Experimental analyses and modeling of pool fires with different ullage heights

In recent years, floating-roof tanks have been widely used for liquid fuel storage in China. However, these tanks suffer from a considerable fire risk owing to the accumulation of highly flammable fuel vapors when the liquid fuel level is low. The risk of pool fires, confined by ullage heights, seriously threatens fuel storage and transportation. Furthermore, the flame characteristics, including burning rates and flame heights, are notably affected by ullage heights. However, the research on pool fires with different ullage heights is few. Hence, this study aims to experimentally study the pool fires for different ullage heights and analyze the key parameters (burning rate and flame height). Additionally, a correlation based on dimensionless analysis is proposed to predict the down-reaching flame height.

This study investigates the effect of ullage heights on pool fires. A series of pool fire experiments were conducted using a transparent quartz glass tray and heptane fuel. Different ullage heights (ranging from 3 cm to 50 cm, measured as the vertical distance from the tank top to the liquid fuel surface) were considered. The burning rate was measured using a Sartorius balance and video cameras were installed to record the burning process and flame heights. Flame heights were calculated by converting the flame videos into binary images. Subsequently, the key parameters were analyzed.

The experimental results reveal the following: (1) The ullage height considerably affects the burning process, particularly at the initial and steady stages. At the initial stage, the burning rate sharply increases for cases with a low ullage height, while a burning rate decrease trend is observed after the rapid increase for cases with a large ullage height. This is mainly caused by the continuous uplift of the flame base, moving away from the fuel surface. (2) At the steady stage, the burning rate first decreases and then increases, followed by a final decrease with the increase of the ullage height. The increase in the distance between the flame base and the fuel surface results in this burning rate decrease trend. The burning rate increases because the flame base enters the tray. (3) Based on the flame shape in cases with a large ullage height in the steady stage, total flame height can be divided into the upper flame height outside the tray and the down-reaching flame height inside the tray. The experimental data demonstrates that the upper flame height decreases as the ullage height increases, whereas the down-reaching flame height shows an opposite trend. (4) A correlation is developed using the experimental data and dimensionless analysis to calculate down-reaching flame height with different ullage heights, in which the characteristic tray diameter is revised by considering the influence of ullage heights and air entrainment.

The findings of this study will contribute to the understanding of the burning behaviors of fires at different ullage heights, with practical implications in providing guidance for quantitative risk assessment in tank fires.