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.

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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
  1104 ( i.e. a concentration of 100 ppm ) from
  stack release
• The plume core is the spatial envelope of this
  critical dilution / concentration

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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

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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

• 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

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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(r j) /Dt div (r Uj - Gj gradj ) Sj
  
• Terms represent transience, convection, diffusion
  and sources respectively
• Equation is cast into finite volume form by
  integrating it over the volume of each cell

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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

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Boundary ConditionsAmbient Wind Specification
1

• Summer
• Ambient temperature - 18 deg.C
• Wind from SW
• Winter
• Ambient temperature - 5 deg.C
• Wind from NNE

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Boundary ConditionsAmbient 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

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Internal SourcesRooftop 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

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Boundary Condition Independent Region
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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

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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

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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

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

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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
51
Stage 1 As Proposed Results Summer wind
vector 8 m/s from SW
52

Stage 1 Simulations Outcome of Predictions - 1

• 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.
• With new pedestrian precincts, moving vehicular
  traffic can no longer be relied upon to generate
  dilution mixing at low elevations
• Pedestrians will be more sensitive to the impact
  of odour-tinged discharge plumes

53

Stage 1 Simulations Outcome of Predictions - 2

• The plume trajectories at higher wind speeds were
  flatter than their low-wind counterparts, and
  dispersed more rapidly.
• 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.

54

Stage 1 Simulations Outcome of Predictions - 3

• 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 .
• Effluent concentrations in excess of recommended
  limit could thus occur at ground level and over
  building surfaces with openable windows.

55

Stage 1 Simulations Outcome of Predictions - 4

• Under low wind conditions in winter, the street
  level concentrations were above the 1104
  acceptable dilution criterion
• Under high wind conditions in summer, the plume
  core just failed to clear roof and upper storey
  regions in adjacent downwind buildings

56
Stage 1 Conclusions

• 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 can dispersion patterns be improved during
  refurbishment ?

57
Stage 2 Simulations Objective

• The Masterly Inactivity Option
• 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

• Removal of third floor and new plant room
• Substitution of original plant room on
  second-floor roof
• Removal of vertical discharge stacks
• Discharge from basement extract arranged through
  louvred sides of plant room.

59
Masterly inactivity as was facility geometry
60

Stage 2 Simulations Predictions for Case 21

• AS WAS GEOMETRY LOW WIND IN WINTER
• Wind vector 0.5 m/s from NNE
• Ambient temperature 50C
• Release temperature 210C
• Horizontal release through plant room louvred
  sides
• Release velocity 0.4 m/s
• Chiller air curtain OFF

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Stage 2 As Was Results Winter wind vector
0.5 m/s from NNE
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Stage 2 As Was Results Winter wind vector
0.5 m/s from NNE
63
Stage 2 As Was Results Winter wind vector
0.5 m/s from NNE
64
Stage 2 As Was Results Winter wind vector
0.5 m/s from NNE
65

Stage 2 Simulations Predictions for Case 22

• AS WAS GEOMETRY HIGH WIND IN SUMMER
• Wind vector 8.0 m/s from SW
• Ambient temperature 180C
• Release temperature 240C
• Horizontal release through plant room louvred
  sides
• Release velocity 0.4 m/s
• Chiller air curtain OFF

66
Stage 2 As Was Results Summer wind vector
8 m/s from SW
67
Stage 2 As Was Results Summer wind vector
8 m/s from SW
68
Stage 2 As Was Results Summer wind vector
8 m/s from SW
69
Stage 2 As Was Results Summer wind vector
8 m/s from SW
70
Stage 2 As Was Results Summer wind vector
8 m/s from SW
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Stage 2 Conclusions - 1

• 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

• The modified discharge arrangements, proposed
  for the refurbishment of the building, result in
  considerable improvements in dilution
  dispersion.
• Under still Winter conditions, street-level
  concentrations would be reduced by two orders of
  magnitude.

73
End of Stage 2 Status 2

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

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Stage 3 Simulations Background

• A venturi-type stack discharge concept has no
  moving parts
• It uses the momentum of the discharge jet to
  induce dilution mixing with the surrounding
  (free) ambient air flow

75
Stage 3 Simulations Objective

• 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

• Discharge pipe diameter
• Discharge nozzle diameter
• Venturi sheath bottom diameter
• Venturi sheath top diameter
• Venturi sheath length
• Internal baffles ?

77
Stage 3 SimulationsVenturi Stack Design
Parameters
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Stage 3 Simulations Venturi Stack design
constraints

• Total height not to exceed 3 m.
• Discharge velocity not to exceed 15 m/s
• Venturi sheath diameter maximum 1.2 m.
• Fixed (high) effluent flowrate
• Single Venturi sheath for each pipe
• ( max. two on roof )

79
Stage 3 SimulationsOutline of Simulations

• 2-D stand-alone venturi stack model
• 2-D (small sector) model from symmetry
  considerations
• BFC grid configuration
• Influence of various design parameters
  investigated, subject to stated constraints
• 36 different arrangements compared

80
Stage 3 SimulationsStack parameters
entrainment rates
81
Stage 3 SimulationsStack parameters
entrainment rates
82

Stage 3 Predictions Results for Stack Design 8

• TYPICAL STRAIGHT SHEATH DESIGN
• Supply pipe diameter 0.8 0 m.
• Supply nozzle diameter 0.49 m.
• Sheath entry diameter 1.20 m.
• Sheath exit diameter 1.20 m.
• Venturi Sheath length 3.00 m.
• Sheath internal baffles ? NO

83
Stage 3 SimulationsVenturi Stack Design 8
84
Stage 3 SimulationsVenturi Stack Design 8
85

Stage 3 Predictions Results for Stack Design 28

• TYPICAL DIFFUSER SHEATH DESIGN
• Supply pipe diameter 0.60 m.
• Supply nozzle diameter 0.36 m.
• Sheath entry diameter 1.20 m.
• Sheath exit diameter 1.60 m.
• Venturi Sheath length 3.00 m.
• Sheath internal baffles ? NO

86
Stage 3 SimulationsVenturi Stack Design 28
87
Stage 3 SimulationsVenturi Stack Design 28
88

Stage 3 Predictions Results for Stack Design 36

• TWIN-BAFFLE STRAIGHT SHEATH DESIGN
• Supply pipe diameter 0.60 m.
• Supply nozzle diameter 0.35 m.
• Sheath entry diameter 1.20 m.
• Sheath exit diameter 1.20 m.
• Venturi Sheath length 3.00 m.
• Sheath internal baffles ? YES

89
Stage 3 SimulationsVenturi Stack Design 36
90
Stage 3 SimulationsVenturi Stack Design 36
91
Stage 3 Conclusions - 1

• 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
• 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

• 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.

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Stage 4 SimulationsObjective

• 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

Effect of Venturi Extract Stack Consequence
Comparisons - 1

• CASE 25
• Wind vector 0.5 m/s from NNE
• Ambient temperature 50C
• Release temperature 210C
• Chiller air curtain ON

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Stage 4 As Planned Venturi Discharge Stack
Winter wind vector 0.5 m/s from NNE
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Stage 4 As Planned Venturi Discharge Stack
Winter wind vector 0.5 m/s from NNE
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Stage 4 As Planned Venturi Discharge Stack
Winter wind vector 0.5 m/s from NNE
98
Stage 4 As Planned Venturi Discharge Stack
Winter wind vector 0.5 m/s from NNE
99

Effect of Venturi Extract Stack Consequence
Comparisons - 2

• CASE 24
• Wind vector 8.0 m/s from SW
• Ambient temperature 180C
• Release temperature 240C
• Chiller air curtain OFF

100
Stage 4 As Planned Venturi Discharge Stack
Summer wind vector 8.0 m/s from SW
101
Stage 4 As Planned Venturi Discharge Stack
Summer wind vector 8.0 m/s from SW
102
Stage 4 As Planned Venturi Discharge Stack
Summer wind vector 8.0 m/s from SW
103
Stage 4 As Planned Venturi Discharge Stack
Summer wind vector 8.0 m/s from SW
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Stage 4 As Planned Venturi Discharge Stack
Summer wind vector 8.0 m/s from SW
105
Stage 3 Conclusions - 1Influence of Venturi
Stack

• 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.
• For the summer high wind vector condition, the
  plume core just brushes downstream rooftops but
  has safe ground level footprint

106
Stage 3 Conclusions - 2Influence of Chiller
Air Curtain

• Horizontal discharge provided by chiller air
  outlet will have a small a positive influence on
  keeping plume clear.
• Bonus effects
• entrainment dilution of release into
  upward-moving air curtain
• buoyant uplift, due to mixing of release with
  warm chiller exhaust flow

107
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 of Study

108
Study Conclusions - 1

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

109
Study Conclusions - 2

• 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 Contactfor further information

• Flowsolve Ltd
• Dr. Paddy Phelps
• 130 Arthur Rd.
• Wimbledon Park
• SW19 8AA
• 0208 944 0940
• cfd_at_flowsolve.com

• Scott Wilson
• Mr. John Gibson
• 127 Friars House
• 157-168 Blackfriars Rd
• SE1 8EZ
• 0207 401 3933
• john.gibson_at_scottwilson.com