for comparison of the SSVNT turbine performance. After the optimal design had been achieved, the SSVNT stage model including volute was built and the turbine stage performance was simulated to validate turbine stage efficiency improvement. For all the cases with or without volute, total temperature and pressure were applied at computation domain inlet. At rotor outlet, the static pressure at a specific radius was applied. The static pressure was assumed to be uniform in the tangential direction and the pressure distribution along the radial direction was given by Eq. (1). Adiabatic and nonslip wall boundary conditions were imposed Display Formula

(1)pr=ρvθ2r

CFD Validation.

To validate the CFD method, first the base VNT with and without nozzle endwall clearance at small nozzle open condition was tested on turbocharger flow bench. CFD analysis of the base turbine with and without nozzle endwall clearance was performed and the simulated results were compared with the test data. In order to accurately control the nozzle endwall clearance, nozzle inserts were manufactured for this test. The nozzle blades were machined out of an annular ring, as shown in Fig. 5. Two nozzle inserts with different blade heights were manufactured to achieve two different nozzle endwall clearance configurations: one with nozzle endwall clearance and the other one without nozzle endwall clearance. In the case with nozzle endwall clearance, the clearance was located on the hub side, as shown in Fig. 6.

The turbine performance was measured in a turbocharger flow bench. A burner was used to heat the high-pressure air from an air supplier and then the high pressure and temperature gas was used to drive the turbine. The compressor, which is on the same shaft of the turbine in the turbocharger, was used to absorb the power generated by the turbine. Static pressure, total pressure, and total temperature were measured at both turbine inlet and outlet. As the outlet temperature of turbine is not uniform, a mixer was added in the downstream of the turbine and the mixed total temperature was measured for turbine efficiency calculation. On the turbine side, both the housing and pipes have insulation to reduce the heat loss from the turbine to the ambient. Figure 7 illustrates the measured parameters and their locations in current turbine performance test on the turbocharger flow bench.

Figure 8 shows the turbine performance comparison between CFD and test result with and without nozzle endwall clearance at small nozzle open condition. Figure 8(a) shows that without nozzle clearance, the simulated turbine mass flow is slightly higher than the tested mass flow at low expansion ratio area. At higher expansion ratio area, the simulated mass flow agrees well with the test data. With nozzle endwall clearance, exhaust gas could flow through the nozzle endwall clearance, so the effective flow passage of the nozzle could be increased. At small nozzle open condition, the throat area of nozzle passage is small and adding endwall clearance leads to relatively large change of total flow passage. As a result, with nozzle endwall clearance, the flow passage increases a lot compared to the case without nozzle endwall clearance. At the tested open condition, the nozzle endwall clearance leads to 48% increase of the flow passage and the test data showed about 40% increase of the flow capacity. The actual increased mass flow rate with nozzle endwall clearance was smaller than the flow passage area increase, which was mainly due to boundary layer effects. The CFD data also showed similar mass flow rate increase with nozzle endwall clearance. In general, CFD predicted mass flow rate well with and without nozzle endwall clearance.

Figure 8(b) shows that without endwall clearance, the predicted turbine efficiency was about two points higher than the test data. There are two possible reasons for the slightly higher efficiency in the CFD simulated performance. One was the adiabatic assumption in CFD. There was no heat transfer between the turbine and the environment. However, in test, there was still some heat loss from turbine to the ambient even with the insulation on the turbine, which could lead to measured turbine efficiency drop. Another reason was the turbine backface loss, which was not included in the current simulation. It had been found that turbine backface leads to some efficiency loss [16]. Nevertheless, the difference between CFD and test result is acceptable. With nozzle endwall clearance, the simulated efficiency was about 1.5 points lower than the tested efficiency at low pressure ratio area, which was because of the slightly smaller simulated mass flow rate. Compared to the case without nozzle endwall clearance, both the CFD and the test data showed about ten points or 22% turbine efficiency reduction with nozzle endwall clearance. Test data showed that nozzle endwall clearance caused the increase of turbine mass flow and the decrease of turbine stage efficiency. Current CFD method was able to predict the change of turbine aero performance due to nozzle endwall clearance quite well. The CFD method can be used to assist design and analysis of the SSVNT in which turbine performance will mostly be affected by the change of the nozzle endwall clearance.

Design of SSVNT.

The idea of the split sliding variable nozzle is dividing a conventional nozzle vane into two parts: one is fixed vane and the other is sliding vane. The fixed vanes have no clearance, which help reduce the nozzle endwall leakage flow and improve turbine stage efficiency. The SSVNT design needs to start with a base nozzle vane shape, which will be divided into two parts. Many different nozzle vanes have been developed for VNT, including straight nozzle vane and aerodynamic nozzle vane [15,17]. Aerodynamic nozzle vane usually could provide less flow loss and more uniform flow at the nozzle outlet location. In the aerodynamic nozzle vane family, both the slim and thick vanes have been developed [18]. In current SSVNT design, the slim and thick nozzle vanes, shown in Fig. 9, were studied as the base nozzle. When the sliding vane moves, the maximum thickness of the integrated nozzle vane also changes, which also contributes for the nozzle passage throat area change. With thicker blade, the nozzle throat area changes faster which can reduce the required travel of the sliding vane to open up flow passage. Reduction of the nozzle travel helps achieving a smaller package of the SSVNT design, which is very important for vehicle application. As a result, the thick nozzle vane was chosen as the base nozzle for the SSVNT design.

The partition surface between the fixed vane and the sliding vane, which could be a flat surface or a curved surface, needs to be carefully optimized. As shown in Fig. 10, a flat surface and a curved surface were studied for the same base nozzle. To get the same maximum flow passage and turbine flow capacity, the curved partition surface has a smaller inlet diameter compared to that of the flat partition surface. In other words, the curved partition surface could achieve smaller turbo package and was applied in current SSVNT design. The start and end positions of the partition surface also need to be carefully studied. As the suction surface of the nozzle has low pressure and high speed, there is usually higher flow loss on the suction side than that on the pressure side, especially at small nozzle open conditions. Flow loss on the pressure and suction surfaces are illustrated in Fig. 11 by the entropy distribution. Due to the higher flow loss on the suction surface, the aerodynamic shape of the suction surface has more effect on turbine performance than the pressure side shape. When designing the partition surface, it is important to maintain an aerodynamic profile on the suction side, especially for the surface downstream of the nozzle throat.

In current SSVNT design, both the nozzle vane's height and count have been maintained the same as the base VNT. Optimization of vane height and count for SSVNT are the subjects of future nozzle design work. The main parameters of the SSVNT and base VNT design at three studied vane positions are listed in Table 1. The nozzle main parameters are illustrated in Fig. 12. At full open condition, the SSVNT design has slightly higher nozzle vane inlet radius than the base VNT. But the difference is very small and the effect on package is not an issue. When closing down the SSVNT, nozzle outlet radius decreases, while in the base VNT the nozzle outlet radius increases. The radial gap between the nozzle and the rotor was carefully chosen at 38% nozzle open condition, which is near the engine exhaust braking condition and is critical for turbine forced response and HCF performance. At 38% open condition, both the inlet and the outlet radius of the SSVNT were reduced by 0.7% compared to base VNT. With the same rotor, the radial gap between the nozzle vane and the rotor in SSVNT was reduced slightly, which may increase the interaction between nozzle and rotor and have negative effect on turbine forced response. However, the optimal SSVNT design was able to suppress the nozzle endwall leakage flow, which had been identified as one of the main excitation source of turbine forced response [11,12]. The overall turbine forced response may not deteriorate with the SSVNT design. Using the same analysis method introduced in Ref. [12], turbine forced response analysis has been performed. It was found that the optimal SSVNT was able to reduce turbine forced response at engine exhaust braking condition. At 13% open condition, the outlet radii of the base VNT continue to increase, but in the SSVNT the outlet radii of the nozzle vane continue to decrease as the sliding nozzle vane moves to small radii position to close the flow passage. In current SSVNT design, R2/R3 was 1.27 at 13% open to allow some radial gap between the nozzle vane and the rotor. The radial gap in the SSVNT at 13% open was still larger than that of base VNT at full open condition. Also, the radial gap in SSVNT was larger than the value recommended in Ref. [15] and current radial gap was considered to be sufficient.

Results of SSVNT
Turbine Performance.

Turbine stage performance at three key operating conditions with different nozzle openings was numerically simulated for the base VNT and the optimal SSVNT turbine. The turbine aero performance comparison was shown in Fig. 13. The mass flow rate chart shows that the SSVNT design maintains almost the same flow range as the base VNT. At full open condition, the SSVNT has the same turbine efficiency as the base VNT. At the same mass flow rate of 38% and 13% open condition, the SSVNT design improves turbine stage efficiency up to 6.6% and 12.1%, respectively. The SSVNT mainly improves turbine efficiency at small nozzle open condition. The smaller the nozzle opening is, the more efficiency improvement the SSVNT has achieved. As the main advantage of the SSVNT is to reduce nozzle endwall clearance and it has been well understood that nozzle endwall clearance leads to more efficiency loss at smaller opening condition [79], the SSVNT results seem to be reasonable.

Nozzle Flow Comparison.

Figure 14 shows the leakage flow field comparison inside nozzle endwall clearance at 13% open condition between the base VNT and the SSVNT. The red line is the streamline of the nozzle endwall leakage flow and the contour is the entropy inside the nozzle flow passage. In the base VNT, there is strong leakage flow through the nozzle endwall clearance from about 15% streamwise location to the nozzle trailing edge on the suction side. The leakage flow is almost perpendicular to the nozzle blade. The leakage flow is a strong secondary flow that interacts with the main flow. As a result, a strong leakage vortex has been formed in the flow passage. In the leakage vortex area, high entropy can be observed. As the leakage vortex moves along the streamwise direction toward downstream, the entropy also increases. In the SSVNT, the fixed vanes do not have endwall clearance. The length of the nozzle endwall clearance in the SSVNT was reduced by 2/3 compared to the base VNT. The reduction of nozzle leakage flow area can reduce the leakage flow. In the SSVNT, the area of the high entropy has been reduced a lot compared to that of the base VNT.

Figure 15 shows the leakage flow rate (LFR) comparison between the base VNT and the optimal SSVNT. The LFR was defined by the following equation: Display Formula

(2)LFR=MleakageflowMstageflow

where Mleakageflow is the flow rate through nozzle endwall clearance and Mstageflow is the turbine stage flow rate. The chart showed that the LFR in the SSVNT was about 1/3 of the base VNT, which was consistent with the endwall clearance length reduction. The large LFR in the base VNT can cause higher flow loss, which is consistent with the larger high entropy area as shown in Fig. 14.

Figure 16 shows the total pressure loss coefficient comparison between the SSVNT and the base VNT at 13% open condition. The total pressure loss coefficient K was calculated according to the following equation: Display Formula

(3)K=P01P02P02P2

where P0 and P are total pressure and static pressure, respectively, subscript 1 is the nozzle inlet location, and subscript 2 is the nozzle outlet location. In the SSVNT, the nozzle outlet location, Rb, was taken from the mixing plane in CFD model, which was illustrated in Fig. 16(a). As shown in Table 1, the gap between nozzle vane and rotor in the SSVNT was smaller than that of the base VNT, which can also result in different total pressure loss. To separate the effect of different nozzle vanes with different radial gaps between the nozzle vane and the rotor, in the base VNT total pressure loss coefficient K was calculated at two positions. The first calculated location was also Rb, the same location as in the SSVNT, so that the overall nozzle flow loss could be compared. The second calculated location was at slightly higher radius Ra. Ra has the same radial distance ΔR to the nozzle vane trailing edge as the SSVNT. Then, the total pressure loss with the same radial distance to the nozzle vane trailing edge could be compared between the SSVNT and the base VNT. The surface positions Ra and Rb were shown in Figs. 16(a) and 16(b).

With the same distance ΔR to the nozzle vane trailing edge, the SSVNT's total pressure loss at Ra is almost the same as the base VNT's total pressure loss at Ra in the main flow from 11% to 90% spanwise location, which shows that the newly designed SSVNT has the same vane profile loss as the base VNT. However, from hub to 11% spanwise location, the base VNT has much higher total pressure loss than the SSVNT, which is mainly due to the increased clearance and leakage flow loss. In current model, all the nozzle endwall clearance was placed on the hub side, so both base VNT and SSVNT showed higher loss on the hub side than the main flow and shroud side. From 90% to 100% spanwise location, the base VNT showed slightly higher boundary flow loss than that of the SSVNT.

From Ra to Rb in the base VNT, the leakage flow continues mixing with the main flow and the mixing effect leads to more flow loss, as shown by the increased total pressure loss from 20% to 50% spanwise location. Comparing the nozzle total pressure loss at the same location Rb, SSVNT shows lower total pressure loss than that of the base VNT. The reduced flow loss inside the SSVNT at this condition is related to two factors: the reduced nozzle endwall leakage flow and the reduced radial gap from nozzle vane trailing edge to the rotor leading edge.

Rotor Flow Comparison.

The previous study had shown that nozzle endwall clearance flow also affects turbine rotor flow field and performance [9]. Here, the rotor performances with SSVNT and base VNT were studied. Rotor efficiency was calculated using the following equation: Display Formula

(4)η=T02T0311Prk1k

where Display Formula

(5)Pr=P02P3

P0 and P are the total pressure and static pressure, respectively. T0 is the total temperature and k is the ratio of specific heat. Subscripts 2 and 3 stand for rotor inlet and outlet, respectively. Rotor efficiency calculation showed that the efficiency of the rotor with SSVNT had been improved by 2 points compared to that of the base VNT at 13% open condition. The rotor flow field was studied to find out the mechanism of rotor efficiency improvement in the SSVNT.

Figure 17 shows the rotor inlet incidence angle comparison between the base VNT and the SSVNT turbine rotor. In the main stream from 30% to 90% spanwise location, both the cases have large rotor inlet incidence angle. From 90% to the shroud side, rotor inlet incidence angle drops due to the boundary layer effect. From hub to about 30% spanwise location, both the VNT and SSVNT rotors have smaller incidence angle than the main stream. This is mainly due to the nozzle endwall clearance flow. As shown in Fig. 14(a), nozzle clearance flow is from the pressure side to the suction and the flow direction is almost perpendicular to the nozzle vane, which is against the impeller rotation direction. When the endwall clearance exists, the flow at nozzle exit has larger radial velocity. As current investigated model only had hub clearance, the base VNT and SSVNT turbine have small incidence angle at rotor inlet from hub to 30% spanwise location. Also because the base VNT has larger LFR than the SSVNT, the incidence angle of the base VNT was smaller than that of the SSVNT near the hub side. The difference in rotor inlet flow angle affects the internal flow and the rotor efficiency.

Figure 18 shows the flow field comparison at about 10% spanwise location between the base VNT and SSVNT. The contours are entropy and the streamlines are the relative velocity. In the base VNT, the exhaust gas flows into impeller smoothly without separation flow due to the small incidence angle at this location. From 30% to 50% streamwise location, a separation flow occurs. This separation flow is related to the low energy flow migration, which will be explained in the following analysis. In the SSVNT, due to large incidence angle, flow separation can be observed at rotor leading edge and low energy fluid accumulated there, as represented by the high entropy. The flow between the SSVNT and the base VNT at middle span and shroud side varies slightly, but the entropy distribution are quite similar. So, the main flow at middle span and shroud side are not shown here.

Figure 19 shows the boundary layer flow comparison near the rotor hub and suction surface between the two turbine rotors with the base nozzle and the split sliding nozzle. In the base VNT near the hub side, the exhaust gas enters the impeller with negative incidence angle. The pressure gradient from the pressure side to the suction side pushes the low energy fluid inside boundary layer toward the suction side as the exhaust gas flows toward downstream. At around 60% streamwise location, the low energy flow reaches the suction side and a stagnation flow is formed there. Due to the centrifugal force inside rotor, the low energy flow is shifted to high radius location on the suction surface, as represented by the streamline in Fig. 19(a). This low energy fluid almost reaches the rotor inlet before being pushed toward downstream by the main flow. As a result, a large recirculation near the boundary layer is formed. The recirculation of the boundary layer leads to the large separation flow at 10% spanwise location as shown in Fig. 18. In the rear part of the rotor, a migration of the boundary layer from hub to shroud side can be observed.

In the SSVNT, the incidence angle of the rotor inlet is larger than that of the base VNT. As a result, the pressure gradient from the pressure side to the suction side in the SSVNT is larger than the pressure gradient in the base VNT. Then, the low energy flow migrates from the pressure side to the suction side sooner in the SSVNT than in the base VNT. A separation flow is formed from around 10% to 40% streamwise location. Due to the centrifugal force, the low energy flow moves in the radial direction along the suction surface. This flow migration only exists in a spanwise location below 20%. Compared to the base VNT, the boundary layer recirculation zone has been reduced a lot in the SSVNT, which helps reduce the flow loss inside the rotor and improve the turbine wheel efficiency with the split sliding nozzle.

Figure 20 shows the loading comparison of the base VNT with SSVNT at hub, middle span, and near shroud position. At hub side of the base VNT, the loading from leading edge to around 65% streamwise location is good; from about 65% streamwise to rotor trailing edge, there is nearly no loading due to the flow separation. In the SSVNT, due to the large incidence angle at rotor inlet, there is more loading at the front of the rotor and there is nearly no loading from about 50% streamwise location to the trailing edge. At the hub side, the loading distribution in the base VNT was slightly better than that of the SSVNT. At middle span and shroud side, both the SSVNT and the base VNT have large loading at leading edge due to large incidence angle, but the SSVNT has slightly higher loading than the base VNT. In SSVNT, the rotor can generate more power than the base VNT, which explains the better turbine rotor efficiency in the SSVNT design.

The paper presents design and analysis of a novel SSVNT to reduce nozzle endwall leakage flow and to improve turbine stage efficiency using CFD aided design. First, the computation method is validated against a specific experiment of a conventional VNT with and without nozzle endwall clearance. The comparison between CFD and test data shows good agreement and demonstrates the fidelity of current method to capture small endwall clearance's effect on turbine performance.

Then, SSVNT design is carried out and the validated CFD is used to simulate turbine performance. The design mainly focuses on the base nozzle shape and partition surface to improve turbine efficiency. A curved partition surface and thick base nozzle shape are chosen for the current SSVNT design in order to meet the package requirement for vehicle application. The performance of the optimal SSVNT is numerically simulated including the clearance between the fixed vane and the sliding vane. Detailed flow field is investigated. The numerical results showed that the SSVNT improves the turbine efficiency by 12.1% at small nozzle open condition while maintaining the same flow capacity and efficiency of the base VNT at large nozzle open condition. There are three main mechanisms for the improved turbine efficiency at small nozzle open condition:

  1. (1)The fixed vanes do not need endwall clearance therefore the total nozzle endwall clearance, as well as the endwall leakage flow loss, can be reduced with the SSVNT design.
  2. (2)In conventional VNT, closing down the nozzle increases the radial gap between nozzle vane and rotor, as well as the flow loss between blade rows; while in SSVNT, closing down the nozzle reduces the radial gap between nozzle and rotor, as well as the flow loss between blade rows.
  3. (3)Reduced nozzle endwall leakage flow can improve turbine rotor efficiency by reducing the migration of low energy flow and the boundary layer loss on the suction surface of the rotor.

It is worth mentioning that the SSVNT has different mechanisms to change nozzle throat area compared to the traditional swing type VNT and a new actuation system for the SSVNT is needed, which is the subject of future research. For future work, optimization of the nozzle including vane count and height can also be carried out to further improve turbine efficiency.

  • b =

    nozzle height

  • c =

    half of total nozzle endwall clearance

  • FNMB =

    full non matching boundary

  • HCF =

    high cycle fatigue

  • k =

    specific heat ratio

  • K =

    nozzle total pressure loss coefficient

  • LFR =

    leakage flow rate

  • M =

    mass flow

  • MFR =

    mass flow rate

  • P =

    pressure

  • Pr =

    pressure ratio

  • SSVNT =

    split sliding variable nozzle turbine

  • T =

    temperature

  • VGT =

    variable geometry turbine

  • VNT =

    variable nozzle turbine

  • η =

    efficiency

  • ρ =

    density

  • 0 =

    stagnation state

  • 1 =

    nozzle inlet

  • 2 =

    nozzle outlet

  • 3 =

    rotor outlet

Bains, N. , 1998, “ Radial and Mixed Flow Turbines Options for High Boost Turbocharger,” Seventh International Conference on Turbocharger and Turbocharging, pp. 35–44.
Watson, N. , and Janota, M. , 1982, Turbocharging the Internal Combustion Engine, Macmillan Education, New York. [CrossRef]
Rogo, C. , Hajek, T. , and Relke, R. , 1983, “Aerodynamic Effects of Moveable Sidewall Nozzle Geometry and Rotor Exit Restriction on the Performance of a Radial Turbine,” SAE Paper No. 831517.
Arnold, S. , 1987, “Schwitzer Variable Geometry Turbo and Microprocessor Control Design and Evaluation,” SAE Paper No. 870296.
Franklin, P. , 1989, “Performance Development of the Holset Variable Geometry Turbocharger,” SAE Paper No. 890646.
Kawaguchi, J. , Adachi, J. , and Kono, S. , 1999, “Development of VFT (Variable Flow Turbocharger),” SAE Paper No. 1999-01-1242.
Hayami, H. , Senoo, Y. , and Hyun, Y. , 1990, “ Effects of Tip Clearance of Nozzle Vanes on Performance of Radial Turbine Rotor,” ASME J. Turbomach., 112(1), pp. 58–63. [CrossRef]
Tamaki, H. , Goto, S. , and Unno, M. , 2010, “The Effect of Clearance Flow of Variable Area Nozzles on Radial Turbine Performance,” ASME Paper No. GT2010-23677.
Hu, L. , Yang, C. , and Sun, H. , 2011, “ Numerical Analysis of Nozzle Clearance Effect on Turbine Performance,” Chin. J. Mech. Eng., 24(4), pp. 618–625. [CrossRef]
Walkingshaw, J. , Spence, S. , and Enrhard, J. , 2012, “ An Experimental Assessment of the Effects of Stator Vane Tip Clearance Location and Back Swept Blading on an Automotive Variable Geometry Turbocharger,” ASME J. Turbomach., 226(6), pp. 751–763.
Tomoki, K. , Goto, S. , Unno, M. , and Iwakami, A. , 2008, “Unsteady Rotor-Stator Interaction of a Radial-Inflow Turbine With Variable Nozzle Vanes,” ASME Paper No. GT2008-50461.
Hu, L. , Sun, H. , Yi, J. , Curtis, E. , Morelli, A. , Zhang, J. , Zhao, B. , Yang, C. , Shi, X. , and Liu, S. , 2013, “Investigation of Nozzle Clearance Effects on a Radial Turbine: Aerodynamic Performance and Forced Response,” SAE Paper No. 2013-01-0918.
Sun, H. , Zhang, J. , and Hu, L. , 2013, “Sliding Vane Geometry Turbine,” Ford Global Technologies, LLC, Dearborn, MI, U.S. Patent No. US20130042608 A1.
NUMECA, 2014, IGG User Manual, v9, ed., NUMECA International, Brussels, Belgium. [PubMed] [PubMed]
Zhu, D. , 1992, Turbocharging and Turbochargers, China Machine Press, Beijing, China.
He, P. , Sun, Z. G. , Chen, H. S. , and Tan, C. Q. , 2012, “ Investigation of Backface Cavity Sealing Flow in Deeply Scalloped Radial Turbines,” Proc. Inst. Mech. Eng. Part A, 226(6), pp. 751–763.
Bains, N. , 2005, Fundamentals of Turbocharging, Concepts NREC, White River Junction, VT.
Arnold, S. , Groskrevtz, M. , and Shahed, S. , 2002, “Advanced Variable Geometry Turbocharger for Diesel Engine Applications,” SAE Paper No. 2002-01-0161.
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References

Bains, N. , 1998, “ Radial and Mixed Flow Turbines Options for High Boost Turbocharger,” Seventh International Conference on Turbocharger and Turbocharging, pp. 35–44.
Watson, N. , and Janota, M. , 1982, Turbocharging the Internal Combustion Engine, Macmillan Education, New York. [CrossRef]
Rogo, C. , Hajek, T. , and Relke, R. , 1983, “Aerodynamic Effects of Moveable Sidewall Nozzle Geometry and Rotor Exit Restriction on the Performance of a Radial Turbine,” SAE Paper No. 831517.
Arnold, S. , 1987, “Schwitzer Variable Geometry Turbo and Microprocessor Control Design and Evaluation,” SAE Paper No. 870296.
Franklin, P. , 1989, “Performance Development of the Holset Variable Geometry Turbocharger,” SAE Paper No. 890646.
Kawaguchi, J. , Adachi, J. , and Kono, S. , 1999, “Development of VFT (Variable Flow Turbocharger),” SAE Paper No. 1999-01-1242.
Hayami, H. , Senoo, Y. , and Hyun, Y. , 1990, “ Effects of Tip Clearance of Nozzle Vanes on Performance of Radial Turbine Rotor,” ASME J. Turbomach., 112(1), pp. 58–63. [CrossRef]
Tamaki, H. , Goto, S. , and Unno, M. , 2010, “The Effect of Clearance Flow of Variable Area Nozzles on Radial Turbine Performance,” ASME Paper No. GT2010-23677.
Hu, L. , Yang, C. , and Sun, H. , 2011, “ Numerical Analysis of Nozzle Clearance Effect on Turbine Performance,” Chin. J. Mech. Eng., 24(4), pp. 618–625. [CrossRef]
Walkingshaw, J. , Spence, S. , and Enrhard, J. , 2012, “ An Experimental Assessment of the Effects of Stator Vane Tip Clearance Location and Back Swept Blading on an Automotive Variable Geometry Turbocharger,” ASME J. Turbomach., 226(6), pp. 751–763.
Tomoki, K. , Goto, S. , Unno, M. , and Iwakami, A. , 2008, “Unsteady Rotor-Stator Interaction of a Radial-Inflow Turbine With Variable Nozzle Vanes,” ASME Paper No. GT2008-50461.
Hu, L. , Sun, H. , Yi, J. , Curtis, E. , Morelli, A. , Zhang, J. , Zhao, B. , Yang, C. , Shi, X. , and Liu, S. , 2013, “Investigation of Nozzle Clearance Effects on a Radial Turbine: Aerodynamic Performance and Forced Response,” SAE Paper No. 2013-01-0918.
Sun, H. , Zhang, J. , and Hu, L. , 2013, “Sliding Vane Geometry Turbine,” Ford Global Technologies, LLC, Dearborn, MI, U.S. Patent No. US20130042608 A1.
NUMECA, 2014, IGG User Manual, v9, ed., NUMECA International, Brussels, Belgium. [PubMed] [PubMed]
Zhu, D. , 1992, Turbocharging and Turbochargers, China Machine Press, Beijing, China.
He, P. , Sun, Z. G. , Chen, H. S. , and Tan, C. Q. , 2012, “ Investigation of Backface Cavity Sealing Flow in Deeply Scalloped Radial Turbines,” Proc. Inst. Mech. Eng. Part A, 226(6), pp. 751–763.
Bains, N. , 2005, Fundamentals of Turbocharging, Concepts NREC, White River Junction, VT.
Arnold, S. , Groskrevtz, M. , and Shahed, S. , 2002, “Advanced Variable Geometry Turbocharger for Diesel Engine Applications,” SAE Paper No. 2002-01-0161.

Figures

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Fig. 1

Vehicle driving cycle on matched turbine map

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Fig. 2

Illustration of SSVNT

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Fig. 3

Illustrations of nozzle clearance and leakage flow of base VNT

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Fig. 4

Mesh and CFD model of a SSVNT design

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Fig. 5

Nozzle insert at small nozzle opening

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Fig. 6

Nozzle endwall clearance on the hub side

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Fig. 7

Illustration of turbocharger flow bench for turbine performance test

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Fig. 8

Comparison of simulated and tested turbine performance: (a) pressure ratio-mass flow and (b) pressure ratio-efficiency

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Fig. 9

Different base nozzle shapes

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Fig. 10

Partition surface design of SSVNT

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Fig. 11

Flow loss comparisons between pressure side and suction side of the conventional VNT

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Fig. 12

Main parameters of nozzle

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Fig. 13

Comparison of turbine aero performance: (a) turbine mass flow and (b) turbine efficiency

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Fig. 14

Nozzle endwall leakage flow comparison at 13% open condition: (a) base VNT and (b) SSVNT

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Fig. 15

LFR comparison at 13% open condition

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Fig. 16

Nozzle vane total pressure loss comparison: (a) SSVNT, (b) VNT, and (c) nozzle total pressure loss

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Fig. 17

Rotor inlet incidence angle comparison

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Fig. 18

Flow separation at 10% spanwise location: (a) base VNT and (b) SSVNT

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Fig. 19

Boundary layer flow comparison on the hub and impeller suction side: (a) base VNT and (b) SSVNT

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Fig. 20

Loading comparison between base VNT and SSVNT: (a) hub, (b) middle-span, and (c) shroud

Tables

Table Grahic Jump Location
Table 1 Nozzle main design parameter comparison

Errata

Discussions

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