Session 4, Unit 7 Plume Rise

Qualitative Descriptions Plume rise h H=hs + h Driving forces Buoyancy Momentum Different phases Initial phase Thermal phase Breakup phase Diffusion phase

Qualitative Descriptions Influencing factors When there is no downwash Exit velocity Stack diameter Stack gas temperature Ambient temperature Wind speed Atmospheric stability Wind shear Downwash

Holland Plume Rise Formula Simple More suitable for power plant For neutral conditions The wind speed ū is adjusted to the stack height. For non-neutral conditions

Briggs Plume Rise Formulas More complicated Buoyancy flux parameter Momentum flux parameter

Briggs Plume Rise Formulas Determination of buoyancy dominated or momentum dominated plumes Calculate (T)c For unstable or neutral (A-D) For Fb <55 For Fb55 For stable (E,F) If T (=Ts-Ta)  (T)c , it’s buoyancy dominated If T (=Ts-Ta) < (T)c , it’s momentum dominated

Briggs Plume Rise Formulas For buoyancy dominated plume under unstable or neutral conditions (A-D) x* = distance at which atmospheric turbulence begins to dominate entrainment For Fb55 m4/sec3, x*=34 Fb2/5 For Fb<55 m4/sec3, x*=14 Fb5/8 xf=distance to the final rise, m xf=3.5x* Final plume rise:

Briggs Plume Rise Formulas For buoyancy dominated plume under stable conditions (E and F) Stability parameter, s Default values for 0.02 K/m for E stability 0.035 K/m for F stability

Briggs Plume Rise Formulas Final plume rise Distance to final rise

Briggs Plume Rise Formulas For momentum dominated plume under unstable or neutral conditions (A-D) For momentum dominated plume under stable conditions (E,F) Calculate both and use the lower one.

Briggs Plume Rise Formulas Gradual rise Distance < distance to final rise (i.e., x<xf) and Buoyancy dominated plume

Briggs Plume Rise Formulas Distance < distance to final rise (i.e., x<xf) and momentum dominated plume Jet entrainment coefficient Unstable conditions (A-D)

Briggs Plume Rise Formulas X=downwind distance with max value of: Xmax=49Fb5/8 for 0<Fb<55 m4/sec3 xmax=119Fb2/5 for Fb> 55 m4/sec3 Stable conditions (E,F) with

Briggs Plume Rise Summary Unstable and neutral Stable Buoyancy Momentum

Buoyancy Induced Dispersion Air entrainment due to “boiling-like action” enlarges the plume Small impact on ground level concentration in most cases The impact can be reflected in  Initial plume size Effective dispersion coefficients

Session 4, Unit 8 Averaging Time, Multiple Sources, and Receptors Chimney, Building, and Terrain Effects

Averaging Time The concentration calculated from the Gaussian equations should represent the averaging time that is consistent with the averaging time of  Short-term:  1 month Long-term: > 1 month

Averaging Time If longer averaging time is desired, use the following power law P=0.17-0.75, suggested value is 0.17

Crosswind Averaging Integrate y from - to  Average over a sector

Crosswind Averaging Average over a sector considering distribution of wind speeds and stability classes ISCLT3 and STAR

Crosswind Averaging Smoothing transition from sector to sector Weighted smoothing function, WS Smoothed average concentration

Multiple Sources The max from each source do not exactly overlap Use of multiple stack factor More accurate method – modeling with a consistent coordinate system

Receptors Receptor grid Cartesian coordinate system Polar coordinate system Single stack, but the origin of the coordinate system is not at the stack base Multiple stacks Presentation of results Concentration isopleths

Example Calculation Chapter 10

Chimney Effects Stack tip downwash Avoid stack tip downwash Low pressure behind stack ū is at the stack top level No plume rise (“plume sink”) Avoid stack tip downwash

Building Effects General description Expanded meaning of “building” Reduce building effects – rule of thumb hs>2.5hb Too conservative for tall thin buildings

Briggs Procedure to Minimize Downwash Five steps: Correction for stack induced downwash Correction for building effects Determine if plume is entrained in the cavity. If entrained, treat it as a ground level source Buoyancy effect Calculate downwind concentration

Cavity Description Cavity length Short buildings (L/H2) L affects cavity length xr Long buildings (L/H>2) L does not affect cavity length xr

Cavity Max cavity width It’s location long x direction Max height

Cavity Concentrations within cavity

Wake Downwind of Cavity Treated as a ground level source Turner method (virtual source) Gifford method Gifford-Slade method (total dispersion parameters) Huber-Snyder method

Sources Downwind of Buildings Briggs method Beyond 3b  no building effect Within 3b  treat them as ground level sources

Complex Terrain Definition Plume behavior in complex terrain Simple terrain Complex terrain Intermediate terrain Plume behavior in complex terrain

Complex Terrain Modeling approaches Briggs Egan Bowne Modified dispersion coefficients ISC3 (COMPLEX 1) – to be discussed later

GEP Stack Height Definition Greater of 65 m HG=H+1.5L (for stacks in existance on Jan 12, 1979, HG=2.5H) Structures to be considered: within 5L In modeling analyses, no credit is given for stack height above the GEP