Method to obtain large scale burning rate of liquids with lab scale tests
Résumé
The aim of the study is to determine the burning rate of various flammable liquids at large-scales from small-scale experiments. However, it is not possible to carry out systematic tests on medium or large-scale to determine the fire properties of each product. The small-scale fire tests in laboratory offer many advantages in terms of cost and time in comparison with the large-scale experiments. But, the question remains the use of these experimental results in fire scenarios. Indeed, some measurements performed in laboratory scale, as the mass loss rate or the radiative fraction, cannot be directly extrapolated due to the scaling effects. It is well known that these latter modify the fire properties as the pool diameter rises from small cups to large pans. According to Tewarson, fuel mass loss rates close to those measured at large-scale could be obtained from small-scale fire tests by increasing significantly the oxygen concentration of ambient air or by increasing the radiative heat flux to the combustible sample. Moreover, the former technique seems to show that an asymptotic value for mass loss rate is reached, as the oxygen concentration becomes more than 0.30 %. This asymptotic value would be comparable to the fuel mass loss rate measured in large-scale fires. In this work, these two experimental techniques are performed in a calorimeter apparatus for eight liquids. Then, a way to use the small-scale experimental data is investigated in order to estimate the fire behavior at large-scale. The analysis of experimental results allows us to propose a first classification on several liquids. This classification is based on the oxygen concentration or the radiative heat flux necessary to obtain the same burning rates as those measured in large-scale fire tests performed in the INERIS facility (fire gallery). A non-dimensional number, the ratio of the heat of gasification to the heat of combustion, is also introduced and seems to show that the fuel classification proposed here always holds. Finally, this approach (classification of fuel thanks to the non-dimensional number followed by one small-scale experiment) is tested with another liquid fuel (ethyl acetate), for which only the experimental data in the fire gallery are available. The conclusions of this study are very encouraging.
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