BLIND PREDICTION OF DISPERSION AND EXPLOSION EXPERIMENTS USING CFD

Transcription

1 WCOGI 2008 College Station, Texas October BLIND PREDICTION OF DISPERSION AND EXPLOSION EXPERIMENTS USING CFD by Olav Roald Hansen 1 and Prankul Middha 2 GexCon 1, Bethesda, MD 2 GexCon AS, Bergen, Norway

2 WHY BOTHER ABOUT VALIDATION? FLACS sometimes used for 100+ million USD decisions Important that validity is acceptable, high priority for FLACS 1980s & 90s; millions invested in validation tests Consider an explosion at an installation, your consultant predicts the following: 0.5 barg Do you believe in this result?

3 WHY BOTHER ABOUT VALIDATION? FLACS sometimes used for 100+ million USD decisions Important that validity is acceptable, high priority for FLACS 1980s & 90s; millions invested in validation tests Consider an explosion at an installation, a consultant predicts the following: 0.5 barg 4 barg Difficult to know; we believe a good validation work must be performed, guidelines must be based on validation and followed by users Or would you rather believe in this?

4 MOST CFD-TOOLS LEAVE VALIDATION TO END USER? Several tuning factors and choices available, like turbulence model and other parameters If user knows the answer, he/she will get the answer Example; general purpose CFD-tool (10 years ago) GexCon to validate explosion, modeling by developer Experiment P=180 mbar Ta=0.9s Td=0.2s P max known and obtained, but when calibrating a model with poor physics, conservation laws seem to be broken. Simulation P=160 mbar Ta=0.009s Td=0.008s Flame and flow exceed the C sound from t ignition Colourful Fluid Dynamics; experimental pressure reached, but conservation laws violated

5 EXAMPLES OF FLACS VALIDATION Ventilation n 1990s: Studies at oil platforms + full scale test site n Beryl Bravo (Mobil) n Oseberg A (Hydro) n Nelson (Enterprise Oil) n Claymore n Spadeadam JIP Most studies demonstrated good correlation between measurements and simulations with FLACS. Photo shows GexCon wind measurements at Nelson Platform Conclusions simulation studies: Geometry details very important, CFD-model that ignored details overpredicted ventilation 100% Lots of interesting observations in Nelson study, chimney effects, reverse flow in wake, dead corners

6 EXAMPLES OF FLACS VALIDATION Grid dependency report 1994 (gas explosion modeling) Important input to grid guidelines n Need for cubical grid cells n Need for minimum resolution n Discretization problems for turbulence with dia 1 CV Adjustments to numerical schemes Input to work to improve sub-grid object representation. insufficient resolution recommended low grid dependency range 1 CV problem

7 EXAMPLES OF VALIDATION Wide range of tests simulated Several M24 tests ~10 MERGE geometries 4x4 BR=0.23 9x9 BR= x15 BR= Validation study, 50 different experiments compared to FLACS. Plot shown was important to identify need for model improvement. 5 different 3D-corner geometries 4 SOLVEX geometries at 2 scales +5 different BG Bang-box geometries 1995 water deluge study, 3 different nozzle pressures for different gas concentrations

8 EXAMPLES OF VALIDATION GexCon has simulated around 100 large-scale experiments n 23 BFETS Phase 2 tests (confinement, congestion, gas conc., ignition point, deluge) n 40+ HSE Phase 3A tests (ignition point, congestion, low conf., deluge & repeatability) n 30 Phase 3A (dispersion+explosion & partial fills & reference tests) n Indirect 98-JIP (ventilation/dispersion), comparisons done by consortium member Blind predictions were carried out, example BFETS Test Validation report reporting simulations of 50 large scale tests (BFETS Phase 2 and HSE Phase 3A) Example of detailed comparisons Predicted/observed pressure in tests of varying confinement (HSE-3A, almost 1000 measurements in plot)

9 EXAMPLES OF VALIDATION Dispersion (and explosion of dispersed cloud) n Basic tests including numerical schemes n SMEDIS evaluation project n 1998 GexCon 50m 3 n Phase 3B (GexCon 50m 3, Advantica 2600m 3 ) n Kit Fox (52 CO 2 releases) n Prairie Grass (37 SO 2 tracer release) n MUST (42 C 3 H 6 tracer releases) n NYC Urban dispersion project n LNG (Burro, Coyote and Maplin Sands) Volume (m^3) dispersion simulation : Flammble volume 7 Observed Simulated rt.fuel file 8 8 : 1m grid 9 9 : 1m grid : 1m grid : 1m grid : 1m grid test number Phase 3B geometry FLACS flammable volume versus experiments in 20 tests FLACS-98 FLACS-99 Basic tests: Standard numerical schemes will not give symmetrical impinging jet Coyote 5 LNG release simulation Kit Fox: 75 large and 6600 small obstacles Manhattan geometry model MUST: 120 shipping containers 1998 study comparing observed and simulated concentration and explosion pressure

10 RECENT BLIND SIMULATIONS CFD-validation against known tests of low value if tuning factors are used (including arbitrary selection of turbulence model and grid) Optimal evaluation of models: Blind simulations of unknown tests Tool used by more than 1 group/person GexCon BLIND prediction activities last 2-3 years H 2 dispersion INERIS 6C (17) Helium dispersion CEA Garage test (1) Shell H 2 refuelling station explosion test (7) Shell H 2 congested pipe-array (1) FZK H 2 workshop ignited jets (1) Dispersion scenarios at Manhattan (6) Coal mine methane explosion tests (2) and many more e.g. during training courses

11 HYDROGEN DISPERSION INERIS 6 HySafe BLIND benchmark November 2005; 17 modelers delivered blind predictions Jan-March 2006; INERIS performed experiment Presented at ICHS2 in San Sebastian in Sept 2007 Scenario: Room 7.4m x 3.8m x 2.7m 4 minute leak 1 g/s 2 hour waiting time after leak Both GexCon and DNV used FLACS (good consistency) Diffusion phase well predicted Experiment Simulations Experiment Simulations H2 concentration H2 concentration Time (s) Time (s)

12 HYDROGEN DISPERSION HySafe blind benchmark From Venetsanos paper ICHS Experiment Simulations Experiment Simulations H2 concentration H2 concentration Time (s) Time (s)

13 CEA HELIUM DISPERSION TESTS n Room dimension 5.76m x 2.96m x 2.4m n 2 g/s helium release in 2 minutes, 3h waiting time n Blind benchmark HySafe (only GexCon) Status: n GexCon did 3 CEA blind predictions with good results n Experiments published at ICHS2

14 FZK IGNITED HYDROGEN JETS 9 different vertical releases (rate/momentum) in two configurations April 2006 > 200 FLACS blind simulations April-June 2006 test performed by FZK (reported January 2007) Presented at ICHS2, San Sebastian, September 2007

15 DHS/PNNL NYC MODEL EVALUATION n GexCon contributed to dispersion modeling activity (6 modelers) n a blind modeling exercise was performed n FOUO => results not available to public n Plot below is from paper by Julia Flagherty of PNNL 6 different modelers Experiment

16 H 2 EXPLOSION IN PIPE-ARRAY Blind simulations performed May 2006 Results presented ICHS2 September 2007

17 MINE EXPLOSIONS (METHANE) 2006 January: Sago mine explosion NIOSH asks GexCon + other CFD-tool to model 6 LLEM-tests (methane) Up to 18m / 300m3 gas clouds in tunnels

18 MINE EXPLOSIONS (METHANE) GexCon predicts blind, the other modeler simulates after seeing test results FLACS blind predictions much closer to experiment

19 HYDROGEN EXPLOSION n Refuelling Station (Experiment by Shell et al.) n Simulated and submitted in November 2006 n Access to results December 2006, presented at ICHS2 n GexCon/HSL simulated blind, the rest after seeing results

20 HYDROGEN EXPLOSION n Shell Refuelling Station n Plot from paper submitted to IJHE 2.7 m 2.7 m 0.4 m 0.5 m KW14 KW13 KW12 KW m KD1 6.0 m 1.45 m K m 3.05 m 1.8 m 1.50 m 2.5 m K3 K4 K5 K6 Height of sensors above ground KD1, K5, K6, K10, K m KW m KW7, KW m KW m 2.25 m K10 K11

21 HYDROGEN EXPLOSION n Shell Refuelling Station n Plot from paper submitted to IJHE 2.7 m 2.7 m 0.4 m 0.5 m KW14 KW13 KW12 KW m KD1 6.0 m 1.45 m K m 3.05 m 1.8 m 1.50 m 2.5 m K3 K4 K5 K6 Height of sensors above ground KD1, K5, K6, K10, K m KW m KW7, KW m KW m 2.25 m K10 K11

22 HYDROGEN EXPLOSION n Shell Refuelling Station n Plot from paper submitted to IJHE 2.7 m 2.7 m 0.4 m 0.5 m KW14 KW13 KW12 KW m KD1 6.0 m 1.45 m K m 3.05 m 1.8 m 1.50 m 2.5 m K3 K4 K5 K6 Height of sensors above ground KD1, K5, K6, K10, K m KW m KW7, KW m KW m 2.25 m K10 K11

23 HYDROGEN EXPLOSION n Shell Refuelling Station n Plot from paper submitted to IJHE 2.7 m 2.7 m 0.4 m 0.5 m KW14 KW13 KW12 KW m KD1 6.0 m 1.45 m K m 3.05 m 1.8 m 1.50 m 2.5 m K3 K4 K5 K6 Height of sensors above ground KD1, K5, K6, K10, K m KW m KW7, KW m KW m 2.25 m K10 K11

24 CONCLUSIONS CFD-tools can produce results of variable quality, most will have calibration buttons If important decisions shall be taken, one must be able to have confidence in results. This can be achieved if Model developer performs a proper validation effort Detailed user guidelines are developed based on the validation Users of the software follow these guidelines GexCon works to achieve these goals To demonstrate confidence FLACS, GexCon frequently joins blind prediction efforts and generally see satisfactory results For more information; please contact us:

25 NEW DEVELOPMENTS FLACS is all the time developing, some of the new things are LNG Pool spread Flammable vapor cloud Virtual reality simulator Chlorine dispersion (railcar in Chicago) Fire models are being developed