Page 72 - Build 161

Fire safety
vertical panel
front panel
temperature measurement points
two video cameras
radiation measuring device 1.3 m
fuel bed loadcell
0.8 m
Figure 1: BRANZ reduced scale-test compartment.
BRANZ predicting ame heat
Figure 3: Flame geometry without an apron (left) and with a 0.3 m apron (right).
Process for calculation method
The general process identi ed for calculating the heat exposure above a ame projecting from an opening was to:
● nominate a re size – called the heat release rate
● calculate the ame height based on the re conditions and opening
size
● calculate the peak heat intensity on the wall using the ame height
and heat release rate.
Predictions from several hand calculation methods and a widely used computer model were compared with the reduced-scale experimental data and other data (both reduced and full scale) from the literature. The computer model provided comparable ame heights to some of the hand calculation methods but had di culties reproducing the re conditions and heat intensity above the opening in some instances. Another aspect still being investigated is what is a suitable re size to use for design calculations.
Figure 2 shows how a progressive increase in the horizontal projec- tion of an apron decreases the received radiation relative to having no apron above the opening. The amount of radiation received decreases over the height of the ame, so not only does the apron physically push the ame away from the face of the building, the building is only exposed to less-intense regions of the projecting ames. The e ect of an apron on the shape of the ame can be seen in Figure 3.
Translating to full-scale designs
One of the issues identified in this research is the difficulty of translating reduced-scale experimental data to full-scale design situations, and this is the subject of ongoing research.
Note A BRANZ study report with details of this research and associated ndings will be available soon from www.branz.co.nz/study_reports.
To provide guidance to fire safety practitioners, the Building Research Levy-funded BRANZ project Limiting fire spread by design includes an investigation of how to predict the heat from ames projecting from openings. This is being achieved through a review of previous work on this problem, conducting a series of reduced-scale experiments and model validation.
A one-third scale compartment (see Figure 1) was used in the experiments with wood crib and liquid hydrocarbon pool res. The e ects of the position of the fuel package in the compartment and the vertical position of the opening in the wall were investigated. As well, the e ects of aprons on the heat intensity received by the external wall above the opening were investigated.
2 1.5
1 0.5 0
No apron
75 mm apron 150 mm apron 300 mm apron
0 0.5 1 1.5 2 2.5 Relative radiant exposure
Figure 2: E ects of aprons on peak heat ux above a ame projecting from an opening.
70 — August/September 2017 — Build 161
FEATURE SECTION
Height above opening so t (m)