Jet fire

There are two jet fire models available in the PHAST, i.e. API and Shell models. The Shell method treats the flame as a tilted cone frustum, whereas the API model treats it as a banana-shaped plumeā€”i.e. tapered at the ends, and bent by the wind. The study uses API model to calculate thermal impacts resulted from jet fires. Therefore, only the API model to be discussed in the appendix.

Fig. E.5 Shape of API RP521 Jet fire [49]

The flame is a jet that may be bent by the wind or by the effects of gravity, and it is described by ten circles (ci to cio) equally spaced along the flame length. Each circle is defined by the downwind co-ordinate x and elevation z of the center of the circle, by the radius r, and by the inclination of the circle from the horizontal; the illustration below shows a portion of the middle of a flame, with four circles:

The flame length, L, is calculated as:

Where, Q is the mass discharge rate (kg/s) and DHcomb is the heat of combustion (=119.9 MJ/kg, for H2). The maximum radius of the flame is given by:

where L is the flame length (m). If the flame is horizontal, then it is assumed not to be deflected by the wind. For a vertical flame, the program first calculates the velocity ratio, Uratio, as a measure of the power of the wind to deflect the jet:

where Uw is the wind speed (m/s) and U0 is the jet velocity. The jet velocity, U0, is calculated as:

Pvapor exp where Q is the mass discharge rate, rvapor is the vapor density at one atmosphere and Rexp is the expanded radius. The expanded radius is calculated as:

\P vaporPU 0

If the velocity ratio is less than 0.0001, then the flame is treated as being perfectly vertical [49]. For larger values of the velocity ratio, the program calculates the increase in x and z between each circle. For each circle, the gradient dz/dx is dZ = 3.2p ^f I -11 (E-29)

dx Uratio VS L 0

where Rexp is the expanded radius, Uratio is the velocity ratio, s is the distance of the circle along the centerline of the flame, and L is the flame length. The radius of the flame as a function of distance along the length is set according to [49]. The radius of the first circle, ri, is set equal to the expanded radius of the jet. For the other circles, the radius, ri, is given by:

where:

and L is the flame length. If the emissive power was not specified in the input data, the program calculates it as described below. The calculations involve several stages. The fraction, Fs, is

where:

MZ 21 1.69

U0 is the jet velocity and MW is the molecular weight of the substance released. If the value calculated is greater than the maximum allowed (set as 0.5), then Fs is set to 0.5. The surface emissive power of the flame, Em, is calculated from [49] as:

comb

total

Where, Fs the fraction of heat radiated, Q is the mass discharge rate, Hcomb is the heat of combustion, and Arotai is the total surface area of the flame. If the calculated value of Em is greater than the maximum surface emissive power set in the Jet Fire Parameters, then the maximum value from the parameters is used instead. The program calculates either the jet velocity or the expanded radius, depending on which items are supplied in the input data:

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