Turbulent Jet Fires Research Articles

Geometrical features and global radiant heat of double turbulent jet flames

In energy industry, double jet fires are often reported to occur with a large release momentum at the leakage exit. The available literature considered the double buoyancy-controlled jet flames, and also neglected the restrictive effect of ground or tank surface. In this paper, an experimental setup is built to model the interaction of double turbulent jet fires with different spacings. Different nozzle diameters and exit velocities are chosen to achieve the transition from flame buoyancy to exit momentum for the dominance of turbulent jet flames. Comparative tests are also conducted between with and without a plate near the nozzle exit. It is found that the flame buoyancy, exit momentum and nozzle spacing significantly affect the flame merging behavior. A new dimensionless parameter coupling the Froude number (Fr) and the spacing is proposed to well fit the merging probability (Pm) of double jet flames dominated by the flame buoyancy (Fr < 105 for propane) and the exit momentum (Fr ≥ 105 for propane), respectively. The interaction of double jet flames leads to a less lift-off height than a single jet flame, and the lift-off height can also be reduced by a plate near the nozzle exit. A correlation coupling the spacing, the exit diameter and Froude number, is developed for the mean flame height of double jet fires. The distributions of radiant heat flux are intensively measured around the double jet flames, which can help to calculate the radiative fraction. The radiative fraction of double jet flames at the full merging state (Pm = 1) follows the law of a single jet flame, and it decreases at the intermittent (0 < Pm < 1) and no merging (Pm = 0) states, as compared to that of a single jet flame. In addition, the available line source radiation model of a single jet flame is revised to predict the thermal radiation of double jet flames in the three different merging states.

Evaluation of flame geometry of horizontal turbulent jet fires in reduced pressures: A numerical approach

Accidental releases of hydrocarbons can result in jet fires, which have the potential to escalate accidental events to catastrophes. The geometry and size of a jet flame are vital parameters deciding the potential of the flames to trigger such a series of accidental events called domino effects. In this study, Computational Fluid Dynamics (CFD) simulations on horizontal turbulent jet fires are carried out using Fire Dynamics Simulator (FDS®). The results are validated with previous experimental results. Eight different fuel exit velocities ranging between 27.5 and 125 m/s, and 5 different ambient pressures: 0.6, 0.7, 0.8, 0.9 and 1 atm, are used for studying two main flame geometrical parameters: jet flame length and flame area. The simulation results obtained for the flame lengths are found to be higher in reduced pressure cases. In the present study, the classic correlation proposed by Quintiere and Grove is extended to reduced pressure conditions by refining the ‘air entrainment strength (C)’ values. These air entrainment strength constants were found to have a linear relationship with the ambient pressures. In addition, it is revealed that low ambient pressure conditions can also result in higher jet flame areas. The previously established relationship between the flame area and the non-dimensional heat release rate (i.e.AF∼Q*4/5) is also verified with the present obtained results, with an introduced proportionality constant (k) showing linear variations with the ambient pressure.

Experimental study, dimensional analysis and an integral model for horizontal buoyant turbulent jet fires under opposing wind

This work demonstrates a comprehensive model for horizontal turbulent jet flame geometries under opposing wind, for which few data or models can be found in the literature. This situation can be a practical scenario of offshore drilling platform with a horizontal flare under opposing wind. The opposing wind pushes the jet flame back, which eventually turns around and upwards, pointing finally downstream in the windward direction. The flame geometries are characterized by four parameters including horizontal length and vertical height from nozzle exit to the location of farthest point that flame envelope could reach before the flame turns around as well as horizontal length and vertical height from nozzle exit to the location of flame tip. Experiments are carried out for horizontal jets discharged from circular nozzles having diameters of 3, 5, 7 and 9 mm and employing propane as fuel with opposing wind speeds from 0.35 to 1.08 m/s. The locations of flame tip and the flame farthest point decrease with increasing opposing wind for a constant heat release rate. Dimensional analysis is derived from a physical model considering the momentum of fuel jet, flame buoyancy due to the flame temperature rise, opposing wind momentum and normalized flow rate at stoichiometric conditions at flame tip. The normalized flame geometry parameters based on the proposed model are well represented by two characteristic length scales derived by accounting for the interaction between excess jet-wind momentum and flame buoyancy as well as that between opposing wind momentum and flame buoyancy. In addition, the normalized mass flow rate at stoichiometric conditions determines the total length of flame trajectory. Moreover, an integral model is deduced by an air entrainment model and top hat profiles for the mass and momentum conservations to predict well the flame geometries of horizontal jet fires under opposing wind.

A physical model on the flame structure of vertical downward turbulent jet fires

This work explored the flame geometrical characteristics of vertical downward turbulent jet fires in still air, whose flame buoyancy was opposite to its initial momentum. Experiments were carried out under varied fuel supply rates with propane as fuel. Three circular nozzles having diameters of 3, 5 and 7 mm were employed. CFD simulation to show the combustion structure inside the flame was conducted in the work, contributing to proposing a new physical model. The inner flow field and average mixture fraction of a downward jet fire have been revealed and the CFD result was validated by experimental data. A downward jet fire can be distinguished in three regions: the downward core flow region where the initial flame momentum is opposed by buoyancy, the flow reversal dome region where the momentum reverses direction, and the upward combusting flow region, which surrounds the core, dominated by buoyancy. Three basic flame morphologic lengths of the downward jet fires were measured, namely the flame downward length below the nozzle, the flame width, and the total flame height. The inherent differences between downward, upward and horizontal jet, caused by the momentum direction, were discussed by comparing the flame sizes of these three types of jet flames. A physical model considering the initial downward volumetric momentum, the flame buoyancy and the air entrainment volume until complete combustion occurs was raised to depict the flame geometric characteristics derived from three characteristic length scales, namely a momentum-buoyancy competition length scale, a flow-rate length scale linked with the source diameter and a total combustion buoyancy length scale. All measured experimental flame lengths were normalized using dimensionless groups (such as LmLq and Sṁf/ρ∞g′LmLm2) derived by these three length scales. The geometric properties of downward jet fires were shown to be well modelled by the proposed non-dimensional groups which were different from previously used length scales in the literature.

Experimental and analytical study on inclined turbulent fire in still air. Part 2. Round fire

This paper reports an experimental and analytical study on non-vertical round turbulent jet fire. A series of propane jet fire tests are performed under different initial flame angles (θ0) and initial fuel velocities. The two-dimensional velocity and temperature fields of the flame are measured, and the flame geometry is determined from video images. Experimental results show that the flame entrainment is enhanced as θ0 decreases. The flame deflection-induced buoyancy flow plays an essential role in the local flame structure and turbulence. A model of round fire combined with a kinetic energy equation is established, in which the flame entrainment coefficient is analytically correlated with the buoyancy and streamline curvature. Model calculations reproduce the flame entrainment and flame trajectory, consistent with the experimental indications. Under the same burner area and heat release rate, the entrainment coefficient of round fire is greater than that of line fire, closely associated with the difference in integral buoyancy triggered by different burner geometries. The results of this paper may help understand the flame motion and predict the flame radiation of turbulent jet fires.

Study on the global and spatial pulsation frequencies of rectangular syngas jet fire with different aspect ratios

Flame pulsation is a significant feature for both laminar and turbulent jet fires. In this study, the pulsation behaviors and the dominating mechanism of rectangular jet fires were explored. A series of experiments were conducted on the rectangular syngas jet fires with fuel flow rates ranging from 5 L/min to 45 L/min and aspect ratios ranging from 2 to 8. Both the global and spatial pulsation frequencies of syngas jet flame were obtained based on the time variations of image correlation coefficient and flame width. The local pulsation frequency of jet flame near the nozzle exit, which is defined as the natural frequency, increases slightly with the fuel flow rate due to the enhanced interaction of the inner vortex with the flame sheet and outer vortices. A transition from natural pulsation to subharmonic pulsation was found to happen when the local Richardson number (RiZ) of the jet flame is close to 1. The global pulsation frequency is dominated by the natural frequency at the nozzle exit for Qf<20 L/min, however, the subharmonic frequency for higher fuel flow rates. Non-axisymmetrical development of buoyancy-induced instability is considered to play a significant role in the rectangular jet fire, leading to the varied spatial pulsation frequency and the elongated flame height at side edges. In addition, the dimensionless global pulsation frequency (St) was found to correlate well with the Froude number (Fr) at the nozzle exit, St=0.42(1/Fr)0.46 for Fr > 10 and St=0.40(1/Fr)0.56 for Fr < 10.

Flame geometry of downward buoyant turbulent jet fires under cross flows: Experiments, dimensional analysis and an integral model

This study investigates the flame geometry of vertically downward facing buoyant turbulent jet fires issuing from a nozzle subject to cross flows, which deflect the jet flow by interacting with downward momentum issuing from the source and the upward buoyant forces owing to the flames. The flames turn eventually upwards as the buoyant forces dominate. The objective of this work is to understand the physics and develop non-dimensional correlations and modeling for the flame geometry of this jet fire configuration never investigated before. For this purpose, experiments are conducted having 4 circular nozzles (diameter at 3, 4, 5 and 7 mm) with propane employed as fuel under various heat release rates. The cross flows are generated by a wind tunnel having a uniform cross flow air speed varying from 0.31 m/s to 2.08 m/s. The horizontal and downward distance from the nozzle to the lowest point of the jet flame, the vertical thickness of the flame at the lowest point, the flame vertical height from lowest point to the flame tip, the flame tip horizontal projection distance are measured. Three characteristic length scales developed by dimensional analysis considering initial downward momentum M0, flame buoyancy g′, inertial cross flow force ua2 together with the normalized air required for complete combustion Sm˙f/ρaua×(ua2/g′)2, are found to represent well the aforementioned properties of the flame geometry. An integral model, considering the mass and momentum conservations, is developed to predict the trajectory and geometric properties of the downward jet flame under cross flow showing good agreement with the experimental results.

Estimating the trajectory length of buoyant turbulent jet flames issuing from a downward sloping nozzle

Downward sloping buoyant turbulent jet fires occur frequently during flammable gas leakage, while its flame trajectory, especially the quantitative length, is not well described. The flame trajectory lengths of downward jet fires with five sloping angles (0°, 15°, 30°, 45°, 60° relative to the horizontal) and two nozzle diameters (8 and 10mm) were experimentally investigated. A novel method to quantify the flame trajectory length is proposed based on the flame outlines, and the results obtained show good agreement with the existing data of horizontal jet fires. For cases with differently inclined angles, the normalized flame trajectory length as a function of nozzle diameter can be correlated with the Froude number Fr1/5 and dimensionless heat release rate Q*2/5, but not as function of the tilting angle as this is not considered in the existing correlations. In order to establish a global model for predicting the flame trajectory length of downward sloping jet fires as function of the tilting angle of the nozzle, a remodified Froude number (Frf*) is proposed, and the correlated result can describe all the current experimental results together with that of horizontal and vertically downward-oriented jet fires in the literature.

Flame geometrical characteristics of downward sloping buoyant turbulent jet fires

The downward sloping buoyant turbulent jet fires occur occasionally and practically as the gaseous fuel leaks, while their morphologic characteristic parameters have not been quantified yet. In this work, a sequence of experiments with three nozzle diameters are performed at five downward sloping angles, 0°, 15°30°, 45°, 60°relative to the horizontal, aiming to examine the flame geometrical characteristics of downward sloping buoyant turbulent jet fires. It is found that the flame dimension, termed as flame horizontal projection and flame downward distance, increases with the increasing heat release rate, and the increasing downward sloping angle diminishes the flame horizontal projection, but enlarges the flame downward distance. Both the flame horizontal projection and flame downward distance normalized by nozzle diameter can be correlated with dimensionless heat release rate Q∗ by 1/2 and 3/5 power, respectively, and the correlated coefficients exhibit a linear relationship with cosθ and sinθ for cases at different downward sloping angles. Additionally, two remodified flame Froude numbers concerning the horizontal and vertical components of initial momentum, i.e. Frf,h∗ and Frf,d∗, are proposed to correlate the non-dimensionally flame horizontal projection Lp,h∗ and flame downward distance Ld∗, respectively, and the correlated results can well unify the current experimental results. A better understanding of jet flame geometry will increase accuracy in the prediction of fire hazards of such fires.

An experimental study on flame envelope morphologic characteristics of downward-orientated buoyant turbulent jet fires

This study experimentally investigates flame envelope morphologic characteristics of downward-orientated buoyant turbulent jet fires, where the initial jet momentum is opposite to flame buoyancy that an anti-buoyancy jet configuration is naturally formed. Such configuration exists practically in gas leakage jet fires, however, has not been quantified. Experiments were conducted employing four circular nozzles with propane as fuel. The flame downward distance from nozzle outlet, flame length and flame width were measured at various fuel velocities for such downward-orientated jets. The corresponding flame length and width for upward jets were also obtained for comparison. Results showed that the three flame dimensions increased with increasing fuel velocity or heat release rate. The ratio of flame downward distance to flame length correlated well to a dimensionless length scale, ξℓ, representing buoyancy to initial momentum of the jets. The flame width and flame downward distance normalized by nozzle diameter correlated well to non-dimensional heat release rate by 1/2 power and 3/5 power, respectively; and their ratio can be approached by a -1/10 power consequently. The ratio of flame width to flame length of downward jets increased with heat release rates, being much higher than that of upward jets which first increased then plateaued (0.145) finally deceased with increasing heat release rate. Both the flame downward distance and flame length of downward jet showed to be much smaller than flame length of corresponding upward jet. A formula was proposed to correlate flame downward distance as a function of flame Froude number (Frf). The flame downward distance was initially smaller then, however, increased faster and finally turned to be larger than flame length with increasing fuel velocity. The critical turning state as flame downward distance equaling flame length (namely flame upper tip descending to nozzle outlet level) of downward-orientated jet can be represented by ξℓ*=3.4 or Frf*=1.

Flame heights and fraction of stoichiometric air entrained for rectangular turbulent jet fires in a sub-atmospheric pressure

This paper investigates the flame height and the fraction of stoichiometric air entrained (n, the basic parameter in flame heights correlation) for rectangular turbulent jet fires at sub-atmospheric pressure, for which no data are available in the literature. Comprehensive experiments are carried out in a naturally sub-atmospheric pressure (Lhasa city, 64kPa) by rectangular nozzles whose aspect ratio varied from 1:1 to 1:71. The flame heights are measured and correlated by the unified formula suggested by Quintiere and Grove which was obtained at standard atmospheric pressure, meanwhile the change of the important parameter, n or Cf (reflecting air entrainment into flame and thus being basic in flame height correlation) in the sub-atmospheric pressure is addressed. It is found that: (a) the flame height in the sub-atmospheric pressure can be well approached by the unified formula suggested by Quintiere and Grove; however (b) the correlation parameter Cf is larger (1.6 times that in standard pressure) or the value of n is smaller (73% of that in standard pressure) in the sub-atmospheric pressure than that in the standard pressure. The relative change of n at the sub-atmospheric pressure is then theoretically deduced from the change of entrainment strength (C1) and flame envelope along with pressure drop. The theoretically deduced ratio of n (or Cf) between these two pressures agrees with the experimentally correlated results. Then, by accounting for the changes of the correlation parameters, a proposed explicit model can generally predict the flame heights in both pressures.

A re-examination of entrainment constant and an explicit model for flame heights of rectangular jet fires

Flame heights of buoyant turbulent jet fires produced by rectangular nozzles whose aspect ratio varied from 1:1 to 1:71 are investigated experimentally in this work. The change of the entrainment constant parameter C1 with aspect ratio is discussed based on the comprehensive data obtained. It is found the value of C1 does not need to be transformed from 0.179 to 0.444 with an increase in aspect ratio from axisymmetric one to linear one as proposed previously in the classic correlation due to limited data, a change which might be misleading. It is revealed to in fact change little with rectangular fire source aspect ratio and can be constantly taken as 0.185. A new explicit model to predict flame heights for given heat release rates of rectangular jet fires is then proposed, which is shown to be in good agreement with the measured values for different source aspect ratios.

Axial temperature profile in vertical buoyant turbulent jet fire in a reduced pressure atmosphere

Characteristics of a vertical jet fire in a reduced pressure atmosphere (at high altitude) have not been quantified in the literatures. In the reduced pressure atmosphere, the air/oxygen density is lower, which in turn affects both combustion and entrainment, and hence the axial temperature profile of a diffusive turbulent jet fire. Experiments have been conducted in this work to investigate the axial temperature profiles of propane turbulent buoyant jet fires produced by nozzles with diameters of 4–10mm in both reduced- (0.64atm) and normal pressure (1atm) atmosphere. It is found that the maximum temperature in the flame zone is a bit higher, the temperature decreases faster vertically and is somewhat lower in the buoyant plume zone in the normal pressure than those in the reduced pressure. The virtual origin is then deduced and clarified to be larger in the reduced pressure, however, it can be correlated non-dimensionally with flame Froude number (Frf) in a 2/5 power law function for both these two pressures. Finally, the normalized axial temperature profile against non-dimensional height above the virtual origin can be still well characterized into three regions in the reduced pressure by the same power law functions as those in the normal pressure.

Air entrainment into buoyant jet flames and pool fires

A simple model for the fire dynamics of turbulent fires is proposed and then used to derive correlations for air entrainment into turbulent jet fires and pool fires. Five parameters are identified to control the fire dynamics: the pool diameter D, the total buoyancy flow B ∞, a buoyancy force per unit mass at the adiabatic stoichiometric temperature ( ΔT ad T ∞ )g , the ambient density ϱ ∞, and the stoichiometric mass air to fuel ratio S. These parameters lead by dimensional arguments to correlations for air entrainment rates, which are further refined by using a simplified physical model. Three distinct regimes for air entrainment are identified in turbulent pool fires: (a) close to pool surface where entrainment rates vary as Z 1 2 (where Z is the vertical distance from the source), (b) near the neck-in area where entrainment rates vary as Z 3 2 , and (c) farther downstream but below the flame tip where entrainment rates vary as Z 5 2 . The proposed correlations are used to plot experimental data from which proportionality coefficients are determined. It is shown, also, that there is a relationship between entrainment rates up to the flame tip and visible flame feights; such a relationship would allow one to estimate entrainment rates by measuring visible flame heights.