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ENVE5103 Lecture 3a

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Title: ENVE5103 Lecture 3a


1
ENVE5103 Lecture 3a
  • Gaussian Dispersion Modelling for a Single Stack
  • Downwind maximum ground level concentration
  • Ground reflection
  • Mixing height limitation
  • Plume rise
  • Stack tip downwash
  • Effects Requiring Special Techniques
  • Building downwash
  • Cavity and wake effects
  • Elevated terrain
  • Fumigation

2
Gaussian Dispersion Modelling for a Single Stack
3
Gaussian Dispersion Modelling for a Single Stack
  • Downwind maximum ground level concentration
  • Ground reflection
  • Mixing height limitation
  • Plume rise
  • Stack tip downwash

4
2-D STEADY DISPERSION MODEL
  • Solution for windspeed of u m/s and continuous
    release of Q g/s of pollutant at x y 0
    (stack location) and z H (the effective
    stack height)
  • H h ??h
  • h physical stack height, ?
  • ?h plume rise

5
GROUND LEVEL CONCENTRATION ALONG CENTER LINE
  • We are most interested in ground level, z0,
    concentrations (where humans and other life forms
    reside),
  • On the center line, y0, (where concentrations
    are at their maximum

6
2-D STEADY DISPERSION MODEL GROUND
REFLECTION
  • From the release height of H above ground,
    dispersion can progress upward towards the mixing
    height. In the downward direction the ground acts
    as a mirror unless the pollutant gets deposited.
  • The effect of the ground can be handled
    mathematically by treating the reflection as
    another point source located below ground (at - H)

7
Figure 4-3 Wark, Warner Davis
  • Use of an imaginary sourceto describe reflection
    at the ground

8
Figure 4-4 Wark, Warner Davis
  • Effect of ground reflection on pollutant
    concentration

9
MAXIMUM GROUND LEVEL CONCENTRATION
  • At z 0 (cwith reflection ) 2(cwithout
    reflection )
  • Not as simple at other z.
  • C(x,0,0) first increases with x due to ground
    reflection but horizontal dispersion (y
    direction) eventually decreases it. (Fig 4-5)
  • The location and magnitude of the maximum
    concentration can be determined from the
    equations above. Fig 4-8 provides a convenient
    tool. Other empirical methods are also available

10
Figure 4-5 Wark, Warner Davis
  • Concentration profiles along the center line of a
    stack plume

11
Figure 4-8 Wark, Warner Davis
  • Maximum Cu/Q value as a function of stability
    class and downwind distance

12
MIXING HEIGHT LIMITATION
  • As the ground represents a lower limit to the
    vertical dispersion, the mixing height represents
    an upper limit. Multiple reflections from the
    ground and the stable layer above need to be
    considered giving rise to
  • Approximation
  • No effect of mixing height for xltxL
  • Completely mixed in the z direction for xgt2 xL
  • Interpolate (on log-log plot) in between xL
    and 2 xL
  • xL corresponds to ?z0.47(L-H)

13
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14
ONE DIMENSIONAL SPREADING MIXING HEIGHT
LIMITATION
  • After a sufficient distance downstream (say ?z
    mixing height) ?the plume can only disperse
    horizontally.
  • If we consider the plume well mixed in the
    vertical direction, we can obtain
  • where L mixing height

15
PLUME RISE
  • H h ??h
  • h physical stack height, ?
  • ?h plume rise due to thermal buoyancy and
    momentum
  • Correlations of various complexity exist between
    plume rise, stack temperature, stack velocity,
    atmospheric conditions etc. (e.g. Hollands,
    equation 6.35 de Nevers)

16
PLUME RISE - HOLLANDS EQUATION
17
PLUME RISE - BUOYANCY AND MOMENTUM FLUXES
18
Table 4-6 Wark, Warner Davis
  • Equations for calculating final plume rise

19
THE U.S. EPA SCREEN(3) MODEL Buoyancy Induced
Dispersion (BID)
  • Entrainment due to shear between plume and
    outside air increases dispersion in the plume.
    SCREEN model uses dispersion parameters that are
    larger than those previously reported from
    Prairie experiments.
  • More noticeable for concentrations near plume
    level, than for ground-level concentrations

20
STACK TIP DOWNWASH
  • For Vs lt 1.5 us
  • (Vs stack gas velocity, us wind velocity at stack
    height)
  • hs physical stack height
  • hs physical stack height, corrected for stack
    downwash
  • Note that maximum downwash correction is 3 stack
    diameters
  • As before, we have H hs ??h

21
Effects Requiring Special Techniques
  • SCREEN3 will automate all the basic Gaussian
    dispersion calculations mentioned above for a
    single source (point, area, flare, volume).
  • In addition, SCREEN3 incorporates special
    techniques for dealing with
  • Building downwash
  • Cavity and wake effects
  • Elevated terrain
  • Fumigation

22
Building downwash
23
BUILDING DOWNWASH AND WAKE EFFECTS
  • Figs. 3-19 and 3-20 demonstrate these. Special
    treatments are included in models.
  • BUILDING DOWNWASH - Simple rule of thumb
  •   downwash unlikely to be a problem if
  • hs ? hb 1.5 Lb
  •  hs stack height
  • hb building height
  • Lb the lesser of either building height or
    maximum projected building width.
  • Good Engineering Practice (GEP) rule for stack
    design.
  •  

24
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26
BUILDING DOWNWASH AND WAKE EFFECTS
  • Downwash procedures documented in ISC Users
    Guide.
  • SCREEN also implements these procedures.
  • CAVITY
  • Calculations based on
  • minimum
  • maximum
  • horizontal distances alongwind
  •    
  • WAKE
  • Near Wake downwind distance lt 10 Lb
  • Far wake downwind distance gt 10 Lb
  •  

27
SCREEN3 User Guide
28
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29
  • Structure Influence Zone (SIZ) For downwash
    analyses with direction-specific building
    dimensions, wake effects are assumed to occur if
    the stack is within a rectangle composed of two
    lines perpendicular to the wind direction, one at
    5L downwind of the building and the other at 2L
    upwind of the building, and by two lines parallel
    to the wind direction, each at 0.5L away from
    each side of the building, as shown below. L is
    the lesser of the height or projected width. This
    rectangular area has been termed a Structure
    Influence Zone (SIZ). Any stack within the SIZ
    for any wind direction is potentially affected by
    GEP wake effects for some wind direction or range
    of wind directions, and shall be included in the
    modelling project. Please see Figure 4.5 and
    Figure 4.6.

30
Figure 4.5 GEP 5L and Structure Influence Zone
(SIZ) Areas of Influence (after U.S. EPA(19)).
31
  • Figure 4.6 GEP 360 5L and Structure Influence
    Zone (SIZ) Areas of Influence (after U.S.
    EPA(24)).

32
  • Building downwash for point sources that are
    within the Area of Influence of a building shall
    be considered. For US EPA regulatory
    applications, a building is considered
    sufficiently close to a stack to cause wake
    effects when the distance between the stack and
    the nearest part of the building is less than or
    equal to five (5) times the lesser of the
    building height or the projected width of the
    building.
  • Distancestack-bldg lt 5L

33
  • For point sources within the Area of Influence,
    building downwash information (direction-specific
    building heights and widths) shall be included in
    the modelling project.
  • The Building Profile Input Program (BPIP or
    BPIP-PRIME) can compute the direction-specific
    building heights and widths once the basic
    building data is entered.

34
Elevated Terrain Analysis
  • Complex Terrain as illustrated in Figure 5.1,
    where terrain elevations for the surrounding
    area, are above the top of the stack being
    evaluated in the air modelling analysis.
  • Simple Terrain where terrain elevations for the
    surrounding area are not above the top of the
    stack being evaluated in the air modelling
    analysis. The Simple terrain can be divided
    into two categories

35
Simple Terrain
  • Simple Flat Terrain is used where terrain
    elevations are assumed not to exceed stack base
    elevation. If this option is used, then terrain
    height is considered to be 0.0 m.
  • Simple Elevated Terrain, as illustrated in Figure
    5.2 is used where terrain elevations exceed stack
    base but are below stack height.

36
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38
Regulatory (MOE) wording for terrain
considerations
  • Terrain data
  • 16. (1) If an approved dispersion model is used
    for the purposes of this Part with respect to any
    point of impingement that has an elevation higher
    than the lowest point from which the relevant
    contaminant is discharged from a source of
    contaminant, the model shall be used in a manner
    that employs terrain data.

39
Fumigation
  • Fumigation occurs when a plume that was
    originally emitted into a stable layer is mixed
    rapidly to ground-level when unstable air below
    the plume reaches plume level. Fumigation can
    cause very high ground-level concentrations.
  • Typical situations in which fumigation occurs
    are
  • 1. Breaking up of the nocturnal radiation
    inversion by solar warming of the ground surface
    (Fig 5.15 deNevers)
  • 2. Shoreline fumigation caused by advection of
    pollutants from a stable marine environment to an
    unstable inland environment and
  • Advection of pollutants from a stable rural
    environment to a turbulent urban environment.

40
Figure 5.15 de Nevers
  • Fumigation

41
Shoreline fumigation
  • A stable onshore breeze carries a shoreline
    source inland
  • Breeze encounters air from solar heated soil,
    mixing from below
  • The mixing from below pulls plume to ground
  • The end result is the same as in fumigation due
    to inversion break-up (Fig. 5.15 deNevers)
    although the effects leading up to it are
    different
  • SCREEN3 addresses both of these situations.
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