PHOENICS User Group Meeting

1. PHOENICS User Group Meeting Benelux User Group Aristo Centre Eindhoven Netherlands May 2005

2. Modelling Discharges from Rooftop Stacks in Confined Environments A CFD presentation by Dr. Paddy Phelps ( Flowsolve Ltd )

3. Presentation encompasses the results of three different projects, performed over a period of time Each project used CFD to satisfy a different objective Eindhoven 2005Outline of Presentation

4. A Classical “Planning Consent” Project Using simulations to determine the dispersion consequences of releases from a “yet to be constructed” facility Predictions assist building services design team to arrive at an effective venting strategy PROJECT # 1

5. A “Diagnose and Remedy” Project CFD simulations used to investigate cause of environmental nuisance or potential hazard, and assist in design of appropriate retro-fit remedial measures PROJECT # 2

6. A “Compare and Contrast” Project Comparing possible extract strategies, for a situation where tall stacks cannot be used for aesthetic / planning reasons PROJECT # 3

7. PREDICTING THE DISPERSION CONSEQUENCES OF FUME RELEASES FROM BUILDING ROOF-TOP STACKS PROJECT # 1

8. 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

9. Emissions Dispersion Study : Industrial Context • A 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 a mix of private and college accommodation, shops, and university laboratories and buildings in the immediate vicinity.

10. Emissions Dispersion Study : Industrial Context • It is planned to construct a large extension to the existing research building, effectively doubling its size

11. Flow Geometry Close-up

12. Emissions Dispersion Study : Industrial Context • The building extension will contain new research laboratories, from which air and fume-cupboard extracts will need to be vented thoughtfully and considerately to atmosphere .

13. Emissions Dispersion Study : Industrial Context • Low levels of allergens may remain in the vented fumes • Whilst not always necessarily toxic, the releases may be tainted by unpleasant aromas • There is a history of local complaints about poor dispersion of “unpleasant smells”

14. Emissions Dispersion Study : Industrial Context • The building extension will create a significant additional impediment to the local ambient airflow • This may have a big influence on the air flow patterns at the vent stack release points

15. Emissions Dispersion Study : 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 ?

16. Flow Geometry Close-up

17. 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

18. Emissions Dispersion Study: Methodology • Use simulation tools to predict trajectory of effluent discharge • Determine dispersion envelope of potentially toxic components in effluent • Confirm any new discharges will not exacerbate existing discharges

19. Emissions Dispersion Study Objectives • Model predictions will provide input to the design of discharge arrangements which will lead to acceptable environmental impact

20. Emissions Dispersion Study Objectives • What constitutes “acceptable environmental impact” ? • The Plume Core is diluted and dispersed to a safe level at nearby • HVAC intakes • Opening windows • Public access areas

21. Criterion of Acceptability • A safe level is taken in this instance to be 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

22. Overview of Release Conditions

23. 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

24. 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 sources

25. Project #1: Emissions Dispersion Study • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Outline of simulations performed • Sample Results Obtained • Conclusions

26. 3-D PLUME DISPERSION MODEL Solution Domain • Solution domain encompasses the principal neighbouring buildings for at least one block on each side of the research facility • Domain 260m by 260m by 66m high

27. 3-D PLUME DISPERSION MODEL Solution Domain • PHOENICS VR object primitives used to represent building blockages, in the absence of CAD models to import • Some bespoke objects created [e.g. for roofs)

28. CFD Model Description - 1 • Representation of the effects of : • blockage due to presence of neighbouring buildings, obstacles • resistance and mixing in tree canopy [summer only] • ambient wind vector and temperature profile • multiple interacting releases (chillers, laboratory extracts etc)

29. 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

30. Roof-top Release Sites • Extract Stacks from Basement BSU • 2 off • Air Chiller Unit discharges • 12 off • Laboratory Extract Stacks • 8 off • Laboratory Discharge Stacks • 3 off

31. Overview of Release Conditions

32. Internal Sources:Rooftop Release Specification Basement Laboratory Extract Stacks • Two 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

33. Project #1: Emissions Dispersion Study • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Outline of simulations performed • Sample results Obtained • Conclusions

34. Environment Parameters Studied • Domain extent • extended down-wind domain • Ambient Wind • summer and winter wind directions • summer and winter temperature effects • Adjacent buildings • Influence of layout and topology • Environmental Factors • Tree canopy height, layout and resistance

35. Release Parameters Studied • Basement Extract Stacks • Release temperature - 24 deg.C • Release velocity - 15 m/s • Air chiller discharges • Release temperature - 24 deg.C • Release velocity - 2.6 m/s • BSX stack height • Reference - 3m. above chiller top • High - 6m. above chiller top • Also try - 9m. & 12m. above chiller top

36. Ambient Parameters Studied • Summer • Ambient temperature - 24 deg.C • Wind from SW • Winter • Ambient temperature - 5 deg.C • Wind from NNE • Wind Speed (Pasquill D stability profile) • High - 8.0 m/s • Low - 2.5 m/s • Still - 0.5 m/s

37. Project #1: Emissions Dispersion Study • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Outline of Simulations performed • Summary of findings • Conclusions

38. Summary of findings - 1 • Under high wind conditions, from SW and NNE directions, plume trajectory is sufficient to clear neighbour buildings with stack at original elevation. • SW (“summer”) wind direction is worse than NNE (“winter”) direction. • Under low wind conditions, raising stack by 3 m. should be adequate

39. Summary of Findings - 2 • Under still SW wind conditions, adjacent chiller discharge air curtains dominate flow pattern in vicinity of release. • BSX stack emission is entrained in complex flow pattern on roof, and dragged down to ground level. Flow reversal at low level spreads plume around side & rear of building. Raising stack 3m. alleviates, but does not eradicate, the problem.

40. Summer “still” wind flow patternOriginal Stack location

41. Run 21 : Summer; still windOriginal Stack location

42. Run 21 : Summer; still windOriginal Stack location

43. Project #1: Emissions Dispersion Study • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Outline of simulations performed • Summary of findings • Conclusions

44. Project #1: Dispersion Study Conclusions • Raising stack by 3m. would ensure adequate dispersion of plumes except under still, summer conditions. However, in mitigation, . . . . • Is the predicted flow reversal at low level in the adjacent road a realistic scenario, or would occasional vehicular traffic in road be sufficient to prevent occurrence ?

45. Project #1: Dispersion Study Conclusions • Such very tall stacks were not an acceptable option and so that (for the time being) was that.

46. Project # 2 Dispersion Problem Diagnosis & Remedy

47. Project # 2Dispersion Problem Diagnosis & Remedy • Problem Definition • Study Methodology • Benefits of using CFD • Description of CFD Model • Outline of Simulations performed • Synopsis of Results • Conclusions

48. Project # 2Problem Definition - 1 • The buildings of interest here are adjacent to the research facility building, whose proposed extension was the subject of the earlier emissions dispersion study. • The buildings, to be referred to as “Building B” and “Building P” are parallel to the road. • A linkage building forms an enclosed courtyard.

49. Overview of Site from South

50. Flow Geometry Close-up