Experimental study on flame merging behaviors from two pool fires along the longitudinal centerline of model tunnel with natural ventilation

A set of experiments was conducted in a model tunnel to study the interaction behaviors of two pool fires. Square heptane pool fires with side lengths of 10 cm, 13 cm and 16 cm were used as the fire sources. In the experiments, two identical fires were located at the longitudinal centerline of tunnel. The fire spacing was set as an integral multiple of the half pool length. The mass loss rates of two fires in tunnel were compared with fires in the open space and it was explained from the viewpoints of the interactive burnings and the heat feedback mechanism. Results showed that as the spacing decreases, the mass loss rate increases first and then decreases. Similar trends can be found in heat fluxes received by objects at the pool surface level. With decreasing the spacing, the longitudinal ceiling flame shapes are divided into four categories, i.e., non-interaction flames, tilted flames without merging, merging ceiling flames and merging vertical flames. The image processing method is used to judge the flame merging and to obtain the position of merging point and the flame length. An expression correlating the distance from the merging point to the ceiling and the normalized spacing is established. By using dimensional analysis and introducing a correlation factor, a correlation for predicting the effective ceiling flame length is developed, involving the heat release rate, pool size, spacing and effective tunnel height. Moreover, a method to estimate the flame radiant fluxes received by the objects located in the lower layer of tunnel with vertical and horizontal orientations is proposed to further reveal the mechanism of heat radiation from two interacting fires in tunnel.