Flame Tilt Research Articles

Simulation of Large-Scale LNG Pool Fires Using FireFoam

Numerical simulations have been conducted for a range of liquefied natural gas (LNG) pool fires on land and water using FireFOAM, the large eddy simulation (LES)–based fire simulation code within the framework of open source computational fluid dynamics (CFD) toolbox OpenFOAM®. The studied pool diameters range from 14–400 m with cross winds from 1.6–9.6 m/s. The code uses the extended eddy dissipation concept (EDC) and a newly developed soot model based on the laminar smoke point concept. For the low-temperature (111.65–200 K) thermodynamic data of natural gas, the nine-coefficient correlations in the NASA thermodynamic database are used. Comparison between the predictions and measurements were carried out for the first four cases where full-scale test data are available. For all cases, the variations of flame length, tilt angle, and surface emissive power with LNG pool diameters are analyzed. New nonlinear correlations for predicting length-to-diameter ratio and tilt angle are also proposed.

Burning rate and flame tilt characteristics of radiation-controlled rectangular hydrocarbon pool fires with cross air flows in a reduced pressure

The mass burning rate and flame tilt evolution for radiation-controlled rectangular hydrocarbon pool fires with cross air flow (ranged from 0 to 3m/s) in a reduced pressure atmosphere at high altitude (altitude: 3650m, 64kPa), are experimentally studied, with rectangular acetone pools with same surface area but different aspect ratios n (n=L/W, ranged in 1–8). It is found that burning rate increases a bit faster with flow speed in reduced pressure. For condition of shorter side facing the flow, the increase of mass burning flux with flow speed is found to be dominated by the three side wall dimensions (except the leading one) in both pressures. However, in case of longer side facing the flow, a stagnant layer theory solution to include ambient pressure (due to the fuel vapor gas phase density is proportional to pressure) and cross air flow speed by taking the shorter side as characteristic length scale is proposed to correlate the data well in both pressures. For the mean vertical flame height and flame tilt angle, characteristic length scales including pool aspect ratio are proposed to apply available classical models to this special condition, which well correlate the obtained data.

Applicability of currently available flare radiation models for hydrogen and syngas

The primary purpose of a flare model is to provide a reasonably accurate prediction of radiant heat flux at any point of interest around the flame in order to be able to define safe release locations. In order to do this, a flare model needs to provide a good prediction of the following critical flame parameters: flame length, flame tilt, and radiant heat fraction, including the impact of wind and release orientation on these parameters. In addition, the model needs to distribute the radiant heat along the flame in a reasonably realistic way and to allow for the transmissivity of the ambient air.Industry standard models for vertical flares include Chamberlain [Chamberlain, Chem Eng Res Des 65 (1987)], API 521 [API STANDARD 521, Pressure‐Relieving and Depressuring Systems, 6th Edition, American Petroleum Institute, Washington DC, 2014], and Brzustowski & Sommer [API STANDARD 521, Pressure‐Relieving and Depressuring Systems, 6th Edition, American Petroleum Institute, Washington DC, 2014]. These models were originally developed primarily based on hydrocarbon data. Several commercially available consequence models allow the use of these models for all flammable materials. Due to the lack of other options, these commercially available consequence software models are often applied to hydrogen, syngas (hydrogen/carbon monoxide mixtures), and other materials that are well outside the intended scope of the models. A review has been performed to evaluate applicability of the Chamberlain, API 521, and Brzustowski & Sommer models to hydrogen and syngas, including comparison with limited published data. As a result of this review, a number of significant concerns have been identified. This has led to initiation of a new test program to collect data specifically for hydrogen and syngas and the subsequent development of new models. © 2014 American Institute of Chemical Engineers Process Saf Prog 34: 141–146, 2015

Stabilization and structure of n-heptane tribrachial flames in axisymmetric laminar jets

A set of tribrachial flames of n-heptane/air is simulated with finite rate chemistry and detailed transport in a realistic laminar jet configuration for which experimental data are available. The flames differ by the temperature of the unburnt mixture and stabilization height, which controls the mixture fraction gradient ahead of the flame front. The simulations reproduce the lift-off heights in the experiments, showing that the flame stabilizes further downstream as the unburnt temperature decreases. For the lowest unburnt temperature, resulting in a weak mixture fraction gradient at the tribrachial point, positive stretch along the rich premixed wing leads to an increase in the rate of chemical reaction in the whole flame. The tribrachial flame burning velocity exceeds that in the unstretched, one-dimensional flame. For the highest temperature, the flame stabilizes closest to the nozzle. Large flame tilt, large mixture fraction gradient, and small radius of curvature lead to a reduction in the heat release rate and the flame propagates slower than its one-dimensional counterpart. The observed behavior is explained with a detailed analysis of the flame geometry, differential diffusion effects, flame stretch, and transport of heat and mass from the burnt gases to the flame front.

EXPERIMENTAL STUDY ON INFLUENCE OF STACK EFFECT ON FIRE IN THE COMPARTMENT ADJACENT TO STAIRWELL OF HIGH RISE BUILDING

In this paper, to study the influence of stack effect on fire in the compartment adjacent to a stairwell, a set of experiments were conducted by varying the pool size, top vent state and bottom vent size in a 1/3 scaled 12-layer-stairwell configuration. The phenomenon of methanol flame tilting in the fire room was observed and studied. Results showed that the flame tilt angle increases with an increase of Ri-1. The temperatures of hot gases in the fire room decrease due to the cooling effect of fresh air induced by stack effect. The mass loss rate of methanol fuel is influenced by fresh air flow sucked into fire room due to stack effect. On the basis of the experimental results, we conclude that the velocity of air flow into fire room is proportional to 1/3 power of the heat release rate in the stairwell.

Influence of staircase ventilation state on the airflow and heat transfer of the heated room on the middle floor of high rise building

Safety and energy efficiency of high rise buildings have attracted public attention in recent decades. In this paper, a set of experiments was conducted in a scaled building model with 12 floors to study the influence of the staircase ventilation state on the flow and heat transfer of the heated room on the middle floor. The airflow, room temperature and fuel burning rate were investigated. It is found that when the window above the heated room is opened, the vents state below the heated room has a significant effect on the airflow and heat transfer in the heated room. When the vents below the heated room are closed, the single-directional air flows into the heated room owing to the stronger stack effect. And the flame tilt angle is larger and the upper hot smoke temperature in the heated room is low. However, when the windows above the heated room are closed, the vents state below the heated room has little influence on the airflow and heat transfer in the heated room. And, there is two-directional air flowing through the door of the heated room The burning rate of heat source is also affected by the staircase ventilation state, and the variation trend varies with the opened window position and pool size.

Correlations for Evaluation of Flame Spread over an Inclined Fuel Surface

The time-dependent flame spread process over thermally thick slabs of polymethyl methacrylate (PMMA) is investigated with particular emphasis on the burning behavior and geometry of the flame. 10 cm wide by 20 cm long samples are ignited and allowed to spread upwards at angles of orientation between pool and ceiling burning. Correlations between the flame length and the fuel mass-loss rate have revealed a delayed transition to turbulence for flames residing on the underside of fuel samples, and an earlier transition to turbulence for flames on the topside of these samples, compared to traditional vertical wall flames. As the fuel inclination increases, the relationship between the flame length and fuel mass-loss rate ranges between a recent theoretical prediction for a laminar wall plume dominated by diffusion and the traditional prediction for a turbulent wall plume dominated by convective mixing. The buoyancy-induced flow field, characterized by the flame tilt angle is shown to correspond to previously-found modifications of heat-flux profiles ahead of the flame front, which control flame spread, and the heat flux to the burning surface of the fuel, which controls fuel mass-loss rates. Other correlations between some of these parameters, such as the flame and pyrolysis lengths are also presented as a function of inclination.

A new mathematical quantification of wind-blown flame tilt angle of hydrocarbon pool fires with a new global correlation model

A new mathematical methodology is brought forward in this paper to objectively quantify the flame tilt angle based on flame appearance intermittency spatial distribution obtained from the flame images series recorded in the experiments, in counteracting the uncertainty in traditional method due to the fact of extensive vibration nature of the flame in a wind. The flame tilt angle is determined based on a proposed sum function of the product of multiplication of p(x, y) as probability of flame appearance at each pixel, by D(x, y) as perpendicular distance from the pixel to a threshold line originated from the flame base center. The flame tilt angle is solved mathematically as the angle of the threshold line when the above sum function at the two sides of the threshold line in the flame intermittency distribution contour are early equal. Experiments are carried out for ethanol and heptane square pool fires with dimensions of 10cm, 15cm, 20cm and 25cm in wind speed of 0–2.5m/s. The tilt angles of the flames in the experiments determined by above mathematical method are compared with previous models reported in the literatures. A new dimensionless global parameter, equating the wind speed by a characteristic uprising velocity of the flame supported by the buoyancy strength of the pool fire, is further proposed, which is shown to better converge and correlate the flame tilt angle data than previous models.

Effects of low air pressure on radiation-controlled rectangular ethanol and n-heptane pool fires

Experiments on rectangular ethanol and n-heptane pool fires were conducted at different altitudes in Hefei (99.8kPa) and Lhasa (66.5kPa). The burners tested had the same fuel area of 900cm2, but with aspect ratio of long side to short side (n=l/w) varied from 1 and 8. The individual and combined influences of low pressure and aspect ratio on burning rate, temperature, puffing frequency, flame height and radiation for the two fuels were interpreted and formulated. First, burning rate was found to be proportional to ambient air pressure under radiation control, the main reason is that radiative heat flux decreased with pressure due to the pressure affecting the soot absorption coefficient. Flame temperature slightly increased, leading to higher flame puffing frequency at low pressure. Flame height was almost insensitive to pressure as H∝p0. Second, for aspect ratio n, flame temperature was constant and independent of fuel type and burner shape. With increasing n, burner wall temperature increased at the long side, and decreased drastically at the short side, especially n=8. This was attributed to the change of flame tilt and heating of the burner side, caused by variation of entrainment motion. Flame puffing frequency was found to increase with n as a function of f∼(ΔT/T∞)(n+1)/2n. The flame was observed to split into small clusters by enhanced asymmetric entrainment, and H decreased with increasing n as H∼(1/n)2/5Q˙4/15. Considering fuel differences, with increasing n, the burning rate of the ethanol pool fire decreased, and n-heptane showed the opposite trend.

Effects of slope on fire spread observed through video images and multiple-point thermal measurements

A series of nine fire experiments was conducted on a large-scale bench with an inclinable plate to study the effects of a slope on fire spread. The temperature and total and radiant heat flux densities were measured at the top of the fuel bed during the upslope fire spread in still air. Measurements were recorded at five different locations ahead of the fire front. The effects of the slope on the flame topology, fire dynamics, and heat transfers ahead of the fire front were observed. The thermal measurements and image analyses showed a change in the fire spread regime and its related behavior with an increase in the slope. This was observed through an elongation of the flame and its contact surface over the upper part of the fuel bed. When the slope increased, the fire topology changed from a U-shape to a V-shape, and the radiation-dominated thermal environment ahead of the flame front was progressively turned into a mixed convective-radiative one, in which convection finally dominated for steep slope configurations. For fire spreading across a 30° slope, this increase in the curvature of the fire front (V-shape) is associated with the appearance of fire whirls rolling along the flanks of the fire front, which indicates significant changes in the fluid mechanics and heat transfers. The span of the heat transfer increased and a significant amount of energy impinged 1 or 2m ahead of the flame front because of the flame tilt (radiation) and hot gas flow (convection). The measurements also indicated a greater amount of energy impinging in the center of the fuel bed than at the edge. This study shows the importance of investigating the fluid mechanics of fire in future wildfire researches.

Entrainment regimes and flame characteristics of wildland fires

This paper reports results from a study of the flame characteristics of 22 wind-aided pine litter fires in a laboratory wind tunnel and 32 field fires in southern rough and litter–grass fuels. Flame characteristic and fire behaviour data from these fires, simple theoretical flame models and regression techniques are used to determine whether the data support the derived models. When the data do not support the models, alternative models are developed. The experimental fires are used to evaluate entrainment constants and air/fuel mass ratios in the model equations. Both the models and the experimental data are consistent with recently reported computational fluid dynamics simulations that suggest the existence of buoyancy- and convection-controlled regimes of fire behaviour. The results also suggest these regimes are delimited by a critical value of Byram’s convection number. Flame heights and air/fuel ratios behave similarly in the laboratory and field, but flame tilt angle relationships differ.

A STUDY OF COMPARTMENT FIRE UNDER FORCED VENTILATION: EXPERIMENTAL AND COMPUTATIONAL FLUID DYNAMICS MODELING

This paper discusses on the dynamic behavior of the flame and smoke inside a compartment fire. The compartment can be referred to a room, road tunnel, duct, compartment or a building. A series of small scales experiments were carried on four rectangular ducts that have the same height (250 mm) but different widths (125 mm, 250 mm, 500 mm and 1000 mm). Fire simulations on the same compartments were performed to investigate the effectiveness of Computational Fluid Dynamics (CFD) in predicting fire phenomenon. CFD was found capable to predict fire phenomenon such as flame shape, flame height and flame tilt angle similar to the experiment.

Metrological Tool for the Characterization of Flame Fronts Based on the Coupling of a Heat Flux Approach with Image Processing Data

The aim of this paper is to present some improvements in the metrology of forest fire flames by coupling image processing and radiative heat flux measurement. A new metrological tool using a visual video camera and a specific multiple thermal sensors is proposed. By means of an appropriate segmentation algorithm and the Direct Linear Transformation, the image processing methodology gives the forward or the backward fire front positions as input data to a radiative heat flux approach. Using a simplified flame model, this technique provides fire front positions versus time, and average values for the flame length, flame depth, flame tilt angle, apparent flame temperature and flame emissivity. The rates of spread, obtained by a linear regression of the determined fire front positions, as well as the thermal and radiative properties of the flame, are compared favorably to those given in the literature.

Numerical study of wind effects on the characteristics of flames from non-propagating vegetation fires

A variant of a three-dimensional two-phase model was proposed to address the central question of how wind affects the geometric properties of flames from steady vegetation fires. The pyrolysis rate and the spatial distribution of burning vegetation were representative of the heat release rates of fires ranging from single tree or tree group “torching” to continuous surface fires. Wind speed, U ∞ , was varied from 0.5 to 8 m s −1. Results revealed that about one third of the heat produced by combustion is released within the fuel bed, and this occurs almost independently of wind speed and vegetation distribution. Predicted values of flame height, flame length and tilt angle were found to be consistent with available literature data. Model results showed that the flame length and height normalized by the flame depth, D, scale with the modified Froude number Fr D = U ∞ 2 / g D and the dimensionless heat release rate. Predicted mean velocity flow streamlines appeared to be nearly straight in both the flaming region and the inert plume, supporting the relevance of a far field analysis to determine the flame tilt angle. This analysis showed that the tangent of the flame tilt angle may be expressed in terms of the Froude number based on the characteristic length scale of the plume. The numerical model was then used to quantify these relationships. The relevance of scaling parameters was supported by both field and laboratory-scale experiments.

Numerical simulation of fire in a tunnel: Comparative study of CFAST and CFX predictions

The mathematical modeling of fire growth and smoke movement in any enclosure is a formidable task. Two types of deterministic models are in vogue, zone models and field models (popularly known as CFD technique). CFAST is a popular zone model used for modeling of fires in enclosures. Likewise, CFX is a general purpose CFD code used for various purposes including modeling of fires. In the present paper, a tunnel of length 150 m having a rectangular cross-section of 80 m 2 has been considered for analyzing the temperature and velocity profiles generated by fire, placed at a distance of 20 m from one end of portal, by both CFAST and CFX. The simulation by CFAST has been carried out by dividing the tunnel into 1, 2, 5, 8, 10, 12 and 15 compartments of equal size, where these compartments are joined by openings or vents having same cross-section as that of the tunnel. In case of tunnel divided into 15 compartments the fire source position lies at the position of vent; CFAST predicted very high temperatures. The simulations have also been carried out by dividing tunnel into unequal sized compartments such that position of fire was at the center of the compartment. It was found that for accuracy of results, location of fire source inside compartment is an important factor. Computational difficulty was experienced when tunnel was divided into more than 15 compartments. In this paper, a comparative study of temperatures predicted by CFAST and CFX has been done. The CFX and CFAST predictions show that smoke temperature changes with a pattern roughly similar to that of heat release rate. The temperature profiles at selected positions cannot be predicted by CFAST unlike CFX. The detailed features like flame tilt, flow field can only be observed from CFX predictions.

Effects of thin-layer boilover on flame geometry and dynamics in large hydrocarbon pool fires

This work aims to estimate the effects of thin-layer boilover on flame geometry and dynamics. A series of large scale experiments (in pools ranging from 1.5 to 6 m in diameter) were performed using gasoline and diesel as fuel. As expected, only diesel showed evidence of this phenomenon. This article presents a summary of the results obtained for flame height, tilt and pulsation. Flame height increases during water ebullition, though the increase is no longer detectable when wind speed exceeds certain values. Correlations previously presented in the literature to predict flame length and tilt were modified in order to fit the results obtained during thin-layer boilover. However, the influence on flame tilt is not as great and the equations for the stationary period seem suitable for the entire fire. Results of flame pulsation during the stationary period fill the gap in the literature for fires between 1.5 and 6 m and fit previous correlations. On the other hand, during ebullition, the flame pulsates faster, as air entrainment is greater and, as one would expect, this effect decreases with pool size. A new equation for estimating pulsation frequency during boilover is proposed.

Model of large pool fires

A two zone entrainment model of pool fires is proposed to depict the fluid flow and flame properties of the fire. Consisting of combustion and plume zones, it provides a consistent scheme for developing non-dimensional scaling parameters for correlating and extrapolating pool fire visible flame length, flame tilt, surface emissive power, and fuel evaporation rate. The model is extended to include grey gas thermal radiation from soot particles in the flame zone, accounting for emission and absorption in both optically thin and thick regions. A model of convective heat transfer from the combustion zone to the liquid fuel pool, and from a water substrate to cryogenic fuel pools spreading on water, provides evaporation rates for both adiabatic and non-adiabatic fires. The model is tested against field measurements of large scale pool fires, principally of LNG, and is generally in agreement with experimental values of all variables.

Effect of Shape and Size of a Fire Source on Fire Properties in Vicinity of a Fire Source in a Tunnel

Experiments were performed to investigate the effects of the shape and size of a fire source on fire properties using a tunnel model with a rectangular cross section and aspect ratio of 1:2. Square and rectangular burners were used as the model fire sources. The variations in the heat release rate (HRR) from 4.5 to 36 kW was proportional to the fire size, and the longitudinal ventilation velocity was also varied in the range from 0 to 0.8 m/s. The flame tilt angle, maximum temperature rise of the smoke layer near the ceiling and its position were adopted as the variables that characterise the fire phenomena in the near field of a fire source in the presence of longitudinal ventilation. New empirical formulae considering the effects of the shape and size of a fire source on these variables were developed.

Fire properties in near field of square fire source with longitudinal ventilation in tunnels

In order to understand fire phenomena in the near field of a fire source in a tunnel, experiments were conducted using three kinds of model tunnels of which the dimensions were 1/10 scale, 1/2 scale and full scale, having rectangular and horseshoe cross-sections. Square fire sources were employed as model fire sources. The aspect ratio of the tunnel cross-section, heat release rate and longitudinal forced ventilation velocity were varied. Empirical formulae for flame tilt, apparent flame height, maximum temperature of the smoke layer and its position were developed based on the results obtained in the 1/10 scale model tunnel. The effect of aspect ratio of tunnel cross-section was incorporated in these models. It was found that the length defined by H 3/2/ b 1/2 became a representative length for investigating fire phenomena in tunnels. It is also confirmed that these empirical formulae are sufficiently applicable for predicting fire phenomena in the near field of square fire source from the comparison with the results in 1/2 and full scale experiments.

Effect Of Cross-Winds To Apparent Flame Height And Tilt Angle From Several Kinds Of Fire Source

Experiments with a single fire source were carried out in an unconfined space to characterize the effects of cross-winds on apparent flame heights and flame tilt angles. Three sets of propane burners were used as model fire sources. One was a small circular burner, the other was large square burners, and the other was rectangular burners, having the aspect ratio ranging from 1 to 6. A refined empirical model on the apparent flame heights of the inclined flames modified by heat release rate and aspect ratio of fire source was presented. We also developed an empirical model of the flame tilt angles based on the balance of mass between the fluxes given by the upward hot current and the cross-winds. These models are correlations with respect to dimensionless heat release rates, Froude numbers and aspect ratios of fire sources. The values of empirical coefficients and exponents of the correlations were derived from the experimental results.