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

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Presentation on theme: "10th PHOENICS User Conference Melbourne, Australia A CFD presentation by Dr. Paddy Phelps ( Flowsolve Ltd ] & Mr. John Gibson [ Scott Wilson Ltd ] May."— Presentation transcript:

1 10th PHOENICS User Conference Melbourne, 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 l Industrial Context l Objectives of Study l Benefits of using CFD l Description of CFD Model l Outline of simulations performed l Sample Results Obtained l Conclusions

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

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

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

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

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

9 Industrial Context l There is a history of local complaints about poor dispersion of unpleasant smells l The new buildings will create a local impediment to airflow, and may significantly alter local air flow patterns l 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 ? 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 ? 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 ? 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 l Industrial Context è Objectives of Study l Benefits of using CFD l Description of CFD Model l Outline of simulations performed l Sample Results Obtained l Conclusions

14 Objectives of Study - 1 l Use simulation tools to predict mixing of rooftop extract releases from the research building with the ambient airflow over the adjacent buildings l 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 ? What constitutes acceptable environmental impact ? l Plume core is diluted and dispersed to a safe level at nearby ä HVAC intakes ä Opening windows ä Public access areas

16 Criterion of Acceptability l A safe level is taken as a dilution level of 1:10 4 ( i.e. a concentration of 100 ppm ) from stack release l The plume core is the spatial envelope of this critical dilution / concentration

17 Overview of Release Conditions

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

20 Benefits of CFD Approach (1) l No scale-up problem l Three-dimensional, steady or transient l Interrogatable predictions l Handles effect of èblockages in domain èrecirculating flow èmultiple inlets and outlets èmultiple interacting releases

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

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

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

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

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

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

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

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

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

30 Boundary Conditions: Ambient Wind Specification [ 2 ] l Wind Speeds Still m/sStill m/s Low m/sLow m/s Medium m/sMedium m/s l Wind Profiles Pasquill D stability profilesPasquill D stability profiles

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

32 Boundary Condition Independent Region

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

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

35 Workscope for Stage 1 As Planned Building Geometry ä Model Construction & Testing ä Initial Scoping Studies Objective : Determine whether proposed arrangement leads to adequate dilution / dispersion of effluent releases under various wind conditions 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 outto 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 geometryto 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 & testingModel construction & testing Parametric study of various design parametersParametric study of various design parameters Objective : to investigate effectiveness of various designs of venturi stack, subject to given constraintsto 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 stackto investigate dispersion effectiveness of as planned post-refurbishment geometry with venturi stack

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

40 Stage 1 Simulations: Initial Scoping Studies As Planned Building GeometryAs Planned Building Geometry Twin Vertical Stack releasesTwin Vertical Stack releases Summer (18 0 C ambient, 24 0 C release);Summer (18 0 C ambient, 24 0 C release); Winter (5 0 C ambient, 20 0 C release)Winter (5 0 C ambient, 20 0 C release) Wind velocities 0.5, 2.5, 5 and 8 m/sWind velocities 0.5, 2.5, 5 and 8 m/s Various wind directionsVarious wind directions

41 Stage 1 Simulation Matrix

42 AS PROPOSED SCHEME; LOW WIND IN WINTER l Wind vector 0.5 m/s from NNE l Ambient temperature 5 0 C l Release temperature 21 0 C l Vertical release through twin stacks l Release velocity ~15 m/s l Chiller air curtain OFF Stage 1 Simulations: Predictions for Case 7

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

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47 PROPOSED SCHEME; HIGH WIND IN SUMMER l Wind vector 8.0 m/s from SW l Ambient temperature 18 0 C l Release temperature 24 0 C l Vertical release through twin stacks l Release velocity ~ 15 m/s l Chiller air curtain OFF Stage 1 Simulations: Predictions for Case 4

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

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52 l The imminent construction of the massive new buildings on the opposite side of the street will irrevocably change the ambient air flow patterns in the vicinity of the release site. l With new pedestrian precincts, moving vehicular traffic can no longer be relied upon to generate dilution mixing at low elevations l Pedestrians will be more sensitive to the impact of odour-tinged discharge plumes Stage 1 Simulations: Outcome of Predictions - 1

53 l The plume trajectories at higher wind speeds were flatter than their low-wind counterparts, and dispersed more rapidly. l At lower wind speeds the core envelope was more erect at the point of release; in the winter case, this was assisted by a contribution from buoyancy-driven uplift. Stage 1 Simulations: Outcome of Predictions - 2

54 l For the as proposed geometry & release configuration, predictions indicated that releases from stacks could be entrained into the recirculation regions in the lee of the downwind buildings. l Effluent concentrations in excess of recommended limit could thus occur at ground level and over building surfaces with openable windows. Stage 1 Simulations: Outcome of Predictions - 3

55 l Under low wind conditions in winter, the street level concentrations were above the 1:10 4 acceptable dilution criterion l Under high wind conditions in summer, the plume core just failed to clear roof and upper storey regions in adjacent downwind buildings Stage 1 Simulations: Outcome of Predictions - 4

56 Stage 1 Conclusions è Need to change release conditions and / or location to get acceptable dispersion at higher wind speeds in summer; low wind speeds in winter. How would dispersion patterns compare if no refurbishment was done ? How would dispersion patterns compare if no refurbishment was done ? How can dispersion patterns be improved during refurbishment ? How can dispersion patterns be improved during refurbishment ?

57 Stage 2 Simulations : Objective The Masterly Inactivity Option: è How would the original discharge concept have coped, given the large- scale buildings being constructed across the Street ?

58 Stage 2 Simulations : Geometry Representation Modifications è Removal of third floor and new plant room è Substitution of original plant room on second-floor roof è Removal of vertical discharge stacks è Discharge from basement extract arranged through louvred sides of plant room.

59 Masterly inactivity: as was facility geometry

60 AS WAS GEOMETRY; LOW WIND IN WINTER l Wind vector 0.5 m/s from NNE l Ambient temperature 5 0 C l Release temperature 21 0 C l Horizontal release through plant room louvred sides l Release velocity ~0.4 m/s l Chiller air curtain OFF Stage 2 Simulations: Predictions for Case 21

61 Stage 2 As Was Results : Winter wind vector 0.5 m/s from NNE

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65 AS WAS GEOMETRY; HIGH WIND IN SUMMER l Wind vector 8.0 m/s from SW l Ambient temperature 18 0 C l Release temperature 24 0 C l Horizontal release through plant room louvred sides l Release velocity ~ 0.4 m/s l Chiller air curtain OFF Stage 2 Simulations: Predictions for Case 22

66 Stage 2 As Was Results : Summer wind vector 8 m/s from SW

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71 Stage 2 Conclusions - 1 è These stage 2 predictions confirm that the dispersion plume consequences would have been unacceptable if the discharge arrangements at the research building had remained unaltered once the major building works are completed opposite...

72 End of Stage 2 Status [ 1 ] è The modified discharge arrangements, proposed for the refurbishment of the building, result in considerable improvements in dilution dispersion. è Under still Winter conditions, street-level concentrations would be reduced by two orders of magnitude.

73 End of Stage 2 Status [ 2 ] è Despite the improvements, concentration levels at ground elevation would still not be acceptable under worst case wind conditions è Dilution of effluent at point of release would ameliorate this situation è A venturi stack arrangement might provide a low cost solution....

74 Stage 3 Simulations : Background è è A venturi-type stack discharge concept has no moving parts è è It uses the momentum of the discharge jet to induce dilution mixing with the surrounding (free) ambient air flow

75 Stage 3 Simulations : Objective è è What dilution level might be achieved using a venturi-type stack discharge concept in the present instance?

76 Stage 3 Simulations : Venturi Stack design parameters è Discharge pipe diameter è Discharge nozzle diameter è Venturi sheath bottom diameter è Venturi sheath top diameter è Venturi sheath length è Internal baffles ?

77 Stage 3 Simulations: Venturi Stack Design Parameters

78 Stage 3 Simulations : Venturi Stack design constraints è Total height not to exceed 3 m. è Discharge velocity not to exceed 15 m/s è Venturi sheath diameter maximum 1.2 m. è Fixed (high) effluent flowrate è Single Venturi sheath for each pipe ( max. two on roof ) ( max. two on roof )

79 Stage 3 Simulations: Outline of Simulations 2-D stand-alone venturi stack model 2-D (small sector) model from symmetry considerations2-D (small sector) model from symmetry considerations BFC grid configurationBFC grid configuration Influence of various design parameters investigated, subject to stated constraintsInfluence of various design parameters investigated, subject to stated constraints 36 different arrangements compared36 different arrangements compared

80 Stage 3 Simulations: Stack parameters & entrainment rates

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82 TYPICAL STRAIGHT SHEATH DESIGN l Supply pipe diameter m. l Supply nozzle diameter 0.49 m. l Sheath entry diameter 1.20 m. l Sheath exit diameter 1.20 m. l Venturi Sheath length 3.00 m. l Sheath internal baffles ? NO Stage 3 Predictions: Results for Stack Design # 8

83 Stage 3 Simulations: Venturi Stack Design #8

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85 TYPICAL DIFFUSER SHEATH DESIGN l Supply pipe diameter 0.60 m. l Supply nozzle diameter 0.36 m. l Sheath entry diameter 1.20 m. l Sheath exit diameter 1.60 m. l Venturi Sheath length 3.00 m. l Sheath internal baffles ? NO Stage 3 Predictions: Results for Stack Design # 28

86 Stage 3 Simulations: Venturi Stack Design #28

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88 TWIN-BAFFLE STRAIGHT SHEATH DESIGN l Supply pipe diameter 0.60 m. l Supply nozzle diameter 0.35 m. l Sheath entry diameter 1.20 m. l Sheath exit diameter 1.20 m. l Venturi Sheath length 3.00 m. l Sheath internal baffles ? YES Stage 3 Predictions: Results for Stack Design # 36

89 Stage 3 Simulations: Venturi Stack Design #36

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91 Stage 3 Conclusions - 1 è The results obtained from using a venturi stack device represent a balance between the discharge velocity, the entrainment rate and the degree of mixing possible in the sheath è The constraints on the installation mean that effluent concentration levels at the point of discharge can only be reduced to around 40%.

92 Stage 3 Conclusions - 2 è Predictions indicate that an entrainment rate of 1.5 ( i.e. stack outflow rate of 2.5 times discharge rate, making mean outlet concentration 40% ) combined with a mean outlet velocity of around 12 m/s is a realistically attainable design target.

93 Stage 4 Simulations Objective To determine the extent of improvement in ground-level footprint which would be obtained by fitting a typical venturi extract device to the discharge stack from the BSU To determine the extent of improvement in ground-level footprint which would be obtained by fitting a typical venturi extract device to the discharge stack from the BSU

94 CASE 25 l Wind vector 0.5 m/s from NNE l Ambient temperature 5 0 C l Release temperature 21 0 C l Chiller air curtain ON Effect of Venturi Extract Stack : Consequence Comparisons - 1

95 Stage 4 As Planned + Venturi Discharge Stack : Winter wind vector 0.5 m/s from NNE

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99 CASE 24 l Wind vector 8.0 m/s from SW l Ambient temperature 18 0 C l Release temperature 24 0 C l Chiller air curtain OFF Effect of Venturi Extract Stack : Consequence Comparisons - 2

100 Stage 4 As Planned + Venturi Discharge Stack : Summer wind vector 8.0 m/s from SW

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105 Stage 3 Conclusions - 1 Influence of Venturi Stack è Using a venturi stack device with the as planned geometry, the plume core clears adjacent buildings and has a safe ground level footprint, for worst case winter wind vector. è For the summer high wind vector condition, the plume core just brushes downstream rooftops but has safe ground level footprint

106 Stage 3 Conclusions - 2 Influence of Chiller Air Curtain l Horizontal discharge provided by chiller air outlet will have a small a positive influence on keeping plume clear. l Bonus effects : entrainment dilution of release into upward- moving air curtainentrainment dilution of release into upward- moving air curtain buoyant uplift, due to mixing of release with warm chiller exhaust flowbuoyant uplift, due to mixing of release with warm chiller exhaust flow

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

108 Study Conclusions - 1 è Construction of large new structures adjacent to research facility will adversely affect airborne plume dispersion patterns. è The worst case ambient conditions were found to be: Still wind conditions (0.5 m/s NNE) in winterStill wind conditions (0.5 m/s NNE) in winter High wind conditions (8.0 m/s SW) in summerHigh wind conditions (8.0 m/s SW) in summer

109 Study Conclusions - 2 è If a venturi-type dilution discharge stack were to be fitted, the plume core would clear regions with opening windows, and give acceptable ground level concentration footprints

110 Study Conclusions - 3 The environmental impact of the laboratory stack releases is a substantial improvement over the existing arrangements, and effluent levels meet exposure criteria which have been accepted at similar research establishments.

111 Points of Contact for further information è Flowsolve Ltd Dr. Paddy Phelps 130 Arthur Rd. Wimbledon Park SW19 8AA è Scott Wilson Mr. John Gibson 127 Friars House Blackfriars Rd SE1 8EZ


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