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
140
150
160
170
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190
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210
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
-0.02
0
0.02
0.04
0.06
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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200
1.5 1.6 1.7 1.8 1.9 2
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 ?
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