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Pulsating Flow Impact on Turbocharger Turbines
Ricardo Martinez-Botas
Srithar Rajoo
Turbocharger Research Group
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Pulsating Exhaust Flow
Internal Combustion Engine: Reciprocating, positive
displacement
Turbocharger Turbine: Rotodynamic, steady flow device
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•Turbine design and matching is largely based on steady flow performance.
Turbine Design Methodology
Design Condition – m, P, T, W, N
Euler Turbomachinery Equations + free Vortex + Continuity + Sweifel Criterion +
losses
Velocity Triangles
Geometries
Final Geometry
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Unsteady Spectra
• Turbocharger is exposed to a wide range of unsteady events 1. Engine Load Transients(~1 Hz) 2. Exhaust pulse (10~100Hz) 3. Blade wake passing 4. Turbulent fluctuations
• Exhaust pulsations sit in an interesting area
•How does unsteady flow influence performance?
•Which components of the turbine can be treated as quasi-steady and which cannot
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Velocity Triangle – Pulsating Flow
Steady Flow Design Condition
incidence
W
U
Cm
0
0.1
0.2
0 6 0 12 0 18 0 2 4 0 3 00 3 6 0ωt (Degrees)
min
st ( k
g/s )
f = 40 Hz
f = 60 Hz
b
1
1
3
3
2
2
EXP
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-100
-80
-60
-40
-20
0
20
40
0 60 120 180 240 300 360
Inci
denc
e An
gle Phase Angle
20Hz 80Hz
Optimum Incidence
Most Energy
-100
-80
-60
-40
-20
0
20
40
0 60 120 180 240 300 360
Inci
denc
e An
gle Phase Angle
20Hz 80Hz
Optimum Incidence
Most Energy
Velocity Triangle – Pulsating Flow CFD
30kRPM
48kRPM
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Turbine Unsteady Performance
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0.00
1.002.00
3.004.00
5.00
6.007.00
8.009.00
10.00
1 1.4 1.8 2.2 2.6 3Pressure Ratio (P01/P5)
Mas
s Fl
ow P
ar. (
kg/s
T 0
1/P01
)
Lean VaneStraight VaneSteady Straight
x 1e-540Hz, 80% Speed, 40deg
0.00
1.002.00
3.004.00
5.00
6.007.00
8.009.00
10.00
1 1.4 1.8 2.2 2.6 3Pressure Ratio (P01/P5)
Mas
s Fl
ow P
ar. (
kg/s
T 0
1/P01
)
Lean VaneStraight VaneSteady Straight
x 1e-560Hz, 80% Speed, 40deg
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
1 1.4 1.8 2.2 2.6 3Pressure Ratio (P01/P5)
Mas
s Fl
ow P
ar. (
kg/s
T 0
1/P01
)
Lean VaneStraight Vane
Steady Straight
x 1e-540Hz, 80% Speed, 70deg
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
1 1.4 1.8 2.2 2.6 3Pressure Ratio (P01/P5)
Mas
s Fl
ow P
ar. (
kg/s
T 0
1/P01
)
Lean VaneStraight Vane
Steady Straight
x 1e-560Hz, 80% Speed, 70deg
Mass flow parameter vs. Pressure ratio
Turbine Unsteady Performance
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-1.00-0.80-0.60-0.40-0.200.000.200.400.600.801.001.201.40
0.2 0.4 0.6 0.8 1 1.2 1.4Velocity Ratio (U/Cis)
Effic
ienc
y (h
t-s)
Lean VaneStraight VaneSteady Straight
40Hz, 80% Speed, 40deg
-1.00-0.80-0.60-0.40-0.200.000.200.400.600.801.001.201.40
0.2 0.4 0.6 0.8 1 1.2 1.4Velocity Ratio (U/Cis)
Effic
ienc
y (h
t-s)
Lean VaneStraight VaneSteady Straight
60Hz, 80% Speed, 40deg
-1.00-0.80-0.60-0.40-0.200.000.200.400.600.801.001.201.40
0.2 0.4 0.6 0.8 1 1.2 1.4Velocity Ratio (U/Cis)
Effic
ienc
y (h
t-s)
Lean VaneStraight VaneSteady Straight
40Hz, 80% Speed, 70deg
-1.00-0.80-0.60-0.40-0.200.000.200.400.600.801.001.201.40
0.2 0.4 0.6 0.8 1 1.2 1.4Velocity Ratio (U/Cis)
Effic
ienc
y (h
t-s)
Lean VaneStraight VaneSteady Straight
60Hz, 80% Speed, 70deg
Efficiency vs. Velocity ratio
Turbine Unsteady Performance
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UNSTEADY PULSE FLOW MEANS TURBINE OPERATES OVER A
WIDE RANGE
OVERALL BEHAVIOUR IS PREDICTED FROM SIGNIFICANT DATA EXTRAPOLATION
PULSE FLOW RELIES ON TURBINE EXTRAPOLATION
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11
1D Simulation Treatment
Time
Pres
sure
Exhaust Manifold (Wave Action) Unsteady pulse at the Turbine node
Steady-state performance
Regulated two-stage Turbo GT-POWER
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Standard Procedure
GAS DYNAMICS INSIGNIFICANT
(ZERO PATH LENGTH)
1D WAVE ACTION MODEL OF ENGINE & EXHAUST
STEADY-STATE GAS TEST STAND PERFORMANCE
ASSUMPTIONS
TURBINE IS A NODE
(ZERO VOLUME)
STEADY STATE PERFORMANCE
RESPONSE
QUASI-STEADY ASSUMPTION
MODEL TURBINE
UNSTEADY PULSE FOLLOWING
EXTRAPOLATED MAP
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Map Extension in Simulation
• 1-D simulation take limited gas stand data and extrapolate • Unsteady pulse Wave Action in the manifold means that the model
relies on a significant amount of the off design extrapolation
Limited data
Extrapolation
Time
Pres
sure
Unsteady range
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NEED FOR EXPERIMENTALLY MEASURED EXTENDED TURBINE PERFORMANCE MAPS
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Steady & Pulsating Flow Test Rig Imperial College London
• Pulse generator has two rotating ‘chopper plates that produce pulses into a single or twin entry turbine
OUTER LIMB INNER LIMB
Rotating chopper plate produces pulsating flow
Eddy Current Dyno 60kW / 60kRPM
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MAP WIDTH: Conventional turbine maps are narrow in range • Efficiency → Velocity ratio, ≈0.6 ÷ 0.8 • Mass flow parameter → Pressure ratio, ≈1.9 ÷ 2.3 (at 100% speed)
0
1
2
3
4
5
6
7
1 1.5 2 2.5 3
MAS
S FL
OW
PAR
AMET
ER
PRESSURE RATIO
50% Speed 60% Speed 70% Speed
80% Speed 90% Speed 100% Speed
50% SPEED
100% SPEED
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 0.2 0.4 0.6 0.8 1 1.2
EFFI
CIE
NC
Y
VELOCITY RATIO 50% Speed 60% Speed 70% Speed
80% Speed 90% Speed 100% Speed
50% SPEED
100% SPEED
Velocity Ratio 50%, 100% speed
0.6 ÷ 0.8
Pressure ratio 50% speed
Pressure ratio 100% speed
1.2 ÷ 1.4 1.9 ÷ 2.3 0
1
2
3
4
5
6
7
1 1.5 2 2.5 3
MAS
S FL
OW
PAR
AMET
ER
PRESSURE RATIO
50% Speed 60% Speed 70% Speed
80% Speed 90% Speed 100% Speed
50% SPEED
100% SPEED
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 0.2 0.4 0.6 0.8 1 1.2
EFFI
CIE
NC
Y
VELOCITY RATIO 50% Speed 60% Speed 70% Speed
80% Speed 90% Speed 100% Speed
50% SPEED
100% SPEED
Maps obtained are 3-4 times wider than conventional (100% speed)
Maps obtained are 3-4 times wider than conventional (100% speed)
Maps Extension
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QUASI-STEADY ASSUMPTION WHAT DOES IT MEAN ?
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Quasi-steady assumption
PR(t) T(t)
TIME
PRES
SUR
E R
ATI
O
t
PR(t)
UNSTEADY PULSE
N(t) Nconst
PRconst Tconst
UNSTEADY CASE AT TIME = t STEADY CASE
Mas
s Fl
ow
Pressure Ratio
ṁquasi
PR(t)
STEADY-STATE MAP
ṁ(t) = ṁquasi ?
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Quasi-Steady Assumption: Efficiency
• Is the unsteady efficiency simply an integration of steady-states at each instance in the pulse ?
TIME
PRES
SUR
E R
ATI
O
t
PR(t)
EFFI
CIE
NC
Y
.
PRESSURE RATIO
UNSTEADY PULSE STEADY-STATE MAP
η(t)q-s
PR(t)
= ?
∑∑=
in
outavgUS tW
tW)()(
,
η
( )∑
∑−
− ⋅=
QSin
QSQSinavgQS tW
ttW)(
)()(,
ηη
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Filling and Emptying vs. Quasi Steady
• To illustrate this effect: The same pressure ratio across the turbine (1.6) produces two different unsteady mass parameters.
• Steady behaviour in between both (in this case).
turbine ‘filled’ steady
turbine ‘empty’
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VOLUME DISPLAYS ‘FILLING AND EMPTYING’ BEHAVIOUR
VOLUTE LENGTH DISPLAYS WAVE ACTION
UNSTEADY MASS FLOW OF TURBINE STAGE
BEHAVIOUR IS NOT QUASI-STEADY
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Turbine Modelling Options
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1-D turbine Model vs. Experiment
Experimental testing schematic diagram 1-D turbine model schematic diagram
• Volute modelled as series of “pipes” with finite length & volume. • Quasi-steady pressure loss boundary to represent the flow restriction due to the
rotor.
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Instantaneous mass flow rate:
1-D turbine Model vs. Experiment
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Modelling unsteady effects in 1D
Swallowing capacity:
1-D turbine Model vs. Experiment
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Modelling unsteady effects in 1D
Integration with Mean Line Model
1-D turbine Model vs. Experiment
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Modelling unsteady effects in 1D
Unsteady power:
1-D turbine Model vs. Experiment
Integration with Mean Line Model
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Modelling unsteady effects in 1D
Unsteady efficiency:
1-D turbine Model vs. Experiment
Integration with Mean Line Model
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INSTANTANEOUS EFFICIENCY DIFFICULT TO DEFINE ACCURATELY AVERAGED OVER A CYCLE, MEASURED UNSTEADY EFFICIENCY DEPARTS FROM THE QUASI-STEADY PREDICTION
UNSTEADY PULSE FLOW PERFORMANCE CANNOT BE
ASSUMED TO BE QUASI-STEADY
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ATTEMPTS TO IMPROVE TURBINE PERFORMANCE UNDER PULSATING FLOW
ONE IDEA
ACTIVE / PASSIVE CONTROL TURBINE
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Operation
tsp PPTcmW η
γγ
−=
−1
01
201 1
20 – 60 Hz pulse Frequency
Concepts
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Experiment
Internal Spring Stiffness = 12.3 N/mm
Picture of the Laboratory Arrangement
x∆
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-10
0
10
20
30
40
50
60
Turb
ine
Act
ual P
ower
(kW
)
One Pulse Cycle (~0.05s)
Case 1-20Case 2-20Case 3-20
A
B
-10
0
10
20
30
40
50
60Tu
rbin
e A
ctua
l Pow
er (k
W)
One Pulse Cycle (~0.05s)
Case 1-20Case 2-20Case 3-20
20 Hz Flow, N turbine ~ 30000 , Natural Oscillation
Results
20Hz Pulsation and ~30000rpm
Settings Cycle Average Power (kW)
Average Power (kW) A
Average Power (kW) B
Case 1 8.91 23.55 1.89
Case 2 8.62 21.83 2.24
Case 3 8.43 18.36 3.64
70deg 8.34 18.47 3.62
65deg 8.39 19.91 2.98
60deg 8.27 21.34 2.05
50deg 8.19 21.98 1.54
40deg 8.22 22.41 1.45
Case 1 8.91 23.55 1.89
Cycle Average Power in Case 1-20 is 6.2% higher than 65deg vane setting.
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ACT SIMULATION
ACT bsfc 210.1g/kWh at amplitude 0.2 and phase shift ~70° 2.1g/kWh improvement over the standard VGT =2% bsfc improvement
bsfc
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