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Most previous work concentrated on the thermal smoke temperature distribution in traditional single tunnel fires. However, few investigations have studied the effects of branch tunnel and longitudinal fire source location on thermal smoke temperature beneath the ceiling. In this paper, a series of experiments were performed in a 1:10 reduced scale branched tunnel to investigate the longitudinal fire location effects on the thermal smoke temperature distribution beneath the ceiling. Thirteen longitudinal fire locations and three heat release rates were considered. The main conclusions are summarized as follows: The thermal smoke temperature near the fire source is obviously different for various longitudinal fire locations, while the temperature in the area far from the fire source is basically the same. The dimensionless longitudinal thermal smoke temperature beyond 0.75 m downstream from the fire source in the main tunnel is not obviously affected by the longitudinal fire location, which can be well predicted by Li's model. However, the attenuation coefficient is obviously larger than that in traditional single tunnel due to the existence of branch tunnel. Moreover, when the fire source moves away from the intersection region to upstream or downstream, the thermal smoke temperature in the branch tunnel shows a decrease. A modified model is proposed to describe the longitudinal temperature decay in branch tunnel taking the longitudinal fire location into account, which is reasonably well fitting with experimental results. The results in this paper are essential for better understanding of thermal smoke temperature in branched tunnel and providing references for fire safety design of branched tunnels.

Summary A series of upward flame spread experiments over two‐sided long thin flax fabric sheets were conducted at different inclination angles from horizontal (0°) to vertical (90°). With the inclination increased to a critical angle, the flame on the lower side of the 4 and 6 cm wide samples can be blown off spontaneously. After the lower‐side blow‐off, the remained flame on the upper side spread steadily to the end of the sample. To clarify the controlling mechanism, the flame spread rates and flame geometry characteristics were investigated and compared in different inclination angles. The maximum flame spread rate and flame length monotonically increased with the inclination angle under the assistance of self‐induced buoyancy. However, the steady flame spread rate and flame length on the remained side dropped significantly after one‐side extinction. The lower side flame was blown off by the large buoyant flow velocity induced from the critical flame length, which is controlled by the small Damköhler number. Flame thickness and flame tilt angle were analyzed at varied inclinations to help understand the heat transfer from the flame to the unburnt fuel surface. Two opposite effects of self‐induced buoyancy were proposed at different ranges of inclination: the thermal assistance regime at the lower inclination angle and the kinetical extinguishing regime at a higher inclination angle. They are dominated by the heat transfer from the flame and local fluid dynamic around the flame base, respectively.

Up to 2021, most previous work focused on upward flame spread over thin solid fuel completely attached to objects or with both sides freely exposed to the air, but did not take the restricted distance (distance between fuel and objects) effects into account. In this paper, the restricted distance effects on upward flame spread over thin solid fuels were investigated using 0.65 mm thick, 120 cm tall and 6.0 cm wide flax fabric sheets under various restricted distances of 1.0–3.5 cm. The essential parameters were monitored and analyzed simultaneously, including flame length, pyrolysis spread rate, surface temperature and ignition time. The main conclusions drawn are as follows: when the restricted distance is no more than 1.5 cm, the flame length on the unrestricted side is larger than that on the restricted side, whereas the variation exhibits the opposite trend when the restricted distance is beyond 1.5 cm. As the restricted distance increases from 1.0 to 3.5 cm, the flame length and flame spread rate first increase and then decrease, reaching a maximum value at 3.0 cm restricted distance, whereas the ignition time shows the opposite trend. The decrease rate of the surface temperature with the distance from the pyrolysis front first drops and then rises as the restricted distance increases, which qualitatively characterizes that the heat flux received by the virgin surface first increases and then decreases with restricted distance. The non-monotonic trends of heat flux received by the virgin surface and consequently the flame spread rate as a function of restricted distance are due to the combined restricted distance effects of the chimney effect, wall radiation and restricting oxygen supply. The results of this paper are not only helpful in better understanding the upward flame spread over a thin flax fabric under restricted distance, but also provide some basic data for fire prevention of thin solid fuels.