OBJECTS IMMERSED IN FLAMES

The simplest and most conservative way to treat the fire exposure boundary condition would be to apply a constant, bounding heat flux to all structural elements in the area of interest. The bounding heat flux boundary condition was developed from data on objects immersed in large hydrocarbon pool fires. The heat flux data for objects immersed in

Surface Temperature (°C)

200 400 600 800 1000 1200 1400 1600 1800

Surface Temperature (°F)

FIGURE 25. Magnitude of the Surface Temperature Corrections on the Measured Total Heat Flux Using a Cooled Gauge (see Equation 56). Radiation (—), Convection with h = 0.015 kW/(m K) (– . . –), and Convection with h = 0.050 kW/(m K) (– – –).

Heat Flux (kW/m2)

fires are presented in this section and used to deter-mine the magnitude of the bounding heat flux. The information in this section is derived primarily from direct or indirect measurements of heat flux taken in open hydrocarbon pool fires with optically thick flames. There is insufficient data available at this time to adequately address the impact of a boundary such as a wall or ceiling on the heat flux conditions to an immersed object. It is expected that the data obtained from optically thick flames in unconfined pool fires is bounding.

Test Data

A series of 30-minute, 9.1 m by 18.3 m hydrocar-bon pool fires (JP-4) conducted by Gregory, Mata, and Keltner⁸¹provided useful temperature and heat flux data at various elevations above the base of the fire. Steel cylinders filled or lined with insulation (referred to as small or large calorimeters, respec-tively) at several locations were used to indirectly measure the net heat flux for objects immersed in the fire. The temperature inside the cylinder was recorded, and the net heat flux was extracted using the inside temperature as a boundary condition. The

measurements were taken at various elevations and angular positions on the calorimeters. The cold-wall (i.e., peak) heat fluxes to the large calorimeter varied between 100 kW/m²and 160 kW/m²at any one loca-tion, with the largest peak heat fluxes observed on the underside and the lowest on the top. Figure 26 shows the average peak heat flux at various angular positions as a function of the external surface tem-perature of the large calorimeter, which increases as a function of time, and the angular position.

The cold-wall fluxes to the small calorimeter varied between 150 kW/m²and 220 kW/m². As with the large calorimeter data, the maximum heat fluxes were observed on the bottom of the calorimeter and the minimum were observed on the top. There was no decrease in the cold-wall heat flux detected over the elevation range (1 to 11 m) sampled.

Russell and Canfield⁸²immersed a steel cylinder in a 2.4 m by 4.9 m JP-5 pool fire in windy condi-tions. The inside surface temperature of the cylinder was directly measured, and the exposure heat flux was determined in the same manner as Gregory, Mata, and Keltner.⁸¹The peak heat fluxes to the surface of the cylinder were measured at various angular positions. The peak heat fluxes ranged from

External Surface Temperature of Large Calorimeter (K)

400 600 800 1000

Average Heat Flux (kW/m2)

0

18 kW/m²on the windward side to 144 kW/m²on the leeward side. The heat fluxes on the top and bottom of the cylinder were 48 kW/m²and 103 kW/m², respectively.

Cowley⁸³summarized the peak heat fluxes measured directly or indirectly to objects immersed in various large-scale pool fires. The values range between 80 kW/m²and 270 kW/m². Table 4 sum-marizes some of this information. Cowley speculates differences between low- and high-volatile fuels with heat fluxes as high as 300 kw/m²are possible in the latter.

Most of the heat flux test data suggest a bound-ing cold-wall heat flux between 150 kW/m²and 170 kW/m². Although some data (small calorimeter) indicate that the peak may be as high as 220 kW/m², these appear to be exceptional.

The heat flux in a flame increases with fire diameter and where the object or flame impinge-ment is located. The upper bound of heat flux can be calculated as follows:

(Eq. 56a) Data Requirements

The flame temperature is needed to perform this calculation.

Data Sources

For pool fires, the radiative fraction can be deter-mined as a function of pool diameter from the SFPE Engineering Guide to Assessing Flame Radiation to External Targets from Pool Fires. This radiative fraction can be substituted into Figure 23 to esti-mate the flame temperature. For noncircular pools with a length-to-width ratio of near one, the equiva-lent diameter of the pool can be estimated using the surface area, A, of the noncircular pool:

(Eq. 56b) Where:

A = Surface area of the fuel package (m²) Assumptions

1. The flame emissivity and surface absorbtivity are equal to 1.0.

2. The impact of a compartment on the heat fluxes at the surface of an immersed object can be ignored.

3. Reduction in net heat flux due to heating of the target is not considered.

Validation

Equation 56a is based on first principles. Heat fluxes calculated using Equation 56a are much larger than measured heat fluxes. For example, Baum and McCaffrey⁶⁸ reported gas temperatures as high as 1250°C in 30 m diameter pool fires. Assuming that the gases are optically thick, emissivity of 1.0, the cold-wall heat flux is 305 kW/m². As seen in Table 4, measured values are less than this value, indicating that the assumed emissivity may be significantly less than 1.0 or the effective gas temperatures providing the radia-tion are lower than measured or reported temperatures.

Peak Heat Flux

Test Pool Size Fuel (kW/m²)

AEA Winfrith⁸⁴ 0.49 x 9.4 m Kerosene 150

US DOT⁸⁴ Not listed Kerosene 138

USCG⁸⁴ Not listed Kerosene 110-142

US DOT⁸⁴ Not listed Kerosene 136-159

Sandia⁸⁴ Not listed Kerosene 113-150

HSE Buxton⁸⁴ Not listed Kerosene 130

Shell Research⁸⁴ 4.0 x 7.0 m Kerosene 94-112

Large cylinder⁸² 9 x 18 m JP-4 100-150

Large cylinder⁸² 9 x 18 m JP-4 150-220

Russell and Canfield⁸³ 2.4 x 4.9 m JP-5 144 TABLE 4. Selected Heat Fluxes to Objects Immersed in Large Pool Fires⁸³

Limitations

The results of this section are limited to Class A (plastic or wood-based) combustible material fires or hydrocarbon pool fires. Gaseous jet flames are beyond the scope of this section because they may produce larger cold-wall (200 to 270 kW/m²) heat fluxes to immersed objects.⁸³

The results are also not applicable to objects that are located near (collocated), but not in, the burning region. Methods of estimating the incident heat flux to collocated objects are available in another Engineering Guide.⁶⁶

HEAT FLUXES FOR