Pasquill-Gifford model

Cases 1 through 10 all depend on the specification of a value for the eddy diffusivity Kr In general, Kj changes with position, time, wind velocity, and prevailing weather conditions. Although the eddy diffusivity approach is useful theoretically, it is not convenient experimentally and does not provide a useful framework for correlation. 

Sutton7 solved this difficulty by proposing the following definition for a dispersion coefficient  

The dispersion coefficients are a function of atmospheric conditions and the distance downwind from the release. The atmospheric conditions are classified according to six different stability classes, shown in Table 5-1. The stability classes depend on wind speed and quantity of sunlight. During the day, increased wind speed results in greater atmospheric stability, whereas at night the reverse is true. This is due to a change in vertical temperature profiles from day to night. 

Surface wind speed (m/s) Daytime insolation3 Thin overcast or 4/8 low cloud 3/8 cloudiness 

3Strong insolation corresponds to a sunny midday in midsummer in England. Slight insolation to similar conditions in midwinter. 

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PasquiU Atmo.spheric Diffusion, Van Nostrand, 1962) recast Eq, (26-60) in terms of the dispersion coefficients and developed a number of useful solutions based on either continuous (plume) or instantaneous (puff) releases, Gifford Nuclear Safety, vol, 2, no, 4, 1961, p, 47) developed a set of correlations for the dispersion coefficients based on available data (see Table 26-29 and Figs, 26-54 to 26-57), The resulting model has become known as the Pasquill-Gifford model. 

The equations for cases 1 through 10 were rederived by Pasquill8 using expressions of the form of Equation 5-37. These equations along with the correlations for the dispersion coefficients are known as the Pasquill-Gifford model. 

One classic Gaussian plume model for smokestack emissions is the Pas-quill-Gifford model, which applies for steady emissions of a chemical over relatively level terrain. If no chemical sinks exist in the air (i.e., no reactions are degrading the chemical) and if there is an unlimited mixing height (i.e., no atmospheric inversion exists, and the plume can be mixed upward indefinitely), the Pasquill- Gifford model can be expressed in the form... 

The Pasquill-Gifford model is applicable for jc < 50 km. Therefore, calculations need only be performed for 50-pm particles where the distance traveled is 0.446 miles or 710 m. For this condition ... 

The Pasquill - Gifford model PUFF is suited for dispersion modeling in instantaneous outflow conditions. It is a dispersion model with normal (Gauss) distribution of concentrations and Lagrange approach, which consists in gas element move monitoring in wind field. The gas cloud spreads in the wind direction. At first the cloud grows and the gas concentration sinks. Later the cloud volume decreases, because more and more gas disperses in insignificant concentrations outside the cloud. 

The flash fire can occur as a result of delayed ignition of a cloud of gas. In this case a time-limited steady release of gas is simulated as series of puffs, each of which is considered as a separate release described by Pasquill-Gifford model. Both atmosphere stability and direction and speed of wind are taken into account, as the gas dispersion depends on these factors. The computation of individual risk for flash fire reflects the fact that the heat flux, the probability of ignition of dispersed gas and exposure time vary in time and space. 

TABLE 26-28 Atmospheric Stability Classes for Use with the Pasquill-Gifford Dispersion Model... 

FIG. 26-54 Horizontal dispersion coefficient for Pasquill-Gifford plume model, Reprinted ffomD. A. Ct owl and J. F. Louvar, Chemical Process Safety, Fundamentals with Applications, Z.9.90, p. 138. Used hy permission of Ft entice Hall)... 

The Gaussian diffusion equation is known as the Pasquill and Gifford model, and is used to develop methods for estimating the required diffusion coefficients. The basic equation, already presented in a slightly different form, is restated below ... 

In the calculations that were made to predict ground level concentrations from a VCM reactor blow off, the Pasquill-Gifford-Holland dispersion model was used as a basis for these estimations. Calculations were made for six different stability classes and ground level concentrations, and at various distances from the point source of emission. 

Figure 5-11 Dispersion coefficients for Pasquill-Gifford plume model for urban releases.

Distance downwind, km for Pasquill-Gifford puff model. 

Pasquill-Gifford plume model At a given downwind distance x, the maximum (average) concentration for a (continuous) passive plume from a point source is... 

When denser-than-air effects are important, use the Britter-McQuaid (plume or puff) models. Otherwise, assume the release is passive and use the Pasquill-Gifford (plume or puff) models. Adjust values for the virtual source correction s) as appropriate. 

As an alternative to estimating empirical equations. Seinfeld (1986) presents equations that are equivalent to the Pasquill-Gifford curves, as well as several other empirical models. An example of an empirical model presented as a set of formulae is that recommended by Briggs (Gifford, 1976), shown in Table 4-7. 

Assume that a certain dump fire emits 10 g/sec of soot particles (less than 0.1 /xm in diameter). You decide to model soot transport using the Pasquill-Gifford approach, treating the dump as a zero-height stack. On this day, an outdoor ceremony is planned at city hall, 2 km directly downwind. 

Basing estimates on a wind field described by a single mean wind vector and an estimate of the atmospheric stability (in effect, the modeler in this situation would simply be using a computer to implement a Pasquill-Gifford-style prediction). 

In their simplest form, these models provide results essentially equivalent to those provided by the Pasquill-Gifford approach. With more sophisticated meteorological data, increasingly sophisticated predictions can be obtained. In the case of the second-order closure integrated puff (SCIPUFF) algorithm used in HPAC, predictions of both the ensemble-averaged puff profile and the range of variation can be provided, as will be discussed in Section 3.2.6. 

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