Ignitability and mixing of Under Expanded Hydrogen Jets

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.





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


Centreline unmixedness = 0.222

Jet spreading rate = 0.111 Virtual origin =

7.14mmMean Centreline decay rate =

Virtual origin =


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




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





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

Ignitability and mixing of Under Expanded Hydrogen Jets

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