Experimental review of the homogeneous temperature assumption in post-flashover compartment fires

Abstract

Traditional methods for quantifying and modelling compartment fires for structural engineering analysis assume spatially homogeneous temperature conditions. The accuracy and range of validity of this assumption is examined here using the previously conducted fire tests of Cardington (1999) and Dalmarnock (2006). Statistical analyses of the test measurements provide insights into the temperature field in the compartments. The temperature distributions are statistically examined in terms of dispersion from the spatial compartment average. The results clearly show that uniform temperature conditions are not present and variation from the compartment average exists. Peak local temperatures range from 23% to 75% higher than the compartment average, with a mean peak increase of 38%. Local minimum temperatures range from 29% to 99% below the spatial average, with a mean local minimum temperature of 49%. The experimental data are then applied to typical structural elements as a case study to examine the potential impact of the gas temperature dispersion above the compartment average on the element heating. Compared to calculations using the compartment average, this analysis results in increased element temperature rises of up to 25% and reductions of the time to attain a pre-defined critical temperature of up to 31% for the 80 th percentile temperature increase. The results show that the homogeneous temperature assumption does not hold well in post-flashover compartment fires. Instead, a rational statistical approach to fire behaviour could be used in fire safety and structural engineering applications.Kirby et al. [15] ran a test series burning wood cribs in a long enclosure with approximate dimensions of 22.9 m long x 5.6 m wide x 2.8 m high. All of the tests were ignited at the rear, except one in which all wood cribs were ignited simultaneously. The results of all tests show that the fire moved relatively quickly from the ignition location to the front of the compartment, where the vent was located. After the fuel in the front of the compartment burned out, the fire progressively travelled back into the compartment and ultimately consumed all the fuel and self-extinguished at the rear. Temperature results of Test 1 from this test series are shown below in Figure 1 at the rear, middle and front of the compartment. structural elements are adversely affected by temperature gradients gives motivation to revisit the homogeneous temperature assumption and further explore its ramifications.While the full implications of the temperature heterogeneity of post-flashover fires are not explored here, it is apparent that post-flashover fires do not reach uniform conditions. The presented results highlight the need to increase the spatial resolution of measurements in fire experiments to capture the full variation within the compartment. Spatially resolved data can lead to a rational statistical approach to fire behaviour when applied to fire safety and structural engineering applications.

Structural Engineering and Fire Dynamics: Advances at the Interface

“…This hypothesis has recently been tested. The results of this work demonstrate that uniform temperature conditions are not present and large variation exists [32].…”

Travelling fires for structural design–Part I: Literature review

“…Stern-Gottfried et al [19] have reviewed this assumption by analysis of existing experimental data from well-instrumented fire tests. Results show that dispersion from the spatial compartment average is significant and that the assumption of uniform temperature conditions does not hold well.…”

“…The well instrumented tests conducted at Dalmarnock [24] and Cardington [25] were shown to have large standard deviations (in excess of 200°C at times) within the temperature field [19]. Additionally, peak local temperatures in these tests were found to vary from 23% to 75% above the compartment spatial averages, and local minimums ranged from 29% to 99% below the averages.…”

Structural response of a MRF exposed to travelling fire

“…Nonetheless, the most important shortcoming of these methods is the assumption of uniform burning and homogenous compartment temperature regardless of the compartment size Rein, 2012a, 2012b) as it is shown that temperature conditions are non-uniform in most compartments (Stern-Gottfried et al, 2010b). Tests have also shown that there is a high degree of temperature variations even within small compartments (Stern-Gottfried et al, 2010a;Welch et al, 2007).…”