e. papanikolaou, d. baraldi joint research centre - institute for energy and transport

ICHS4, San Francisco, 12-14 September 2011

1

E. Papanikolaou, D. BaraldiJoint Research Centre - Institute for Energy and [email protected]://www.jrc.ec.europa.euhttp://iet.jrc.ec.europa.eu

Comparison of modelling approaches for CFD simulations of high pressure

H2 releases

mailto:[email protected]

http://www.jrc.ec.europa.eu/


2

Outline

1. Notional nozzle concept2. Notional nozzle approaches investigated3. Experimental description4. Simulations set up5. Results6. Conclusions7. Ongoing work


3

Notional nozzle concept

Under-expanded jet release

Level 1: “reservoir” conditions

Level 2: real orifice

Level 3: notional nozzle location

• Does not necessarily exist in a physical sense • Assumptions made:

• atmospheric pressure• uniform velocity profile

Need for reasonable computer run-times, for engineering applications

The jet flow of a high pressure unintended release is complex: a supersonic region in the near field and a subsonic region downstream. Numerical modelling of the near

field is quite demanding

Replacement of the actual release nozzle by a notional, occupying an area with the same flow rate as the actual one


4

Notional nozzle approaches investigated1. Birch et al. (1984)

Conservation of mass, isentropic change, ideal gas, temperature at notional nozzle equal to atmospheric

2. Birch et al. (1987)Conservation of mass, conservation of momentum, isentropic change, ideal gas, temperature at notional nozzle equal to atmospheric

3. Ewan and Moodie (1986)Conservation of mass, isentropic change, ideal gas, temperature at notional nozzle equal to the one at the actual release nozzle

4. Schefer et al. (2007)Conservation of mass, conservation of momentum, isentropic change, real gas (Abel-Noble equation of state), temperature at notional nozzle equal to atmospheric

Notional nozzle approaches that take into account the conservation of energy have been also proposed such as Yücel & Ötügen (2002), Molkov et al. (2009) and Xiao et al. (2011)


5

Experimental description

• High momentum horizontal (free-surface) H2 experiments in the HYKA test facility of the Institute for Nuclear and Energy Technologies of FZK

• Selected experiment for investigation: H2 release from 1 mm diameter at stagnation pressure of 98.1 bar and stagnation temperature of 14.5 ºC

• H2 concentration and flow velocity were measured on the jet symmetry axis at 1.5 m and 2.25 m from the release


6

Simulations set up: conditions at the actual and notional nozzle

Pressure (kPa)

Density (kg/m3)

Temperature (K)

Sonic Velocity (m/s)

Ideal gas* 5165 5.25 239 1178

Real gas* (Abel-Noble) 4932 4.89 235 1216

Table 1: Conditions at the release (actual nozzle)

Approach Diameter (10-3 m)

Area (10-5 m2)

Density (kg/m3)

Temperature (K)

Velocity (m/s)

Birch (1984)* 7.14 4.00 0.085 288 1293

Birch (1987)* 5.78 2.62 0.085 288 1972

Ewan&Moodie* 6.81 3.64 0.103 239 1178

Schefer 5.86 2.69 0.085 288 2029

Table 2: Conditions at the notional nozzle

* A discharge coefficient equal to 0.91 was used to match the calculated mass flow rate to the experimental


7

Simulations set up: dispersion calculations• ANSYS-CFX version 12.1

• 4 notional nozzle approaches

• “real” pipe with diameter equal to the experimental (D=1mm) and length equal to 10D to evaluate the level of accuracy with a coarse mesh (shock region not resolved)

• Equations of mass, momentum and energy (total enthalpy) conservation

• 4 commonly used turbulence models: standard k-, SST, RNG k- and baseline k- (BSL)

• All simulations run as transient cases for 5 s

• High resolution scheme for discretization of advection terms

• 2nd Order Backward Euler for discretization of transient terms

• Ideal gas law for the notional nozzle cases, Redlich Kwong equation for the “real” pipe cases

• Inlet with boundary conditions for velocity and temperature from the notional nozzle approaches or a given mass flowrate (equal to the experimental) for the “real” pipe cases.

• All simulations had a common incoming level of turbulence (intensity of 5%) assigned to the inlet


8

Simulations set up: dispersion calculations

• Computational domain: 15 m × 10 m × 10 m

•Minimum and maximum timestep was 10-8 s and 10-3 s for the notional nozzle cases and 10-8 s and 10-4 for the “real” pipe cases

• Unstructured mesh: 250.000 nodes for notional nozzle cases and 135.000 nodes for “real” pipe cases

Solution domain for all cases


9

Results: H2 concentration at 1.5 m and 2.25 m from the release

MEV: Mean Experimental Value

STD: Standard Deviation

30% - 50% over or under prediction of MEV

Comparison between approaches

In general, Birch 1987, Schefer and “real” pipe cases perform better

Comparison between turbulence models

General tendency for higher predictions with k- , followed by RNG k- , BSL and lastly SST


10

Results: Flow velocity at 1.5 m and 2.25 m from the release

Comparison between approaches

Majority of predictions lie within the 30% over/under prediction of MEV

Highest values predicted by approaches with highest release velocity (Schefer, Birch 1987)

Comparison between turbulence models

General tendency for higher predictions with k- , followed by either RNG k- or BSL and lastly SST


11

Results: Contour plots of H2 concentration – Birch 1984 & Birch 1987Case: Birch84 – BSL

H2 mass in flammable cloud: 0.957 gFlammable volume: 0.189 m3

Case: Birch84 – k-H2 mass in flammable cloud: 1.51 g

Flammable volume: 0.308 m3

Case: Birch84 – RNGH2 mass in flammable cloud: 0.919 g


Case: Birch84 – SSTH2 mass in flammable cloud: 0.803 g


Case: Birch87 – BSLH2 mass in flammable cloud: 0.549 g


Case: Birch87 – k-H2 mass in flammable cloud: 0.73 g


Case: Birch87 – RNGH2 mass in flammable cloud: 0.494 g


Case: Birch87 – SSTH2 mass in flammable cloud: 0.438 g


0.5 m


12

Results: Contour plots of H2 concentration – Schefer and EwanCase: Schefer – BSL

H2 mass in flammable cloud: 0.542 gFlammable volume: 0.11 m3

Case: Schefer – k-H2 mass in flammable cloud: 0.787 g


Case: Schefer – RNGH2 mass in flammable cloud: 0.513 g


Case: Schefer – SSTH2 mass in flammable cloud: 0.443 g


Case: Ewan – BSLH2 mass in flammable cloud: 1.083 g


Case: Ewan – k-eH2 mass in flammable cloud: 1.698 g


Case: Ewan – RNGH2 mass in flammable cloud: 1.174 g


Case: Ewan – SSTH2 mass in flammable cloud: 0.937 g



13

Conclusions

The conclusions are relevant to the selected experiment, the available data and the simulations’ set up. To make general comments, more experimental conditions should be investigated

Birch 1987 and Schefer approaches produce more accurate results for the H2 concentration

Including the conservation of momentum in the approach increases the accuracy of the results

The coarse mesh of the “real” pipe cases produced accurate enough results for both H2 concentration and

flow velocities on the symmetry axis

Further investigation is necessary on both axial and radial distances from the release


14

Ongoing work

Assessment of approaches with:

• different initial conditions (stagnation properties, release diameter)

• both axial and radial experimental measurements of H2 concentration and flow velocity

Effect of:

• Grid resolution. Grid independence for notional nozzle approaches.

• Turbulence intensity at the source (5%, 10%).

• Discretization schemes


15

Thank you for your attention


16

Results: Contour plots of H2 concentration – Schefer and “real” pipe

Case: Schefer – BSLH2 mass in flammable cloud: 0.542 g


Case: “real” pipe – BSLH2 mass in flammable cloud: 0.575 g


e. papanikolaou, d. baraldi joint research centre - institute for energy and transport

Documents