Flame Tilt Angle Research Articles

Interaction of Heat Release Rate, Flame Tilt Angle and Flame Length of a Compartment Fire Under Stack Effect

This paper is a re-examination to quantify the flame tilt angle of fires induced by stack effect from a heat and mass transfer perspective. In previous studies, the critical transition and length of the deflection flame were formulated. In the current study, with different kinds of fuels, the flame tilt angles under different strength of stack effect are compared. Results show that under stack effect, the flame tilt angles are quite close at the quasi-steady state within a narrow range regardless of the heat release rate of fire and the position of the open window in a stairwell, which indicates that the inertial force of air flow induced by the stack effect equilibrates dynamically with the thermal buoyancy induced by fire plume. However, the velocity of air flow drawn into the fire compartment is directly related to the strength of stack effect, which is mainly influenced by the heat release rate of fire. A new correlation for the flame tilt angle under the influence of stack effect is developed by taking the dynamic equilibrium into account. The tangent value of the flame tilt angle is linearly proportional to the 4/15 power of the heat release rate of fire. Additionally, a correlation to predict the heat release rate with the titled flame length and flame tilt angle is formulated.

Diffusion flame morphology with or without near-wall in cross-winds: Experiments and a correlation based on momentum-buoyancy length scale

This paper investigates the flame morphological characteristics (flame height, flame horizontal length and flame tilt angle) of diffusion combustion with or without near-wall constraint in cross-wind. Experiments are carried out using propane as fuel for 10, 15, 20 cm square burners at various heat release rates. Two air entrainment conditions are performed: with near-wall (simulated by a mica board) constraint or in free space (without near-wall). The cross-wind is provided by a wind tunnel up to 3.5 m/s. The evolution of flame height, flame horizontal length and flame tilt angle are measured for a total of 240 test conditions. It is found that the flame height of near-wall condition is higher while the flame tilt angle is lower than that of free condition, both changing monotonously with cross-wind speed. The flame horizontal length is first relatively smaller then larger for near-wall condition than that of free condition with increase in cross-wind speed, however, for both air entrainment conditions it increases first then decreases with cross-wind speed. Three independent length scales are proposed based on the analysis of competition of combustion buoyancy and cross-wind momentum and the inherent physics. General formulas are proposed, based on dimensionless parameters derived according to the derived length scales, to represent the flame morphological characteristics for both with or without near-wall conditions in cross-wind.

Influence of bottom wall on characteristics of jet diffusion flames under cross-wind

This study investigated the influence of different rear bottom wall lengths on the blow-out limit, flame shape, flame mean length, and flame tilt angle of jet diffusion flames under cross-wind conditions. The experimental results reveal that the bottom wall had a significant effect on the flame blow-out limit. The flame blow-out velocity was maximum only when the front bottom wall existed, and all jet diffusion flames had similar blow-out velocity under the influence of the rear bottom wall. Once the length of the rear bottom exceeded a certain value, the jet diffusion flames were prone to lift-off. Moreover, the jet diffusion flames adhered to the bottom wall, which is attributed to the Coanda effect. The flame length significantly increased under the influence of the bottom wall. Additionally, under the influence of the front bottom wall, the flame length was maximum when the cross-wind velocity was higher. Under the influence of different rear bottom wall lengths, the flame tilt angle was divided into two variation regions. Among them, the flame tilt angle affected only by the front bottom wall was minimum. The flame tilt angle was maximum when the rear bottom wall length was 20 cm. Finally, the empirical formula of the jet diffusion flame tilt angle was obtained by analogy analysis and exhibited good correlation with the experimental data.

An investigation of flame tilt transition inside the compartment with horizontal opening in cross wind

This paper experimentally investigated the flame tilt transition of the heptane pool fires inside a compartment with horizontal opening in cross wind, which has not been reported till now. A series of experiments were carried out with a 0.4 m cubic compartment with fully opening at the top. The cross wind produced by a small-scale wind tunnel ranged from 0 to 3 m/s, which was parallel to the horizontal opening. Temperature field inside the compartment was obtained by the forty-nine thermocouples. Results showed that the flame inside the compartment with horizontal opening had a tilt transition, which would tilt from the windward side towards the leeward side with the increase of cross wind velocity, and keep vertical at a certain velocity value. The critical cross wind velocity for the flame tilt transition was obtained. Temperature differences between windward and leeward side presented the decrease and then increase trend with the cross wind velocity increasing. Moreover, the critical cross wind velocity was found to be proportional to DhlQ˙* on the basis of the dimensional analysis, and the empirical correlations of flame tilt angle were proposed to predict the flame tilt transition inside the compartment with fully horizontal opening in cross wind.

The effect of preheating on fire propagation on inclined wood by multi-spectrum and schlieren visualisation

A systematic visualisation system that can image the visible flame, invisible hot gas and the wood surface temperature, was applied to study self-sustained fire propagation in a wood rod at different inclination angles. It was found that the burned wood rods at positive inclination angles presented longer burning lifetimes and charring lengths than those at negative and horizontal angles. Three physical phenomena were found to determine the fire sustaining and propagation: 1. Underneath hot gas layer; 2. Flame attachment phenomenon; 3. Impact point of piloted imping heat flux. Longer underneath hot flow was observed at positive inclination angles. The underneath hot gas flow could preheat the adjacent wood making them more readily for fire propagation. The flame attachment length and flame tilt angle had been investigated and quantified with enhanced thermal images. It was found that the flame is more prone to attach to the rod at positive angles. The impact point of piloted impinging heat flux was analysed by multiple imaging systems. Higher positive inclinations mean that a larger part of the rod is contacting with the impinging heat flux. The new insights gained are beneficial for fire safety in construction especially for the fire propagation at early stage.

An extensive numerical study of the burning dynamics of wildland fuel using proposed configuration space

Physics-based flame models capable of predicting small-scale fire behaviors reduce computational power needed for predicting fires of large- and giga-scale. However, classical model correlations are often developed for ‘free fires’ without considering vegetation around. These models may result in inaccurate fire modeling due to wrong ‘prior’ flame shape estimated from θ ~ wind speed. To overcome this defect, three-dimensional small-scale fires with fireline intensity of 100 KW/m are numerically simulated using large eddy simulation. Fire behaviors such as flame tilt angle and heat transfer mechanisms are extensively studied using a newly proposed configuration space {NC, CdLAI}. The former one represents the ratio between fire to wind power, and the latter one considering the vegetation effect is for the first time introduced in flame models. Using the configuration space, two model correlations for flame tilt angle and radiative heat power reaching the unburnt fuels are proposed. The flame tilt angle θ is directly related to CdLAI (CdαsσsHF/2), while inversely related to NC (2gI/ρ0Cp,0T0U03), in contrast to the model proposed for radiative heat power. Comparisons with several classical models evidenced the capability of new flame models in predicting both free and non-free fires. The limits of the validity of the newly proposed models are also discussed.

Experimental Investigation on Flame Characterization and Temperature Profile of Single/Multiple Pool Fire in Cross Wind

An experimental study was carried out to investigate the flame characterization and temperature profile for single and multiple pool fire with the influence of cross wind. There were 13 test cases in total, categorized into circle and rectangle fuel pans, with diameter (or equivalent diameter) ranged from 50 mm to 300 mm. Kerosene was used for the fuel of pool fire. Some K-type thermocouples were arranged around the flame to monitor the flame temperature, while the flame tilt angle was measured based on the photograph of flame for different case. Firstly, it can be found that there are three phases, including preheating, steady burning and extinguishing phase, during the flame evolution. The maximum temperature near the fuel surface is ∼1040 K, which is higher than that of flame plume (∼600 K), in the steady burning phase of circle single pool fire (D=300 mm), while the average burning rate is ∼1.525 g/s. In addition, the burning rates of all cases were measured and compared with the current predicted method. Typically, the flame morphology of single/multiple pool fire at different cross wind speed (ranging from 0 to 3.5 m/s) was analyzed, and it is found that the results for single pool fire agree with Thomas model and AGA model well, which are not suitable for multiple pool fire. Finally, the temperature profile of different case was measured with various wind speed.

Investigation of sidewall height effect on the burning rate and flame tilt characteristics of pool fire in cross wind

In the current study the combining effect of sidewall height and cross air flow on the mass burning rate, flame height and flame tilt behavior of pool fire has been experimentally investigated. The mass burning rate increases as the cross wind velocity increases. The presence of sidewalls also influences the mass burning rate, as it is increased at lower cross wind velocities and decreased as this velocity further increases. A dimensionless parameter is introduced to illustrate the sidewall height effect on the mass burning rate. The effect of sidewall height on burning rate, flame height, and flame tilt angle of pool fire is further investigated. The flame height is an exponential function of a new dimensionless parameter Fr˜. Based on the experimental results, new empirical correlations are proposed to predict flame height and flame tilt angle α in relation to the combining effects of sidewall height and cross wind.

Physical model of wildland fire spread: Parametric uncertainty analysis

For the physical model of wildland fire spread, errors or discrepancies in the prediction of spread rate may arise from uncertain, imprecise or improper determinations of the model parameters due to unreasonable assumptions, rough approximations, or inaccurate measurements. In this study, a parametric uncertainty analysis is made on a typical model of upslope fire spread that is consistent with many other models in the framework of the heat transfer theory and the major equations of sub-models. The variation ranges of the model parameters are determined based on the experimental data and assumptions, thereby the rate of fire spread (ROS) is used as a target variable of model calculation to evaluate the response of the model to parametric uncertainties. It is found that the values of ignition temperatures and averaged flame temperatures have significant impacts on the predicted values of ROS under lower slopes. Although the impacts decrease with the increase of slope, they are still non-negligible under higher slopes. Especially under lower slopes, the assumed values of both temperatures have significant effects on model prediction. The error in estimating the flame length has a remarkable influence on model prediction, and especially, the flame length variation of the same magnitude results in consistent relative errors under different slopes. A larger fluctuation range around a fixed average flame length results in a greater deviation from the predicted results, while for the same fluctuation range of flame length, the relative errors under higher slope angles are distinctly greater than those under lower slopes. In contrast, the errors for evaluation of the flame tilt angle have negligible effects on model predictions. The flame emissivity has a remarkable effect on model prediction and thus should be adjusted to better characterize the variation of flame intensity under different slopes. Besides, the prediction of ROS is very sensitive to the evaluation of fuel consumption efficiency, especially for higher slope angles. Simulation results indicate that larger heat convection coefficients are required for achieving better prediction results for higher slope angles, which agrees well with the previous experimental finding that with the increase of slope angle, a kind of flame-induced convection plays a more and more important role in fire spread.

Impact of wind on in-situ burning behavior of spilled oil on open water

Spilled oil may cause serious environmental pollution and ecological destruction. Thus, an efficient oil spill response is essential to reduce and minimize the impacts seen from an oil spill. In-situ burning (ISB) has been proven to be one of the most convenient and efficient treatment methods for oil spills on sea. When in-situ burning operation is carried out on sea, the wind is usually available. Spilled oil pool fire flame will be blown to tilt with an angle from the vertical direction and the flame length will be stretched. This effect of flame on downstream will be significantly enhanced. It may even cause the burning of spilled oil out of control or hazard the operators near site. Therefore, it is meaningful to study the burning behavior of oil pool fire on open water under wind.A small-scale oil pool fire on open water experiment system was designed, which could simulate the boundary conditions of sea surface ISB. A series of experiments were carried out to study the effects of oil pool diameter, oil layer thickness and wind speed on fire flame length and flame tilt angle. Flame characteristics and temperature of burning system were recorded. This study observes burning behavior to evolve from short pool fire to long declining phase which transient through boilover stage. Dimensionless flame length in pool fire stage increases with wind speed. As expected the flame tilt is strongly influenced by wind speed, tilt angle increase with wind speed. The observed experimental behavior is captured as improvement in the flame length and flame tilt angle models.This study helped better understanding fire behavior in evolving environmental conditions, and the most importantly developed model would help guide better assessment of fire mitigation strategies.

Experimental and modelling study on flame tilt angle of flame spread over jet fuel under longitudinally forced air flows

The flame spread above oil surface is affected by the ambient wind, including the velocity and direction of the wind. In current paper, the experiments of flame spread above jet fuel of RP-3 with different pool widths are conducted to find out the variations of flame tilt angle under the influence of longitudinally forced air flows. It is found that as the longitudinally forced air flow speed increases from 0 to 2.57 m/s, the flame tilt angle increases from 0° to about 75°, regardless of directions of air flows. The new non-dimensional parameter, characteristic size ratio λ=d/D is introduced into the Welker et al.’s model. The tangent value of the flame tilt angle is directly proportional to αRi-βd/Dγ under both opposed and concurrent flame spreads. The mechanism of flame inclination under the effect of longitudinally forced air flow is analyzed according to the coupling of combustion and gas flows. The flame inclination is principally determined by the coupled effects of vertical buoyancy and horizontally forced air flow.

Effects of wind flow and sidewall restriction on the geometric characteristics of propane diffusion flames in tunnels

Diffusion flame characteristics are important for predicting and controlling the energy transfer to fuel surface and surrounding targets during an undesirable energy source releasing in a tunnel. To explore the effects of wind flow and sidewall restriction on the geometric characteristics of turbulent diffusion flames in tunnels, experiments of propane burner fires were performed in a bench-scale wind tunnel. Two different fire source positions, i.e., centerline fire and sidewall fire, were considered. The energy (heat) release rate of the fire source and the ventilation velocity were varied in the range of 15–105 kW and 0–3 m/s, respectively. The flame shape, flame tilt angle and flame drag length were investigated. The results suggest that the flame tilt angles of two fire source positions both increase faster first and then increase slowly with increasing ventilation velocity, and the flame tilt angle of the sidewall fire is smaller than that of the centerline fire due to the inhibition of the frictional drag force. With the increase in ventilation velocity, the flame drag length of the centerline fire is first smaller than, and then approaches, and eventually exceeds that of the sidewall fire owing to the varying force mechanism. Finally, correlations of flame drag length were proposed and compared with previous experimental results.

Experimental investigation of LNG release underwater and combustion behavior under crosswinds

Liquefied natural gas (LNG) trade has increased globally; therefore, assessments of the hazards of its accidental release and associated consequences must be conducted to ensure LNG security during marine transportation. This study aims to understand the dynamic behavior occurring when an LNG jet is released underwater and the combustion behavior of a flammable vapor cloud on the water’s surface with airflows from 0 to 4m/s. A series of controlled LNG vertical jet release experiments were conducted using a cryogenic storage tank with three orifices. Various instruments were employed to measure the flow rate and pressure in pipelines during different leakage scenarios, and the emanating LNG vapor clouds were immediately ignited with the mass loss rate of 0.016, 0.037, and 0.049kg/s in three orifices, respectively. The flame behavior was recorded by a video camera. With respect to flame length rapidly decreased with crosswinds of 0−2m/s and then gradually decreased to a constant value with the velocities increase to 4m/s. A dimensionless number of Ri was employed to analyze the relative magnitude between the buoyancy force and transverse flow. The flame tilt angle was found in accordance trend with flame length for first increased and further reach to become constant at Ri−1 increase to 2. As existing correlations provide an overestimation, a new correlation was established to describe the flame length as a function of crosswind speed, and a good prediction was found for measured tilt angles with the correlated values.

Study on characteristics of fire plume in building facade window under lateral blow.

The overflow of the flame plume from the window is the main cause of the vertical spread of the fire on the facade of the building. This paper considers the geometry of the window by taking measures to prevent the flame from propagating along the vertical wall. In this paper, a residential building is taken as an example to evaluate the flame plume characteristics through experimental tests and numerical simulations. The objective of the present study is to study the flame plume characteristics under the air blow on the outer window side of the building. The theoretical equations of the flame tilt angle, non-dimensional temperature and non-dimensional velocity are derived. A series of experimental tests were carried out in a reduced-scale building model corresponding to the changes of lateral blow ventilation velocity. Reduced scale numerical simulations were conducted to verify the experiments. Results showed that the flame tilt angle increases with ventilation velocity increases. Meanwhile, the experimental results were compared with the reduced-scale tests and numerical simulations. These showed a good agreement between experimental results and numerical simulations. All these findings provide theoretical basis for building fire prevention outside window.

On the influence of nozzle geometry on jet diffusion flames under cross-wind

Four nozzle types were used to investigate the blow-out limit, flame shape, flame mean length, and flame tilt angle of jet diffusion flames under cross-wind conditions. The experimental results revealed that the nozzle geometry had a minor influence on the flame blow-out limit when the fuel jet velocity was low. As the fuel jet velocity increased, the nozzle geometry had a significant influence on the flame blow-out limit, the blow-out velocity of the triangle nozzle was the maximum, and the rectangular nozzle was minimum. The nozzle geometry had a significant influence on the down-wash and lift-off of the flame under cross-wind conditions. The triangle nozzle was prone to the appearance of the down-wash phenomenon, and the rectangular nozzle flame easily exhibited the lift-off phenomenon. Additionally, the effect of the nozzle geometry on the flame blow-out limit and flame shape was explained from a moment of inertia viewpoint. The flame length of the rectangular and cross nozzles monotonously increased with the increase of the momentum flux ratio of the fuel jet to the cross-wind. However, two growth curves occurred as the flame length of the circle and triangle nozzles increased with the momentum flux ratio of the fuel jet to the cross-wind. Additionally, for the jet diffusion flames under cross-wind conditions, the nozzle geometry exerted a smaller effect on the flame tilt angle.

On the Use of Semi-empirical Flame Models for Spreading Chaparral Crown Fire

Flame geometry plays a key role in shaping fire behavior as it can influence flame spread, radiative heat transfer and fire intensity. For wildland fire, a thorough understanding of relationships between flame geometry including flame length, flame height and flame tilt can help advance the derivation of comprehensive models of wildfire behavior. Within the fire community, a classical flame modeling approach has been the development of semi-empirical models. Many of these models have been derived for surface fuels or for pool fire configurations. However, few have sought to model flame behavior in chaparral crown fires. Thus, the objective of this study was to assess the applicability of existing semi-empirical models on observed chaparral crown fire geometry. Semi-empirical models of flame tilt, flame height and flame length were considered. Comparison with experimental observation of crown fuel layer flame height showed good agreement between two-fifths power law that relates flame height to heat release rate. Predictions of flame tilt were obtained from application of semi-empirical power-law correlations relating flame tilt angle to Froude number. Observed flame tilt values exhibited low correlation with predicted values. Thus, two new power-law correlations were proposed. Coefficients for new models were obtained from regression analysis.

Experimental study and physical analysis of flame geometry in pool fires under relatively strong cross flows

This paper investigated the influence of horizontal cross flows on the flame geometry in pool fires. Four square gaseous burners with dimensions of 8 cm, 10 cm, 15 cm, and 20 cm were used employing propane as fuel with various heat release rates. The cross flows were provided by a wind tunnel with air speed ranging from 0 m/s to 6.0 m/s in 0.5 m/s intervals. Five basic quantities of the flame geometry were measured comprehensively, namely, the horizontal flame length Lx, the vertical flame height H and the flame base drag Ldrag as well as two definitions of the flame tilt angle θ1 (originating from pool center) and θ2 (originating from flame base center). The horizontal flame length Lx and the flame base drag Ldrag are found to firstly increase then decrease, while the vertical flame height H decreases and the flame tilt angles θ1, θ2 increase monotonically, with increasing of cross flow air speed. The flame is characterized into two regions, i.e., one similar to boundary layer near the pool surface followed by the other similar to free flow rising above ground. This explains the aforementioned different behaviors of horizontal flame length Lx and vertical flame height H under different cross flow air speeds. In addition, none of correlations previously proposed describes all the flame geometry parameters comprehensively in both horizontal- and vertical directions under relatively strong cross flows. A physical model is developed based on the characteristic length scale derived by combining cross flow air speed, the characteristic volumetric air entrainment, the turbulent flame buoyancy and the air required for stoichiometric combustion. The basic new element is the flame buoyancy (ΔTfTag=g′)in combination with the cross flow air speed, which determines a characteristic length scale, Uw2/g. The proposed model correlates well the above five basic quantities of flame geometry in pool fires under cross flows. This approach allows us predicting the flame geometry in both horizontal- and vertical directions of pool fires for relatively high cross flow air speeds having cross flow (wind) Froude number Uw2gD>0.13.

Experimental study on the characteristics of flame merging and tilt angle from twin propane burners under cross wind

Abstract Multiple fires burning in a windy environment is a special fire phenomenon when multiple flames from separate fuel sources burn simultaneously within a certain space range. It is commonly observed in urban, industrial fires and wildland. To improve the fire safety and achieve optimum energy management, it is important to study the combustion characteristics of multiple fires in wind. This paper investigates the characteristics of flame merging and tilt angle. Twin propane burners aligned parallel to the cross wind with the same energy (heat) release rate were used as energy sources. The experimental results show that with increasing spacing there are three categories of flame merging characteristics, i.e., continuous merging, intermittent merging and non-merging. A new mathematical methodology is introduced to quantify the flame merging possibility in the presence and absence of wind, in counteracting the limitation and uncertainty in traditional method due to the fact of extensive vibration nature of the flames in a wind. Based on the momentum-balance equation, a piecewise function of the flame tilt angles is established in different merging states. Finally, the proposed formulae of flame tilt angles are validated using the literature and experimental data, which show the broad applicability.

Optimal Safe Layout of Fuel Storage Tanks Exposed to Pool Fire: One Dimensional Deterministic Modelling Approach

Fire hazard is one of the main risks associated to fuel storage tanks in petroleum and in the petrochemical industries. Such a hazard includes pool fires in the storage tanks or in the bunds, fire propagation from the source tank to target tanks both in absence and in presence of wind, and also the cascading/domino effect due to confined and unconfined vapour cloud explosion and or BLEVE associated with the source tank. In the present work, a radiation shield of flowing water has been introduced at a distance from the source fuel storage tank to prevent the domino effect originating from this source tank, under fire, to the target fuel storage tanks in a tank farm. A simple one dimensional model has been developed from the steady state energy balance to simulate the safe distances (i.e. rim–rim distance) between fuel storage tanks containing class-I fuel (e.g. gasoline), both in presence and absence of a water-shield under no-wind and cross-wind conditions. The model predictions have shown that the maximum safe inter-tank separation distance of 28.42 m is anticipated at a wind velocity of 6 m/s, compared to 16.34 m in no-wind condition, beyond which the centroid of the parallelepiped (a solid-flame geometry) falls outside the base of the tilted flame geometry causing flattening of flame and a very sluggish increase in the flame tilt angle as the inverse of the Richardson number in the presence of wind velocity vector increases. Furthermore, the present one dimensional mathematical model has also been extended to show that introduction of a water-shield with an appropriate thickness (δopt) is able to prevent the propagation of radiation heat flux under both no-wind and cross-wind conditions to a much lower distance close to 8.34 m between tanks (measured from the centre of the source tank), than those predicted from the existing empirical models; viz. Point Source model and Shokrie-Beyler’s model.

Burning rate, flame geometry and temperature of convection-controlled circular diesel oil pool fire under air crossflow conditions.

Hazardous fires in fuel storage tanks may result in casualties of great impact. Efforts are being done to master the physics of the phenomena, aiming both avoidance and damage minimization. Flame characterization is an important approach studied by many researchers. This article presents the results of an experimental investigation of diesel oil (S500 type) pool fires under air crossflow conditions. A reduced scale model properly instrumented was tested in an aerodynamic channel. Measurement techniques include analysis of infrared images of pool fires, mass burning rates and flame geometry as a function of air crossflow velocity (which ranged from 0 m/s - quiescent air - to 4.0 m/s). Mass burning rates showed an oscillatory behavior as the crossflow velocity was raised. Regarding flame geometry, for an increasing air crossflow, firstly flame tilt angle increased, flame length decreased and flame height decreased until about 1.5-2.0 m/s, while after that velocity, flame geometry became nearly steady. Geometry results were also compared with thirteen correlations from literature for geometric parameters obtaining good agreement for some of them. The IR-measured temperatures showed that the flame region (reacting region) became hotter and the plume region (non-reacting region) became colder as the air crossflow velocity was increased.