In this study, changes in the droplet formation mechanism and the law of droplet length in a two-phase liquid–liquid system in 400 × 400 μm standard T-junction microchannels were experimentally studied using a high-speed camera. The study investigated the effects of various dispersed phase viscosities, various continuous phase viscosities, and two-phase flow parameters on droplet length. Two basic flow patterns were observed: slug flow dominated by the squeezing mechanism, and droplet flow dominated by the shear mechanism. The dispersed phase viscosity had almost no effect on droplet length. However, the droplet length decreased with increasing continuous phase viscosity, increasing volume flow rate in the continuous phase, and the continuous-phase capillary number Cac. Droplet length also increased with increasing volume flow rate in the dispersed phase and with the volume flow rate ratio. Based on the droplet formation mechanism, a scaling law governing slug and droplet length was proposed and achieved a good fit with experimental data.

Urban tunnels are generally narrow and fire smoke can hardly diffuse. In the present study, numerical simulation is used to analyze the diffusion of high temperature smoke produced by fire inside a specific tunnel (the Kaiyuan tunnel). The results are compared with similar data relating to other tests to determine the validity of the numerical method. Moreover, the critical velocity obtained by numerical simulation of 5 MW, 20 MW, and 50 MW fires in curved and linear sections of the considered tunnel is compared with the values obtained using empirical formulas. The results show that, for the tunnel ventilation design, it is necessary to consider the fan pressurization at different sections and the fan pressurization should be higher at curved sections than that at linear sections. The safety of personnel escaping under different critical velocity values in the linear section has also been considered. On the basis of our findings, if only relying on natural ventilation, people can escape safely for the case of small fires, whereas for medium and large fires, it is necessary to turn on mechanical ventilation in time (and in order to avoid the danger caused by rapid diffusion of smoke, the timing of mechanical ventilation should be carefully tuned).

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