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Ignitability and mixing of Under Expanded Hydrogen Jets
Adam RugglesIsaac Ekoto
Combustion Research Facility,Sandia National Laboratories,Livermore, CA, USA
ICHS Technical Seminar14th September, 2011
Simple Engineering solution to Determine Ignitability
Compressible flow replaced with a ‘Notional Nozzle’
Downstream part of leak behaves as an atmospheric jet
Ignitability described by the ‘Flammability Factor’
Notional Nozzles
Replace compressible shock structures with an atmospheric equivalent
ØTρT PT UT TT
Ømρm Pm Um Tm
Atmospheric JetsObey self similarity laws. Mean and rms scalar fields can easily be reconstructed.
Non dimensionalRadial Coordinate
Nor
mal
ised
mea
n co
ncen
trati
on
Gaussian curve
Non dimensionalRadial Coordinate
Nor
mal
ised
rms
conc
entr
ation
4th order polynomial curve
Richards and Pitts, 1993
mean scalar rms scalar
Flammability FactorEqual to the integral of the mole fraction PDF between the flammable limits.
Birch et al, 1981
Mol fraction (xi)0 1
0
1
Prob
abili
ty
LFL
UFL
11
0 idxP
UFL
LFL idxPFF
Can also be calculated using mean and rms values with an intermittency model.
Simple Engineering solution to Determine Ignitability
Compressible flow replaced with a ‘Notional Nozzle’
Downstream part of leak behaves as an atmospheric jet
Ignitability described by the ‘Flammability Factor’
Simple Engineering solution to Determine Ignitability
Compressible flow replaced with a ‘Notional Nozzle’
Downstream part of leak behaves as an atmospheric jet
Ignitability described by the ‘Flammability Factor’
Can it be applied to Hydrogen?
High Pressure H2 delivery system
Ø127mm
345m
m
Stagnation Chamber(up to 60:1 supply pressure)
Stagnation temperatureand pressure monitoredfor feedback control
Nozzle profiles adapted from ASME MFC-3M-2004
High Pressure H2 delivery system
Pr = 10Ø = 1.5mm
Reconstructing the downstream Scalar field
Reconstructing the downstream Scalar field
Centreline unmixedness = 0.222
Jet spreading rate = 0.111 Virtual origin =
7.14mmMean Centreline decay rate =
Virtual origin =
Reconstructing the downstream Scalar field
Mean Centreline decay rate (K) = 0.105 (Lit. Value)Virtual origin = 24.74mm
gradientrKYCL
111
ajrr 0
aeffeffrr Gives metric to assess Nozzle modelsrε, ideal = 0.438mm
Richards and Pitts, 1993
Reconstructing the downstream Scalar field
Richards and Pitts, 1993
2,0
59exp52.9
,
Yzz
rzY
432
,0
81.24048.11609.935.023.052.9
,
Yzz
rzY
jzz
r
,0
Predicting the Flammability Factor
Schefer et al, 2011
Predicting the Flammability Factor
Jet Radius (mm)
Jet
Axi
al L
eng
th (
mm
)
0 10 20 30 40 50 6080
110
140
170
200
230
FF = 0.9 FF = 0.1
Using Notional Nozzle models to predict effective radius and gas density
Model Effective nozzle radius (mm)
Jet density (Kg/m3)
rε (mm) rε /rε, ideal
Birch et al (1984) 1.80 0.0805 0.475 1.084
Ewan and Moodie (1986) 1.70 0.0971 0.492 1.123
Yuceil and Otugen (2002) 1.15 0.1391 0.399 0.911
Birch et al (1987) 1.50 0.0805 0.396 0.904
Harstad and Bellan (2006) 2.70 0.0837 0.726 1.658
rε, ideal = 0.438mm
aeffeffrr
Combined with Abel Nobel
Using Notional Nozzle models to predict the 10% ignitability contour
Using Notional Nozzle models to predict the 10% ignitability contour
Simple Engineering solution to Determine Ignitability
Compressible flow replaced with a ‘Notional Nozzle’
Downstream part of leak behaves as an atmospheric jet
Ignitability described by the ‘Flammability Factor’
Couple very well
Dependent upon model accuracyNeeds to determine virtual origins
What is Happening at the Nozzle?
Mach Disc Ø = 1.3mm
What is Happening at the Nozzle?
Is air and H2 mixing outside of Mach Disc?Do Notional Nozzle models require an air entrainment aspect?
Jet Radius (mm)
Jet
Axi
al l
eng
th (
mm
)
-5 0 5
0
2
4
6
8
10
12
14
16
18
20
Jet Light up Boundary
Kernel never gives sustained flame
Kernel always gives sustained flame
Birch et al, 1981Schefer et al, 2011Swain et al , 2007
Local ExtinctionFlame Speed Vs Flow SpeedTurbulent Time scale Vs Chemical Time scale
Ongoing work
Be able to predict virtual originslooking at compressible shear layers
Improve thermodynamic values usingbetter equation of state
Nozzle model development
Develop insight/model into Jet light up boundaries
Ascertain why no H2 flow with Ø1mm nozzle can have a sustained flame
Ignitability
Ignitability and mixing of Under Expanded Hydrogen Jets
Adam RugglesIsaac Ekoto
Combustion Research Facility,Sandia National Laboratories,Livermore, CA, USA
ICHS Technical Seminar14th September, 2011