Effect of ullage on burning behavior of small-scale pool fires in a cavity

TLDR: In this article, the effect of ullage height on steady mass burning rates in methanol pool flames in a cavity was studied and two burner diameters were used to study the effect.

Abstract: Experiments have been conducted to study the effect of ullage height on steady mass burning rates in methanol pool flames in a cavity. Two burner diameters are used. At low ullages, the flame dynamics are found to be effective in altering the mass burning rates. From baseline case with almost zero ullage, as the ullage is increased, mass burning rate decreases. It produces a local minimum at a given ullage based on the burner internal diameter. After this point, the mass burning rate increases with increasing ullage and reaches an almost uniform value. Numerical simulations are used to complement the results of the experimental study. Low ullage cases have been simulated using a validated numerical model that uses global single step chemistry, partial equilibrium for carbon-dioxide oxidation and optically thin approximation based radiation model. An axisymmetric domain has been employed. Even though the mass burning rates have been over-predicted by the numerical model, the variation trend has been captured quite well. Results from the numerical model reveal that for very low ullage, flame is phenomenally steady and mass burning rate is higher as the diffusion flame anchors around the rim. As the ullage is increased, a transient flame is seen to anchor around the rim and due to increased flame stand-off, the mass burning rate decreases. When the ullage is further increased, due to axial flapping of the flame that partially covers the burner, oxygen is transported into the burner, causing a recirculation pattern within the burner and partial premixing of fuel vapor and oxygen. As a result, the mass burning rate increases.