Smoke Layer Research Articles

Numerical study on the airflow characteristics and smoke control condition in stair area of subway stations

As the key facility connecting different floors in subway stations, the smoke control in the stair area has been one of the most important goals of fire protection design. But the detailed airflow field and smoke spread process are rarely discussed. With numerical simulation, this work investigated the airflow velocity distribution and critical condition for smoke control in two critical openings of the stair area, which include the opening beneath the ceiling screen and the opening on the top of the stair. The results indicate that (1) In spatial dimension, the airflow velocity distribution shows a nonuniformity within each opening and a difference between two openings. (2) In the temporal dimension, the airflow velocity shows a normal distribution characteristic. The strong turbulence leads to a dynamic airflow field with an unstable smoke spread process. (3) For the critical opening beneath the ceiling screen, the airflow velocity is dominated by thermal buoyancy or inertia force in different areas. The practical smoke control criterion is to ensure the smoke layer is higher than the edge of the ceiling screen. (4) For the critical opening on the top of the stair, airflow velocity shows an exponential decay distribution. The critical condition could be predicted with a virtual fire source within the stair. This work hopes to present the necessity of dynamic and microscopic modeling of the airflow - smoke flow interaction for an accurate prediction of the smoke control condition in the stair area of subway stations.

Performance-Based Design Assessment of Temperature Tenability Condition in a Hotel Atrium

Fire safety risk assessment to prevent loss of human life and property is necessary for complex structures where the traditional prescriptive designs are inapplicable. Maintaining tenable temperature conditions is essential for the safe evacuation of building occupants. This study considered British Standards (BS) 7974 and the Code of Practice for Fire Precautions in Buildings to analyze the temperature tenability conditions in the hotel atrium through a performance-based design approach. Further, the Computational Fluid Dynamics (CFD) code, Pyrosim, a Graphical User Interface (GUI) of the Fire Dynamics Simulator (FDS), was utilized to simulate a constant Heat Release Rate (HRR) steady state design fire of 5 Megawatts (MW) igniting from the hotel brasserie (ground floor) within the atrium. The researcher used the big design fire of 5MW to generalize the results to similar fires in similar structures. The objective was to conduct a fire safety risk assessment to prevent loss of property and fatalities by activating a smoke management control system with a ‘what if’ condition when the fire suppression system failed to start. Temperature data was collected through the statistical steady state profile, smoke layer temperatures, thermocouples, and 2-dimensional slice temperature recordings. The ground floor, the fire source, was found to have untenable conditions for human survival with temperatures above 2250C, as confirmed by thermocouple and steady-state temperature profile readings. Other floors demonstrated tenable conditions for human survival with temperatures below 400C due to the effect of natural vents and mechanical extractors, which extracted the hot smoke at a rate of 15m/s. The temperature recordings from this study compared well with those from Chew and Liew’s (2000) Computational Fluid Dynamics results, which had fire igniting within the atrium, without sprinklers, and had mechanical extraction fans and a constant Heat Release Rate of 5 Megawatts (MW). The results indicated that the smoke control management system maintained tenable temperature conditions within the hotel atrium, even without the fire suppression system through sprinklers. Considering the design fire size was 5 MW, which was significantly big, the results could be generalized for other hotels or structures with similar designs.

Experimental Study on the Smoke-Logging Phenomenon Owing to Sprinkler Operation During Initial Fire

This study establishes basic data for the reasonable evaluation of evacuation safety and investigates the smoke-logging phenomenon owing to the operation of sprinklers during the initial fire. The spray droplet characteristics of a sprinkler and water mist head were examined, and the descending air current of the smoke layer was analyzed according to the type of fire source (ethanol, polyurethane) and head. A regression equation based on the findings was derived using the prediction equation for the descending air current of the smoke layer presented in a previous study.

Influence of the water spray particle diameter and flow rate on the smoke flow and the back-layering in tunnels with longitudinal ventilation

When there is a fire in a tunnel, the heat released by the combustion is difficult to dissipate and the temperature inside the tunnel would rise rapidly. Longitudinal ventilation (LV) has been commonly adopted as the smoke control strategy in tunnel fires, which is essential to safeguard the tunnel structure and the trapped people. Besides the smoke control method by LV, the water mist system (WMS) can provide a powerful cooling effect for the high-temperature smoke in the tunnel. This article aims to investigate the impact of WMS on the smoke back-layering (BL) length with LV and the mutual effect process between the smoke spread and the WMS. The findings show that the BL length is restricted more effectively by the combined action of water mist and LV. Upon activation of the WMS, the water mist’s effect inhibits the smoke’s ability to spread upstream and causes the smoke layer to settle down, which disturbs the stability of the smoke layer. An unstable layer of smoke is more likely to be blown downstream by longitudinal ventilation. All water mist particle diameters show a significant reduction in smoke back-layering with an increased flow rate. The BL length would gradually reduce as the particle diameter is decreased. The tendency of flow rate and particle diameter on the BL length shows similar effects to the critical velocity ( Vcr). The Vcr is reduced with the rising flow rate and increased with expanding particle diameter.

Experimental study on the descent distance of fire smoke at the corridor corner

In this paper, a series of experimental studies were conducted in a L-shape scaled corridor to investigate the fire induced smoke descent behavior at the corridor corner. The effects of fire source location and fuel pool size on the smoke descent distance were studied. The variation of the thickness of smoke layer in the L-shaped corridor and the temperature attenuation under the ceiling were elucidated. Results showed that the smoke layer height decreased significantly at the corner. The descent distance increased with the increase of fuel pool size. The temperature of the incoming smoke decreased as the distance between the fire source and the corner increased, the Ri number of smoke layer also decreased. The effects of gas temperature, velocity and thickness on smoke settling distance were analyzed by experiments. A prediction model for the smoke descent distance at the corridor corner was proposed.

An Investigation of Non‐Spherical Smoke Particles Using CATS Lidar

AbstractSmoke particles originating from biomass burning events are typically assumed to be spherical, yet non‐spherical smoke particles are also reported from in situ observations. The spatial and temporal distributions of non‐spherical smoke particles, which could have impacts on passive‐ and active‐based satellite aerosol retrievals, are not yet well understood. In this analysis, using NASA's Cloud Aerosol Transport System (CATS) lidar data during the biomass burning season over Africa and South America from 2015 to 2017, we studied the frequency and distribution of non‐spherical smoke particles. A supplemental smoke aerosol typing algorithm was developed to identify aerosol layers containing non‐spherical smoke particles which could otherwise be misclassified as dust or dust mixture using the CATS standard aerosol typing algorithm. Approximately 30% of smoke layers over Africa and South America are non‐spherical (depolarization ratio >0.1) and align with dry biomes of low soil moisture values. Conversely, spherical smoke layers (depolarization ratio <0.1) are in moist regions. The modified algorithm with improved discrimination of non‐spherical smoke detection using CATS depolarization ratio was further verified with the National Oceanic and Atmospheric Administration Hybrid Single‐Particle Lagrangian Integrated Trajectory model, Aerosol Robotic Network Ångström exponent retrievals, and National Centers for Environmental Prediction/National Center for Atmospheric Research Reanalysis soil moisture data. This study highlights the limitations of current aerosol typing algorithms and the potential of algorithms employing ancillary data to improve aerosol typing such as multi‐wavelength volume depolarization ratio measurements or synergy with passive sensors to further discriminate between aerosol types from spaceborne elastic backscatter lidar.

Experimental investigation of impact of fire size on heat transfer and flame behavior of initial stage unsteady pool fires inside a cubical enclosure

The ventilation equipment for enclosed spaces or office rooms is specified according National Building Code of India published by the Bureau of Indian Standards. Natural ventilation peri-odically together with mechanical ventilation is recommended to remove pollutants. The need to study fire and smoke behavior inside a completely closed room with air intake and exhaust vents becomes important in case of low or no mechanical ventilation service. An experimental study on unsteady heptane pool fires of different sizes in their initial stages was conducted in a cubical fire test chamber of 27 m3 inside dimensions. The compartment was naturally ventilated with a typical configuration of a vertical intake on a side wall and an exhaust vent at the ceiling leading into a duct. Three circular pans of diameters 0.34, 0.47 and 0.61 m were employed to generate the fire with n-heptane as fuel on a bed of water. Temperatures, wall heat fluxes and mass loss rate were measured. The flame was visualized using a video camera through a tempered view glass. The total heat transfer to the ceiling and wall increased with the increase in fire size as the flames became taller in the initial stages (3-4 minutes) with sig-nificant increase in case of 500 kW fire. The smoke layer was observed at about mid height (1.5 m) above floor. The leaning behavior of flames was seen due to naturally induced air inflow. The wall heat flux of about 50 kW/m2 obtained indicate hazardous environment for further flame spread. A fourfold increase in mass loss rate was observed with just 2.5 times increase in fire size inside the ceiling vented compartment.

Fire Hazard Analysis on Different Fire Source Locations in Multi-Segment Converging Tunnel with Structural Beams

To investigate the fire risk in a complex tunnel with varying cross-sections, sloped structures, and dense upper cover beams, this study considered four fire source positions: the immersed tube section, confluence section, highway auxiliary road section, and four-lane sections of the main line. It also considered four beam spacings: 1 m, 1.8 m, 3.6 m, and 7.2 m. The Fire Dynamics Simulation Software FDS was utilized to create a comprehensive tunnel model. The analysis focused on temperature and visibility changes at a 2 m height under a 20 MW fire condition for different fire source positions. These changes were then compared with critical danger values to assess the safety of evacuating personnel within the tunnel. Subsequently, this study proposed corresponding emergency rescue strategies. The findings indicated that when the beam grid spacing exceeded 3.6 m, the upper dense beam gap showed a robust smoke storage capacity, leading to a reduced distance of high-temperature smoke spread. However, this increased smoke storage disrupted the stability of the smoke layer, resulting in a heightened smoke thickness. The location of the ventilation vent at the entrance of the immersed tunnel section caused a non-uniform ventilation flow under the girder, deflecting the smoke front towards the unventilated side and decreasing visibility in the road auxiliary area. In comparison to scenarios without a beam lattice, the presence of a beam lattice in the tunnel amplified fire hazards. When the beam lattice spacing was 3.6 m or greater, the extent of the hazardous environment, which is unfavorable for personnel evacuation, expanded. With the exception of the scenario where the fire source was located in the highway auxiliary roadway, all other conditions surpassed 150 m, which is roughly one-third of the tunnel length. Consequently, more targeted strategies are necessary for effective evacuation and rescue efforts.

On smoke layer thickness in short naturally ventilated tunnels with arched roof

Understanding the characteristics of smoke layer thickness is vital for accurate fire detection, efficient smoke exhaust and safe evacuation in tunnel fire. Nevertheless, smoke flow in tunnels with arched ceiling may show different characteristics from the tunnels with flat roof which is usually studied. Theoretical analysis was conducted on the smoke flow characteristics and thickness under the curved roof of natural ventilation tunnel, and further, numerical simulations were performed to elucidate and verify the mechanism. The results show that smoke mass flow rate is positively correlated with heat release rate(HRR) and section radius. However, the smoke layer thickness is not significantly affected by HRR, and is positively related to the section radius. Based on the mass conservation, a practical prediction model for smoke layer thickness under arched ceiling was derived, and an empirical equation for estimating the total smoke mass flow rate was also proposed. The results presented fairly good agreement with simulations and experimental data from previous works.

Wildfire smoke triggers cirrus formation: lidar observations over the eastern Mediterranean

Abstract. The number of intense wildfires may increase further in upcoming years as a consequence of climate change. It is therefore necessary to improve our knowledge about the role of smoke in the climate system, with emphasis on the impact of smoke particles on the evolution of clouds, precipitation, and cloud radiative properties. Presently, one key aspect of research is whether or not wildfire smoke particles can initiate cirrus formation. In this study, we present lidar observations over Limassol, Cyprus, from 27 October to 3 November 2020, when extended wildfire smoke fields crossed the Mediterranean Basin from Portugal to Cyprus. We found strong evidence that aged smoke (organic aerosol particles) originating from wildfires in North America triggered significant ice nucleation at temperatures from −47 to −53 ∘C and caused the formation of extended cirrus layers. The observations suggest that the ice crystals were nucleated just below the tropopause in the presence of smoke particles serving as ice-nucleating particles (INPs). The main part of the 2–3 km thick smoke layer was, however, in the lower stratosphere just above the tropopause. With actual radiosonde observations of temperature and relative humidity and lidar-derived smoke particle surface area concentrations used as starting values, gravity wave simulations show that the lofting of air by 100–200 m is sufficient to initiate significant ice nucleation on the smoke particles, leading to ice crystal number concentrations of 1–100 L−1.

Reduced Pressure Effect on Smoke Layer Plug-Holing Behavior of Tunnel Fires with a Naturally Ventilated Vertical Shaft

Reduced Pressure Effect on Smoke Layer Plug-Holing Behavior of Tunnel Fires with a Naturally Ventilated Vertical Shaft

Annual cycle of aerosol properties over the central Arctic during MOSAiC 2019–2020 – light-extinction, CCN, and INP levels from the boundary layer to the tropopause

Abstract. The MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition was the largest Arctic field campaign ever conducted. MOSAiC offered the unique opportunity to monitor and characterize aerosols and clouds with high vertical resolution up to 30 km height at latitudes from 80 to 90∘ N over an entire year (October 2019 to September 2020). Without a clear knowledge of the complex aerosol layering, vertical structures, and dominant aerosol types and their impact on cloud formation, a full understanding of the meteorological processes in the Arctic, and thus advanced climate change research, is impossible. Widespread ground-based in situ observations in the Arctic are insufficient to provide these required aerosol and cloud data. In this article, a summary of our MOSAiC observations of tropospheric aerosol profiles with a state-of-the-art multiwavelength polarization Raman lidar aboard the icebreaker Polarstern is presented. Particle optical properties, i.e., light-extinction profiles and aerosol optical thickness (AOT), and estimates of cloud-relevant aerosol properties such as the number concentration of cloud condensation nuclei (CCN) and ice-nucleating particles (INPs) are discussed, separately for the lowest part of the troposphere (atmospheric boundary layer, ABL), within the lower free troposphere (around 2000 m height), and at the cirrus level close to the tropopause. In situ observations of the particle number concentration and INPs aboard Polarstern are included in the study. A strong decrease in the aerosol amount with height in winter and moderate vertical variations in summer were observed in terms of the particle extinction coefficient. The 532 nm light-extinction values dropped from >50 Mm−1 close to the surface to <5 Mm−1 at 4–6 km height in the winter months. Lofted, aged wildfire smoke layers caused a re-increase in the aerosol concentration towards the tropopause. In summer (June to August 2020), much lower particle extinction coefficients, frequently as low as 1–5 Mm−1, were observed in the ABL. Aerosol removal, controlled by in-cloud and below-cloud scavenging processes (widely suppressed in winter and very efficient in summer) in the lowermost 1–2 km of the atmosphere, seems to be the main reason for the strong differences between winter and summer aerosol conditions. A complete annual cycle of the AOT in the central Arctic could be measured. This is a valuable addition to the summertime observations with the sun photometers of the Arctic Aerosol Robotic Network (AERONET). In line with the pronounced annual cycle in the aerosol optical properties, typical CCN number concentrations (0.2 % supersaturation level) ranged from 50–500 cm−3 in winter to 10–100 cm−3 in summer in the ABL. In the lower free troposphere (at 2000 m), however, the CCN level was roughly constant throughout the year, with values mostly from 30 to 100 cm−3. A strong contrast between winter and summer was also given in terms of ABL INPs which control ice production in low-level clouds. While soil dust (from surrounding continents) is probably the main INP type during the autumn, winter, and spring months, local sea spray aerosol (with a biogenic aerosol component) seems to dominate the ice nucleation in the ABL during the summer months (June–August). The strong winter vs. summer contrast in the INP number concentration by roughly 2–3 orders of magnitude in the lower troposphere is, however, mainly caused by the strong cloud temperature contrast. A unique event of the MOSAiC expedition was the occurrence of a long-lasting wildfire smoke layer in the upper troposphere and lower stratosphere. Our observations suggest that the smoke particles frequently triggered cirrus formation close to the tropopause from October 2019 to May 2020.

Study on Smoke Ventilation Performance According to the Opening Rate in a Corridor-Type Apartment

In case of household fire in in a corridor-type apartment, when the apartment door is opened smoke generated from the fire flows along the corridor, which is an evacuation passage, thereby considerably affecting evacuation safety. It is essential to prepare smoke control measures for the evacuation safety of occupants. In this study, a model corridor was constructed to examine the smoke ventilation performance according to the opening ratio of windows installed in an apartment corridor, and a small-scale empirical fire experiment was conducted to determine the changes in smoke concentration, thermal flow temperature, and smoke layer thickness. Additionally, the smoke ventilation performance was examined in the presence or absence of a sprinkler system under the same conditions. Comprehensively considering the results obtained in the experiment, it is desirable to secure a 100% opening rate of the corridor on the 15th floor and below without sprinklers in corridor-type apartments, exempt from smoke control equipment, and in accordance with related standards.

Numerical study on plug-holing of double fires in tunnel with centralized smoke exhaust

In tunnel fires, smoke is the main fatal factor causing casualties. The technical parameters of tunnel smoke extraction are particularly critical. However, the parameters related to smoke control have rarely been experimentally and quantitatively studied in a tunnel caused by two adjacent fire sources. Numerical simulations were performed to study the plug-holing phenomenon in the centralized smoke exhaust tunnel with multi-fire sources. A 1:10 small-scale tunnel (length 25 m; width 1.2 m; height 0.8 m) was built using FDS. The effects of different fire heat release rates Q̇, fire source separation distances, and smoke exhaust velocities (v) on the plug-holing were considered. Firstly, the smoke layer height beneath the exhaust vent gradually increases as the exhaust velocity becomes higher and then remains almost unchanged until the plug-holing phenomenon occurs. Additionally, the smoke layer height becomes lower as the fire source separation distance increases for the fires with constant heat release rate. For all Q̇ values, the critical plug-holing velocity increases with the increasing separation distance S and Q̇. In addition, by modifying the critical Froude number (Frc), a predictive model of the critical plug-holing velocity considering the dimensionless fire source heat release rate and separation distance is obtained. Furthermore, the modified critical Froude number model, accounting for two adjacent fires with different separation distances, is established. The correlation between the new model and the simulation results shows quite good agreement, indicating that the proposed model can provide a satisfactory prediction for the critical smoke exhaust velocity in plug-holing conditions of double fire sources. This work can provide valuable guidance for tunnel design concerning fire safety.

Visually activated sprinklers applied in atria to mitigate the interaction of water spray with the smoke layer

This research investigates the fire suppression effectiveness of sprinklers in a large atrium of a public transport hub in Taipei, Taiwan, during a fire using FDS. The thermally-activated sprinkler had the shortcoming of delayed activation, with a thick smoke layer accumulating at the ceiling. Thereafter, the interaction of sprinkler spray with the thick smoke layer would cause a large amount of smoke falling to the floor level, significantly impacting safe egress. The novelty of this study is to couple a sprinkler system with a vision-based fire flame detection system (VFDS) having the advantage of locating the fire instantly to suppress fire. A combination of different atrium heights, fire sizes, and scenarios for a total of 27 cases were investigated. The research results have indicated that VFDS-activated sprinklers can suppress an atrium fire at the early stage, greatly reduce the amount of smoke generation and smoke accumulation. In consequence, the interaction of water spray and smoke layer is largely mitigated to prevent early fall of smoke. When the fire is suppressed, the atrium can maintain a clear height that is crucial to safe egress. The research finding of VFDS-activated sprinklers system is significant for fire safety of buildings with large atria.

Theoretical and numerical study on smoke descent during tunnel fires under natural ventilation condition

The smoke stratification and the smoke descent along a tunnel are of the utmost importance for personnel evacuation. The paper investigates the smoke descent along a tunnel during a naturally ventilated tunnel fire. A theoretical model is developed to predict the smoke depth below the ceiling along the tunnel. A series of numerical simulations of full-scale tunnel fires are conducted to compare with the developed model, and some coefficients such as the entrainment coefficient are determined from the simulation results. The concepts of critical moment and critical distance are proposed to characterize the smoke descent along the tunnel. The results show that as the smoke spreads longitudinally, the smoke depth below the tunnel ceiling continuously increases. The temperature decay along the tunnel due to heat losses and air entrainment at the smoke layer interface is considered as the main parameter for the smoke descent. After the vitiated air returns back to the fire source, the smoke stratification in the entire tunnel will be significantly reduced. The smoke layer depth along the tunnel based on the temperature distribution is relatively stable in the process of smoke development, which is not sensitive to the HRR, but influenced by the tunnel width, and this method could only be used before the critical moment. The outcomes of this study could provide references for a better understanding of smoke movement in naturally ventilated tunnels and provide technical guidelines for fire safety designers.

Experimental study on fire characteristics of sidewall fires in a long-narrow compartment with multiple lateral openings

This experimental study delves into the fire characteristics within a long and narrow compartment equipped with multiple lateral openings. The investigation encompasses aspects such as radiant heat flow, temperature distribution, and smoke layer height. The presence of lateral openings introduces asymmetry in suction, resulting in augmented radiative heat flow on the rear side of the fire source compared to the front and left sides. Smoke stratification remains stable, with minimal temperature influence from the fire source power at one opening and negligible impact at the other. Employing a customized two-dimensional method for thermal estimation, our experimental data display lower results than previous predictions due to structural disparities. The utilization of the N-percentage rule unveils two discernible regions in smoke layer height behavior: stable and decreasing, a pattern consistent across varying heat release rates. These insights offer valuable predictive tools for discerning fire characteristics in elongated, restricted compartments with multiple lateral openings. Consequently, this study enhances comprehension and informs safety measures within such configurations, contributing to the broader field of fire safety engineering.

Towards characterizing full-scale furniture fires in a two-storey house: Gaseous species concentrations during a ventilation-limited fire

This work examines the time evolution and spatial distribution of five combustion gases during a large-scale ventilation-limited furniture fire experiment. Past research has largely focused on measurement of major gases collected in an exhaust hood or at limited locations during gas or liquid fuel fires in a single room or single-storey layout. As such, there is a dearth of temporally resolved gas species data, measured in-situ at various locations in a two-storey structure. The aim of this study is to provide a comprehensive set of gas concentration data collected throughout a two-storey, multi-compartment, completely sealed structure during a furniture fire. Higher concentrations of CO, NOx, HCN and VOCs were measured in areas close to the fire and in the smoke layer, while a uniform distribution of gases was recorded further away from the fire, namely on the upper floor. The results are critical to our understanding of gaseous species evolution during ventilation-limited fires and provides additional insights for tenability and other fire safety measures within a residential context.

Assessing performance of LEDSA and Radiance method for measuring extinction coefficients in real-scale fire environments

Two photometric measurement methods (Radiance method and LEDSA) were compared against the established MIREX measurement apparatus under controlled laboratory conditions to assess their capability of measuring extinction coefficients in real-scale fires on a temporal and spatial scale. LEDSA is a tomographic technique based on direct measurements of light intensity from individual LEDs using commercially available DSLR cameras. By discretizing the domain into horizontal layers with homogeneous smoke density, values of the extinction coefficient can be computed using an inverse model based on Beer Lambert’s law. The Radiance method involves measuring the contrast of light and dark areas in images and/or video footage. It was originally developed to investigate the descent of the smoke layer in high-temperature fire events. In this work, the extinction coefficient was deduced from measurements on a contrast board by a straightforward analytical approach. Both methods were shown to yield similar extinction coefficient results in line with the MIREX for an EN 54-7 TF5 n-heptane fire. The Radiance method is able to generate accurate patterns but not values for a TF2 wood smouldering fire, while LEDSA is generally able to reflect the MIREX measurement values, yet requires higher computational effort.

Characteristics of the spatial and temporal evolution of the environmental parameters for belt fire in underground coal mine roadway

The high temperature and toxic gases produced by burning belt in mine are a serious threat to the miners’ safety. A full-scale fire experiment was conducted in an exercise roadway, as a basis for which a physical model was established. The spatial and temporal evolution of the environmental parameters was obtained in the U-shaped working face, and the effect of wind speed on the fire parameters was studied. It was found that the smoke concentration and temperature from large to small are the downwind side of the transport roadway, working face and return airway. The structure of smoke layer near the working face is disorganized, which is related to the local turning structure of the roadway. The smoke reverse occurs later than the spread of smoke downstream of the fire area. The thickness and maximum temperature of counterflow layer decreases linearly with the increase of ventilation speed. Increasing the critical ventilation speed can control the smoke reserve and increase the risk downstream of the fire area. The harmfulness of wind speed to smoke reserve and spread in single roadway and complex network is then discussed. In addition, the influencing factors of fire evolution in confined space are put forward.