10th PHOENICS User Conference Melbourne, Australia

1. 10th PHOENICS User ConferenceMelbourne, Australia A CFD presentation by Dr. Paddy Phelps ( Flowsolve Ltd ] & Mr. John Gibson [ Scott Wilson Ltd ] May 2004

2. Predicting the Dispersion Consequences of Gaseous Releases from a University Research Facility in an Urban Environment A CFD presentation on behalf of the authors by Dr. John Ludwig ( CHAM Ltd )

3. Outline of Presentation • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Outline of simulations performed • Sample Results Obtained • Conclusions

4. Consequences of release dispersion from an urban university research facility • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Outline of simulations performed • Sample Results Obtained • Conclusions

5. Industrial Context • The research facility is housed in a pre-existing building on the campus of a city-based UK University. • The site is a built-up area with private and college accommodation, shops, hospital and university buildings in the immediate vicinity.

6. Industrial Context • The building contains research laboratories, from which air and fume-cupboard extracts need to be vented thoughtfully and considerately to atmosphere . • Whilst not necessarily toxic, the releases can occasionally be tainted by unpleasant aromas ...

7. Industrial Context • The building, at present a two-storey building with roof-top plant room, is to be refurbished. • A third storey and a new plant room are to be added. • Gaseous discharges from the labs will be vented from tall stacks

8. Industrial Context • Opposite the research building, a major campus re-development project is under construction . • New buildings will include a massive “Glass Pavilion” and linked “Plant Wall” • Some adjacent street areas will become pedestrian precincts

9. Industrial Context • There is a history of local complaints about poor dispersion of “unpleasant smells” • The new buildings will create a local impediment to airflow, and may significantly alter local air flow patterns • “Pedestrianisation” of street areas will remove vehicular air stirring and increase awareness of ground-level concentrations

10. Industrial Context • Will releases from the roof-top stacks of the research building have adequate dilution / dispersion consequences ? • Could effluent plumes impinge upon openable windows or HVAC intakes in nearby buildings, or public access areas ? • If a hazard to the public exists, what is the extent, and how may it be eradicated ?

11. Flow Geometry Close-up

12. Overview of Site “as planned”

13. Consequences of release dispersion from an urban university research facility • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Outline of simulations performed • Sample Results Obtained • Conclusions

14. Objectives of Study - 1 • Use simulation tools to predict mixing of rooftop extract releases from the research building with the ambient airflow over the adjacent buildings • Provide input to the design of discharge arrangements which will lead to acceptable environmental impact

15. Objectives of Study - 2 • What constitutes “acceptable environmental impact” ? • Plume core is diluted and dispersed to a safe level at nearby • HVAC intakes • Opening windows • Public access areas

16. Criterion of Acceptability • A safe level is taken as a dilution level of 1:104 ( i.e. a concentration of 100 ppm ) from stack release • The plume core is the spatial envelope of this critical dilution / concentration

17. Overview of Release Conditions

18. Overview of Release Conditions

19. Consequences of release dispersion from an urban university research facility • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Outline of simulations performed • Sample Results Obtained • Conclusions

20. Benefits of CFD Approach (1) • No scale-up problem • Three-dimensional, steady or transient • Interrogatable predictions • Handles effect of • blockages in domain • recirculating flow • multiple inlets and outlets • multiple interacting releases

21. Techniqueallows for rapid and cost-effective assessment of alternative remedial strategies Benefits of CFD Approach (2)

22. Consequences of release dispersion from an urban university research facility • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Outline of simulations performed • Sample Results Obtained • Conclusions

23. Solution Domain 3-D PLUME DISPERSION MODEL • Solution domain encompasses the principal neighbouring buildings for at least one block on each side of the research facility • PHOENICS VR objects (plus some bespoke objects for roofs) used for blockages, in absence of CAD models to import • Domain 216m by 243m by 68m high

24. CFD Model Description - 1 Representation of the effects of • blockage due to the presence of internal obstacles • multiple inlets and outlets for air/effluent releases • Influence of buoyancy on plume trajectory • ambient wind velocity, temperature and turbulence profiles

25. CFD Model Description - 2 Dependent variables solved for : • pressure (total mass conservation) • axial, lateral and vertical velocity components • air / effluent mixture temperature • effluent concentration in mixture • turbulence kinetic energy • turbulence energy dissipation rate Independent Variables: • 3 spatial co-ordinates (x,y,z) and time

26. CFD Model Description - 3 • The set of partial differential equations is solved within the defined solution domain and on a prescribed numerical grid • The equations represent conservation of mass, energy and momentum • The momentum equations are the familiar Navier-Stokes Equations which govern fluid flow

27. CFD Model Description - 4 • The equations may each be written in the form D(rj) /Dt + div (r Uj - Gjgradj) = S{j} • Terms represent transience, convection, diffusion and sources respectively • Equation is cast into finite volume form by integrating it over the volume of each cell

28. CFD Model Description - 5 • “Guess and correct” solution procedure used iteratively to until “convergence” of scheme • Around 1500 “sweeps” of domain required for convergence • Typical nodalisation level - 500,000 • Convergence therefore involves formulation and solution of around 6 billion simultaneous linked differential equations

29. Boundary Conditions:Ambient Wind Specification [ 1 ] • Summer • Ambient temperature - 18 deg.C • Wind from SW • Winter • Ambient temperature - 5 deg.C • Wind from NNE

30. Boundary Conditions:Ambient Wind Specification [ 2 ] • Wind Speeds • Still - 0.5 m/s • Low - 2.5 m/s • Medium - 8.0 m/s • Wind Profiles • Pasquill D stability profiles

31. Internal Sources:Rooftop Release Specification Laboratory Extract Stacks (2 off) • Vertical stacks • Stack diameter 0.95 m. • Height 3 m. above plant room roof • Release temperature - 24 deg.C • Release velocity - 15 m/s • Flowrate 2.84 m3/s each stack

32. Boundary Condition Independent Region

33. Consequences of release dispersion from an urban university research facility • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Outline of simulations performed • Sample results Obtained • Conclusions

34. Overview of Workscope Model build and 25 Simulations performed in 4 “stages”

35. Workscope for Stage 1 “As Planned” Building Geometry • Model Construction & Testing • Initial Scoping Studies Objective : • Determine whether proposed arrangement leads to adequate dilution / dispersion of effluent releases under various wind conditions

36. Workscope for Stage 2 Original “As Was” Building Geometry : “Worst Case” Wind Vector combinations Objective : • to predict consequences of new neighbour buildings on plume dispersion if no refurbishment is carried out • to provide base case data for comparison with “as planned” post-refurbishment geometry

37. Workscope for Stage 3 2-D “Venturi” Stack Design Model • Model construction & testing • Parametric study of various design parameters Objective : • to investigate effectiveness of various designs of venturi stack, subject to given constraints

38. Workscope for Stage 4 “As Planned” Geometry + Venturi Stack “Worst Case” Wind Vector combinations Objective : • to investigate dispersion effectiveness of “as planned” post-refurbishment geometry with venturi stack

39. Consequences of release dispersion from an urban university research facility • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Outline of simulations performed • Sample Results Obtained • Conclusions

40. Stage 1 Simulations: Initial Scoping Studies • “As Planned” Building Geometry • Twin Vertical Stack releases • Summer (180C ambient, 240C release); • Winter (50C ambient, 200C release) • Wind velocities 0.5, 2.5, 5 and 8 m/s • Various wind directions

41. Stage 1 Simulation Matrix

42. Stage 1 Simulations: Predictions for Case 7 “AS PROPOSED” SCHEME; LOW WIND IN WINTER • Wind vector 0.5 m/s from NNE • Ambient temperature 50C • Release temperature 210C • Vertical release through twin stacks • Release velocity ~15 m/s • Chiller air curtain OFF

43. Stage 1 “ As Proposed ” Results :Winter wind vector 0.5 m/s from NNE

44. Stage 1 “ As Proposed ” Results :Winter wind vector 0.5 m/s from NNE

45. Stage 1 “ As Proposed ” Results :Winter wind vector 0.5 m/s from NNE

46. Stage 1 “ As Proposed ” Results :Winter wind vector 0.5 m/s from NNE

47. Stage 1 Simulations: Predictions for Case 4 “PROPOSED” SCHEME; HIGH WIND IN SUMMER • Wind vector 8.0 m/s from SW • Ambient temperature 180C • Release temperature 240C • Vertical release through twin stacks • Release velocity ~ 15 m/s • Chiller air curtain OFF

48. Stage 1 “ As Proposed ” Results :Summer wind vector 8 m/s from SW

49. Stage 1 “ As Proposed ” Results :Summer wind vector 8 m/s from SW

50. Stage 1 “ As Proposed ” Results :Summer wind vector 8 m/s from SW