Experimental and numerical analysis of the effects of fire size on the sooting radiative structure of medium-scale heptane pool fires

Abstract

The principle aim of this article is to perform a detailed analysis of the impact of the fire size on the radiative structure of lab-scale heptane pool fires. The article first reports the comparison of a new exhaustive set of measurements for a 30 cm diameter ($D$) heptane pool fire with a heat release rate (HRR) of $\dot{Q}=45$.82 kW with numerical predictions obtained from a Large Eddy Simulation (LES)-based model. The modeling strategy combines the RCFSK as a gas/soot radiative property model, a two-equation acetylene/benzene soot model and flamelet/presumed filtered density function approaches to model the interactions between turbulence, chemistry, soot and radiation. The model accurately reproduces all the experimental data, including temperature statistics, mean soot volume fraction (SVF), radiative loss to the surroundings and heat feedback to the pool surface. Second, these results are combined with previously-published experimental data and LES relative to a 15 cm diameter heptane pool with a five times lower HRR of $\dot{Q}=8$.89 kW. The experimental and numerical results reveal for the first time that, at normalized locations $(r^{\star }=r/R,z^{\star }=z/{\dot{Q}}^{2/5})$ with $R=D/2$, temperature statistics are virtually independent of the HRR whereas the SVF and the soot radiative emission term depend very little on it, both scaling with ${\dot{Q}}^{-1/10}$. A consequence is that the evolution of the radiant fraction while increasing the fire size is driven by a competition between a weak increase in the radiative emission fraction as ${\dot{Q}}^{1/10}$ and a reduction in flame transparency. These two processes are found to be balanced, explaining why the radiant fraction of the two pool fires remains almost constant. Eventually, it is found that for both 15 and 30 cm heptane pools, neglecting the radiative contribution of the heptane fuel vapor leads to a non-negligible overprediction of the radiative heat feedback, this overprediction increasing with the pool diameter.

Novelty and Significance Statement

This article uses an analysis combining theory, experiments and simulations to demonstrate for the first time that, in medium-scale heptane pool fires (diameter $D$ and HRR $\dot{Q}$), temperature statistics at normalized locations, $r^{\star }=r/R,z^{\star }=z/{\dot{Q}}^{2/5}$ with $R=D/2$, are virtually independent of the HRR whereas soot volume fraction statistics and the soot radiative emission term depend very little on it, both scaling as ${\dot{Q}}^{-1/10}$. These findings explain why the radiant fraction of medium-scale sooting pool fires remains almost constant over a wide range of pool diameters, the weak increase in the radiative emission fraction being balanced by a reduction in flame transparency. The second novelty is to quantify the effects of the fire size on the contribution of the fuel vapor to the radiative feedback. The third major contribution is an exhaustive validation of the LES model in 30 cm heptane pool fire.