1 Validation of CFD Calculations Against Impinging Jet Experiments Prankul Middha and Olav R. Hansen, GexCon, Norway Joachim Grune, ProScience, Karlsruhe, Germany Alexei Kotchourko, FZK, Karlsruhe, Germany September 11, 2007

2 Motivation  CFD calculations increasingly used for quantitative risk assessments  Validation of tool primary requirement  Important to focus on “realistic” scenarios while carrying out validation of CFD tool  Need to reproduce the complex physics of the accident scenario  Validation of tools for combined release and ignition scenarios  Recent experiments performed at FZK present an opportunity to perform “real” validation against a complex experiment  Possibility to develop risk assessment methods for hydrogen applications  (Caution: Not large scale)

3 Experimental Details (1)  Release of hydrogen in a ”workshop” setting followed by ignition  Nine different release scenarios  Total hydrogen inventory fixed (10 g)

4 Experimental Details (2)  Two different geometrical configurations  Released H 2 ignited using at two different ignition positions (0.8 and 1.2 m above the release nozzle) Plate Geometry Hood Geometry

5 CFD Tool FLACS (1)  Solution of 3D compressible Navier-Stokes equations using a finite volume method over a cartesian grid  Implicit method (SIMPLE algorithm) for pressure correction  2 nd order scheme in space and 1 st order scheme in time (2 nd order available)  Standard k-  model with several important modifications  Model for generation of turbulence behind sub-grid objects  Turbulent wall functions for adding production terms to the relevant CV across the boundary layer  Model for build-up of proper turbulence behind objects of a particular size (about 1 CV) for which discretization produces too little turbulence  A “distributed porosity concept” which enables the detailed representation of complex geometries using a Cartesian grid  Large objects and walls represented on-grid, and smaller objects represented sub-grid  Necessary as small details of “obstacles” can have a significant impact on flame acceleration, and hence explosion pressures

6 CFD Tool FLACS (2)  Combustion Model  Flame in an explosion assumed to be a collection of flamelets  1-step reaction kinetics, with the laminar burning velocity being a measure of the reactivity of a given mixture  A “beta” flame model normally used that gives the flame a constant flame thickness (equal to 3-5 grid cells)  Burning velocity model:  A model that describes the laminar burning velocity as a function of gas mixture, concentration, temperature, etc. Le effects accounted for H 2.  A model describing quasi-laminar combustion (increase in burning rate due to flame wrinkling, etc.)  A model that describes S T as a function of turbulence parameters (intensity and length scale) and laminar burning velocity (based on Bray et al.)

7 Purpose of Simulations  Simulations performed prior to experiments with the primary purpose of aiding the design of experiments, if possible:  Identify scenarios for ignition (cloud size & reactivity)  Optimal ignition position and time  Expected overpressures => Avoid un-interesting tests, optimise use of resources  Secondary purposes:  Evaluate prediction capability (topic of current presentation)  Demonstrate efficiency of calculations  Development of risk assessment methods  Presented at LPS, Houston  Connection with HyQRA (HySafe) and IEA Task 19

8 Representation of geometry and grid Grid used: 5 cm standard grid (2.5cm for explosion) Stretch outside interesting region Refine towards leak (21mm and 4mm leaks)

9 Dispersion Simulations: Plate geometry Small flammable volume with plate only Small nozzle (4mm) => ”no flammable cloud”

10 Dispersion Simulations: Plate geometry

11 Dispersion Simulations: Hood geometry Flammable cloud inside confinement for low momentum Small nozzle (4mm) => ”no flammable cloud”

12 Dispersion Simulations: Hood geometry

13 Dispersion Results: Comparison with Experiments 100mm nozzle 21mm nozzle Concentration dependence on distance from nozzle Plate Geometry

14 Dispersion Results: Comparison with Experiments 100mm nozzle (0.7 g/s) 21mm nozzle (3.0 g/s) Lateral distribution of concentration Plate Geometry

15 Dispersion Results: Comparison with Experiments Photograph of plume vs. Predicted shape Plate Geometry, 21mm nozzle (3.0 g/s)

16 Dispersion Results: Comparison with Experiments Concentration dependence on distance from nozzle Hood Geometry, 21mm nozzle

17 Dispersion Results: Comparison with Experiments Concentration dependence on distance from nozzle Hood Geometry, 100mm nozzle

18 Dispersion Results: Comparison with Experiments Photograph of plume vs. Predicted shape Hood Geometry, 21mm nozzle (3.0 g/s)

19 Explosion Simulations (Pre-calculations) ”Worst-case” explosion overpressures (quiescent) Plate geometry Hood geometry  Ignition of non-homogeneous clouds

20 Possible to scale overpressures with cloud size ?  Aim: Development of QRA methodology  Concept of ”equivalent stoichiometric cloud size”  Obtained using reactivity- and expansion-based weighting  Expected to give similar explosion loads as the real cloud Cloud Size Overpressures

21 Explosion Results: Comparison with Experiments ExperimentsSimulations Ignition 1.2m from release nozzle (Calculations performed subsequent to experiments to match ignition position)  Possible different time of ignition for 100mm hood leads to higher simulated pressure

22 Explosion Results: Comparison with Experiments Experiments Simulations Ignition 0.8m from release nozzle (Calculations performed subsequent to experiments to match ignition position)  Local pressure transient around ignition influences simulated pressures near ignition location

23 Conclusions  Leak scenarios well predicted in general  Less interesting scenarios simplified somewhat with respect to grid definition to save time, which led to some underprediction  Predicted pressure levels with FLACS similar to those observed in experiments  Possible to scale predicted overpressures with equivalent gas cloud size  Work important to build confidence in CFD tools for QRA calculations

24 Acknowledgements  FZK and coauthors for interesting experiments and access to experimental data  Look forward to larger scale controlled studies in similar setups  European Union for support through the NoE HySafe  Norwegian Research Council for support for hydrogen modelling activities