The Adjacent Room Field Trees

Temperature Profiles of Each Tree for Different Fires

Specific details of the temperature vs. height profiles (for different fires) are discussed below. Temperature profiles plotted in Figures 5.6 – 5.9 indicate:

• The upper layer height of the adjacent room is higher than the fire room.

A doorway with a soffit separated the fire room from the adjacent room. The soffit increases the depth of the upper layer as it acts as a barrier that the hot gases must ‘rise’ over. Therefore, since the adjacent room does not have a soffit, the upper layer does not increase in depth. The temperature gradient where the upper layer begins in the adjacent room starts at approximately 1850 mm from the floor for all trees in the adjacent room, whereas the temperature gradient for the fire room begins at approximately 1100 mm.

• All trees in the adjacent room show a constant temperature up to a height of 1850 mm. The temperature is low up to this height as high density ambient

air occupies this space, while the less dense hot gases are exiting above.

• A temperature gradient exists up to the ceiling for all trees beyond 1850 mm. The central fires exhibited a constant temperature profile above

generates turbulence, which in turn promotes mixing. Also, without the presence of a soffit, the residence time for the hot gases is less than for the fire room, therefore a uniform upper layer temperature is unable to be formed. The temperature profile in the adjacent room above 1850 mm is typical of a ceiling jet.

• The temperature at 300 mm for the centrally located fires increases marginally as fire size increases. Temperatures increase for tree 6 by about

10oC between the ‘coldest fire’ (55 kW) and the ‘hottest fire’ (110 kW corner). There is a gradual decay in temperature differences for each tree that is further away until tree 9 is reached, where the difference between the ‘hottest fire’ (110 kW corner) and the ‘coldest fire’ (55 kW) is about 6oC. The increase in temperature at this height is likely to be a result of radiation from the fire, not from hot gases raising the temperature of the lower layer.

• The temperature of the upper layer at 2375 mm is highest at tree 6 and decreases for each tree along the adjacent room. The temperature is highest

at tree 6 as this tree is closest to the fire room. The temperature decreases along the length of the adjacent room. The reduction in temperature along the adjacent room results from heat transfer through the ceiling.

• The temperature drop across the thermocouples closest to the ceiling along the adjacent room is greatest for the hottest fire (110 kW corner), and lowest for the coldest fire (55 kW). The temperature difference between

the 2375 mm thermocouples from tree 6 to tree 9 for the 55 kW fire is approximately 30oC, whereas temperature difference for the 110 kW for the corner fire for the same thermocouples is approximately 75oC, with the 110 kW and 160 kW fires being in between these values. This drop in temperature indicates heat transfer across the ceiling. From basic heat transfer principles, if the temperature gradient is increased, heat flux will increase. The temperature gradient across the ceiling for the 110 kW corner fire is greater than that for the 55 kW fire, since the exterior of the compartment is exposed to ambient conditions which do not vary significantly between experiments. Therefore, the temperature drop along the trees for the adjacent room in the upper layer for the 110 kW corner fire, is greater than the 55 kW fire.

Temperature Variations for Each Tree

Specific details of the temperature variation from the mean for each tree (for different fires) are discussed below. Temperature variations graphed in Figures 5.15 – 5.18 indicate:

• The temperature variations for the central fires increase with increasing fire size. As previously discussed in section 5.5.2, an increase in fire size

increases temperature variations. This trend continues in the adjacent room. • Temperature variations increase for trees closer to the fire. Tree 6 shows

the greatest temperature fluctuations, with trees 7, 8, and 9 reducing in temperature variations respectively. The temperature variation of tree 9 is approximately 50 % less than tree 6 (generally), with trees 7 and 8 showing temperature fluctuations between the values of trees 6 and 9. This reduction in temperature variation along the length of the adjacent room is a result of the mixture of entrained air and fire gases becoming more uniform.

• Temperature variations for all trees increase by up to 100 % or more at and beyond 1850 – 2050 mm. Temperatures increase in this region.

Temperature variations are low below this height The temperature variations increase as a result of the hot upper layer moving through the adjacent room, shearing and mixing with ambient air below the hot upper layer, creating a non-uniform mixture of hot and cold gases.

Overall Comparisons between Field Trees in Each Room per Fire

Specific details of the overall variation between each field tree in each room per fire are discussed below. Overall comparisons graphed in Figures 5.20, 5.22, 5.24 and 5.26 indicate:

• The temperature profile for all trees is constant up to a height of 1600 mm. The constant temperature profile shows that mostly cooler ambient air is

below 1600 mm; the cooler ambient air is denser than the outgoing lower density hot gases.

• A temperature gradient emerges for all trees and for each fire at a height of approximately 1850 mm. The increase in temperature illustrates how the

hotter, more buoyant gases are leaving the compartment. The temperature gradient continues up to the ceiling, suggesting a ceiling jet.

• Tree 6 records the highest temperature nearest the ceiling for each fire, with a reduction in temperature along the compartment for trees 7, 8, and 9. As hot gases leave the compartment, cooler air is being entrained along the

length of the compartment as a result of the shearing and mixing between the hot upper layer and cool lower layer, as well as heat transfer into the ceiling. Therefore, as it approaches the opening the upper layer decreases in temperature.

• The 110 kW corner fire produces the highest temperature near the ceiling. As discussed earlier in section 5.5.2, the 110 kW corner fire continues

to generate the highest temperature profile (as a result of restricted air entrainment), particularly near the ceiling. In fact, as the fire size increases (or in the case of the corner fire – entrainment is decreased) the temperature difference increases between trees at the 2375 mm thermocouple. For example, the temperature difference between tree 6 and tree 7 for the 55 kW fire is approximately 20oC, whereas for the 110 kW corner fire the temperature difference between the same trees is approximately 35oC.