Flame Merging Research Articles

Flame interaction and tilting behavior of two tandem adjacent hydrocarbon turbulent diffusion flames in crosswind: An experimental quantification and characterization

This study presents an experimental investigation into the flame interaction and tilting behaviors of two tandem adjacent hydrocarbon turbulent diffusion flames in crosswind, which has not been well quantified in the past. Comprehensive experiments involving 204 tests were carried out employing two 15 cm square burners of various distances and two liquid fuels of representative soot production abilities, in crosswinds up to 6 m/s generated by a wind tunnel. The flame tilt angles were measured to quantify their interactions for flame merging and non-merging conditions. Results showed that the flame merging criterion extended under crosswind, especially for n-heptane fire with longer flame length. The tilt angles of merged flames were found to be smaller than that of single diffusion flame (SDF) and upstream diffusion flame (UDF) with the same crosswind speed for non-merging condition at relative larger separation distance. For flame non-merging condition, downstream diffusion flame (DDF)'s flame tilt angle values were lower than that of UDF due to the first flame shelter effect, and became smaller for larger separation distance at a given crosswind. According to their dynamic behaviors for non-merging condition, the ratio of flame tilt angle of DDF to that of UDF against a modified Fr* number fell into four regimes. The effective crosswind speed for DDF under crosswind momentum prevailed condition was proposed on account of the effects of the fire separation distance, the UDF's flame height and buoyancy flux. Non-dimensional formula was proposed to characterize their flame tilt angles for merging and non-merging flames respectively.

Study on flame merging behavior and air entrainment restriction of multiple fires

Fire is a typical thermal runaway process of uncontrolled energy release. The possible ignition of adjacent leaked fuel might induce multiple fires burning simultaneously. A series of numerical simulations have been conducted to study the flame merging behavior and air entrainment restriction effect from square propane fire arrays. The simulation results were first systematically compared to experimental data, showing that the flame characteristics including the flame height, flame centerline temperature, and velocity distributions can be successfully reproduced numerically. Then, the flame merging phenomenon was studied by varying fire spacing, the number of fires, and the heat release rate of an individual fire in the array. Results show that the critical flame merging spacing of the fire array equals to 0.4 times the flame height of a single fire, which is independent of the fire array size. More importantly, a parameter denoting the extent of air entrainment restriction α was introduced, which enables a characterization of the air entrainment restriction effect of a fire array. A maximum air entrainment restriction model was established, which is shown only related to the number of fires in the array.

Experimental study on flame merging probability and pulsation frequency of annular hydrocarbon pool fires with various inner and outer diameters

Hydrocarbon pool fires can cause serious fire accidents. This study focused on the flame-merging behavior and pulsation frequency of annular hydrocarbon pool fires with various inner and outer diameters. Experiments were performed with annular pool fires having outer diameters (D) in the range of 0.15–0.55 m and inner diameters (d) in the range of 0.05–0.45 m. The fuel used was n-heptane. The flame-merging probability and pulsation frequency of annular pool fires with various inner and outer diameters were examined. With the development of the pool fire, the initially produced “annular flame” finally merged. According to the dimensional analysis, a fitted correlation of the piece function with different heat-release rates and annular-pool sizes was proposed for describing the flame merging and pulsation frequency, and the proposed correlation in this work is suitable for no wind conditions.

Enhanced DDT mechanism from shock-flame interactions in thin channels

We show experimentally and numerically that when a weak shock interacts with a finger flame in a narrow channel, an extremely efficient mechanism for deflagration to detonation transition occurs. This is demonstrated in a 19-mm-thick channel in hydrogen-air mixtures at pressures below 0.2 atm and weak shocks of Mach numbers 1.5 to 2. The mechanism relies primarily on the straining of the flame shape into an elongated alligator flame maintained by the anchoring mechanism of Gamezo in a bifurcated lambda shock due to boundary layers. The mechanism can increase the flame surface area by more than two orders of magnitude without any turbulence on the flame time scale. The resulting alligator-shaped flame is shown to saturate near the Chapman–Jouguet condition and further slowly accelerate until its burning velocity reaches the sound speed in the shocked unburned gas. At this state, the lead shock and further adiabatic compression of the gas in the induction zone gives rise to auto-ignition and very rapid transition to detonation through merging of numerous spontaneous flames from ignition spots. The entire acceleration can occur on a time scale comparable to the laminar flame time.

Quantitative Analysis of the Influence of Air Entrainment Restriction Degree on Burning Characteristics of Two Parallel Rectangular Pool Fires in Still Air

Flame merging is deemed to enhance the burning intensity and make the fire more destructive. This paper presents an experimental study on merging behaviors of two same rectangular heptane pool fires with long sides parallel. The pan aspect ratio was set 2–4 and the spacing was changed. The burning rate and flame height were measured. As the spacing decreases, the flame shape was divided into five regions, i.e., (I) no interaction, (II) tilt but non-merging, (III) intermittent merging, (IV) upper flames fully merging but lower flames separated and (V) flames merging from the pan base. The results showed that both the burning rate and flame height increase within the stages I–IV and then decrease in stage V. A normalized parameter ψ is introduced to characterize the air entrainment restriction. A unified correlation between burning rate and ψ is then developed. Connecting with the theoretical force analysis, the criteria of merging from the base and beginning merging are determined as ψ = 0.33 and ψ = 0.61. Then a piecewise correlation of the merging flame height is established. The proposed correlations for burning rate and flame height are verified using present and literature data and their scope of application is further expanded into square pool fires.

Physical models of flame height and air entrainment of two adjacent buoyant turbulent jet non-premixed flames with different heat release rates

This paper investigated the flame height and air entrainment of two adjacent buoyant turbulent jet non-premixed flames with the same base dimension whilst having different heat release rates (HRRs). Propane was used as fuel. The HRR of one of the burners was fixed at 8, 16, 24, 32 and 40 kW, respectively. By increasing the HRR of the other burner, the interaction behaviors of two jet flames were studied under various burner spacings. A total of 15 categories of HRRs combinations of two jet flames were conducted in this work. Two flames heights are different to lead the heat pressure on both sides of the flame to be non-axisymmetric. And the interaction law, air entrainment mechanism etc. become more complicated than two same HRRs flames. Results showed that with increasing burner spacing, the flame merging behaviors can be divided into three stages. The flame height firstly deceases to a minimum value, then slightly rises again and finally maintains stable due to the weak interaction of non-merging flames. Two effective weighting factors are supposed to quantize the maximum and minimum flame heights to derive a flame height model which then is validated using the experimental data in literatures. The merging point height increases with increasing HRR and burner spacing. Based on the air entrainment mechanism of two interaction flames, a general method for calculating the global volume air entrainment is presented to build a physical entrainment model. Above flame height and physical entrainment models are suitable for the calculation with two flames combinations with the same and different HRRs, which greatly extends the application scope of interaction fires. In addition, two creative methods of calculating flame height and global air entrainment volume have been provided a reference for the research on the combustion behaviors of multiple flames with different HRRs combination.

Predicting radiative heat fluxes from two buoyant turbulent diffusion flames from burning propane under cross wind

Experimental investigations were carried out to estimate the radiative heating that occurs downstream of wind-driven buoyant turbulent diffusion flames produced by twin square propane burners aligned parallel to the cross wind. During the experiments, the heat release rate, cross wind velocity, and burner edge spacing were changed. Considering the effect of wind on flame shapes, a new triangular prism flame model was established to predict the radiant heat flux received by downstream objects from two buoyant turbulent diffusion flames under cross wind. The flame radiations to downstream ground were measured by five radiometers, and the relevant parameters of the flame model were analyzed. The results show that the radiative heat flux decays with increasing flame spacing and intensifies with increasing wind velocity. A piecewise function for the flame drag length in various flame merging stages is established. The effect of wind velocity and flame spacing on flame emissivity is relatively limited, and flame emissivity is formulated as a function of the heat release rate. The view factor between the triangular prism flame model and external targets is calculated based on the contour integral method. Finally, prediction results between the proposed triangular prism flame model and conventional models are compared against experimental data. The results confirm that the new triangular prism flame model can better predict the thermal radiation from two interacting flames in the wind.

Experimental study on vertical temperature distribution of symmetry axis of two identical rectangular pool fires with long sides parallel

The axial temperature distribution of fire plume is an important subject in the field of fire research. While the gas temperature distribution induced by multiple fires is still not clear. This paper presents an experimental study on axial temperature distribution of two identical rectangular pool fires with long sides parallel. The pool aspect ratio and spacing were varied. Results showed that corresponding to different flame merging states including merging from the base, merging above the base and no merging, the axial temperature distribution appears three different change laws. Regarding the case of merging from the base as a large area fire, correlation of axial temperatures from various pool aspect ratios and spacings is proposed by introducing a virtual origin characterizing the effect of fire geometry. For the cases of merging above the base, taking the merging point as the boundary and determining the fire geometry and heat release rate of the upper assumed single fire, a piecewise function for the axial temperature distribution upon the merging point is established. The axial temperature characterizing the flame merging is further determined as 930 °C for two rectangular heptane pool fires, which can be used to judge whether the flames merge or not.

Experimental study on flame height of two oil tank fires under different lip heights and distances

The combustion behavior of two oil tank fires will be influenced by the interaction of their respective flames. This paper presents the experiments to investigate the evolution of the visible flame height of two oil tank fires under different diameters, lip heights and distances. The results show that when the tank spacing is small, the vortex structure plays an important role in the flame behavior showing an enhancement of the flame height. The flame height increases with the increase in lip height and reaches a local maximum value. The flame merging characteristic is influenced by the burner size and shape. A dimensionless model of the visible flame height of two oil tanks fire has been proposed, taking the tank diameter, lip height, and tank spacing into consideration. The correlation is validated using the experimental data.

Experimental study on flame/flow dynamics in a multi-nozzle gas turbine model combustor under thermo-acoustically unstable condition with different swirler configurations

This paper presents an experimental study on the combustion stability characteristics and flame/flow dynamics in a multi-nozzle, lean premixed prevaporized (LPP), swirl-stabilized gas turbine model combustor with different swirler configurations in the presence of self-excited combustion instability. The flame structure was characterized using high-speed OH⁎ chemiluminescence imaging and the flow field across the centerline of three interacting flames was measured by high-speed planar Particle Image Velocimetry (PIV). Two sets of the swirler configurations were considered in this paper, featuring different combinations of swirl rotational directions. The first one consisted of three co-rotating swirlers, whereas the central swirler in the second configuration was replaced by a swirler with counter-rotation direction. These two configurations were termed as COS and CNS combustors respectively in the rest of the paper. It was found that these two combustors exhibited similar stable and unstable operating domains in terms of different equivalence ratios and inlet air velocities. At the same test condition, the amplitude of dominant instability of COS combustor was 130 dB, which was stronger than that of CNS combustor (120 dB). Phase-averaged PIV measurements showed that both COS and CNS combustor featured three recirculation zones downstream the swirlers, and high axial velocity was present after the merging of the adjacent flames. These two flow structures varied periodically, but in different manners for COS and CNS combustors. Phase-averaged OH⁎ chemiluminescence images indicated that the flame was primarily anchored in recirculation zones close to the swirler and most of the heat release was found to occur in flame interaction regions, where large-scale reaction intensity variations occurred. Furthermore, a greater phase delay between heat release rate and acoustic pressure was observed in CNS combustor, which contributed to a weaker instability comparing with that in the COS combustor.

Flame merging: Investigating the spread of wildland fires

Flame merging: Investigating the spread of wildland fires

Ignition process of gasoline compression ignition (GCI) combustion in a small-bore optical engine

The partially premixed charge-based gasoline compression ignition (GCI) combustion is a practical strategy to achieve high engine efficiency and low NOx and soot emissions. While the impact of single and double injections of GCI combustion on engine performance and engine-out emissions are well known, the in-cylinder ignition process is still not clearly understood. In this study, the natural combustion luminosity imaging diagnostics is conducted in an optically accessible, small-bore single-cylinder diesel engine to quantify the ignition kernel development and flame growth rate of single and double injection strategies, which is supported by electronically excited hydroxyl (OH*) imaging. The injection timings for single injection and the second injection timings for double injection were varied from 20° to 26°CA bTDC to evaluate their influence on ignition process. The results show that the ignition process of single injection comprises multiple auto-ignition kernel development from which the isolated flame growth occurs. The flames then merge together and quickly fill the entire combustion volume during the main heat release. While the initial auto-ignition kernel formation for double injection is similar to single injection, the isolated flame growth is not clearly defined due to the increased mixture homogeneity. The faster fuel–air mixing of double injection strategy also brings the timing of flame merging process forward as compared to single injection. For tested conditions of this study, the ignition process of GCI combustion is found to be insensitive to the injection timing variations.

Simulations of flame propagation during the ignition process in an annular multiple-injector combustor

PurposeIgnition process is a critical issue in combustion systems. It is particularly important for reliability and safety prospects of aero-engine. This paper aims to numerically investigate the burner-to-burner propagation during ignition process in a full annular multiple-injector combustor and then validate it by comparing with experimental results.Design/methodology/approachThe annular multiple-injector experimental setup features 16 swirling injectors and two quartz tubes providing optical accesses to high-speed imaging of flames. A Reynolds averaged Navier–Stokes model, adaptive mesh refinement (AMR) and complete San Diego chemistry are used to predict the ignition process.FindingsThe ignition process shows an overall agreement with experiment. The integrated heat release rate of simulation and the integrated light intensity of experiment is also within reasonable agreement. The flow structure and flame propagation dynamics are carefully analyzed. It is found that the flame fronts propagate symmetrically at an early stage and asymmetrically near merging stage. The flame speed slows down before flame merging. Overall, the numerical results show that the present numerical model can reliably predict the flame propagation during the ignition process.Originality/valueThe dedicated AMR method together with detailed chemistry is used for predicting the unsteady ignition procedure in a laboratory-scale annular combustor for the first time. The validation shows satisfying agreements with the experimental investigations. Some details of flow structures are revealed to explain the characteristics of unsteady flame propagations.

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.

Characteristics of multiple pool fires in a tunnel with natural ventilation.

Tunnel fires are usually accompanied with major casualties and properties losses, and multiple fires in a tunnel may cause intense burning behavior like fire merging, which could be more destructive and hazardous. Therefore, the characteristics and hazards of multiple pool fires in a tunnel were investigated under natural ventilation in this study through a series of experiments. Experimental results showed that the characteristics of one-dimensional fire arrays were different from those of square fire arrays in open space. Due to the extra heat feedbacks (e.g. from adjacent fire sources, tunnel wall and hot smoke), the burning rate was found being enhanced. When there is more than one fire source, it was observed that the flames deflect towards the center of one-dimensional fire arrays, leading to flame merging when fire spacing decrease to some extent, which would result in the increased hazards of the heptane fires. An empirical model was developed to predict the global dimensionless average burning rates based on modified fire area ratio, which characterizes the fuel load and incorporated fire spacing, fire number and the fuel pan size. The research results in this paper provide a theoretical guide on understanding the behaviors of multiple fire resources in tunnels.

Characterisation of flame surface annihilation events in self excited interacting flames

This paper investigates the non-linear flame dynamics of two interacting, premixed, V-shaped flames by characterising the two-dimensional topology of flame annihilation events when the separation distance, S, between them is reduced and large-scale flame merging occurs. The equivalence ratio was varied to promote self-excited oscillations, with the oscillation frequency, heat release phase, and stability limits shown to be dependent on S. High-speed OH-PLIF measurements show that these changes are correlated with the break-up of the shear layers into structures that lead to large-scale flame annihilation events. In isolated flames the shear layers break-up independently, but as S is reduced the shear layers combine leading to large-scale flame merging resulting in the roll up of a single large-scale vortex structure altering the flame annihilation events compared with the case of isolated flames. A flame front event tracking algorithm is developed to characterise the two-dimensional topology and identify the number and spatial location of flame front annihilation events, which shows a strong correlation between these events and the fluctuating heat release rate. Compared with stable flames for the same S, it is found that self-excited instabilities do not significantly increase the number of annihilation events but rather affects their spatial distribution and phase within the oscillation cycle. It is also shown that flame merging significantly increases the probability of flame front annihilation events which alters the phase of the fluctuating heat release rate.

Effect of Lewis number on premixed laminar lean-limit flames stabilized on a bluff body

In this paper, we present a study on the effect of Lewis number, Le, on the stabilization and blow-off of laminar lean limit premixed flames stabilized on a cylindrical bluff body. Numerical simulations and experiments are conducted for propane, methane and two blends of hydrogen with methane as fuel gases, containing 20% and 40% of hydrogen by volume, respectively. It is found that the Le > 1 flame blows-off via convection from the base of the flame (without formation of a neck) when the conditions for flame anchoring are not fulfilled. Le ≤ 1 flames exhibit a necking phenomenon just before lean blow-off. This necking of the flame front is a result of the local reduction in mass burning rates causing flame merging and quenching of the thin flame tube formed. The structure of these flames at the necking location is found to be similar to tubular flames. It is found that extinction stretch rates for tubular flames closely match values at the neck location of bluff-body flames of corresponding mixtures, suggesting that excessive flame stretch is directly responsible for blow-off of the studied Le ≤ 1 flames. After quenching of the neck, the upstream part forms a steady and stable residual flame in the wake of the bluff body while the downstream part is convected away.

Interaction of two parallel rectangular fires

This paper investigates the flame interaction of two parallel rectangular fires. Multiple propane burners were used to produce parallel fires for experiments. Experimental observations combined with analytical analysis provide a physical interpretation on the mechanism of flame interaction. Three types of flame interaction regimes, namely the “Independent regime”, “Intermittent regime” and “Consistent regime”, are quantitatively identified in terms of the flame merging probability (Pm) extracted from image analysis. These three regimes are further divided into five distinct interaction states including (a) State I, independent flames with no interaction, (b) State II, independent flames that have interaction, (c) State III, flames that have intermittent touch with each other, (d) State IV, partly merged flames, and (e) State V, completely merged flames. It is found that for a given fuel size parameter, the data of Pm in different interaction states versus the dimensionless flame spacing s/zc (s: flame spacing, zc: a characteristic length) cluster well and can be well fitted by Boltzmann function, and the function parameters have definite physical significance. For different interaction states, the dimensionless flame height Zf/Zf0 (Zf0: the flame height of one single fire) is correlated with s/zc. Especially, it is found that when s/zc falls into State II, Zf/Zf0 declines to be a constant due to the competition of two distinct effects induced by flame interaction. The dependence of Zf/Zf0 upon s/zc in different interaction states is fully illustrated. Finally, another dimensionless flame height Zf/D (D: a characteristic size of fuel bed by regarding the two flames as a whole) is formulated as a function of heat release rate for different interaction states.

Experimental study on merging behaviors of two identical buoyant diffusion flames under an unconfined ceiling with varying heights

This paper presents an experimental study on the flame merging behaviors of two identical square propane burners with the side length of 15 cm burning under an unconfined ceiling. The total heat release rate of the two burners, burner edge spacing and ceiling height above the burner were varied. The flame merging probability, both the longitudinal and transverse ceiling flame extensions respectively along and perpendicular to the line connecting the centers of the two burners were measured. Based on the dimensional analysis, a piecewise linear function for the flame merging probability was developed and the criteria of the beginning and fully merging of the ceiling flames were thus determined, which provides a guidance of the safety distance between combustibles in buildings. The flame length is defined as the half of the total ceiling flame length in this work. Due to the interaction of two burners, the longitudinal and transverse flame lengths are different. Experimental results showed that as the spacing increases, the longitudinal flame length increases while the transverse flame length decreases. Based on the energy distribution model developed for the vertical and ceiling flames, correlations for predicting the merging ceiling flame lengths in the two directions were respectively established as the functions of the total heat release rate, burner size and spacing, ceiling height and the free flame height of two burners with zero spacing. The proposed models for the ceiling flame lengths can help to better understand the fire ignition and spread probability and flame radiation properties.

Experimental study on flame characteristics of propane fire array

This paper presents an experimental analysis on the flame characteristics of multiple fires burning by tests of square fire array consisting of even number of equidistant burners, using propane as the fuel. We propose to use the temperature along the centerline of fire array, instead of visual observations, to identify flame merging and examine the characteristics of flame structure under different states of flame merging. It is revealed that with decreasing flame spacing, the flames undergo a physical transition in the centerline temperature of the fire array, induced by the formation of the fuel-rich region in the lower region above the burner surface. Analyses on fuel-rich region and axial temperature, in a quantitative way, provide physical details of the changes of flame structure when the flames that burn on discrete fuel beds undergo physical transition to become one single flame with decreasing flame spacing. It is inferred from the temperature data that the air entrainment is more restricted with increasing merging level of flames. A new method in terms of the centerline temperature of fire array is proposed to determine the critical heat release rate for the onset of flame merging. It is found that the heat release rate Q˙D∗, using the fire array size D as the characteristic length scale, can be used to correlate the flame height of a merged fire array. It is further indicated that a general linear form L/D=3.7Q˙D∗2/5−c can be used as a general correlation of flame height for fire arrays in different merging levels, in which the parameter c characterizes the deviation from the Heskestad curve under different flame spacings. For non-merged fire array, the flame heights of separate flames mainly depend on the heat release rate, while the flame spacing and the number of flames have minor effects.