In tunnel fire, smoke is a great threat to successful emergency escapes, and its spreading patterns at high altitude tunnel may differ. Therefore, the impact of ambient pressure on the stability of smoke layers and maximum smoke temperature under ceiling in ventilated tunnels were investigated in this study. Results show that the temperature of smoke layer is negatively correlated with the longitudinal velocity and ambient pressure. Besides, the influence of longitudinal velocity at different ambient pressures on the shear velocity between the smoke and air layers was further studied. Using Richardson number (Ri) and Froude number (Fr), smoke flow can be classified into three patterns, namely stable and obvious smoke stratification (Ri>3.2 or Fr < 0.38), a stable smoke layer but with blurred interface (2.3 < Ri < 3.2 or 0.38 < Fr < 0.53) and a completely unstable smoke stratification (Ri < 2.3 or Fr > 0.53). Furthermore, a model has been developed to predict maximum smoke temperature under the tunnel ceiling, which agrees well with previous studies at standard atmospheric pressure. The results of the present work provide information for tunnel structure protection and safe evacuation in fire accidents at different ambient pressures.

The effects of parallel curtain walls on the characteristics and mass loss rate of the upward flame spread over polymethyl methacrylate (PMMA) have been experimentally studied. The experimental research variables were the sample size and separation distance of the curtain wall. In the experimental setup, a PMMA plate was attached to one of the curtain walls. The results were analyzed to assess the effect of the curtain wall separation distance on the flame height. The special condition of two curtain walls with only a small distance between them was also analyzed. Analysis of experiments with systematically varied distances between the curtain walls has provided insight into factors such as air entrainment and the chimney effect. The results show the flame height evolution trend with the separation distance, and a new correlation to predict the global mass loss rate of the PMMA plate under the influence of parallel curtain walls, which can potentially be used in curtain wall design through optimization of the separation distance given fire safety requirements and practical needs. Graphical Abstract: [Figure not available: see fulltext.]. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.

This paper describes the quantitative study on the effects of tunnel longitudinal ventilation velocities on the critical excess heat release rate and intermittent flame ejecting behaviors of carriage fires. Experiments were carried out in a 1/8 model tunnel with the dimensions of 22 m x 1.2 m x 0.8 m (length x width x height). 442 repeatable test conditions contained gas fuel (propane) and solid fuel (corrugated paperboard), including variations on fuel supply rates, tunnel opening sizes, and longitudinal ventilation velocities. The intermittent flame ejecting behaviors were expressed as the flame ejection probability in carriage fires, which was characterized by image statistical processing. The results showed that the flame ejection probability was positively correlated with the fire heat release rate, but it was negatively correlated with the ventilation factors of carriage opening and longitudinal ventilation velocities. New non-dimensional correlations between the critical excess heat release rate and the longitudinal ventilation velocities are proposed. Moreover, a piecewise equation is proposed to describe flame ejection probability that is affected by longitudinal ventilation velocities for the first time. All the proposed correlations are found to fit well with the experimental data, which established an essential foundation to quantify the critical heat release rate and intermittent flame ejecting behaviors of carriage fire in various longitudinal ventilation velocities. Finally, the some limitations of propane gas fuel are analyzed. Moreover, this paper depicted the flame ejecting behaviors induced by gas fuel and solid fuel, and reveal the difference and relationship between them.

This work focused on the study of smoke plug-holing phenomenon with the effect of lateral smoke extraction system in tunnel fires. The reduced tunnel model was used to conduct experiments with various test conditions, the smoke plug-holing phenomenon at extraction vents with different aspect ratios were analyzed. It was found that, the smoke temperature rise under tunnel ceiling firstly decreased as the increasing the lateral smoke extraction velocities, and then show a trend of increase or remained stable, which can be explained by the plug holing phenomenon occurs as lateral smoke extraction velocity reaches a certain value. As the aspect ratio of the lateral smoke extraction vent increases, the smoke extraction velocity required for the smoke layer occurs plug holing phenomenon also increases correspondingly, and the previous classic empirical description model for the smoke plug-holing criterion is no longer suitable for tunnel fires with lateral smoke extraction. Then, a revised Froude number is proposed to determine smoke plug-holing phenomenon in a tunnel under the effect of lateral smoke extraction, the updated criterion can be used to evaluate smoke plug-holing phenomenon of tunnel fires with lateral smoke extraction system. The critical Froude numbers were approximately between 1.5 and 1.75. Meanwhile, with the aspect ratio of lateral smoke extraction vent increased, the critical Froude number also increased accordingly, the increase in the aspect ratio of lateral smoke extraction vent, the occurrence of smoke plug-holing phenomenon will be further reduced.

Ceiling thermal impingement fire spread induced by indoor uncontrollable energy combustion occurred frequently and caused many casualties and property damage. The sub-atmospheric pressure environment has a great impact on uncontrollable energy combustion characteristics of hazardous combustible gaseous fuel fire. China west high-altitude cities develop rapidly due to the economic development, and fire accidents frequently occur. This paper focused on experimentally studying the ceiling fire impingement hazard characteristics induced by wall-attached fires under sub-atmospheric pressures, the various sub-atmospheric pressures (from 55 kPa to 100 kPa), energy heat release rates (from 0.25 kW to 2.5 kW), and source-ceiling heights (0.475 m, 0.38 m, 0.285 m, 0.19 m and 0.095 m) were selected as an variable. The ceiling flame extension lengths of facing and lateral directions were measured and analyzed. Result showed that, the dimensionless ceiling flame extension area is higher in sub-atmospheric pressures. There are many differences between them due to the lower buoyancy in various sub-atmospheric pressures. By accounting for the air entrainment change characterization, it is found that the air entrainment of wall-attached propane jet fire is weaker in sub-atmospheric pressures. The influence of pressure on the flame expansion area is related to the 4/5 power of the ratio of the entrainment coefficient ((alpha/alpha(100))(4/5)). And a new model was finally obtained to correlate ceiling flame extension area induced by wall-attached fires with various sub-atmospheric pressures. This work provides can help understand ceiling impingement flow and hazard characteristics under various sub-atmospheric pressures. (C) 2020 Elsevier Ltd. All rights reserved.

Laboratory tests were firstly conducted in a reduced scale channel to investigate the conditions for flame ejections when the carriage was at the centre of the channel. The ejected flames were recorded by two cameras and the temperature inside the enclosure and on the ceiling were measured. The lower critical heat release rate (HRR) which would result in intermittent flame ejection and upper critical HRR which would lead to continuous external flame were analysed with regards to their variations with the ventilation factor. Correlations for the longitudinal and transverse flame extension lengths and flame extension area beneath the ceiling were proposed. Subsequently further experimental investigations were conducted to study the effect of sidewall constraint on flame extension by changing the position of the carriage along the transverse direction to vary the distance between the sidewall and the carriage opening, which was also systematically varied. With the decrease of the sidewall-to-opening distance, the longitudinal flame extension length was found to increase whereas the transverse flame extension length decreased. The changes are most significant when sidewall-to-opening distance was relatively small. These trends are different from those observed in previous wall-attached fires or corner fires in channels, where the flame extension length beneath the ceiling firstly increased with the decrease of sidewall-source distance, and then decreased slightly when the fire source was attached to the wall. New correlations were proposed to account for the effect of sidewall-to-opening distance for longitudinal and transverse flame extension lengths under the ceiling. They captured well the measurements for all the present cases and some published cases not used in their derivation, demonstrating the potential for fire safety engineering applications. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Radiant heat flux of fire plume is an essential parameter indicating the hazards of a fire. In this work, fire experiments of a wall-attached fire with different aspect ratios were conducted. In addition, the effect of the sidewall on mean flame height and radiant heat flux of the rectangular fire was investigated. The sidewall causes a decrease in air entrainment; therefore, the mean flame height is observed to increase. The radiant heat flux increased owing to the increase in flame height and thermal radiation from the heated sidewall. Experimental results of the mean flame height are consistent with those predicted by an existing model. This validated the reliability of the experimental setup for a rectangular wall-attached fire. Furthermore, a model based on the increase in flame height and thermal radiation from the heated sidewall was developed to estimate the radiant heat flux from a rectangular wall-attached fire. The error in prediction was less than 15% for this model.

The present study investigated experimentally the effects of relative strong crosswinds on the flame geometry characteristics (flame height and flame horizontal length) from carriage fires in a tunnel, which have not been quantified previously. Overall 144 test conditions were involved for various heat release rates, crosswind speeds and opening sizes. It was found that, with increase in crosswind speed till relative strong conditions, the flame height first decreased then remained nearly unchanged, while the flame horizontal length first increased then decreased. Finally, the flame is blown-off at strong crosswind conditions. A physical model is proposed to interpret and characterize the evolution behavior of these flame geometry characteristics with crosswind till relative strong condition. Two non-dimensional parameters, the wind Froude number and the dimensionless excess heat release rate, were found based on the analysis of the controlling mechanisms including the flame tilt and air combustion with excess fuel outside the carriage. The experimental results were shown to be well represented by the proposed parameters. This study provides quantitative data and basic understanding of the flame geometry characteristics under relative strong crosswinds from carriage fires in a tunnel.(c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.