PHOENICS User Group Meeting

1
PHOENICS User Group Meeting

• Benelux User Group
• Aristo Centre
• Eindhoven
• Netherlands

May 2005
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Modelling Discharges from Rooftop Stacks in
Confined Environments

• A CFD presentation
• by
• Dr. Paddy Phelps
• ( Flowsolve Ltd )

3
Eindhoven 2005Outline of Presentation

• Presentation encompasses the results of three
  different projects, performed over a period of
  time
• Each project used CFD to satisfy a different
  objective

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PROJECT 1

• 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

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PROJECT 2

• 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

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PROJECT 3

• A Compare and Contrast Project
• Comparing possible extract strategies, for a
  situation where tall stacks cannot be used for
  aesthetic / planning reasons

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PROJECT 1

• PREDICTING THE
• DISPERSION CONSEQUENCES
• OF FUME RELEASES
• FROM BUILDING ROOF-TOP STACKS

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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

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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.

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Emissions Dispersion Study Industrial Context

• It is planned to construct a large extension to
  the existing research building, effectively
  doubling its size

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Flow Geometry Close-up
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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 .

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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

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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

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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 ?

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Flow Geometry Close-up
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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

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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

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Emissions Dispersion Study Objectives

• Model predictions will provide input to the
  design of discharge arrangements which will lead
  to acceptable environmental impact

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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

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Criterion of Acceptability

• A safe level is taken in this instance to be 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
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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

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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

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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

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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

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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)

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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)

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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

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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

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Overview of Release Conditions
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Internal SourcesRooftop 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

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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

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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

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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

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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

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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

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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

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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.

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Summer still wind flow patternOriginal Stack
location
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Run 21 Summer still windOriginal Stack
location
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Run 21 Summer still windOriginal Stack
location
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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

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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 ?

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Project 1 Dispersion Study Conclusions

• Such very tall stacks were not an acceptable
  option
• and so that
• (for the time being)
• was that.

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Project 2

• Dispersion Problem Diagnosis Remedy

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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

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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.

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Overview of Site from South
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Flow Geometry Close-up
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Project 2Problem Definition - 2

• Under adverse ambient conditions, traces of
  malodorous releases from laboratories in the
  Buildings B and / or P are apparently
  detectable at the upper floors on the courtyard
  side of the building linking the two . . . . .

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Project 2Problem Definition - 3

• Are the odorous releases originating from
  Building B, or Building P, or both ?
• Do they indicate that there are hazardous
  dispersion consequences from these roof-top
  releases ?
• If a hazard to the public exists, what is the
  extent, and how may it be eradicated ?

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Project 2Dispersion Problem Diagnosis
Remedy

• Problem Definition
• Study Methodology
• Benefits of using CFD
• Description of CFD Model
• Outline of Simulations Performed
• Synopsis of Results Obtained
• Conclusions

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Project 2Study Methodology

• Use CFD simulations to predict plume trajectories
  issuing from rooftop extract release points on
  Buildings B P
• Identify offending source(s)
• Provide input to design of modified fume extract
  arrangements, to reduce impact by reducing
  effluent concentration at source

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Project 2Dispersion Problem Diagnosis
Remedy

• Problem Definition
• Study Methodology
• Benefits of using CFD
• Description of CFD Model
• Outline of Simulations Performed
• Synopsis of Results Obtained
• Conclusions

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Project 2Benefits of using CFD

• Using concentration marker variables in CFD
  simulations allows identification of individual
  contributions to effluent concentrations at
  particular locations, as well as cumulative
  effects
• Sources can be activated singly or together

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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

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Project 23-D Plume Investigation Model

• Domain size - 250m by 400m by 60m .
• Typical nodalisation level - 350,000
• Target area is windows at upper level of link
  building, at 14.9m ASL
• Problem will be worst in Winter , when wind is
  directed from sources to target. Summer
  prevailing wind is in opposite direction

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Project 2Release Site Details

• Building B Extract Arrangement
• Vertical release from stack pipe
• Release temperature - 24 deg.C
• Release velocity - 10 m/s
• Building P Extract Arrangement
• Horizontal release ( N, S, E, W ) from capped
  vertical pipe
• Release temperature - 24 deg.C
• Release velocity - 3 m/s

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Project 2Dispersion Problem Diagnosis
Remedy

• Problem Definition
• Study Methodology
• Benefits of using CFD
• Description of CFD Model
• Outline of simulations performed
• Synopsis of Results Obtained
• Conclusions

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Project 2Outline of Simulations Performed

• Simulations performed in 3 stages
• Stage 1 - As is release concept both releases
  active still and low wind velocities
  summer and winter conditions
• Stage 2 - Effect of single release, either from
  Building B or Building P
• Stage 3 - Effect of revisions to release
  arrangements

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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

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Project 2Findings of Stage 1 Study

• Concentrations of effluent from releases are in
  excess of target level (lt 100 ppm) at courtyard
  walls
• Winter conditions worse than summer
• Low wind conditions worse than still wind
  conditions
• This contrasts with the releases from the
  adjacent facility, which were worst under still,
  summer conditions)

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Maximum Concentration of effluent at South wall
and at any wall in courtyard
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Winter , Low WindBoth ReleasesEffluent
Spread Elevation 14.6m. asl
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Project 2Objectives of Stage 2 Study

• Thus far, have addressed simultaneous
• as is emissions releases from the stacks
• on Buildings B and P.
• Now need to know
• What is the contribution of each to the effluent
  levels in the courtyard ?

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Stage 2 simulationsSummer, 0.5 m/s windPeak
effluent levels at courtyard walls
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Stage 2 simulationsWinter, 2.5 m/s wind Peak
effluent levels at courtyard walls
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Project 2Conclusions of Stage 2 Study

• The effluent at the south face of the courtyard,
  where the problem has been reported, originates
  predominantly from the releases from Building P
  roof rather than from Building B roof.
• More effective venting arrangements are required
  to remove the nuisance / hazard

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Project 2Objectives of Stage 3 Study

• Replace present horizontal outlet for Building
  P emissions by vertical discharge from stack at
  same elevation
• Investigate possibility of reducing emissions
  from Building B by modifying stack discharge
  arrangements

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Project 2 Stack modification options for
Building B - 1

• Building B discharge stack is already at
  maximum permissible elevation
• Try venturi sheath approach to dilute the
  effluent discharge with fresh ambient air prior
  to discharge
• Implementation would not require major
  modification to existing arrangement

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Project 2Venturi mixer design concept

• 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

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Project 2 Venturi Stack design parameters

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

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Principle of Venturi Discharge Stack
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Venturi Stack Dispersion Results for Typical
Discharge Stack Geometry
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Stage 3 Simulations Venturi Stack design
constraints

• Total height not to exceed given limit
• Discharge velocity not to exceed 15 m/s
• Venturi sheath diameter limited by space
• Fixed (high) effluent flowrate
• Single Venturi sheath needed for each pipe

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Stage 3 Simulations Venturi Stack design
constraints

• 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 by a limited amount.

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Project 2 Stack modification options for
Building P

• Building P stack currently a 4-way horizontal
  discharge
• Exposed and highly visible location rules out an
  elevated venturi sheath approach
• Seek some improvement by adopting a vertical
  discharge arrangement

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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

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Project 2Conclusions of Project - 1

• Emissions from the roof stacks of the two
  buildings can build up to significant levels in
  the courtyard between them
• Build-up occurs over a variety of ambient
  conditions, being worse in winter and at higher
  wind speeds
• Re-ingestion to the buildings could occur through
  windows which open onto the courtyard

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Project 2Conclusions of Project - 2

• Changing to a vertical release for the Building
  P releases, coupled with use of a venturi stack
  for the Building B releases, can reduce
  courtyard concentrations to acceptable levels
  under summer conditions.
• This is the highest risk period when windows onto
  the courtyard are likely to be open for
  ventilation purposes

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Project 2Conclusions of Project - 3

• Concentrations of effluent from roof stack
  releases remain in excess of the recommended
  limit (100 ppm) in the courtyard under winter
  conditions.
• In mitigation, windows giving onto the courtyard
  are then less likely to be open

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Project 2Conclusions of Project - 4

• Comparatively simple modifications to roof stack
  release arrangements can still result in
  significant reductions in effluent levels in the
  courtyard under winter conditions.

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Project 2Conclusions of Project - 5

• HOWEVER
• If it were feasible to raise the stack on
  Building P a little (say 2 m.) above its
  present elevation, it might be possible to
  fit a short venturi nozzle.
• In which case . . . . .

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Winter, 2.5 m/s wind extended stack venturi
Peak effluent levels at courtyard walls
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Project 2Conclusions of Project - 6

• IN SHORT . . .
• Effluent release concentrations at the upper
  courtyard levels could be dropped to acceptable
  levels of around 1104 dilution in winter
• BUT ONE WAS NOT ALLOWED
• TO RAISE THE STACK
• BY THE NECESSARY 2 METRES . . .

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Project 2Conclusions of Project - 7

• AND SO THERE
• FOR THE PRESENT
• THE MATTER RESTED

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Project 3

• A
• COMPARE AND CONTRAST
• PROJECT

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Project 3Compare Contrast Discharge
Strategies

• Problem Definition
• Study Methodology
• Benefits of using CFD
• Description of CFD Model
• Summary of Results Obtained
• Conclusions

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Project 3Problem Definition - 1

• The two earlier projects had a common thread
• Acceptable dilution dispersion of releases could
  be achieved under worst case ambient conditions
  only when the stack height was raised beyond an
  acceptable level .

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Project 3Problem Definition - 2

• Some alleviation could be achieved by dilution at
  point of release using a venturi device to
  entrain surrounding air
• Unfortunately the ability of this concept to make
  real savings is limited by design constraints,
  most usually
• height impairs mixing efficiency
• nozzle velocity noise limits entrainment

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Project 3Problem Definition - 3

• Previous Flowsolve attention to venturi stacks
  has been conditioned by simplicity
• free entrained air for dilution
• no moving parts
• but as we have seen, it has its limitations.
• Meanwhile problems go unresolved

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Project 3Problem Definition - 4

• Now we have a possible solution
• The Tri-Fan stack from
• STROBIC AIR CORPORATION

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Project 3Problem Definition - 5

• This device combines
• a mixing plenum, through which ambient air is
  drawn by a fan to dilute the fume stream
• two venturi nozzles, through which the mixture is
  then forced
• these 2 jets are then mixed, to induce
  entrainment of external air
• two separate entrainment zones

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Overview of Strobic Tri-Stack
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Overview of Strobic Tri-Stack
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Project 3Compare Contrast Discharge
Strategies

• Problem Definition
• Study Methodology
• Benefits of using CFD
• Description of CFD Model
• Summary of Results Obtained
• Conclusions

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Project 2Study Methodology - 1

• Use CFD simulations to predict plume trajectories
  issuing from rooftop extract release points on
  the original research building of Project 1
• Compare and contrast the dispersion efficiency,
  under identical adverse weather conditions, of
  the Tri-Stack arrangement, and two vertical stack
  arrangements

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Project 2Study Methodology - 2

• The Contenders
• Strobic Tri-Stack
• Discharges at 21.75 m elevation
• Original Throttled Stack
• Discharges at 22.9 m elevation
• Raised Throttled Stack
• Discharges at 25.9 m elevation

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Project 3Compare Contrast Discharge
Strategies

• Problem Definition
• Study Methodology
• Reference Case Definition
• Description of CFD Model
• Summary of Results Obtained
• Conclusions

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Project 2Reference Case Definition - 1

• Dispersion of Basement Extract fumes from stacks
  located at front of research facility
• Worst case wind vector scenario from earlier
  study, to give tough case for comparison
• Downwind plume trajectory passes directly over
  adjacent buildings

102
Project 2Reference case Definition - 2

• Ambient Wind Vector Specification
• Still wind in summer
• Wind speed 0.5 m/s
• Wind direction - from release directly over
  Buildings B and P downwind
• Air temperature 28 0C

103
Project 2Reference case Definition - 3

• Release Scenario
• 6 m3/s effluent gas entering each stack via a 1m
  diameter duct
• Vertical straight stacks throttled to give 15
  m/s exit velocity
• Release temperature 24 0C

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Project 2Reference case Definition - 4

• Tri-Stack Operating Data
• 6 m3/s effluent gas inlet feed
• 6.75 m3/s plenum air feed
• 15.69 m3/s entrained ambient air
• 29.39 m/s nozzle velocity
• Release temperature 24 0C

105
Project 3Compare Contrast Discharge
Strategies

• Problem Definition
• Study Methodology
• Reference Case Description
• Description of CFD Model
• Summary of Results Obtained
• Conclusions

106
Project 3CFD Model Details

• 3-D PLUME DISPERSION MODEL
• As described in earlier study (Project 1)
• Solution domain 400 x 300 x 120 m high
• Tri-Stack represented as discrete distributed
  sources internal details of plenum, fan and
  entrainment cap not solved for

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Tri-Stack Discharge Arrangement
108
Original Stack Discharge Arrangement
109
Original Stack Discharge Arrangement
110
Raised Stack Discharge Arrangement
111
Raised Stack Discharge Arrangement
112
Project 3Compare Contrast Discharge
Strategies

• Problem Definition
• Study Methodology
• Reference Case Description
• Description of CFD Model
• Summary of Results Obtained
• Conclusions

113
Discharge Strategy Comparison

• Comparison of 100ppm
• plume core envelopes

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100 ppm core envelopeOriginal Stack
115
100 ppm core envelopeRaised Stack
116
100 ppm core envelopeStrobic Tri-Stack
117
100 ppm core envelopeStrobic Tri-Stack
118
Discharge Strategy Comparison

• Comparison of effluent
• concentrations on adjacent surfaces

119
Effluent concentrations on surrounding
structures Original Stack
120
Effluent concentrations on surrounding
structures Raised Stack
121
Effluent concentrations on surrounding
structuresStrobic Tri-Stack
122
Discharge Strategy Comparison

• Comparison of plume
• cross-sections at vertical slices through release
  plane

123
Comparison of 1000ppm plume sections
124
Comparison of 5000ppm plume sections
125
Comparison of 20,000ppm plume sections
126
Discharge Strategy Comparison

• Comparison of plume
• cross-sections at horizontal slices at various
  elevations

127
1000 ppm contour sections at elevation 27m
128
1000 ppm contour sections at elevation 46m
129
1000 ppm contour sections at elevation 61m
130
Project 3Compare Contrast Discharge
Strategies

• Problem Definition
• Study Methodology
• Reference Case Description
• Description of CFD Model
• Summary of Results Obtained
• Conclusions

131
Project 3Conclusions of Project - 1

• The forced induction and additional dilution of
  the effluent at the point of release, which are
  afforded by the Tri-Stack device, give rise to
  clearly better dilution dispersion under the
  extreme conditions of the test case than can be
  obtained from the original or 3m-extended
  ordinary stacks

132
Project 3Conclusions of Project - 2

• The Tri-Stack would appear to offer great
  advantages over throttled or natural
  venturi-enhanced systems when stack heights are
  limited by planning or aesthetic constraints

133
Lest we forget ..

• Our thanks to
• David Glynn Flowsolve
• John Gibson Scott Wilson
• Phil Milne-Smith Critical Airflow Controls
• Paul Tetley Strobic Air Corporation
• and not forgetting
• the University Authorities

134
Thank you for your attention

• Thats
• all
• for
• now
• ...