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

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Presentation on theme: "PHOENICS User Group Meeting Benelux User Group Aristo Centre Eindhoven Netherlands Netherlands May 2005."— Presentation transcript:

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

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

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

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

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

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

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

8 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

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

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

14 Emissions Dispersion Study : Industrial Context l The building extension will create a significant additional impediment to the local ambient airflow l 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 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

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

19 Emissions Dispersion Study Objectives l 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” ? l The Plume Core is diluted and dispersed to a safe level at nearby ä HVAC intakes ä Opening windows ä Public access areas

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

22 Overview of Release Conditions

23 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

24 Benefits of CFD Approach (1) 4No scale-up problem 4Three-dimensional, steady or transient 4Interrogatable predictions 4Handles effect of èblockages in domain èrecirculating flow èmultiple inlets and outlets èmultiple interacting sources

25 Project #1: Emissions Dispersion Study 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

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

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

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

29 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

30 Roof-top Release Sites è Extract Stacks from Basement BSU 2 off2 off l Air Chiller Unit discharges 12 off12 off l Laboratory Extract Stacks 8 off8 off l Laboratory Discharge Stacks 3 off3 off

31 Overview of Release Conditions

32 Internal Sources: Rooftop Release Specification Basement Laboratory Extract Stacks Two Vertical stacksTwo Vertical 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

33 Project #1: Emissions Dispersion Study 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 Environment Parameters Studied l Domain extent extended down-wind domainextended down-wind domain l Ambient Wind summer and winter wind directionssummer and winter wind directions summer and winter temperature effectssummer and winter temperature effects l Adjacent buildings Influence of layout and topologyInfluence of layout and topology l Environmental Factors Tree canopy height, layout and resistanceTree canopy height, layout and resistance

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

36 Ambient Parameters Studied l Summer Ambient temperature - 24 deg.CAmbient temperature - 24 deg.C Wind from SWWind from SW l Winter Ambient temperature - 5 deg.CAmbient temperature - 5 deg.C Wind from NNEWind from NNE l Wind Speed (Pasquill D stability profile) High m/sHigh m/s Low m/sLow m/s Still m/sStill m/s

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

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

39 Summary of Findings - 2 l Under still SW wind conditions, adjacent chiller discharge air curtains dominate flow pattern in vicinity of release. l 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 pattern Original Stack location

41 Run 21 : Summer; still wind Original Stack location

42

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

44 Project #1: Dispersion Study Conclusions è 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 ?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 è 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 # 2 Dispersion Problem Diagnosis & Remedy è Problem Definition l Study Methodology l Benefits of using CFD l Description of CFD Model l Outline of Simulations performed l Synopsis of Results l Conclusions

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

49 Overview of Site from South

50 Flow Geometry Close-up

51 Project # 2 Problem Definition - 2 l 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.....

52 Project # 2 Problem Definition - 3 l Are the odorous releases originating from “Building B”, or “Building P”, or both ? l Do they indicate that there are hazardous dispersion consequences from these roof- top releases ? l If a hazard to the public exists, what is the extent, and how may it be eradicated ?

53 Project # 2 Dispersion Problem Diagnosis & Remedy l Problem Definition è Study Methodology l Benefits of using CFD l Description of CFD Model l Outline of Simulations Performed l Synopsis of Results Obtained l Conclusions

54 Project # 2 Study Methodology l Use CFD simulations to predict plume trajectories issuing from rooftop extract release points on Buildings “B” & “P” l Identify offending source(s) l Provide input to design of modified fume extract arrangements, to reduce impact by reducing effluent concentration at source

55 Project # 2 Dispersion Problem Diagnosis & Remedy l Problem Definition l Study Methodology è Benefits of using CFD l Description of CFD Model l Outline of Simulations Performed l Synopsis of Results Obtained l Conclusions

56 Project # 2 Benefits of using CFD l Using concentration marker variables in CFD simulations allows identification of individual contributions to effluent concentrations at particular locations, as well as cumulative effects l Sources can be activated singly or together

57 Project # 2 Dispersion Problem Diagnosis & Remedy l Problem Definition l Study Methodology l Benefits of using CFD è Description of CFD Model l Outline of simulations performed l Synopsis of Results l Conclusions

58 Project # 2 3-D Plume Investigation Model l Domain size - 250m by 400m by 60m. l Typical nodalisation level - 350,000 l Target area is windows at upper level of link building, at 14.9m ASL l Problem will be worst in Winter, when wind is directed from sources to target. Summer prevailing wind is in opposite direction

59 Project # 2 Release Site Details è “Building B” Extract Arrangement Vertical release from stack pipeVertical release from stack pipe Release temperature - 24 deg.CRelease temperature - 24 deg.C Release velocity - 10 m/sRelease velocity - 10 m/s è “Building P” Extract Arrangement Horizontal release ( N, S, E, W ) from capped vertical pipeHorizontal release ( N, S, E, W ) from capped vertical pipe Release temperature - 24 deg.C Release temperature - 24 deg.C Release velocity - 3 m/sRelease velocity - 3 m/s

60 Project # 2 Dispersion Problem Diagnosis & Remedy l Problem Definition l Study Methodology l Benefits of using CFD l Description of CFD Model è Outline of simulations performed l Synopsis of Results Obtained l Conclusions

61 Project # 2 Outline of Simulations Performed Simulations performed in 3 “stages” Simulations performed in 3 “stages” Stage 1 - “As is” release concept; both releases active; “still” and “low” wind velocities; summer and winter conditionsStage 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 2 - Effect of single release, either from “Building B” or “Building P” Stage 3 - Effect of revisions to release arrangementsStage 3 - Effect of revisions to release arrangements

62 Project # 2 Dispersion Problem Diagnosis & Remedy l Problem Definition l Study Methodology l Benefits of using CFD l Description of CFD Model l Outline of Simulations Performed è Synopsis of Results l Conclusions

63 Project # 2 Findings of Stage 1 Study è Concentrations of effluent from releases are in excess of target level (< 100 ppm) at courtyard walls è Winter conditions worse than summer è “Low” wind conditions worse than “still” wind conditions è This contrasts with the releases from the adjacent facility, which were worst under “still, summer” conditions)

64 Maximum Concentration of effluent at South wall and at any wall in courtyard

65 Winter, “Low” Wind Both Releases Effluent Spread: Elevation 14.6m. asl

66 Project # 2 Objectives of Stage 2 Study Thus far, have addressed simultaneous “as is” emissions releases from the stacks on Buildings “B” and “P”. on Buildings “B” and “P”. Now need to know : è What is the contribution of each to the effluent levels in the courtyard ?

67 Stage 2 simulations: Summer, 0.5 m/s wind Peak effluent levels at courtyard walls

68 Stage 2 simulations: Winter, 2.5 m/s wind Peak effluent levels at courtyard walls

69 Project # 2 Conclusions of Stage 2 Study è 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. è More effective venting arrangements are required to remove the nuisance / hazard

70 Project # 2 Objectives of Stage 3 Study l Replace present horizontal outlet for “Building P” emissions by vertical discharge from stack at same elevation l Investigate possibility of reducing emissions from “Building B” by modifying stack discharge arrangements

71 l “Building B” discharge stack is already at maximum permissible elevation è Try venturi sheath approach to dilute the effluent discharge with “fresh” ambient air prior to discharge l Implementation would not require major modification to existing arrangement Project # 2 Stack modification options for Building B - 1

72 Project # 2 Venturi mixer design concept è A venturi-type stack discharge concept has no moving parts è It uses the momentum of the discharge jet to induce dilution mixing with the surrounding (free) ambient air flow

73 Project # 2 : 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 ?

74 Principle of Venturi Discharge Stack

75 Venturi Stack Dispersion Results for Typical Discharge Stack Geometry

76 Stage 3 Simulations : Venturi Stack design constraints è Total height not to exceed given limit è Discharge velocity not to exceed 15 m/s è Venturi sheath diameter limited by space è Fixed (high) effluent flowrate è Single Venturi sheath needed for each pipe

77 Stage 3 Simulations : Venturi Stack design constraints 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 by a limited amount.

78 l “Building P” stack currently a 4-way horizontal discharge l Exposed and highly visible location rules out an elevated venturi sheath approach l Seek some improvement by adopting a vertical discharge arrangement Project # 2 Stack modification options for “Building P “

79 Project # 2 Dispersion Problem Diagnosis & Remedy l Problem Definition l Study Methodology l Benefits of using CFD l Description of CFD Model l Outline of Simulations Performed l Synopsis of Results è Conclusions

80 Project # 2 Conclusions of Project - 1 è Emissions from the roof stacks of the two buildings can build up to significant levels in the courtyard between them è Build-up occurs over a variety of ambient conditions, being worse in winter and at higher wind speeds è Re-ingestion to the buildings could occur through windows which open onto the courtyard

81 Project # 2 Conclusions of Project - 2 è 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. è This is the highest risk period when windows onto the courtyard are likely to be open for ventilation purposes

82 Project # 2 Conclusions of Project - 3 è Concentrations of effluent from roof stack releases remain in excess of the recommended limit (100 ppm) in the courtyard under winter conditions. è In mitigation, windows giving onto the courtyard are then less likely to be open

83 Project # 2 Conclusions of Project - 4 l Comparatively simple modifications to roof stack release arrangements can still result in significant reductions in effluent levels in the courtyard under winter conditions.

84 Project # 2 Conclusions of Project - 5 Project # 2 Conclusions of Project - 5 HOWEVER è 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.....

85 Winter, 2.5 m/s wind: extended stack + venturi Peak effluent levels at courtyard walls

86 Project # 2 Conclusions of Project - 6 Project # 2 Conclusions of Project - 6 IN SHORT... l Effluent release concentrations at the upper courtyard levels could be dropped to acceptable levels of around 1:10 4 dilution in winter BUT ONE WAS NOT ALLOWED TO RAISE THE STACK BY THE NECESSARY 2 METRES...

87 Project # 2 Conclusions of Project - 7 Project # 2 Conclusions of Project - 7 AND SO THERE [ FOR THE PRESENT ] THE MATTER RESTED

88 Project # 3 A “COMPARE AND CONTRAST” PROJECT

89 Project # 3 Compare & Contrast Discharge Strategies è Problem Definition l Study Methodology l Benefits of using CFD l Description of CFD Model l Summary of Results Obtained l Conclusions

90 Project # 3 Problem Definition - 1 l The two earlier projects had a common thread è Acceptable dilution dispersion of releases could be achieved under worst case ambient conditions only when the stack height was raised beyond an acceptable level.

91 Project # 3 Problem Definition - 2 l Some alleviation could be achieved by dilution at point of release using a venturi device to entrain surrounding air l Unfortunately the ability of this concept to make real savings is limited by design constraints, most usually height : impairs mixing efficiencyheight : impairs mixing efficiency nozzle velocity: noise limits entrainmentnozzle velocity: noise limits entrainment

92 Project # 3 Problem Definition - 3 l Previous Flowsolve attention to venturi stacks has been conditioned by simplicity “free” entrained air for dilution “free” entrained air for dilution no moving partsno moving parts but as we have seen, it has its limitations. l Meanwhile problems go unresolved

93 Project # 3 Problem Definition - 4 l Now we have a possible solution The Tri-Fan stack from STROBIC AIR CORPORATION

94 Project # 3 Problem Definition - 5 This device combines l a mixing plenum, through which ambient air is drawn by a fan to dilute the fume stream l two venturi nozzles, through which the mixture is then forced l these 2 jets are then mixed, to induce entrainment of external air l two separate entrainment zones

95 Overview of Strobic Tri-Stack

96

97 Project # 3 Compare & Contrast Discharge Strategies l Problem Definition è Study Methodology l Benefits of using CFD l Description of CFD Model l Summary of Results Obtained l Conclusions

98 Project # 2 Study Methodology - 1 l Use CFD simulations to predict plume trajectories issuing from rooftop extract release points on the original research building of Project #1 l Compare and contrast the dispersion efficiency, under identical adverse weather conditions, of the Tri-Stack arrangement, and two vertical stack arrangements

99 Project # 2 Study Methodology - 2 The Contenders l Strobic Tri-Stack Discharges at m elevationDischarges at m elevation l “Original” Throttled Stack Discharges at 22.9 m elevationDischarges at 22.9 m elevation l “Raised” Throttled Stack Discharges at 25.9 m elevationDischarges at 25.9 m elevation

100 Project # 3 Compare & Contrast Discharge Strategies l Problem Definition l Study Methodology è Reference Case Definition l Description of CFD Model l Summary of Results Obtained l Conclusions

101 Project # 2 Reference Case Definition - 1 l Dispersion of Basement Extract fumes from stacks located at front of research facility l “Worst case” wind vector scenario from earlier study, to give tough case for comparison l Downwind plume trajectory passes directly over adjacent buildings

102 Project # 2 Reference case Definition - 2 Ambient Wind Vector Specification l “Still” wind in summer l Wind speed 0.5 m/s l Wind direction - from release directly over Buildings “B” and “P” downwind l Air temperature = 28 0 C

103 Project # 2 Reference case Definition - 3 Release Scenario l 6 m 3 /s effluent gas entering each stack via a 1m diameter duct l Vertical straight stacks “throttled” to give 15 m/s exit velocity l Release temperature = 24 0 C

104 Project # 2 Reference case Definition - 4 Tri-Stack Operating Data l 6 m 3 /s effluent gas inlet feed l 6.75 m 3 /s plenum air feed l m 3 /s entrained ambient air l m/s nozzle velocity l Release temperature = 24 0 C

105 Project # 3 Compare & Contrast Discharge Strategies l Problem Definition l Study Methodology l Reference Case Description è Description of CFD Model l Summary of Results Obtained l Conclusions

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

107 Tri-Stack Discharge Arrangement

108 “Original” Stack Discharge Arrangement

109

110 “Raised” Stack Discharge Arrangement

111

112 Project # 3 Compare & Contrast Discharge Strategies l Problem Definition l Study Methodology l Reference Case Description l Description of CFD Model è Summary of Results Obtained l Conclusions

113 Discharge Strategy Comparison Comparison of 100ppm plume core envelopes

114 100 ppm core envelope: “Original” Stack

115 100 ppm core envelope: “Raised” Stack

116 100 ppm core envelope: Strobic Tri-Stack

117

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 structures: Strobic 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 # 3 Compare & Contrast Discharge Strategies l Problem Definition l Study Methodology l Reference Case Description l Description of CFD Model l Summary of Results Obtained è Conclusions

131 Project # 3 Conclusions of Project - 1 Project # 3 Conclusions of Project - 1 è 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 # 3 Conclusions 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: l David Glynn [ Flowsolve ] l John Gibson [ Scott Wilson ] l Phil Milne-Smith [ Critical Airflow Controls ] l Paul Tetley [ Strobic Air Corporation ] and not forgetting l the University Authorities

134 Thank you for your attention “That’s all for now …...”


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