Experimental study on the dynamic evolution of merged flame under the thermal effect of multiple fire sources in a shaft

Special flame merging behaviour and complex combustion characteristics can be formed under the coupling effect of tangential flow and heat transfer of multiple pool fires (MPFs) in a shaft. In this study, quadruple pool fires (DPFs) evolutionary experiments in shaft were carried out, and the influence of pooled spacing and side slit width of the shaft on flame merging behaviour were analyzed. The primary influencing mechanism of merged flame evolution was expounded from heat transfer and thermal feedback between fire sources. The shape of conical merged fire whirls that can completely fill the gap between the oil pools with flame is confirmed, and its cone angle is determined by the position of outer edge of oil pools. In the formation of merged fire whirls, the critical conditions are that the distance between fire sources is less than three times the pool diameter and the width of side slits is 1/4 of the side length of the shaft, respectively. Compared with the single pool fire whirls with the same liquid surface area, the thermal flow field of DPFs in shaft is relatively unfavorable to the formation of fire whirls. The formation time of merged fire whirls is nearly five times that of the single pool fire whirls, and the flame height can be reduced by nearly half.The evolution process of the DPFs that can form merged fire whirls exhibits the multi-stage morphological characteristics over time and all of these stages possess different heat transfer mechanisms. Before forming the merged fire whirls, intermittent merging among pool fires is a necessary stage. And the duration of this stage is relatively less affected by the width of slit on the side of shaft. The transverse force of asymmetric tangential flow on the flame is difficult to be completely overcomed by merged fire whirls. Because of the inclination of merged flame, the overall vertical height is unsuitable as a direct characterization parameter of combustion intensity.