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Investigations of ventilation in an immersed tunnel have recently drawn greater research attentions; however, analyses on the influence of vent design and tunnel width on ventilation performance have rarely been addressed. For the sake of the security of evacuees in an immersed tunnel fire, the influence of three vent designs and two immersed tunnel widths on mechanical ventilation performance during tunnel fires were numerically investigated using large eddy simulation. The pollutant gas flow characteristics in the tunnel after a fire were analyzed, and the pollutant gas exhaust efficiency based on the mass conservation of carbon monoxide in the smoke was proposed in this study. By comparing the smoke propagation, smoke distribution, and exhaust efficiency between three different vent designs, it was determined that the Top Vent Design has the best smoke exhaust effect, and the Sidewall Vent Design (with an activated vertical smoke screen) has a better smoke exhaust effect than the Sidewall Vent Design. The influences of the tunnel width and heat release rate of the fire on the ventilation effect were also investigated.

• A novel mechanical board-coupled ventilation shaft was introduced. • Smoke extraction pattern would shift due to the coupling-effect between the shaft and the fan. • A model to analyze the dynamic of the synergistic smoke extraction was established. • The novel shaft can significantly improve both smoke and heat extraction performance. Ventilation shafts are widely equipped in extra-long road tunnels. Due to the plug-holing phenomenon, it may be impossible to improve smoke extraction performance solely by increasing extraction velocity in a traditional ventilation shaft. Taking reference from practices in mine ventilation shafts, a novel mechanical board-coupled ventilation shaft (MBCS) was introduced. A total of 120 experiments in a 1:20 reduced-scale tunnel model were conducted to validate the effectiveness of the novel shaft. The effects of shaft height, board location, and extraction velocity were investigated, and the interaction dynamic mechanism of synergistic smoke extraction between fan-driven and shaft-driven was revealed. The results indicate that the novel MBCS can achieve better smoke extraction performance as the extraction velocity increases. Besides, compared to the traditional MBCS in our previous study, the smoke extraction efficiency of the novel MBCS improves by 19.8 %, the heat extraction performance improves by 75.7 %. Moreover, based on theoretical and dimensional analysis, a model to analyze the dynamic mechanism of synergistic smoke extraction was established, which may provide positive insights for smoke control and extraction design both in extra-long road tunnels and mine ventilation shafts. [ABSTRACT FROM AUTHOR]

In this study, a horseshoe cross-sectioned, short tunnel was considered. To simulate fire scenarios, a rectangular fire pool was placed in centreline of the tunnel floor. At different cross-sections of the tunnel, jet fan groups were installed and each fan group includes three jet fans. In order to simulate the fire induced smoke, the species transport model was used. Fires with 10 and 50 MW were considered for the simulations. The effects of LVS (Longitudinal Ventilation System) on the smoke movement and temperature distribution were numerically investigated by using the commercial CFD code FLUENT. Especially, simulations were conducted for the worst scenarios. After the fire induced smoke filled a big part of the tunnel, in order to determine the optimal ventilation approaches, combination of different fan groups were operated with various pressure jumps. From the simulations, it was observed that the smoke propagation was faster than the long tunnel studies found in the literature. It was also observed that the smoke velocity increases toward the tunnel entrance in the backlayer. In order to prevent this uncontrolled smoke movement, the best charge velocities and operation order of jet fan groups were determined numerically. It was observed that the operation order of fan groups, pressure jump values of fans and start time of the fans are important parameters to extinguish. [ABSTRACT FROM AUTHOR]

Abstract: Full-scale and reduced-scale measurements along with numerical simulations have been carried out in order to increase fire safety in car parks, with particular focus on the smoke back-layering (SBL). SBL is the distance covered by the smoke upstream of the ventilation flow with respect to the fire source. It has been measured in both full- and reduced-scales and the results are compared to the literature. The investigated car park (full- and reduced-scale) has been simplified: ceiling and walls are plain and the final layout is similar to a very wide road tunnel. Horizontal and vertical beams supporting the ceiling are not considered. Therefore the results obtained are only valid for large closed parks with a flat ceiling and uni-directional smoke and ventilation patterns within the investigated heat release rate range. The study serves double purposes: Empirical formulae can be obtained and the best model for SBL can be chosen from the existing correlations for road tunnel fires to be applied to car parks with the above described simplified geometries. If the results of the reduced-scale car park model are in accordance with the full-scale ones, the reduced-scale model can be used for more complex car park layouts, which is a very favorable solution in terms of both time and costs. In addition to the SBL investigation, unexpected flow phenomena at the entrance and extractors, respectively, are also explained by reduced-scale trials. [Copyright &y& Elsevier]

Abstract: In case of fire in an atrium, a smoke and heat control (SHC) system can be designed to improve safety inside the atrium. An important design criterion is the smoke free height in the atrium. This smoke free height is the result of a number of parameters, of which the fire heat release rate (HRR), the SHC extraction mass flow rate, the position of the extraction device or opening and the size and position of openings for make-up air are of primary importance. In the present paper, an extensive numerical study, based on computational fluid dynamics (CFD), is presented in which these parameters are varied. The presence of a downstand or balcony is not covered in the study at hand. From the results, a correlation is presented for adhered spill plumes in atria without downstand, relating the smoke extraction rate to the smoke free height in the atrium. The smoke layer in the atrium is illustrated to become multi-dimensional beyond a certain threshold value of the smoke extraction rate and existing correlations, which do not take this phenomenon into account, are not conservative. The position and exact size of the make-up air inlet openings are shown not to affect the observations, as long as the openings are sufficiently large. [Copyright &y& Elsevier]

• The availability and stability of the BCSs in deeper buried tunnel is explored. • BCS is effective and has stable smoke extraction performance in deeper buried tunnels. • Smoke extraction efficiency of the BCS will be improved when the HRR increase. • Correlation function for CO concentration and temperature in the shaft is deduced. • Dimensionless volume flow rate ( V s ∗) of the BCS under different HRR and h D is deduced. In theory, if the plug-holing phenomenon can be effectively eliminated, the application of natural ventilation with vertical shaft will no longer be limited to shallow buried tunnels, or even extended to deep buried tunnels. Researches have validated that the board-coupled shaft (BCS) can efficiently eliminate the plug-holing phenomenon and promote the smoke extraction performance in a shallow buried tunnel. The purpose of this paper is to explore whether the BCS is effective and has stable smoke extraction performance in deep buried tunnels. Experiments were conducted in a 1/20 scale model tunnel, and the height of the board-coupled shaft applied in this study is 5 times the height of the tunnel height. The influence of heat release rate (HRR) and board location on the smoke extraction performance was investigated. Experimental results indicated that the BCS is effective and has stable smoke extraction performance for the deep buried tunnel applied in this study. Besides, it can be found that the smoke extraction efficiency of the BCS will be improved and the improvement in the smoke extraction performance with BCS is stable when the power of the fire source increases. Also, the optimal board location for the BCS to achieve the peak smoke extraction performance was obtained and the peak smoke extraction performance of the BCS is about 1.451–1.457 times of the traditional shaft. Beyond that, two dimensionless correlations were developed under the general condition that the first shaft cannot extract the entirety of the smoke, one is the correlation function for analyzing the smoke temperature difference and the CO concentration in the shaft; the other is the correlation for predicting the dimensionless volume flow rate under different HRRs and board locations. These studies may contribute positive significance for promoting the application of natural ventilation with vertical shafts. [ABSTRACT FROM AUTHOR]