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Pulsating Flow Impact on Turbocharger Turbines

Ricardo Martinez-Botas

Srithar Rajoo

Turbocharger Research Group

Pulsating Exhaust Flow

Internal Combustion Engine: Reciprocating, positive

displacement

Turbocharger Turbine: Rotodynamic, steady flow device

•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

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

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

-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

Turbine Unsteady Performance

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

-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

UNSTEADY PULSE FLOW MEANS TURBINE OPERATES OVER A

WIDE RANGE

OVERALL BEHAVIOUR IS PREDICTED FROM SIGNIFICANT DATA EXTRAPOLATION

PULSE FLOW RELIES ON TURBINE EXTRAPOLATION

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

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

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

NEED FOR EXPERIMENTALLY MEASURED EXTENDED TURBINE PERFORMANCE MAPS

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

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

QUASI-STEADY ASSUMPTION WHAT DOES IT MEAN ?

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 ?

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

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’

VOLUME DISPLAYS ‘FILLING AND EMPTYING’ BEHAVIOUR

VOLUTE LENGTH DISPLAYS WAVE ACTION

UNSTEADY MASS FLOW OF TURBINE STAGE

BEHAVIOUR IS NOT QUASI-STEADY

Turbine Modelling Options

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.

Instantaneous mass flow rate:

1-D turbine Model vs. Experiment

Modelling unsteady effects in 1D

Swallowing capacity:

1-D turbine Model vs. Experiment

Modelling unsteady effects in 1D

Integration with Mean Line Model

1-D turbine Model vs. Experiment

Modelling unsteady effects in 1D

Unsteady power:

1-D turbine Model vs. Experiment

Integration with Mean Line Model

Modelling unsteady effects in 1D

Unsteady efficiency:

1-D turbine Model vs. Experiment

Integration with Mean Line Model

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

ATTEMPTS TO IMPROVE TURBINE PERFORMANCE UNDER PULSATING FLOW

ONE IDEA

ACTIVE / PASSIVE CONTROL TURBINE

Operation

tsp PPTcmW η

γγ

−=

−1

01

201 1

20 – 60 Hz pulse Frequency

Concepts

Experiment

Internal Spring Stiffness = 12.3 N/mm

Picture of the Laboratory Arrangement

x∆

-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.

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