numerical study of tunnel slope effect on hydrogen dispersion€¦ · 2, released from 700 bar cgh2...

Numerical study of tunnel slope effect on hydrogen dispersion

I.C. Tolias, N. Koutsourakis, A.G. Venetsanos

National Centre for Scientific Research Demokritos

Digital Stakeholders WorkshopHyTunnel-CS project

4-5 May 2020

v Motivation

v Examined scenario

v Numerical details

v Simulations results

v Conclusions

Tunnel slope effect – Numerical analysisOutline

Digital Stakeholders Workshop, 4-5 May 2020

v Buoyancy:§ Main safety asset of H2

§ Will it help H2 to disperse faster in sloped tunnels?

v Literature review (mainly on tunnel fires)§ The ‘stack-effect’, due to buoyancy, affects the flow & dispersion field (Wan

et al. 2015)

§ Adverse consequences may exist, esp. for ventilated descending tunnels (Du et al. 2018)

§ In longitudinally Lambda-shaped tunnels, H2 is trapped (Mukai et al. 2005)

§ Confusion may be created about how to act in emergency (Zhao et al., 2019)

Tunnel slope effect – Numerical analysisMotivation


v Tunnel§ Horseshoe cross-section§ 200 m length, 7.1 m max height§ Two cars (4.2 x 1.8 x 1.3 m)

• H2 release beneath the 2nd car§ Slope: 0.0%, 2.5%, 5.0% (descending)

Tunnel slope effect – Numerical analysisExamined scenario


Cross-section area = 60 m2

7.1 m

9.2 m 0.5 m

2 m

5.1 m7.1 m

9.2 m 0.5 m

2 m

5.1 m

9.2 m9.2 m 0.5 m0.5 m

2 m

5.1 m

2 m2 m

5.1 m5.1 m

v H2 release§ 6 kg of H2, released from 700 bar CGH2 car tank§ Release position: At the bottom of the car (0.2 m from the ground)§ Release direction: Towards the ground§ Two release diameters

• 4 mm• 2 mm

§ Sonic velocity (chocked flow) -1300 m/s

Tunnel slope effect – Numerical analysisExamined scenario


!

v ADREA_HF CFD codev H2 release

§ Blow-down: Flow rate decreases with time§ Notional nozzle approach

• Fictitious nozzle decreases with time§ Duration: 100 s (4 mm), 400 s (2 mm)

v Computational grid§ 4 cells at the release§ Uniform grid around release§ Total number of active cells:

• 1,1 M (4 mm case)• 1,9 M (2 mm case) - Symmetry

v Domain

Tunnel slope effect – Numerical analysisNumerical details


v Turbulence model§ Standard k-ε turbulence model with buoyancy terms§ Initial k about 0.0025 m2/s2

v Discretization schemes§ Spatial: MUSCL (2nd order)§ Temporal: 1st order backwards differences

• CFL number: 4 (4 mm case), 15 (2 mm case)

v Slope modelling§ Change of gravitation orientation § Pressure initialization for inclined tunnels

Tunnel slope effect – Numerical analysisNumerical details


v Initial stages§ 4 mm case

• H2 spreads over the streetreaching tunnel walls

• 4 tongues are created which transfer most of H2 upwards• Car geometry responsible

§ 2 mm case• H2 spreads less (horizontally)

due to lower flow rate• Elevates quickly surrounding

the car

§ Slope has negligible effect on initial stages

Tunnel slope effect – Numerical analysisSimulations results


10% v/v H2

v Intermediate stages§ 4 mm & 2 mm case: Impinging-like flow above the car



4 mm(0% slope)

10% v/v H2

2 mm(0% slope)

v Intermediate stages§ Slope effect (4 mm case)



10% v/v H20% slope(4 mm)

5% slope(4 mm)

v Intermediate stages: Concentration field (2 mm case)



0% slope

5% slope

>75 %

• Slope has small effects around the car

v Propagation of H2 with time: 4 mm case (release duration: 100 s)Slope 0% Slope 5%

v Propagation of H2 with time: 2 mm case (release duration: 400 s)


60s

80s

100s

80s

120s

140s

200s


Tunnel slope effect – Numerical analysisSimulations resultsv Flow and concentration field at part of tunnel at 60 s (4 mm

case)§ Full-height recirculations are formed§ Flow field similar to that of a fire

v Flammable cloud (4% - 75%)



v H2 cloud 10% - 75%



v H2 cloud 32% - 42%



v Concentration time-series§ Unstable period in the first half of the release

(until 50s and 200s respectively)§ There are moments where slope-cases

have higher values§ Concentration increase for all cases just before the end of the release

(100s and 400 s respectively) due to hydrogen re-settlement.



v The ventilation helps in the shorter dispersion of hydrogen

Tunnel slope effect – Ventilation influenceFirst results (On-going work)

No ventilation Av. velocity of 1 m/s

Flammable cloud at 10s

(2 mm case)

Contours at 20s(just at the right side of the car – not at the middle of the tunnel)

v Hydrogen dispersion in tunnel presents similarities with smoke dispersion

v Inclination has small effect on the velocity and concentration field around the car. It affects hydrogen dispersion at larger time and spatial scales where interesting phenomena occur.

v There are cases/places/times where adverse effects may exist at inclined tunnels. For example, volume of flammable cloud can be bigger in case of inclination

v Hydrogen clouds of concentrations above 10%, which matter more in case of an ignition, are only very slightly affected by the slope

v Release diameter (2 mm vs 4 mm):§ Reduction of max. flammable cloud in 2 mm case is around 38%§ Reduction of max. of concentrations above 10% in 2 mm case is around

75%

Tunnel slope effect – Numerical analysisConclusions


This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking (JU) under grant agreement No 826193. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and United Kingdom, Germany, Greece, Denmark, Spain,Italy, Netherlands, Belgium, France, Norway, Switzerland.

Acknowledgements

numerical study of tunnel slope effect on hydrogen dispersion€¦ · 2, released from 700 bar cgh2...

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