Mitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)67

Advanced Aerodynamic Design Technologies forHigh Performance Turbochargers

TAKAO YOKOYAMA*1

KENICHIRO IWAKIRI*2

TOYOTAKA YOSHIDA*2 TORU HOSHI*3TADASHI KANZAKA*2

SEIICHI IBARAKI*１

In recent years, the fuel efficiency of automobiles has been enhanced through thesupercharged downsizing of engines for passenger vehicles. To meet increasingly stricter fuelefficiency regulations, however, turbochargers that have a wider operation range and are highly
efficient in comparison with existing ones are strongly required. For this reason, Mitsubishi Heavy
Industries, Ltd. (MHI) worked on the clarification of the internal flow phenomenon and design
improvement through large-scale flow analysis of the turbine in consideration of the engine
exhaust pulsation and visualization of the flow inside the compressor recirculation flow path, withthe purpose of enhancing the performance of the compressor and the turbine that govern theperformance of the turbocharger. In addition, a design optimization method was applied to plan aconcept to enhance the efficiency of the compressor impeller and execute design improvement of
the piping for a two-stage turbocharger. As a result, we enhanced the efficiency of a variablecapacity turbine at minimal flow, the efficiency of a twin-scroll turbine and the extension of theoperation range and efficiency of a compressor.

|1. Introduction
As a measure against global warming, the reduction of CO2 greenhouse gas is being
promoted all over the world. Strict fuel consumption regulations have also been imposed onpassenger vehicle engines, and are planned to be strengthened further in the future. One of themethods to enhance the fuel efficiency of an engine is supercharged downsizing using aturbocharger. The use of this method has quickly expanded to gasoline engines and diesel engines.
In addition, the use of variable geometry turbines, electric compressors and two-stage
turbochargers is increasing, and supercharging systems are becoming increasingly sophisticated inorder to obtain high supercharging efficiency and high response in a wide range of engine operation
points. Therefore, in addition to the enhancement of efficiency of single turbocharger units and the
expansion of the operation range, the enhancement of the performance of the entire supercharging
system is also required.
MHI has been working on enhancing the efficiency of turbochargers utilizing numerical flow
analysis and internal flow measurement. However, it was necessary to analyze the flow structure indetail for the further enhancement of efficiency. For this reason, we worked on a detailed analysisof the internal flow using whole turbine flow analysis of a variable capacity turbine in the small
nozzle opening region, unsteady flow analysis of a twin scroll turbine under an engine exhaust
pulsation condition and stereo PIV (particle image velocimetry) measurement capable of measuring
unsteady three-dimensional flow velocity in a two-dimensional plane. In addition, a new concept
geometry to improve performance was established based on each obtained loss structure, and theenhancement of efficiency was verified using analysis and performance tests. Furthermore, we
worked on short-time design optimization of complicated three-dimensional flow fields using an
ANN-DE (artificial neural network - differential evolution) optimization method and an Adjoint
method. This paper presents these efforts for the enhancement of turbocharger performance.
*1*2

Chief Staff Manager, Fluid Dynamics Research Department, Research & Innovation Center
Fluid Dynamics Research Department, Research & Innovation CenterMitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)68|2. Large-scale flow analysis
2.1 Variable geometry turbine
A variable geometry turbine has a rotatable nozzle vane with a throat area that is variable on
the upstream side of the turbine rotor blade and therefore can control the turbine flow
characteristics. A turbine can be driven in a wide range of engine rotation speeds from the low
rotation speed region. In recent years, however, the enhancement of the performance at minimal
flow is required in order to obtain the supercharging pressure from a lower engine rotation speed.
When the turbine efficiency remains high on the high flow side, then supercharging pressure
control becomes difficult at a high engine rotation speed in some cases, and therefore an increase of
the maximum flow and the suppression of the turbine efficiency on the high flow side are also
necessary.
When the flow rate is minimal, the opening of the nozzle vane becomes very small and theeffect of the leak flow from the nozzle side clearance on the nozzle main stream becomes large. In
addition, deviation between the flow speed at the nozzle outlet and the circumferential speed of therotor blade expands and a large-scale separation flow is generated at the leading edge of the rotor
blade, resulting in a complicated three-dimensional flow. We worked on a detailed analysis of theflow field using flow analysis in consideration of the narrow space of the side clearance targeting
all areas including the scroll, nozzle vane and turbine rotor blade.
Figure 1 shows the flow line in the small nozzle opening region and the flow speed vector
distribution at the leading edge of the rotor blade. In the case of an existing nozzle shown in Figure1(a), the flow did not spread inside the rotor blade because of the minimal flow, and tends to bebiased to the shroud by centrifugal force. For this reason, the blade load is not maintained by theentire blade face and the turbine efficiency decreases. In addition, because the flow speed at therotor blade outlet becomes high on the shroud side and low on the hub side, the generated mixing
loss increases due to the non-uniform flow distribution. Focusing on this flow, we invented a
nozzle structure with a taper located on the shroud side. This concept changes the direction of theflow at the rotor blade inlet to the hub side as shown in Figure 1(b), and it was confirmed that theflow inside the rotor blade spreads to the hub side. Figure 2 shows the results of the performance
test for the verification of this concept. It could be confirmed that this concept enhances the turbine
efficiency by 2.2% in the small nozzle opening and minimal flow region in comparison with theexisting nozzle, while suppressing the turbine efficiency on the high flow side.

Figure 1 Flow line and flow speed vector distribution at leading edge of rotor blade(small nozzle opening)Mitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)69

Figure 2 Results of VG turbine performance test

2.2 Twin scroll
One of the technologies for the enhancement of turbine output at a low engine speed is a twin
scroll. The twin scroll effectively utilizes exhaust pulsation from the engine by axially separating aturbine scroll cross section into two and connecting the two to different engine cylinders. On theother hand, the twin scroll has an asymmetric cross section because of the constraint of size
reduction, and there is the issue where the inflow efficiency on the rear side decreases incomparison with the front side. This issue must be improved because an efficiency difference
during partial inflow causes performance deviation between engine cylinders.

Figure 3

Turbine scroll internal flow in rear side inflow

On an engine, because exhaust pulsation flows into the two separated scrolls alternately,strong unsteady flow occurs on the upstream of the turbine rotor blade where they join together.
We worked on unsteady flow analysis using the exhaust pulsation as a boundary condition andunderstood the flow issue where a loss caused by leak flow on the front side occurred, particularlyin the case of rear-side inflow as described in a previous paper.1 Figure 3 shows the loss
distribution and the flow distribution of the rear cross section (inflow side) and the front crosssection (non-inflow side) in the rear side inflow. It could be confirmed that leak flow from the rearside to the front side occurred and a large-scale loss region was generated on the front side. In thecase of a twin scroll, exhaust pulsation flows into the rear side and the front side alternately andMitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)70

generates a difference in pressure between the two sides, resulting in the occurrence of leak flow to
the low pressure side. We focused on the flow structure and invented an outlet shape that can
suppress leak flow from the rear side to the front side, and obtained an estimated enhancement inefficiency of 1.9% in flow analysis. As shown in Figure 4, an evaluation of the turbine heatinsulation efficiency was executed in a performance test, and it was confirmed that the performancein the rear-side inflow was enhanced by 2.1% as expected. The new shape also improves the flowbalance and the efficiency balance between the front-side inflow and the rear-side inflow, andperformance enhancement on an engine can be expected. In this way, unsteady flow analysis inconsideration of exhaust pulsation was verified to be effective.

Figure 4 Results of twin-scroll turbine performance test

|3. Three-dimensional internal flow measurement
A casing treatment is used as one of the compressor operating range extention devices. The
casing treatment consists of a slit installed near the impeller throat on the compressor cover, arecirculation flow path, and several struts that forms the flow path. In the low flow rate condition
near the surging limit, part of the flow into the impeller recirculates through the slit to the upstream
pipe to increase the flow on the upstream of the impeller and suppress leading edge separation. Thiseffect allows the improvement of the pressure characteristics and the reduction of the surging limit
flow. Figure 5 compares compressor performances with the number of struts in the recirculation
flow path when changed. It is known that the surging flow varies depending on the number of
struts, and it is necessary to analyze the flow in the recirculation flow path to understand thephenomenon. Because the flow near the casing treatment is a mix of normal flow and reverse flow
and has a strong swirling flow due to the effect of the impeller, the flow has a strong
three-dimensionality. Therefore, we used stereo Particle Image Velocimetry (PIV), which canmeasure three-dimensional flow components in a two-dimensional plane to work on a detailed
analysis of the internal flow.2

Figure 5 Results of casing treatment performance test

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Figure 6 shows the targeted compressor. In this measurement, three dimensional velocity
components at the exit of the circulation flow path were measured for each of (1) the three-strut
configuration and (2) the thirteen-strut configuration. Figure 7 shows the color contour of the axial
flow velocity at the exit of the circulation flow path and the in-plane flow vector. The flow velocity
region in blue represents the circulation from the slit. It was confirmed that circulation flow
occurred with both the three-strut configuration and the thirteen-strut configuration. In the case of
the three-strut configuration, large swirling flow occurred in the center of the flow path. This
indicates that swirling flow passing through the slit from the impeller remains even at the outlet ofthe circulation path. In the case of the thirteen-strut configuration, on the other hand, it was
confirmed that the swirling flow in the flow path was suppressed. Consequently, it was verified thatthe number of struts affected the swirling flow in the circulation flow.

Figure 6 Measurement of casing treatment internal flow

Figure 7 Measurement results of casing treatment internal flow

Based on the results of the internal flow measurement described above, we invented a new
device that controls the swirling flow.3 Figure 8 shows a bird's eye view of the new device. The
new device has a guide vane near the circulation path outlet of existing casing treatment. The guidevane is installed in the direction opposite that of the impeller rotation and controls the swirling flow
occurring in the circulation flow path.

Figure 8

Bird's eye view of new device

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Figure 9 shows the distribution of the Mach number around the impeller tip at near surging
point. In the case of the existing casing treatment, the stall of the blade was suppressed, but the
distribution of the Mach number on the negative pressure surface of the blade was non-uniform in
the circumferential direction. In the case of the new device, the velocity difference between thesuction surface and pressure surface of the blade was large, and therefore the distribution of theMach number was uniform in the circumferential direction and the blade load could also be
maintained. The surge limit flow was evaluated by a performance test and it was confirmed that the
new device reduces the surge limit flow in the entire region from a low pressure ratio to a highpressure ratio. Consequently, the new device was verified to be effective.

Figure 9

Internal flow and performance of new device|4. Design optimization method
4.1 Neural network and optimization algorithm
For engines of passenger vehicles, a supercharging pressure even at a low engine rotation
speed is required in order to compensate for the lowered engine torque due to downsizing.
Accordingly, it is necessary to reduce compressor surge flow and improve the efficiency near
surging. The design of the flow field near surging requires a significant amount of time because theenhancement of efficiency is difficult due to separation and stall of the flow inside an impeller, andat the same time the suppression of the decrease in efficiency and the reduction of the flow on the
choke side is also required.
MHI has already established a design optimization method capable of quick performance
evaluation and optimized shape extraction, combining the performance and strength evaluation
using ANN (artificial neural network) and the individual search using DE (differential evolution).
As described in a previous paper,4 improvement in efficiency was carried out by using a 1-pitch
impeller. This time, we reconsidered the design parameters and the evaluation function, andworked on the expansion of the operation range through design optimization of multiple operation
points.
Figure 10 compares impeller shapes and internal flow between before and after
optimization. While the shape before optimization has a straight leading edge, the shape after
optimization has a three-dimensionally bent blade leading edge. A comparison of loss distribution
near the tip indicates that the loss region on the suction surface was significantly reduced by the
optimization.
Figure 11 shows the results of a performance test. The results indicate that the pressure ratio
characteristics were improved, the surge limit flow was reduced and the compressor operating
range was extended by the optimization. Consequently, the operation range extension method
through optimization using ANN and DE was verified to be effective.

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Figure 10 Comparison of compressor impeller shape andinternal flow between before and after optimization

Figure 11 Comparison of performance test results between
before and after optimization of compressor impeller

4.2

Adjoint method

Because of the constraint of the engine layout, the adoption of a two-stage turbocharger, etc.,
the shape of intake and exhaust pipes that affects the turbocharger performance has become
sophisticated and has caused the reduction of the operation range and the degradation of efficiency.
The shape of pipes depends on the engine specifications of customers, and therefore flow issues
such as pressure loss vary on a case-by-case basis. For this reason, we worked on design
optimization of a compressor scroll and two-stage turbocharger pipes using the Adjoint method.
The Adjoint method (adjoint variable method), one of the deterministic optimization
methods, optimizes a shape by obtaining sensitivity of the design variable to the objective function.
This method cannot search globally for an optimum solution in comparison with a probabilistic
optimization method (such as an evolutionary algorithm described above), but can obtain gradientsof objective functions for all design variables at one time (can perform optimization using all
contact points that represent the shape as design variables) and it has a high calculation efficiency
that advantageously allows optimization in a short amount of time.
(1) Compressor scroll
Figure 12 shows the shapes of the target compressor scroll before and after optimizationand the results of internal flow analysis. This compressor scroll has a layout with a discharge
port that is bent 90 degrees in the rotation axis direction. Such a layout has been increasing inrecent years, but the bent part causes the generation of flow distortion and an increase ofpressure loss. The optimized shape has a reduced curvature of the bent part and therefore theflow is improved. In addition, the optimized shape that the cross-sectional shape of the scroll
changes three-dimensionally in the circumferential direction allows the recirculation flow

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generated in the scroll to be smoothened and the recirculation loss to be reduced. As a result, it
was expected that the optimized scroll, in comparison with its original shape, would reduce thepressure loss coefficient in the entire operation range and enhance the efficiency. Figure 13
shows the performance test results. The performance enhancement effect of the optimized scroll
was also verified in a test where the efficiency was enhanced by up to 1.3% while the operation
range remained unchanged.

Figure 12 Comparison of compressor scroll shape and internal flow between before andafter optimization

Figure 13 Comparison of performance test results
between before and after optimization ofcompressor scroll(2)

Two-stage turbocharger pipes
A two-stage turbocharger, which is a combination of two turbochargers of ahigh-pressure stage and a low-pressure stage, can provide supercharging pressure at a wide
range of engine rotation speeds. Because two turbochargers are housed in a narrow and small
space in the engine compartment, there is the issue where performance is degraded by themultiple high-curvature bent parts of the pipes as shown in Figure 13. Therefore, the shapes of
the two-stage turbocharger pipes were optimized using the Adjoint method.

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Figure 14 compares the shapes and the internal flows between before and after
optimization. Section 2, which leads from the outlet of the high-pressure stage turbine to theconfluence of the bypass valve, had a sharp bend just downstream of the high-pressure stage
outlet and separation occurring thereat. However, the separation was suppressed by the
optimization that secured the length of the high-pressure turbine outlet and the expansion of thearea of the bent part. Section 3, which is the confluence of the bypass line, generated strong
swirling flow in the clearance when the bypass valve was closed. However, the swirling flow
was suppressed by the optimization that reduced the curvature of the high-pressure stage side
confluence. Figure 15 shows the performance analysis results. The effectiveness of the pipe
shape optimization using the Adjoint method was verified in the analysis where the pressure
loss coefficient was reduced by 2%, 45% and 48% at Sections 1, 2 and 3, respectively, under
the condition of the valve being closed, and the overall efficiency was enhanced by 1.9%.

Figure 14 Comparison of two-stage turbine shape and internal flow between before andafter optimization

Figure 15 Comparison of performance analysis results between
before and after optimization of two-stage turbine|5. Conclusion
We worked on a detailed analysis of internal flow and the improvement of design usinglarge-scale flow analysis, three-dimensional internal flow measurement, and design optimization inorder to enhance the performance of turbochargers. In the large-scale flow analysis, the loss
mechanism on an actual engine was understood and enhancement in efficiency was attained
through the setting of a wide range and detailed areas and in consideration of the unsteadyconditions of the engine exhaust pulsation. In the three-dimensional internal flow measurement of acasing treatment, the flow structure of a complicated three-dimensional flow field where normal,

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reverse and swirling flows were mixed was visualized, which led to the development of newdevices. In design optimization, we worked on design improvement of compressor impellers andtwo-stage pipes that had many design variables, resulting in the improvement of the internal flow ina short amount of time and enhancement in efficiency.
Because the trend of strengthening regulations related to engines for passenger vehicles willcontinue into the future, passenger vehicle manufacturers are assuming various developments
including the motorizing of powertrains, and it is expected that necessary supercharging
technologies will continuously change. We are willing to promote the swift and innovative
development of turbochargers through analysis capabilities that continue to evolve year after year,
internal flow measurement technologies that are advancing and design optimization methods tofurther develop aerodynamic design technologies.
The ANN-DE design optimization method was developed in cooperation with the Von
Karman Institute in Belgium. We would like to express our appreciation to Professor Emeritus Van
den Braembussche, Assistant Professor Verstraete and Doctor Alsalihi for their contributions.

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