Surge modeling in GTP

A status report on the method to simulate compressors close to- and in surge

Michael Vallinder

GM Powertrain Sweden

in co-operation with Gamma Technologies

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0.028 0.03 0.032 0.034 0.036 0.038 0.04

Mass flow rate [kg/s]

Co

mp

ress

or o

utl

et p

ress

uer

[kP

a]

• What is surge, exactly?

• Measured surge

• Necessary changes to GT-POWER

• Compressor extrapolation

• Simulated surge with GT-POWER

• Conclusions and further work• Further improvements• Conclusions

What is surge ?

• Surge is caused by the compressor blades’ inability to uphold the air flow. The reason is the same as for an airplane wing stalling, the pressure gradient over the blade is to large and this will cause separation of the flow on the backside of the compressor blade (wing)

• Surge can happen partially at one blade passage or over the entire compressor wheel

• When surge happens at a one or two blade passages only, the temperature at the compressor inlet will start to rise. This is a good indication of beginning of surge

• Once the flow has separated from the compressor blades the pressure ratio over the entire compressor causes the flow to reverse backwards trough the compressor which then acts as a turbine

Measured surge

HFM measurements close to compressor inlet

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0

0.02

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0.08

0.1

0.12

0.14

1.5 1.6 1.7 1.8 1.9 2

Time [sec]

Mas

s flo

w r

ate

[kg/

s]

15Hz

Missing part because of

limitations in the HFM

Measured surge

Pressure profile at the compressor outlet

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Time [sec]

Com

pres

sor

outle

t pre

ssur

e [k

Pa]

Measured surge

Steady operating point close to surge

Pressure recovery

“Discrete” shift to negative mass flow rate, same

speed and pressure ratio

Negative mass flow rate

Pressure ratio drops as mass

flow rate is negative

Mass flow rate increases rapidly

• Measured surge plotted in the compressor map as the white line

Necessary changes to GT-POWER

• According to measurements the mass flow trough the compressor is negative in a part of the surge cycle• The compressor map in GT-POWER needs to be

extended to negative mass flow rates

• The compressor object must be able to handle negative mass flow rates in a correct way• Implementing “turbine” equations in the compressor

object during negative mass flow rates• The implementation of the new “turbine” equations

in the compressor was performed by Gamma Technologies

Compressor extrapolation

pr0

Surge point

PR

mc

Measured surge definition

X3

polynomial

X2 polynomial

Original measured compressor map

Speedline

Compressor extrapolationImport .cmp in MATLAB Extrapolate Export new .cmp to GT-POWER

Output from MATLAB

Compressor plot in GT-POWER with the new .cmp file

Simulated surge with GT-POWERMeasured with HFM close to

compressor

Simulated surge loop from GT-

Power

•Simulated surge has the same

frequency of 15Hz as measured

•Same pressure amplitude

•Only mass flow which is a little more

fluctuating than measured

Further work

• Investigate the dampening factor in more detail• Compare mass flow fluctuations during pulsating

“engine” flow• Add a time-based criteria to fully control transition to

surge

• Further develop the extrapolation routine• Validate the extrapolation routine

Conclusions• GT-POWER can be used to model a compressor close to- and in

surgeWith the changes made to the compressor object and the compressor .cmp map file

• Transition to surge corresponds well to measured data• A surge cycle in GT-POWER is very similar to measurements

• Pressure amplitude is correct• Frequency is correct• Mass flow behavior is on the whole correct with just minor

fluctuations• It is not an easy task to perform a surge simulation, it relies on test

data of the compressor and a lot of hours to extrapolate the compressor map correctly

• Open up doors to many new interesting ways in performance simulations with GT-POWER!

Questions ?

Michael.Vallinder@se.gm.com

Thanks to Tom Wanat, John Silvestri and Tom Morel for their incredibly fast and good support on this project !

Backup slides

Backup slides

pr0

• Compressor extrapolation theory• The compressor characteristics provided by the compressor manufacturer are only given in the

useful range of the compressor between the surge and choke lines• The compressor maps needs to be extended beyond the surge line down to negative mass

flow rates, this can be achieved with a set of different theories• Compressor characteristics left of the surge line can be estimated using a third order

polynomial [18]• Compressor characteristics at negative mass flow rates can be estimated using a second

order polynomial [6]• Pro which is the pressure ratio at zero mass flow can be estimated using the radial

equilibrium theory [19]• Its also necessary to calculate the compressor torque beyond the surge line in order to

define an efficiency [7]

Surge pointpr

mc

Measured surge definition

X^3 polynomial

X^2 polynomial

Original measured compressor map