Experimental study on the flame merging and ceiling impingement behavior of transversely located double fire sources in an urban utility tunnel

The investigation of the fire characteristics and propagation behaviour in the urban utility tunnel is of great practical importance, especially for the conditions with double fire sources. In this work, a 1/8th scaled urban utility tunnel model was built to conduct the experiments to characterize the flame merging and ceiling impingement behaviour of transversely located double fire sources. Two rectangular fire sources with the same dimension were used, and their heat release rate (HRR) and fire source spacings were varied to consider the typical scenarios. Results show that for the smaller HRR, the tunnel ceiling and sidewalls have little influence on the flame merging of double fire sources. With the increase of HRR, the flame gradually impinges on the tunnel ceiling and forms a stable ceiling jet flame extension, and the ceiling and sidewalls of the tunnel can promote the merging of the flames of the double fire sources. Besides, with the increase of the fire source spacings, the flame merging probability P-m can be divided into three stages, that is, (i) complete merging stage, (ii) intermittent merging stage and (iii) complete separation stage. On this basis, the predicting relation of P-m was obtained by using the piecewise function.