Maximum Flame Length Research Articles

Release of high-pressure hydrogen from type III tank in a fire scenario: Analysis and prediction of jet flame length and thermal response characteristics

In this paper, the thermal response and jet flame behaviour of a typical type III high-pressure hydrogen storage tank with nominal working pressure (NWP) and volume of 70 MPa and 48 L were analysed at fire scenarios. The tank was discharged at a critical internal pressure 77.4 MPa, which was ca. 110 % of NWP. The pressure drop rates of type III tanks were greater than 1 MPa/s during the initial 15 s after venting. The length and width of jet flame were 4.93 and 1.65 m at the critical moment, which fluctuated downward over time. A thermokinetic model was employed to predict the variation of hydrogen gas state parameters and the maximum length of jet flame. The calculation results indicated that the maximum flame length of hydrogen released from a valve of 2 mm was around 2–6.5 m under a pressure range of 10–80 MPa. The error between the predicted results and the experimental values was less than 5 %. It was found that the flame length was enlarged by an increase of aperture. The length of flame increased by 6 m after the aperture changed from 2 to 5.08 mm at the equivalent pressure.

Research on explosion venting characteristics of CH4/H2/Air mixture in square explosion vessels

Hydrogenation of methane (CH4) is an important means of achieving energy savings and large-scale hydrogen (H2) transportation. Explosion venting is considered to be an effective method to reduce the risk of gas explosions. To investigate the parameters of the explosion characteristics of CH4/H2/Air premixed fuel under venting conditions, the explosion characteristics and the dynamic propagation law of venting flame in CH4/Air mixtures with 5% concentration were studied in 20 L and 40 L square explosion venting vessels. The development process of flames inside vessels, the evolution characteristics of explosive venting flames, and the changes in explosion pressure were tested using a high-speed camera and pressure sensors. The maximum explosion pressure, the maximum explosion pressure rise rate, and the maximum flame velocity increased first and then decreased with the addition of H2. However, the maximum flame length increased continuously with the increase of H2. With the increase of hydrogenation, the maximum flame area showed an exponential increase. At equivalence ratio less than 1, the effect of hydrogenation on the flame area was much attenuated than that at equivalence ratio was greater than 1.

Effects of hydrogen addition on the explosion venting characteristics of hydrogen-propane-air mixtures

To study the effect of hydrogen fraction (η) in stoichiometric hydrogen-propane-air mixtures ranging from 0 to 1.0 on overpressure and external flame temperature, experiments on the vented explosion are conducted in a 1-m long cylindrical tube with a vent on the top surface. The explosion venting process is recorded by the high-speed shadowgraph imaging system, pressure testing system, and infrared thermal imager. The internal peak overpressure is proportional to the hydrogen fraction, and the external pressure peak (PExt) induced by the second explosion in the external flow field shows a similar trend. The higher external impulse occurs in higher hydrogen fractions and farther distances. The maximum flame length drops first and then rises with the addition of η. The maximum surface temperature of the external flame peaks at η = 1, followed by that of η = 0, η = 0.3, η = 0.9, and η = 0.6, respectively. Besides, the relative-safety radii, free of thermal radiant damage, are obtained.

Experimental study on flame length and pulsation behavior of n-heptane continuous spill fires on water

Continuous spill fires pose significant risks to the storage and transportation industries of liquid fuels. To investigate the flame length and pulsation behavior of continuous spill fires, large-scale spill fire experiments were conducted on water to simulate the burning fuel on a water surface that could occur during firefighting of tank fires or due to leaked fuels on water during transportation, with varying discharge rates (3, 6, 9, 14 mL/s) and ignition delays (0, 10, 20, 30 s). The results suggest that the flame length increased initially with the spread area and reached the maximum at the end of the spreading stage, then followed by a decrease at the shrinking stage and kept nearly constant at the steady stage. For the steady flame length, a new correlation (Ls/D=3.1(Q˙*)0.42) was established based on dimensionless analysis, which was higher than that for pool fires, highlighting the important differences between spill and pool fires. This developed model was also used to predict the maximum flame length with a maximum deviation of 19.2%, as the heat transfer mechanism and air entrainment are expected to be similar at the shrinking and steady stages. The flame pulsation frequency was determined through the fast Fourier transform (FFT) method, which shows a gradual decrease during the spreading stage while remaining higher than that of pool fires at the steady stage. Subsequently, two new correlations (fi=0.525g/D and fs=0.724g/D) were derived from experimental data to predict the pulsation frequency at both spreading and steady stages. The higher frequencies during the steady stage can have a significant influence on the radiation impact to adjacent equipment or personnel. This study not only provides insights into the flame and pulsation behaviors of spill fires but lays a solid foundation for the thermal hazard and risk assessment analysis in the liquid fuel storage and transportation industries.

Time Series Analysis on Flame Length of Micron Coal Dust Explosion with Ignition Delay Time

To discuss Pm variation law with t in large scale coal dust explosion equipment, shock wave pipe is used and a 60 m linear explosion test system is built. P m of dust explosion is measured by flame collection system. ARIMA (p,q,d) time series analysis model of t and P m is built, differential methods are used to eliminate upward trend of the maximum flame length data. By calculating the autocorrelation, the partial correlation coefficient, and the value of AIC and SBC, autoregression and moving average orders are got. Model parameters are estimated by least squares method. 20 groups of flame length prediction show that: ARIMA model have high precision, prediction value of optimum ignition delay time is consistent with measured value, which realizes reasonable analysis on fluctuation characteristics of P m. This model can reasonably predict the up and down fluctuations of the local flame length data, reflecting the superiority of the model, which is not achievable by general prediction models. The results of this study can greatly reduce the number of coal dust explosion tests, save material costs and provide a new idea for studying the relationship between ignition delay time and coal dust explosion flame length.

Effect of N2 and CO2 on explosion behavior of hydrogen-air mixtures in non-premixed state

In this paper, the suppression effects of non-premixed inert gas (N2, CO2) on hydrogen-air explosion behaviors are experimentally studied. The effects of N2 and CO2 addition on flame propagation, explosion pressure and flame length are analyzed. The results show that N2 and CO2 can effectively reduce the explosion hazard in non-premixed state. The mixing process of non-premixed gases leads to the development of turbulent flame, but the suppression of inert gas on explosion exceeds the promotion of turbulent flame. Under the action of N2 and CO2, the energy of the flame decreases, and the front contour becomes blurred. When the fuel zone is only set at the ignition end, with the suppression of N2 and CO2, the maximum explosion pressure is reduced by 35.1% and 50.6%, respectively. It is worth noting that the high-frequency periodic oscillation and pressure soaring appear in the pressure histories, which are attributed to the rapid phase transition of water. Furthermore, the two inert gases can significantly reduce the chemical reaction rate and heat release rate during the explosion, which is reflected in the decrease of the maximum flame length. It is found that N2 and CO2 have different explosion suppression effects and mechanisms, but CO2 has better suppression performance on hydrogen-air explosion.

Influencing factors of coking coal dust explosion pressure and flame and effect of inert dust on its explosion suppression

Coking coal is a precious resource in the world and an important raw material for the production of steel, but it is easy to cause explosion accidents in the process of coking coal mining, which is very detrimental to safe production. In order to reveal the influencing factors of coking coal dust explosion intensity and the suppression effect of inert dust on coking coal dust explosion, an experimental study was carried out in this paper. The results show that the particle size and the mass concentration of coal dust have a great influence on the explosion pressure and flame. By analyzing the suppression effects of NaCl, KCl, and NH4H2PO4 on coking coal dust explosion, it is got that NH4H2PO4 has the best explosion suppression effect. When the mass percentage of NH4H2PO4 mixed into coking coal dust increases to 60%, the maximum explosion pressure decreases by 0.47 MPa, and the maximum flame length decreases by 0.50 m. As the particle size of NH4H2PO4 decreases, the explosion intensity continue to decrease. When the particle size of NH4H2PO4 is 0 ~ 25 μm, and the mass percentage of NH4H2PO4 mixed into coking coal dust is 50%, the explosion doesn’t occur anymore.

Vented hydrogen-air explosions at elevated static activation pressures

The flame behaviors and pressure characteristics of vented hydrogen-air explosions at elevated static activation pressures (Pstat) were investigated using a 20 L spherical vessel. For vented flame, it was found that the maximum flame lengths and widths raised with the increase in venting diameters (Dv) and equivalent ratios (Φ) by dominating the concentration of vented unburned gas, while they were insensitive to the change of Pstat. Based on the variation law, non-dimensional formulas for predicting maximum flame length and width were established and the relative error was within 50 %. For internal pressure, the pressure curves were characterized with notably oscillations and an empirical formula coupled with NFPA 68 was proposed with a relative error within 25 %. For external pressure, the mechanism of the external explosion was revealed with the assistance of BOS images. The intensity of external explosion mainly depended on the external flow field intensity and the concentration of the unburned gas. The peaks of external explosion pressure and the pressure recession indices reached their maximum at Dv = 125 mm due to the conflicting effect of the increased Dv.

Influence of air gap ratio of the Chinese historical wooden window on the vertical flame spread performance

The fire protection of historical buildings with wooden structures is a topic that has attracted much attention over the last few years. As one of the critical structures of ancient buildings, wooden windows have high fire hazards. This study aims to clarify the influences of air gap ratio on the fire propagation mechanism of wooden windows by experimental investigating flame characteristics such as mass loss rate (MLR), flame spread rate (FSR), total burning duration (TBD), and maximum flame length (MFL), and propose corresponding fire protection strategies. In the experiment, the typical wooden window structure in ancient buildings was selected as the research object. The wooden windows with different porosity were installed on the vertical sample rack to carry out the combustion experiment, and the area air gap ratio ranged from 32.9% to 64.2%. The experimental results show a clear relationship between MLR and the area air gap ratio. TBD, FSR and MFL are related to the vertical linear air gap ratio of the wooden window structure. FSR first increases and then decreases with the linear air gap ratio, and the peak value of FSR appears on the sample with a vertical linear air gap ratio of 69.6%.

The effect of initial furnace temperature on combustion gas and PM exhausted from unsteady pool flame

In this study, PM exhausted from unsteady pool flame of iso-octane on a shallow dish under various furnace temperature conditions (25 – 700 °C) was investigated as fundamental study of fuel film combustion. Flame appearance during from ignition to combustion end under various furnace temperature conditions was observed. And relationship between maximum flame length and furnace temperature was obtained. Moreover, total masses of CO, CO₂ and PM emitted during flame formation period were measured. As the result, it was also found that total mass of PM exhausted during flame formation period increased with furnace temperature raise.

Effect of vent area on the vented methane-air deflagrations in a 1 m3 rectangular vessel with and without obstacles

The effects of vent area on pressure and flame evolution in a 1 m3 vessel with and without obstacles were investigated. Five pressure peaks of Popen, Pout, Pext, Phel, and Pvib, which correspond to vent activation, flame venting, external explosion, Helmholtz oscillation, and the coupling of acoustic waves and flames, respectively, were observed without obstacles when the vent coefficient Kv ≤ 6.25. However, the pressure peaks of Pext and Pout from the external explosion and flame venting disappeared when Kv > 6.25. Moreover, the pressure peaks of Popen and Pvib produced by vent activation and acoustically enhanced combustion become dominant when Kv ≥ 12.5. With the installation of obstacles, the pressure peak Pvib induced by acoustically enhanced combustion disappeared. The other four pressure peaks were observed when Kv < 6.25. The pressure peaks induced by the external explosion and the Helmholtz oscillation disappeared when Kv ≥ 12.5. Nevertheless, the pressure peak resulting from the obstacles was observed and became dominant at Kv ≥ 12.5. It takes less time for the external flame to propagate to the maximum displacement in the tests with obstacles than that without obstacles, and the maximum flame length for the former is smaller than the latter.

Characteristic evaluation of aluminum dust explosion venting with high static activation pressure

Aluminum dust explosion venting experiments with static activation pressure ranging from 70 kPa to 280 kPa were conducted in the 20 L spherical chamber with venting diameters from 20 mm to 125 mm. For the case of high activation pressure, the maximum flame length in experiments is 1.64 m, which is close to the prediction of standard, while the maximum width is significantly smaller than the prediction. New fitting formulas in a laboratory scale for flame length and width are obtained, which provide a reference for venting safety area design, and the effect of activation pressure, dust concentration and venting area are taken into account. By the comparison, the external pressure prediction of EN14491 shows a good performance and NFPA68 has an opposite trend with experiments. Based on the experimental results, it is found that the the external pressure attenuation is inversely proportional to the distance, which is different with the coefficient of the distance suggested by the standard EN 14491.

Experimental Analysis of Limited Distance Effects on Self Induced Blow Off and Heat Transfer in Upward Flame Spread Over Thin Fabric Fuels

Most previous work concentrated on the one-sided upward flame spread under limited conditions. However, few investigations have studied the effects of limited distance between wall and fuel surface on two-sided upward flame spreading. In this paper, a series of upward flame spread experiments were conducted using 0.65 mm thick, 130 cm tall and 4.0 cm wide flax fabric sheets with various limited distance of 1.0 cm–3.5 cm. The flame shape, flame length, flame standoff distance, pyrolysis spread rate and surface temperature were measured and analyzed. An unexpected but interesting behavior of one-sided flame blow off was observed in upward flame spread. When limited distance is no more than 1.5 cm, the one-sided flame blow off behavior occurs on unlimited side, whereas this behavior occurs on limited side for 2.0 cm–3.5 cm limited distance. This result could be qualitatively explained by Damkohler number Da, which could be characterized by two parameters of induced flow velocity and heat flux received by virgin surface. Moreover, the maximum flame length, steady flame length, maximum spread rate and steady spread rate first increase and then decrease as the limited distance. This non-monotonicity trend is determined by heat flux profiles in the preheating zone, which is due to the combined limited distance effects of restricting oxygen supply, chimney effect and wall radiation. The surface temperature profile ahead of pyrolysis front is used to qualitatively describe the heat flux received by virgin surface. The results in this paper are essential for better understanding of self induced flame blow off mechanism and upward flame spread under limited distance conditions.

An investigation on the aluminum dustexplosion suppression efficiency and mechanism of a NaHCO3/DE composite powder

A NaHCO3/diatomaceous earth (DE) composite powder suppressant with unique clustered structures is prepared by high-pressure impact method, using DE as the carrier and NaHCO3 as the loaded chemical suppressant. The purpose is to obtain affordable, environmentally friendly, high-efficiency power explosion suppression materials. The suppression efficiencies of the NaHCO3/DE composite powder suppressant on aluminum dust propagation and explosion pressure are tested. The results show that as the content of the NaHCO3/DE composite powder suppressant increases, the maximum flame length gradually reduces and the suppression efficiency gradually increases. Addition of 60 wt% of the NaHCO3/DE composite powder suppressant suppress aluminum dust flame propagation and addition of 100 wt% of the NaHCO3/DE composite powder suppressant can fully suppress aluminum dust explosion. Comparison with pure NaHCO3 and pure DE reveals that NaHCO3/DE composite powder suppressant is more effective in suppressing aluminum dust flame propagation and aluminum dust explosion pressure than either of the two powders alone. The suppression mechanism of the NaHCO3/DE composite powder suppressant is established: On the one hand, reaction is suppressed by the decomposition of NaHCO3 and the product of this decomposition; on the other hand, as DE is rich in porous structures, when the loaded NaHCO3 powder separates from DE, the porous structures will not only limit flame propagation, but will also well adsorb the substances generated from explosion reaction.

Effectiveness and mechanism of carbamide/fly ash cenosphere with bilayer spherical shell structure as explosion suppressant of coal dust.

This study aims to eliminate or suppress explosion accidents in powder preparation and storage systems of coal dust. The modified fly ash cenosphere (FAC) was used as an inner spherical shell carrier with carbamide impregnated. Carbamide/FAC composite suppressant with bilayer spherical shell was prepared by vacuum freeze-drying. The flame propagation and explosion performances of coal were tested, respectively. The results showed that the surface-loaded carbamide particles had a particle size of ∼4 nm. With the increasing content of carbamide/FAC composite suppressant, the maximum flame length gradually decreased, and the explosion suppression effect was gradually improved. Nearly complete explosion suppression of coal could be achieved with an added amount of 40 wt% carbamide/FAC composite suppressant. The carbamide loaded in carbamide/FAC composite suppressant mainly acted in the 0-τ1 stage to weaken (dP/dt)max. FAC mainly but not solely acted on the stage of τ1-τ2 to decrease Pmax. Carbamide particles were highly efficient and responsive. The suppression mechanism of carbamide/FAC composite suppressant was proposed, which was micro-nano multiscale complementary effect and deceleration-depressurization coupling effect.

Improving effect of boron carbide on the combustion and thermal oxidation characteristics of amorphous boron

Boron carbide (B4C) is one of the main products from the primary combustion of boron (B)-based propellants and has a significant influence on the secondary combustion of B. To systematically evaluate its effects on the secondary combustion of B, mixtures of B4C and B in different mass ratios were prepared. To study the ignition temperatures and combustion flames of the samples, a xenon lamp ignition experimental system and a flame shape test system were designed, respectively. A thermogravimetry–differential scanning calorimetry–Fourier transform infrared spectroscopy combined thermal analysis system was used to study the thermal oxidation characteristics and analyze the gaseous products of the samples. The results indicate that B4C reduces the heat absorption at the beginning of the ignition, but subsequently prevents the rapid rise of sample temperature. During the stable combustion stage, the maximum flame length under optical density 10−4 (OD4) filter was 20.4 mm, and the maximum flame length under 580 nm + OD4 filters (represents the combustion of B element) was 16.7 mm. The samples contained a small amount of HBO2 and H3BO3, which led to slight mass loss during the low temperature section of the thermal oxidation process. During the high temperature section, the oxidation of B and B4C caused considerable mass gain. The gaseous products of the thermal oxidation process include CO2, CO, and H2O. In general, the B content of 60% was the most beneficial to decrease the oxidation temperature, increase the combustion intensity, and improve the heat-releasing ability of the samples.

DIMZAL: A Software Tool to Compute Acceptable Safety Distance

The scope of this work is to present a multidisciplinary study in order to propose a tool called DIMZAL. DIMZAL forecasts fuelbreak safety zone sizes. To evaluate a safety zone and to prevent injury, the Acceptable Safety Distance (ASD) between the fire and firefighters is required. This distance is usually set thanks to a general rule-of-thumb: it should be at least 4 times the maximum flame length. A common assumption considers an empirical relationship between fireline intensity and flame length. In the current work which follows on from an oral presentation held at the VII International Conference on Forest Fire Research in Coimbra in 2014, an alternative way is proposed: a closed physical model is applied in order to quantize the ASD. This model is integrated in a software tool, which uses a simulation framework based on Discrete EVent system Specification formalism (DEVS), a 3D physical real-time model of surface fires developed at the University of Corsica and a mobile application based on a Google SDK to display the results

Cattle Grazing, Forest Loss, and Fuel Loading in a Dry Forest Ecosystem at Pu'u Wa'aWa'a Ranch, Hawai'i1

ABSTRACTWe used aerial photographs to measure long‐term loss of forest cover in dry to mesic forests at Pu'u Wa'aWa'a on the island of Hawai'i. The aerial extent of dense forest decreased 62 percent between 1954 and 1994, and the area covered by grassland increased by 237 percent. We then used field data and modeling to evaluate the consequences of removing cattle to the composition, biomass, and flammability of the grass community. Field studies of exclosures demonstrated that grazing reduced biomass of kikuyu grass (Pennisetum clandestinum) from 770 to 229 g/m2, and canopy height from 26.8 to 6.1 cm. Grazed kikuyu was unable to carry fast‐spreading or intense fires (maximum flame length 70 cm, covering 1.4 ha after one hour), while ungrazed kikuyu could carry catastrophic fires (flame lengths 3.0 m, covering &gt;75 ha after one hour). In addition, removing cattle from drier areas allowed the spread of fountain grass (Pennisetum setaceum), which created a higher risk of fire due to its architecture and low fuel moistures.

Cattle Grazing, Forest Loss, and Fuel Loading in a Dry Forest Ecosystem at Pu'u Wa'aWa'a Ranch, Hawai'i1

We used aerial photographs to measure long-term loss of forest cover in dry to mesic forests at Pu'u Wa'aWa'a on the island of Hawai'i. The aerial extent of dense forest decreased 62 percent between 1954 and 1994, and the area covered by grassland increased by 237 percent. We then used field data and modeling to evaluate the consequences of removing cattle to the composition, biomass, and flammability of the grass community. Field studies of exclosures demonstrated that grazing reduced biomass of kikuyu grass (Pennisetum clandestinum) from 770 to 229 g/m2, and canopy height from 26.8 to 6.1 cm. Grazed kikuyu was unable to carry fast-spreading or intense fires (maximum flame length 70 cm, covering 1.4 ha after one hour), while ungrazed kikuyu could carry catastrophic fires (flame lengths 3.0 m, covering >75 ha after one hour). In addition, removing cattle from drier areas allowed the spread of fountain grass (Pennisetum setaceum), which created a higher risk of fire due to its architecture and low fuel moistures.

Scaling of wake-stabilized jet diffusion flames in a transverse air stream

This experimentally based study considers the scaling of the flame length, the cross-stream dimension of the plume of combustion products, and the overall combustion efficiency of wake-stabilized jet diffusion flames. The study focuses on scaling with respect to the burner tube diameter (ds), but relationships include scaling with the jet exit velocity (Vj) and the transverse air velocity (U∞). Phenomenological models are used to correlate the data presented. The flame length (Lf) is shown to correlate well by relating the time-scale for the flow to advect the length of the flame (Lf/U∞) to the time scale of the external flow with respect to the fuel jet dimension (ds/U∞). A consequence of this model is that Lf∞ds, and Lf either increases or decreases depending on whether the operating conditions are greater or less than a critical value of ds/U∞. The maximum flame length occurs at the critical value of ds/U∞. The proposed model for flame length predicts that Lf actually scales with (pjVj)1/2 ds/U∞, but the (pjVj)1/2 term has yet to be validated. The characteristic cross-stream dimension of the plume (dp*), defined by the square root of its cross-sectional area, is shown to correlate with non-reacting plume models that use buoyancy forces and air stream momentum to give dp*∝g13Vj13U∞x23ds23 where g is the gravitational constant and x is the downstream position. The combustion inefficiency is shown empirically to scale inversely with ds1/3.