Experimental study on the fire shape and maximum temperature beneath ceiling centerline in utility tunnel under the effect of curved sidewall

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To study the effect of curved sidewall on the fire shape and maximum temperature beneath ceiling centerline, a series of experiments were conducted in a scaled concrete utility tunnel model (at a ratio of 1:4 to the real size) with the centerlines of 0, 15, 30, 45 and 60 cm. The flame height, temperature beneath ceiling centerline under different conditions were observed to analyze the influence of the curved sidewall in the utility tunnel. The results showed that: ① The flame shape can be bent by curved ceilings, while the flame did not touch the curved ceiling. This can be explained by Bernoulli principle. When flame touched the curved ceiling, a correlation for the transverse flame length was obtained in terms of flame height under free condition and the correlation was consistent with previous research. ② The characteristic temperature of smoke layer was employed to correct the ambient temperature considering the acceleration of smoke in the utility tunnel. ③ On this basis, relevant equations for maximum temperature beneath ceiling centerline were established. Referring to the maximum temperature under the inclined ceiling, the formula (Eq. (26)) was established if d ≠ 0. The concept of characteristic angle was employed to equal the buoyancy effect on jet under the curved ceiling. Referring to Li’s model, the formula (Eq. (28)) was established if d = 0 cm. These formulas above could predict the maximum temperature beneath the ceiling centerline with the relative error within 15% and 10%, respectively.