Fire Heat Release Rate Research Articles

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.

Can heavy rainfall affect the burning and smoke spreading characteristics of fire in tunnels?

Extreme rainfall events are increasingly common under the current trend of global warming. This work investigates how a heavy rainfall on one exit affects the fire burning and the smoke spread in tunnels. Several reduced-scale tests are designed with various rainfall intensities (up to 60 mm/h, equivalent to 232 mm/h in nature), raindrop sizes (1.0-1.5 mm, equivalent to 4-6 mm in nature), and tunnel fire heat release rates (2.1-6.7 kW, equivalent to 2-6 MW in real scale). Experiments show that heavy rainfall on one exit can induce a longitudinal airflow inside the tunnel, and the induced airflow is caused by the increased pressure at the rainfall exit. The airflow pushes the flame tilting towards the no-rainfall portal, and the correlation models of the flame length and flame inclination are characterized by considering the induced airflow and rainfall. The rainfall-induced airflow has a limited effect on the burning rate of pool fires, but it can change the ceiling temperature and limit the smoke back-layering toward the rainfall portal. In contrast, the ceiling temperature distribution towards the no-rainfall portal is found not sensitive to rainfall, which can be well described by an empirical model.

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.

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.

Reduced pressure effects on smoke temperature, CO concentration and smoke extraction in tunnel fires with longitudinal ventilation and vertical shaft

Nowadays, high-altitude tunnels at reduced pressure are emerging in the world's road network. A better understanding of the temperature and CO concentration distribution as well as the shaft extraction performance during tunnel fires with combined longitudinal ventilation and shaft exhaust under reduced ambient pressure is particularly crucial for the tunnel fire protection design. Series of tunnel fire simulations were carried out with ambient pressures of 50 kPa–101 kPa, fire heat release rates (HRRs) of 3 MW and 10 MW, and longitudinal ventilation velocities of 0 m/s to 2 m/s. The results show that the reduced ambient pressure enhances the difference in vertical and longitudinal distribution of CO concentration and smoke temperature, while the longitudinal ventilation gradually weakens this difference in longitudinal distribution. The natural exhaust shaft reduces the difference in vertical distribution but increases the difference in longitudinal distribution. As the reduction of ambient pressure, the smoke extraction efficiency of the shaft will increase slowly. Longitudinal ventilation can enhance the shaft's smoke extraction performance, but the general smoke extraction efficiency of the shaft deteriorates with the increasing longitudinal ventilation velocity. This study provides a fundamental reference for tunnel natural ventilation system under different ambient pressures, particularly in high-altitude tunnel fires.

Predicting real-time fire heat release rate by flame images and deep learning

The heat release rate (HRR) is the most critical parameter in characterizing the fire behavior and thermal effects of a burning item. However, traditional fire calorimetry methods are not applicable due to the lack of equipment in most fire scenarios. This work explores the real-time fire heat release rate prediction by using fire scene images and deep learning algorithms. A big database of 112 fire tests from the NIST Fire Calorimetry Database is formed, and 69,662 fire scene images labeled by their transient heat release rate are adopted to train the deep learning model. The fire tests conducted in the lab environment and the real fire events are used to validate and demonstrate the reliability of the trained model. Results show that regardless of the fire sources, background, light conditions, and camera settings, the proposed AI-image fire calorimetry method can well identify the transient fire heat release rate using only fire scene images. This work demonstrates that the deep learning algorithms can provide an alternative method to measure the fire HRR when traditional calorimetric methods cannot be used, which shows great potential in smart firefighting applications.

Physical modeling and machine learning of ceiling maximum temperature rise induced by tandem heat sources with unequal heat release rates in a natural ventilation tunnel

The present work involved experimental studies of physical modeling as well as machine learning prediction of the characteristics of ceiling maximum temperature induced by adjacent heat sources with unequal fire heat release rates in a tunnel. The following are considered as variables in this study: separation distances between the adjacent heat sources, and unequal heat release rates between upstream fire sources (UFS) and downstream fire sources (DFS). It was found that, with the increasing separation distance between adjacent fires, the ceiling maximum temperature gradually decreases for the given total heat release rate (Q˙TFS). Meanwhile, with the increase in the ratio (Q˙UFS/Q˙TFS) of the upstream fire heat release rate (Q˙UFS) to the total heat release rate, the ceiling maximum temperature rise of the tunnel presents nonmonotonic changes: first a decrease and then an increase. From the dimensional analysis of adjacent tandem fires, a prediction model of ceiling maximum temperature rise induced by different separation distances and Q˙UFS/Q˙TFS ratios was proposed. The model has a good correlation with the experimental and previous research results. Furthermore, to perform the calculation quickly and facilitate rapid engineering design, we use machine learning methods to derive the maximum temperature rise of the tunnel ceiling. The fire heat release rate is predicted using a machine learning algorithm. The three parameters (Q˙UFS/Q˙TFS, ΔTmax, Hd) were paired with the maximum temperature rise of the ceiling and trained using the GRU neural network model. The results show that the machine learning algorithms has a good agreement with the verification experimental value, cmparisons with established physical models are also discussed. The research results can serve as a reference for an intelligent tunnel firefighting system.

An integrated method for assessing passenger evacuation performance in ship fires

In order to assess effectively the passenger evacuation performance in ship fires, a quantitative method combining the computational fluid dynamics (CFD)-derived fire data and AnyLogic evacuation simulation is proposed in this paper. In this developed method, the speed reduction factor, instantaneous untenable conditions and fractional effective dose (FED) model are utilized to consider the effects of smoke visibility, heat and toxic gases in ship fires on the passenger walking speed, activity space and human health conditions. In order to demonstrate this proposed method, passenger evacuation analysis is performed in a hypothetical zone of the ship with obstacles for fast t-squared fires. The results indicate that the relationship between the required safe egress time (RSET) and fire heat release rate (HRR) is non-monotonic. However, the total FED increases with the increase of the fire HRR and the sequence of survivors. Compared with two existing methods, this proposed method is able to assess more rigorously the passenger evacuation performance in ship fires. The existing ASET/RSET method is more conservative, and the existing FED method with the Pathfinder underestimates the FED in ship fires. This work will provide a more accurate method for ship fire safety assessment and performance-based fire protection design.

Experimental study on temperature decay and smoke control in tunnel fires with combination of multi-point smoke exhaust and longitudinal ventilation

This paper investigates the longitudinal temperature distribution characteristics beneath the ceiling and smoke control of the tunnel with combination of multi-point smoke exhaust and longitudinal ventilation. Experiments are carried out in a model tunnel with dimensions of 60 m × 1.3 m × 0.67 m. The longitudinal wind speed is set to 0–0.5 m/s, the exhaust air volume to 0.34–1.09 m³/s and the fire heat release rates (HRR) are 30.7 kW and 61.4 kW. The results show that the tunnel fire with combination of multi-point smoke exhaust system and longitudinal ventilation mode (the exhaust fan is installed on the upstream side of the tunnel) causes the offset of flame, and the maximum temperature point beneath the tunnel ceiling appears in the upstream direction from the fire source. The large smoke exhaust air volume will cause smoke layering plug-holing at the smoke exhaust vents, and the temperature of the vents where plug-holing occurs is lower. Longitudinal ventilation combined with the top multi-point smoke exhaust system has a better smoke control effect, but the longitudinal wind speed cannot be greater than 0.5 m/s. The temperature decay beneath the tunnel ceiling is in accordance with the exponential attenuation law in a tunnel with combination of multi-point smoke exhaust system and longitudinal ventilation mode (the exhaust fan is installed on the upstream side of the tunnel), and the longitudinal temperature decay rate of the side close to the exhaust fan is smaller than that of the other side.

Investigation of the Temperature Beneath Curved Tunnel Ceilings Induced by Fires with Natural Ventilation

The distribution characteristics of the temperature below ceilings in curved tunnel fires have not been quantitatively studied. A small-scale tunnel was constructed in this work to study the maximum temperature and longitudinal attenuation of the temperature below ceilings induced by fires in a curved tunnel with natural ventilation. Different tunnel turning radiuses and fire heat release rates were taken into account. The results show that the distribution characteristics of temperature below the tunnel ceiling is hardly affected by the tunnel turning radius in a scenario where the flame plume impinges on the ceiling (strong-plume-driven). The fire-induced maximum temperature and longitudinal attenuation of temperature in curved tunnels are comparable to those of straight tunnels. Improved correlations are proposed for the condition of a strong-plume-driven ceiling jet, and the measured value of the temperature of the experiment collapsed well. This work may enhance the understanding of temperature distributions in curved tunnel fires.

Temperature variation inside a corridor-like enclosure under limited ventilation

This paper investigates the temperature variation inside a corridor-like enclosure (for example, high-speed train compartment) with persistent flame ejection through an opening (for example, window). The fire heat release rate and opening sizes are varied with 28 different experimental conditions. An exponential function is built and found to be able to describe the longitudinal temperature distribution, in which the decay rate has a power function with the window aspect ratio. The maximum temperature shows a considerable difference for various heat release rates (HRR), due to the mass flow rate of the fuel supply through the opening is comparable to or greater than the air mass inflow rate. A general model characterizing the longitudinal temperature distribution of various opening sizes and HRR is proposed with heat release to heat loss ratio inside the compartment, HRR and window aspect ratio, which shows a good agreement with the experimental results. This study provides important and new contributions over previous knowledge and has potential practical applications concerning the corridor-like enclosure fire safety and occupant evacuation.

Numerical study of the combustion regimes in naturally-vented compartment fires

Numerical simulations are performed to investigate the classical compartment fire problem involving a single door opening. Three different configurations were considered, namely a full-scale (ISO 9705), an intermediate and a small-scale enclosure with various opening heights and widths. A large number of numerical simulations was carried out using the CFD code Fire Dynamics Simulator (FDS)(version 6.7.0). Based on the variation of the average temperature inside the compartment, three combustion regimes were identified namely well-ventilated, transitional and under-ventilated regime. This variation also allowed us to identify the boundaries between regimes. Furthermore, by adopting a non-dimensional representation of the fire heat release rate inside the compartment as a function of the Global Equivalence Ratio (GER), a clear demarcation between these combustion regimes was obtained. A linear correlation has been established between the maximum heat release rate inside the compartment and the ventilation factor. The latter is expressed as Q̇inmax=850AH. A linear relation between the maximum air flow rate ṁin and the ventilation factor AH was found, i.e. ṁinmax=0.46AH where A is the door surface area and H its height.