Fire Heat Research Articles

Study on the maximum excess temperature and temperature distribution under the influence of lateral smoke exhaust in tunnel fires

This paper numerically investigates the temperature characteristics of tunnel fire under the ceiling with the effect of lateral smoke exhaust. In the past, analyses of maximum excess temperature under the ceiling and temperature distribution with extraction flows have been conducted. However, the lateral exhaust has a different impact on temperature than the ceiling exhaust. Consequently, several simulations were run in a tunnel (60m long, 9m wide, and 6m high). The lateral exhaust velocity is set by 0 – 10 m/s with a fire heat release rate of 5 – 20 MW. The temperature beneath the tunnel ceiling is measured by 9 rows of thermocouples. The results show that there is a linear relationship between the maximum excess temperature and the lateral exhaust velocity. The longitudinal temperature distributions at different transverse positions are distinct because of the lateral smoke exhaust especially when the hot smoke passes by the vent. The transverse temperature difference expands with the lateral exhaust velocity increases and the heat release rate decreases. Several modified models are proposed to provide references for predicting the temperature characteristics of tunnel fires with lateral smoke exhaust.

Experimental study on the effect of mechanical ventilation on cable tray fires in compartments

This study deals with the effect of ventilation flow rate and the ventilation configuration on the behavior of a cable tray fire in a confined and mechanically ventilated enclosure. Results are based on large-scale fire tests performed within the framework of the OECD Nuclear Energy Agency (NEA) PRISME projects. The fire source, identical for all tests, consisted in five horizontal cable trays measuring 3 m in length, stacked vertically and filled with halogen-free flame retardant cable-type, non-powered cables. The installation consists of two rooms connected by an open doorway. Two ventilation configurations were considered with two different arrangements of the inlet and outlet branches. The analysis demonstrates the effects of ventilation flow rate and air intake position on the fire heat release rate (HRR). Decreasing the ventilation flow rate reduces the maximum HRR and delays combustion on the trays, which burn one after the other rather than simultaneously. This result is the consequence of oxygen vitiation near the cable trays, which reduces the horizontal flame spread on each cable tray. The analysis also discusses the phenomenon of low frequency oscillatory behavior of fire in a ventilated enclosure. The results highlight that the conditions of occurrence of oscillations combine two criteria: a low ventilation flow rate to favor the release of unburnt gases and an appropriate configuration of the ventilation air intake to generate ignition of premixed gases.

Experimental study on vertical temperature distribution of the two-layer smoke flow in tunnel during construction

For a tunnel fire during construction, the formation of two-layer smoke flow underneath the ceiling accelerates the smoke descent threatening the personnel evacuation. As an important indicator of smoke diffusion, the gas temperature distribution inside tunnel is worthy of analysis in depth. However, the vertical temperature profiles within this kind of two-layer smoke flow inside tunnel have not been experimentally studied. To fill the gap, this paper conducts a series of fire experiments in a 1/20 reduced-scale tunnel model. Multi-variables including fire heat release rate, fire position, lifting-up height of fire source, inclined shaft slope, are considered. The visible flame shape, O2 volume fraction and gas temperature are recorded and measured. The burning of the propane gas persists steadily for 20 mins without the occurrence of self-extinction. The structural evolutions of the smoke layer are predicted by FDS simulation and traced by the laser sheet. The smoke descent process in the main tunnel is divided into three stages. The effects of fire source position, inclined shaft slope, fire heat release rate and lifting-up height on the gas temperature distributions inside tunnel are analyzed. The measured temperature rise and the perpendicular distance from the ceiling are normalized by ΔTmax and LT, respectively. At various longitudinal positions (1.82 ≤ │X-dm│/Hef ≤ 10.68), the normalized temperature distributions follow the same curve, independent of the values of Qm, dm and hm. The self-similarity of the vertical temperature distribution within the two-layer smoke flow are sufficiently proved. It is found that ΔT/ΔTmax with z/LT shows a two-regional variation. Based on over 1,740 temperature data from 36 experimental cases, an experimental equation is proposed to describe the gas temperature rise in the region between the fire source and the closed end. The proposed model shows good agreement with the measured data, which can help the researchers to better assess the smoke layer height inside tunnel.

Pipeline leak jet fire thermal characteristic induced by hydrogen-blended natural gas: Physical modeling of temperature profile based on virtual origin theory

Hydrogen-blended natural gas has widely used in energy fuel systems in industrial and domestic applications. During storage, transportation and usage of the fuel, temperature characteristics make a key contribution to energy leak gas fire accidents. In this manuscript experimentally, the buoyant turbulent diffusion axial temperature profile of a hydrogenated methane jet fire is experimentally investigated under free fire and wall fire. Different hydrogen addition ratios, nozzle diameters, and fire heat release rates were considered in the experimental conditions. Temperature profiles and virtual origins were recorded and analyzed. It is found that the wall fire temperature is somewhat higher than that in the free fire at a fixed height. And the vertical temperature of the jet fire increases with the increase of the hydrogen addition ratio, while the flame height decreases. With the addition of hydrogen, the virtual origin is increased, and virtual origin of wall fire is clearly larger than that of the free fire. Due to the restriction of air entrainment from the sidewall and the hydrogen addition, the physical mechanism of restricted air entrainment was analyzed. In addition, a non-dimensional model of the virtual origin was proposed and analyzed for different conditions. Finally, the normalized vertical temperature profile in the continuous flame region, intermittent flame region and buoyant plume region based on the virtual origin and mirror-approach are still well characterized for air entrainment under all conditions. The results of this paper may demonstrate the potential for the risk assessment of hydrogen-blended natural gas fire.

MEMBRANE ACTION OF REINFORCED CONCRETE SLABS IN FIRE

AbstractFor RC slabs supported in 2 ways, when deflection increases, membrane stress occurs within the cross section of the slab, and as a result the load‐bearing capacity of the slab is improved due to this membrane action. In this paper, the results of load‐bearing fire test are introduced that targets a composite flooring system composed of a flat RC slab in which two‐tier reinforcements are arranged and an unprotected beam. The RC slab to which the bending collapse load based on the yield‐line theory was given sustained load‐bearing capacity even after being subjected to 3 hours and more of standard fire heating.

Vulnerability assessment of storage tanks exposed to simultaneous fire and explosion hazards

• Vulnerability of storage tanks under simultaneous fire and explosion is addressed. • A quantitative method for vulnerability assessment is developed. • The effect of pre-fire exposure on the scaled safety distance of tanks is analyzed. • The method can be used for rapid quantitative risk assessment of domino effects. Storage tanks are vulnerable to the synergistic effect of fire heat radiation and explosion shock wave during domino accidents in hydrocarbon storage tank farms. The available vulnerability analysis model for estimating the damage probability of secondary targets subjected to accident escalation vectors is a critical step for quantitative risk assessment of domino effects. However, due to the complexity of combined loadings, research on the vulnerability assessment of atmospheric storage tanks involved in the synergistic effect of fire and explosion is limited. This study aims to establish a method for quantitative evaluation of the vulnerability of storage tanks exposed to simultaneous fire and explosion hazards. The developed method addresses the coupling effect of high temperature caused by fire and instantaneous strong impulse induced by explosion on the damage probability of atmospheric storage tanks. Application of the proposed method to different cases highlight that pre-fire exposure has significant effects on the damage probability and the scaled safety distance of the tank under subsequent explosion load. The proposed approach can be used for rapid quantitative risk assessment of domino effect involving the synergistic impacts of fire and explosion.

Numerical Study on Smoke Diffusion and Smoke Control in Urban Underground Curved Tunnel

Effects of the radius and fire heat release rate on the smoke propagation and smoke control in curved tunnel are studied by the numerical methods. Six radiuses as 70 m, 150 m, 200 m, 250 m, 300 m and 400 m and two different fire sizes as 5 MW and 30 MW are involved. Numerical results show that comparing with the smoke flow in the straight tunnel, smoke temperature distributions across the cross-section is not symmetry with the centerline in the curved tunnel, and the smoke diffusion close to the outside boundary appears to be fast. At the higher level of the curved tunnel, such as 3 m above the tunnel floor, the smoke temperature would decrease with the distance from the fire increasing. However, at the lower level of the tunnel, the temperature would increase firstly and then decrease, maximum temperature would occur in the bend region of the curved tunnel. When the fire heat release rate is big, this temperature might be high enough to injure the people. Smoke ventilation system would run in time under this condition. Critical velocity in curved tunnel is larger than that in the straight tunnel.

Experimental study and machine learning on the maximum temperature beneath tunnel ceiling induced by adjacent tandem fires in longitudinally ventilated tunnel

This study experimentally investigated the effects of longitudinal ventilation on the maximum temperature rise under tunnel ceilings induced by adjacent tandem fires. Because the tunnel is a narrow building structure, the fire source may ignite the surrounding flammable materials (such as adjacent vehicles, etc.), causing the simultaneous burning of multiple fires. The experiments were conducted in a 1:8 reduced-size tunnel. The ceiling temperature were measured for 60 repeatable test conditions, including five separation distances between adjacent fire sources at various heat release rates with different longitudinal wind speeds. As regards the effect of fire separation distance, the maximum ceiling temperature rise was recorded and measured for five different fire separation distances; as the fire separation distance increased to 0.9 m for a given test condition, the maximum ceiling temperature rise gradually decreased. Meanwhile, the maximum ceiling temperature rise decreased as the crosswinds increased. A new correlation was proposed between the maximum ceiling temperature rise as a function of the change in fire plume due to the fire separation distance and the Froude number representing flame tilting caused by crosswinds. Meanwhile, machine learning algorithm is used to predict the fire heat release rate. The model was shown to be able to effectively explain the experimental results and can provide a reference for examining the evolution of tunnel fire situations.

Experimental study on the impact of asymmetric heavy rainfall on the smoke spread and stratification dynamics in tunnel fires

This work examines the impact of heavy rainfall on the smoke spread and stratification dynamics in tunnel fires with a reduced-scale (1:15) experimental platform. Scaled tests vary the rainfall intensity (up to 60 mm/h, equivalent to 232 mm/h in nature), raindrop size (1.0–1.5 mm, equivalent to 4–6 mm in nature), and fire heat release rate (2.1–6.7 kW, equivalent to 2–6 MW in real scale). We found that the asymmetric rainfall on one exit can induce the longitudinal airflow inside the tunnel due to the dissipation of the raindrop momentum and the rain-induced air entrainment. The velocity of such a longitudinal airflow increases with increasing rainfall intensity, and smaller raindrops tend to induce faster longitudinal airflow. The spread and stratification of hot smoke are sensitive to the induced airflow. In the absence of rainfall, there was a symmetrical temperature distribution from the fire source due to the symmetrical air entrainment. Under rainfall, the temperature distribution of tunnel fires gradually becomes less symmetrical, and the smoke stratification interface becomes unclear. With the increase in rainfall intensity, the phenomenon of smoke back-layering appears, and the smoke layer height decreases. Under a small rainfall intensity, raindrop size has a higher impact on the smoke stratification near the fire source. This study aims to attract more attention to tunnel safety under the dual disaster of fire and extreme weather and support the emergency response.

Maximum ceiling temperature and longitudinal distribution in a corridor-like enclosure with opening

This paper reports a quantitative study on the effects of a side opening on the transition between the fuel-controlled and the ventilation-controlled condition and temperature distribution in a corridor-like enclosure fire. A side opening was introduced to simulate breakage of glass window in case of fire. Experiments were carried out in a 1/8 model tunnel with the dimensions of 3.14 m×0.42 m×0.368m(length×width×height), including 56 test conditions arising from variations of heat release rates (HRR) and opening sizes. The results showed that the transition was correlated with the fraction of the heat released to the heat convected through the opening and the ratio of the heat lost through enclosure boundary to the heat convected through the opening. The maximum temperature rise in the fuel-controlled condition was positively correlated with the fire heat release rate, but it was negatively correlated with the opening aspect ratio. There were two combustion stages in the ventilation-controlled condition, one with flame oscillation inside the corridor-like enclosure and the other with persistent flame ejection through the opening. The maximum temperature rise in the stage of flame oscillation inside the compartment was positively correlated with the air mass supply through the opening and negatively correlated with the heat loss through the corridor-like enclosure boundary. Non-dimensional correlations between the maximum temperature rise and the heat release rates (HRRs), opening sizes and the heat loss were proposed. Moreover, a piecewise equation was proposed to predict the longitudinal decay along the centerline of the corridor-like enclosure with an opening. All the proposed correlations were found to fit well with the experimental data, which established a basic relationship to quantify the (i) temperature variation, (ii) critical heat release rate and (iii) transition for various opening sizes and HRRs in a corridor-like enclosure fire. These new findings and the proposed non-dimensional model provide the basis for understanding fire growth and temperature distribution inside a corridor-like enclosure with opening.

Independence Effects of Heat and Ash on Forest Soil Nematode-Trapping Fungi Communities

Heat input and ash residues are main components of vegetation fire disturbances. Understanding the distinct impacts of heat and ash on soil microorganisms is crucial to comprehend and predict the induced changes in soil ecosystem composition and dynamics following different types of fire disturbances. This study’s main goal was to track the dynamic shifts in the community of soil nematode-trapping fungi (NTF) following the separate disturbances of heat and ash by means of a specifically designed experiment conducted in the field. Four simulated fire treatments, one treatment reproducing natural burning, and one control treatment were taken into account. Every ten days following the disturbance, soil samples from each treatment were collected, and soil NTF and physicochemical characteristics were measured. The results showed that: (1) Heat drastically decreased the number of strains and heavily altered the NTF community, but it also encouraged the emergence of new NTF species. Instead, no overt changes were observed in the treatment that just experienced the addition of ash on the soil. (2) When compared to treatments that received only heat input, the lower strain count of NTF recovered more quickly in the natural burning treatment which was affected by both heat and ash input. These findings suggest that the disruptive effect of fire heat on soil NTF biomass may be counterbalanced by the emergence of new species and the repairing capabilities of new nutrients introduced by ash residue. In the future, both conventional and cutting-edge techniques should be considered in research designs to better understand the ecological role of fire disturbances occurring at different intensities, as well as the mechanisms that make soil ecosystems resilient to fire, in particular the role of new species.

Toxicity of smoke from upholstery materials in under-ventilated fires

Toxicity of smoke is an important parameter when evaluating the hazards associated with upholstery materials to be used in buildings and transportation vehicles. Fires in enclosed spaces with limited ventilation such as rooms, carriages and fuselages cause smoke accumulation and increased exposure of occupants to its toxic components. The paper focuses on experimental measurements of smoke toxicity in under-ventilated fires and presents data for various ventilation conditions. The results indicate the influence of ventilation on fire heat output, the composition and amount of produced smoke. The data can be used in fire modelling, fire investigation and evaluation of fire toxicity hazards in a particular field of application.

The role of helicity and fire–atmosphere turbulent energy transport in potential wildfire behaviour

Background Understanding near-surface fire–atmosphere interactions at turbulence scale is fundamental for predicting fire spread behaviour. Aims This study aims to investigate the fire–atmosphere interaction and the accompanying energy transport processes within the convective boundary layer. Methods Three groups of large eddy simulations representing common ranges of convective boundary layer conditions and fire intensities were used to examine how ambient buoyancy-induced atmospheric turbulence impacts fire region energy transport. Key results In a relatively weak convective boundary layer, the fire-induced buoyancy force could impose substantial changes to the near-surface atmospheric turbulence and cause an anticorrelation of the helicity between the ambient atmosphere and the fire-induced flow. Fire-induced impact became much smaller in a stronger convective environment, with ambient atmospheric flow maintaining coherent structures across the fire heating region. A high-efficiency heat transport zone above the fire line was found in all fire cases. The work also found counter-gradient transport zones of both momentum and heat in fire cases in the weak convective boundary layer group. Conclusions We conclude that fire region energy transport can be affected by convective boundary layer conditions. Implications Ambient atmospheric turbulence can impact fire behaviour through the energy transport process. The counter-gradient transport might also indicate the existence of strong buoyancy-induced mixing processes.

Formation of silicated layer of austenitic pipe steel with different pipe sizes

Tubular furnaces of hazardous production facilities are unique technical devices, and serve for open fire heating of various process media to relatively high temperatures. Some of the furnaces, such as pyrolysis furnaces, hydrogen conversions are also reactors that serve to decompose the hydrocarbon feedstock into separate components. The most important element of the structure is a tubular coil. The effect of high temperatures and working media on the coil metal inevitably leads to metal degradation by forming various defects, changing the structure and mechanical properties for the worse. One of the most unfavorable moments is considered saturation of the surface layers of the metal with carbon or another - carburization of the metal. When the metal is carburized, its embrittlement occurs and the probability of brittle destruction increases. All this can lead to frequent shutdown of furnaces for repairs and significant costs. One way to contain the active diffusion of carbon into the metal may be to silicate the surface of the coil elements using different silicating mixtures, since silicon can effectively delay carbon incorporation. In this work, studies are carried out on the peculiarities of the formation of a silicated layer in fragments of pipes with dimensions of 160x6 mm, 32x2 mm, 57x4 mm made of steel 08Kh18N10T depending on the holding time in the silicating mixture.

Study on the maximum and longitudinal distribution of ceiling gas temperature in a naturally ventilated tunnel: The effect of longitudinal fire location

The ceiling gas temperature is an important evaluation indicator for determining tunnel fire hazards. The maximum and longitudinal distributions of the ceiling gas temperature are studied numerically with consideration of the effect of longitudinal fire locations, fire heat release rate, tunnel aspect ratio, and total length of the tunnel. Results show that as the fire moves from the middle of the tunnel to the upstream (left opening), the flame tilts to the downstream (right opening) due to the induced longitudinal flow inside the tunnel. The tilting angle increases as the fire approaching the tunnel opening. Correspondingly, the position of the resulting maximum ceiling gas temperature is shifted from right above the fire source to the downstream side. An offset distance is used to quantify this phenomenon and an empirical equation is proposed to calculate this distance. Meanwhile, the resulting maximum ceiling gas temperature decreases. An empirical equation is also proposed to predict the evolutionary trend of maximum ceiling gas temperature under the effect of the longitudinal fire locations. In addition, asymmetric longitudinal distributions of ceiling gas temperature were observed in the upstream and downstream directions. The ceiling gas temperature decays exponentially in both directions with a lower decay rate on the shorter side (the side with a shorter distance between the fire and the nearest opening). Corresponding correlations for related parameters are proposed for the shorter and longer side, respectively.

Flame characteristics and heat flux profile of a tank surface caused by horizontal gas leak jet fire impingement

Jet fire accidents caused by leaks in gas pipelines or gas tanks often occur, which may trigger a domino effect. The present work experimentally investigated the characteristics and heat flux profile of flames on a tank surface, with different leak hole-tank spacing and fuel initial velocity. The results show that, for any given leak hole diameter, the height and width of the impingement flame increase with an increasing heat release rate, whereas they decrease with an increase in the leak hole-tank spacing. The impingement flame height correlated well with the characteristic length Lm, which was used in free jet fires. Based on the balance of the momentum flux to the buoyancy flux of the horizontal jet fire, a new characteristic length scale and a dimensionless heat release rate was proposed. Meanwhile, some of the numerical results were also analyzed and the non-dimensional functional correlation for the flame width of the impinging area was proposed with the fire heat release rate and flame Froude number Frf. Finally, considering the effect of the unburned portion of the fuel with jet fire impinging the tank, a new correlation was proposed to describe the heat flux and temperature profile of the tank surface.

Experimental investigation of the effects of a sidewall and cable arrangement on a horizontal cable tray fire in an open atmosphere

AbstractThe work deals with the influence of a sidewall and cable arrangement on the behavior of a fire involving horizontal cable trays in the framework of fire safety assessments in nuclear installations. The analysis is based on large‐scale fire tests performed in the open atmosphere in the frame of the OECD Nuclear Energy Agency (NEA) PRISME 3 project and on the corresponding simulations applying the FLASHCAT model for predicting the fire heat release rate. The fire configuration consists of five horizontal trays filled with PVC‐insulated power cables. The parameters investigated are the presence or absence of a sidewall and the cable arrangement (loose or tight bundles). The results show that the presence of a sidewall increases the fire HRR in comparison to a scenario without a sidewall. This effect is due to the increase of the flame spread velocity on the lower trays. Regarding cable arrangement, a tight configuration in bundles reduces the fire HRR in comparison to a scenario with a loose arrangement. This result is due to the reduction of the fire heat release rate per unit of area (HRRPUA) as well as the flame spread velocity. The performance of the FLASHCAT model in predicting the effects of the sidewall and the cable arrangement was also assessed on the basis of the fire tests and satisfactory agreements are reported. The presented analysis demonstrates that the fire scenario with horizontal cable trays against a sidewall and with a loose cable arrangement represents a conservative scenario for fire risk assessment. In addition, on the basis of these experiments, the effect of cable arrangement is more substantial than the sidewall effect.

Flame evolution and radiation hazards of rectangular fires between two parallel walls

The manuscript investigated experimentally the influence of two parallel walls on the flame geometric parameters and radiative heat flux distributions along the wall from fires on rectangular burners. Four sets of burners with equal area and different aspect ratios were used. The burner aspect ratios, separation distance between two parallel walls and the fire heat release rates were systematically varied in the tests. Measurements were conducted for the flame height evolution and radiation hazards. The results were analysed to build the change trends of the vertical flame height and radiant heat flux with the change of the separation distance between two parallel walls. A normalized flame height equation incorporating the separation distance was proposed. The results also revealed that the radiant heat fluxes along a vertical target do not change monotonously. Comparison between the measurements and the radiant heat fluxes calculated by some published empirical models revealed relatively large discrepancies. A view factor based formula was hence proposed by assuming the flame shape as a triangular prism based on the probability flame contours for relatively larger burner aspect ratios (n ≥ 3) and found to correlate well with the measurements.

Fire gilding investigation on early medieval copper-based jewellery by focused ion beam (FIB) on FEG-SEM

The study of ancient gilding is often problematic, as the gilding layers are soft and prone to deformation during sample preparation. In this respect, focused ion beam (FIB) milling on a field emission gun scanning electron microscope (FEG-SEM) provides poorly invasive in situ sampling. The operating process is here detailed and applied on gilded medieval copper-based elite jewellery (10th century) from Prague Castle. Obtained cross-sections and slices of gilded samples were investigated up to nanometer scale without gilding layer deformation. By coupling structural observation with elemental X-ray analysis (EDS) and electron beam diffraction (EBSD), FIB FEG-SEM provided new data on the physical–chemical characteristics of the gilded layer. The gilding has a two-layer metallurgical structure containing a quaternary Au (Hg, Cu, Ag) alloy corresponding to the gold solid solution fcc phase, and a submicrometric inner sublayer formed by a Au-Cu phase, relative to the interdiffusion of copper from the substrate during the fire gilding process. The applied temperature for gilding can be estimated at c. 400 °C. Sintering of the mercury-gold amalgam globules during the fire heating is highlighted. The precise characterisation of the gilding layers provided useful comparative parameters for identifying fire gilding skill levels and assessing the overall quality of the archaeological pieces.

Study on smoke temperature induced by two fires in a naturally ventilated tunnel

The temperature of smoke flow beneath the tunnel ceiling induced by two fires in a naturally ventilated tunnel is studied. The two fires are located in the lateral direction of the tunnel, and the distance between two fires is varied. Both the maximum temperature rise and the temperature distribution beneath the tunnel ceiling are investigated. For temperature distribution in the tunnel longitudinal direction, there is decrease in temperature with the increasing burner separation distance at the near fire region, while at the region far away from the fire location, the smoke temperature gets very close. Results show that the longitudinal temperature distribution exhibits an exponential decay law along the tunnel ceiling. A formula is established to predict the longitudinal temperature distribution and results show that there is good agreement between the numerical data and formula calculations. Comparison of temperature distribution in the tunnel longitudinal direction and transverse direction shows that at separation distance ofL=0m, there is obvious decrease in temperature with the increasing distance from the zero point both in the tunnel longitudinal direction and transverse direction, while at separation distance ofL=6m, there is obvious increase in temperature at the near fire location in the transverse direction, and at other regions, the temperature is very close in both directions. The maximum temperature rise in the tunnel longitudinal direction decreases with the increasing burner separation distance. A model by considering the fire heat release rateQ̇, the effective tunnel height Hef and the burner separation distance L is proposed to express the maximum temperature rise, and results show that the model can well predict the numerical data. The findings obtained in this work could provide reference for the design of tunnel fire safety with two fire sources.