Pulsating Flame Research Articles

An experimental study on combustion performance and flame spread characteristics over liquid diesel and ethanol-diesel blended fuel

Ethanol has gradually become a commonly used additive in diesel to reduce the carbon foot print of the combustion products. In the process of production, transportation and storage, once the accidental leaked ethanol-diesel blend fuel is ignited, the flame will spread very fast and cause great threaten to people's lives and properties, the combustion properties of ethanol-diesel blends is thus a matter of great concern to us. A set of experiments was conducted to study the effects of fuel depth and ullage height on liquid flame spread of diesel and 5% ethanol-diesel. Results showed that as the fuel depth increases under ullage effect, the flame spread rate increases first and then maintain constant. For 5% ethanol-diesel, the flame spread rate decreases monotonously and then tends to be unchanged with ullage height due to the higher combustion efficiency and oxygen content of ethanol-diesel. For deep pools, the surface velocity increases with fuel depth. Moreover, the flash flame pulsation frequency is weakly affected by the fuel depth and presents a negative relationship with ullage height. The study on these issues may have the potential to benefit the current safety production, utilization and management of the energy of ethanol-diesel blends.

Estimation of Heat Release Rate and Fuel Type of Circular Pool Fires Using Inverse Modelling Based on Image Recognition Technique

A computational model is developed to estimate the heat release rate and fuel type of circular pool fires by analyzing the videos with recognizable flames. The model can be used in both static and mobile platforms. The pool diameter, mean flame height, heat release rate and fuel type are estimated using image recognition technique on flame videos and inverse modelling with traditional fire dynamics theories. A set of experimental videos from different sources are used to validate the model. During image recognition, the model isolates the flame from the non-flame elements in each frame using the “automatic seed placement and region growing” method. The method is found to be effective in non-flame elements removal and improves the flexibility of the model to wide range of flame videos. To automatically determine the pool diameter, fast Fourier transform (FFT) is involved to identify the flame pulsation frequency that is used as the input of inverse modelling. It is found that a sampling duration of 10 s to 20 s gives the most reliable predictions to the flame pulsation frequency for the current set of videos. A shorter duration is not sufficient for FFT to recognize the correct main frequency of the signal while a longer duration increases the low frequency components caused by the unsteady flame. Compared to the traditional fire dynamics theories with power indexes of input variables less than 1, the inverse modelling enlarges the error in the modelling results. Therefore, the main weakness of current model is perhaps the enlarged uncertainty led by the inverse modelling conducted with the traditional theories which are empirical, although the current predictions to the experiments are acceptable. Moreover, the current model divides the pool fires into three categories and based on the predicted pool diameter and heat release rate the fuel type can be estimated, which might benefit the hazard analysis in certain circumstances. Finally, a cross-platform comparison shows that the mobile devices can be considered for fire applications although it is still less powerful than personal computers.

ANALYSIS OF THE FLAME PULSATION SIGNALS USING A SHORT-TIME FOURIER TRANSFORM

The main aim of the diagnostics of combustion process is ensuring its stability and efficiency. The most important aspect related to the monitoring of the combustion process is a non-invasive acquisition of information from flame and subsequently subjecting it for further processing. Such method of research allows to evaluate the course of the process and determine the characteristic conditions under which the combustion process is stable or not. The article presents the application of short-time Fourier transform for the analysis of flame pulsation signals. The aim of the research was to find an area especially sensitive to the change of combustion process conditions.

An Experimental Investigation on Flame Pulsation for a Swirl Non-Premixed Combustion

Flame pulsation has a significant effect on combustion, and understanding its oscillatory behavior is important to the combustion community. An experiment was performed to analyze the pulsation characteristics of a swirl non-premixed flame under various parameters. The results showed that as fuel mass flow rate increased, the puffing frequency increased due to the decreased flame radiation fraction, and the puffing amplitude became smaller resulting in a more stable flame. With an increase in combustor pressure, the flickering frequency declined because of the increasing soot radiation, while the flickering amplitude uniformly increased, leading to more deteriorative flame stability. With an increment in mass flow rate of primary air, the puffing frequency decreased due to the enhanced mixing between fuel and primary air. Also, the puffing amplitude had an oscillating relationship with the mass flow rate of primary air. When the exit velocity of the injector was increased, the flickering frequency diminished nearly linearly because of the improving swirl intensity, and the flickering amplitude was approximately unaffected by injector exit velocity. Moreover, the measured puffing frequencies summarized over all cases varied within the range of 3–22 Hz, the predicted values from theoretical models based on non-swirl flame also fell within this range. The puffing frequency of swirl combustion was more sensitive to the variation in operating conditions than that of non-swirl combustion. Additionally, the obtained correlations indicated that the Strouhal number St was proportional to Fr−1.4 (the Froude number) and Re−2.9 (the Reynolds number), respectively.

Experimental and numerical study of premixed flame penetration and propagation in multichannel system

ABSTRACTExperimental and numerical results on premixed flame penetration and subsequent propagation in a multichannel burner are presented. The burner consists of the set of planar straight quartz channels which transverse sizes can be varied. It is found that, depending on mixture flow rate, equivalence ratio and channels transverse sizes a big variety of combustion regimes can be observed. These regimes include burner-stabilized flames, upstream propagating flames, and flames stabilized under the burner external surface. The placement of different combustion regimes in equivalence ratio/flow rate plane is plotted by means of experimental and numerical studies. In wide range of parameters, the flame pulsations consisting of repetitive stages of flame ignition, upstream propagation, and quenching take place. Results of numerical simulations obtained in the framework of simplified thermal-diffusion model are found to be in a good qualitative agreement with experimental data and allow to explain experimental findings.

On the Analysis of Flame Pulsation and Fire Induced Vent Flow Oscillatory Behaviors in Confined Enclosures with Horizontal Openings

Abstract This paper undertakes a comprehensive analysis of flame pulsation behavior and vent flow oscillatory behavior in a confined enclosure. Experiments are carried out in a small scale confined compartment with a horizontal opening, and several configurations characterized by different pool diameters and ceiling vent sizes are considered. Flame pulsation and horizontal vent flow oscillatory behaviors are investigated during the experiments, and visualization recordings were collected. Results indicate that the flame pulsation frequency ( f fire ) can be obtained by way of the Fast Fourier Transform (FFT) method (with recorded RGB images being transformed into pseudo-gray images based on Ostu’s method) and the vent flow oscillatory frequency ( f vent ) can also be obtained using a similar processing method. The frequencies are found to vary in accordance with variation of the horizontal opening size. Through theoretical analysis, the relationship between the flame pulsation frequency and vent flow oscillatory frequency is investigated. Moreover, it is expected that the comprehensive frequency term would alter in alignment with the parameters of the horizontal opening, and this sufficiently fitted with the experimental data.

Validity evaluation on temperature correction methods by thermocouples with different bead diameters and application of corrected temperature

Accurate measurement of flame temperature by using bare thermocouples is a traditional and still a challenging problem. Three measurement methods, including the double- and triple-thermocouple correction methods and extrapolation method, have been developed to seek accurate gas temperatures, but the validity of these methods has been rarely evaluated for application in fire experiments. All the three methods use the temperature readings of multiple thermocouples with different bead diameters. This paper presents a systematic evaluation of validity for these methods by conducting fire temperature measurements. A turbulent propane pool fire was used. The temperatures along the fire plume centreline were measured at ten vertical heights of 10–55 cm, for which four thermocouples with different bead diameters were used at each height. Analysis shows that the flame turbulence imposes a significantly negative effect on the double- and triple-thermocouple correction methods. However, the extrapolation method can correct the fire gas temperature regardless of temperature pulsation related to flame turbulence. In addition, a new radiation model that incorporates the corrected flame temperature is proposed well against the radiant heat flux measurement. Moreover, it is found that the flame pulsation frequency can be roughly estimated by the temporal derivation of the corrected flame temperature.

Flame propagation behaviours in nano-metal dust explosions

Flame propagation behaviours in 40nm titanium, aluminium and iron dust clouds were investigated by high-speed photography. 40nm titanium, aluminium and iron dust flames were all characterized by discrete single glowing burning particles with smoothly spherical front. “Micro explosions” caused by the fragment ejection occurred in both 40nm titanium and aluminium particles burning processes. At nano scales, the heat and mass transfer regimes were absolutely different with micro particles, which resulted in different propagation velocities. The average pulsating flame propagation velocities of 40nm titanium, aluminium and iron dust clouds were 0.565m/s, 0.189m/s and 0.035m/s, respectively. From SEM analysis, it was inferred that 40nm titanium, aluminium and iron particles were burnt in liquid, gas, and solid-phase, respectively. From XPS analysis, it was found that N2 played a significant role in titanium particles combustion. 40nm titanium combustion products contained 43% TiO2 (Ti4+), 27% Ti2O3 (Ti3+), 21% TiO (Ti2+) and 9% TiN (Ti3+). While 40nm aluminium combustion products just contained 100% Al2O3 (Al3+), and 40nm iron combustion products contained 49% Fe2O3 (Fe3+), 26% Fe3O4 (Fe3+/Fe2+), 15% FeO (Fe2+) and 10% iron nonoxides. Inerted by nitrogen for preventing nano‑titanium dust explosions was inadvisable.

Thermal-diffusional Instability in White Dwarf Flames: Regimes of Flame Pulsation

Abstract Thermal-diffusional pulsation behaviors in planar as well as outwardly and inwardly propagating white dwarf (WD) carbon flames are systematically studied. In the 1D numerical simulation, the asymptotic degenerate equation of state and simplified one-step reaction rates for nuclear reactions are used to study the flame propagation and pulsation in WDs. The numerical critical Zel’dovich numbers of planar flames at different densities (ρ = 2, 3, and 4 × 107 g cm−3) and of spherical flames (with curvature c = −0.01, 0, 0.01, and 0.05) at a particular density (ρ = 2 × 107 g cm−3) are presented. Flame front pulsation in different environmental densities and temperatures are obtained to form the regime diagram of pulsation, showing that carbon flames pulsate in the typical density of 2 × 107 g cm−3 and temperature of 0.6 × 109 K. While being stable at higher temperatures, at relatively lower temperatures, the amplitude of the flame pulsation becomes larger. In outwardly propagating spherical flames the pulsation instability is enhanced and flames are also easier to quench due to pulsation at small radius, while the inwardly propagating flames are more stable.

Effects of particle size distributions on PMMA dust flame propagation behaviors

Experiments of polymethyl methacrylate (PMMA) dust clouds with same Sauter diameters were conducted to reveal the effects of particles size distributions on PMMA dust flame propagation behaviors. High-speed photography was used to capture the flame propagation behaviors and microstructures. The results showed that the combustion behaviors of PMMA dust clouds with different mass fractions of 100nm, 5μm and 30μm PMMA dust particles were complicated. The macroscopical developments of the flames were determined by the major mass proportion dust particles. The flame front became smoother and the average pulsating flame propagation velocity was faster with more proportion of smaller PMMA dust particles. The flame temperatures were detected by a fine thermocouple comprising 25μm-diameter Pt-Pt/Rh13% wires. It was found that the faster the flame propagated, the higher maximum flame temperature was. The maximum temperatures of mixture B and E dust clouds could maintain longer time due to the larger mass fractions of smaller particles. The thermal conversion processes of mixture A–E dust clouds were dominated by the external heat transfer sources, including radiation from burned region and heat convection between particles and gases. And pyrolysis/devolatilization controlled the overall mixture B–E dust clouds combustion processes. Flames of mixtures were coupled with the premixed gas flame of the smaller particles, the diffusion flame of the agglomerates accompanying local premixed flame around split agglomerates, and the diffusion flame accompanying local premixed flame in high pyrolyzates concentration areas of larger particles. The combustion mechanism was determined by the dominant mass fraction dust particles. Smaller particles not only accelerated flame propagation but influenced the flame structure and combustion mechanism.

Premixed Flame Pulsations in Obstructed Channels with both Ends Open

Prior research indicated extremely fast acceleration, with potential detonation triggering, of premixed flames propagating in semi-open obstructed pipes (a pipe filled with a combustible premixture is open at one end; ignition occurs at the closed end such that a flame spreads towards the opening). However, industrial applications oftentimes have both ends open, with ignition occurring at one of them. The latter configuration constitutes the focus of our work. Specifically, flame propagation through a comb-shaped array of obstacles in a pipe with both ends open is studied by solving two-dimensional fully-compressible hydrodynamic and combustion equations with Arrhenius chemical kinetics. The blockage ratios BR=1/3, 1/2, 2/3 (with respect to the pipe radius) are considered, with oscillations of the burning rate observed in all cases. The oscillations are nonlinear but periodic; being steady for BR=2/3, 1/2, they slightly damp for BR=1/3. Increase in the BR facilitates nonlinearity and promotes the oscillation period but reduces the average burning rate. These oscillations can be treated as fluctuations around a quasi-steady solution, being thereby in agreement with recent experiments and modelling of flames in open obstructed pipes that yielded steady flame propagation prior to an onset of flame acceleration. Overall, a conceptual difference between open (oscillations) and semi-open (acceleration) obstructed pipes is identified. It is presumably attributed to the fact that, in a semi-open pipe, the entire fame-generated jet flow is pushed towards a single opening, whereas with both ends open this jet flow is distributed between two flows, towards both openings, thereby weakening flame spreading. Keywords: premixed flames, oscillating flames, equi-diffusive flames, obstructed channels, computational simulation, fire safety.Acknowledgements: This work is supported by the National Science Foundation (NSF) through the CAREER Award #1554254 (V.A.)

Large eddy simulation of a turbulent swirling premixed flame coupling the TFLES model with a dynamic wrinkling formulation

Dynamic models that take advantage of the known resolved scales to automatically adjust the model parameters have proved to be very effective in large eddy simulations (LES). Global (uniform parameter evolving only with time) and local (parameter evolving both in space and time) dynamic formulations for the flame wrinkling factor are combined with the Thickened Flame (TFLES) model and simulations of the semi-industrial PRECCINSTA burner studied experimentally by Meier et al. (2007) are performed for the stable and unstable configurations. The global formulation predicts a time-dependent model exponent that remains close to 0.5 for the stable flame and oscillates strongly around 0.8 for the pulsating flame. The local formulation adapts the model parameter locally and automatically damps the wrinkling factor in the vicinity of walls, contrary to the global formulation requiring a wall law. The usual non-dynamic approach with an appropriate parameter is found to capture flow statistics of the stable flame with good accuracy, both in terms of Favre and quasi-Reynolds averages. However, the self-excited mode of the pulsating flame is predicted only with the dynamic formalism. The fractal dimension of the unstable flame is found to vary locally and depends on the phase within the period of oscillation. Dynamic models may then play an important role in the prediction of combustion instabilities.

Pulsating behaviors of flame spread across n-butanol fuel surface

Pulsating behaviors of flame spread over n-butanol fuel are investigated with various initial temperatures under quasi-quiescent condition. The flame pulsating is an alternate process of diffusion combustion (crawling phase) and premixed combustion (jumping phase). Non-dimensional pulsating velocity Vfmax-VfminVf varies directly proportionally with the square root of non-dimensional initial temperature (THb-T0)/THb. The fundamental mechanisms of flame pulsation are physically interpreted based on heat and mass transfer in both liquid- and gas-phase flows. In liquid phase, a thermal vortex that is driven by the combined effects of Marangoni force and buoyancy force is formed ahead of the flame tip to enhance fuel evaporation rate. In gas phase, the existence of a gas-phase recirculation cell is indispensable for flame pulsation. Counter-current buoyancy dominates hot gas expansion effect, which produces a gas-phase recirculation cell ahead of the flame tip in pulsating flame spread regime. Otherwise, the uniform flame spread regime is achieved. The critical temperature between the uniform and pulsating flame spread regimes of n-butanol is predicted to be 27.9°C, which is acceptably consistent with the experimental value of 30.2°C. Finally, the coupling process between flame tip and subsurface convection flow front is achieved. Three steps are included in the coupling process of both movements: (a) flame crawling and subsurface convection flow crawling, (b) flame jumping and subsurface convection flow crawling, and (c) flame crawling and subsurface convection flow jumping.

Effects of turbulent intensity on nano-PMMA flame propagation behaviors

Experiments with different ignition delay times (td) corresponding to different residual turbulent intensities after dispersion were conducted to reveal the effects of turbulent intensity on nano-PMMA flame propagation behaviors. The residual vertical root-mean-square (RMS) turbulent velocities and RMS vorticities measured by a particle image velocimetry system (PIV) at td = 0.9 s, 1.0 s and 1.1 s were 0.22 m/s, 0.16 m/s, and 0.15 m/s and 50.40/s, 38.23/s, and 36.68/s, respectively. It was indicated that the residual turbulent intensity in the combustion space decayed with increasing ignition delay time. One-hundred nanometer PMMA dust flames with a nominal concentration of 450 g/m3 were approximately spherical in shape and propagated continuously after ignition at different ignition delay times. As time proceeded, the flame front of td = 0.9 s was still smooth, whereas the flame fronts of td = 1.0 s and 1.1 s were irregular due to the dominant effects of flame self-instability against the residual turbulence. The average pulsating flame propagation velocities of td = 0.9 s, 1.0 s and 1.1 s were 0.67 m/s, 0.54 m/s and 0.44 m/s, respectively. The pulsating level was enhanced by extending the ignition delay time due to the weaker residual turbulent intensity compared with the turbulence induced by the dust flame self-instability. In addition, the suspended particle size distribution was measured by a Phase Doppler Particle Analyzer (PDPA). It was found that the effective suspended particles existed as agglomerates of fine particles rather than as the primary particles themselves.

Combustion and exhaust emission characteristics of low swirl injector

The present experimental study aims to investigate the combustion and emission characteristics of the flow through a low swirl injector (LSI). An experimental study was carried out on the flame structure, the temperature distribution and the exhaust emission of low swirl pre-mixed combustion under the condition of different swirl number and different fuel composition. In order to qualitatively analyze the flame structure, the velocity distribution of the non-reacting flow through the LSI was measured using the particle image velocimetry (PIV) technique. Experimental results indicated that: (i) the LSI can generate a blue lift-off “W” type flame which consists of four clusters of flames connected together and holds up a long yellow pulsating flame, (ii) the blue flame structure converts the “W” type flame into the “broom” type flame and the distance between the front of the flame and the nozzle shortens with increasing swirl number, (iii) there exist high temperature region flanked by two peaks on the temperature profiles in the blue flame while uniform higher temperature in yellow pulsating flame, (iv) the NOx and CO emission level of the LSI mainly depends on the gas composition and thermal load.

The effect of Soret diffusion on stability of rich premixed hydrogen–air flames

The effect of thermal (Soret) diffusion on the structure, propagation speed and diffusional-thermal stability of rich hydrogen–air freely propagating planar premixed flames is investigated. Both the detailed mechanism and transport model is employed to study numerically the structure and stability of this flame. Warnatz detailed reaction mechanism is used. The obtained results, first of all, confirm classical asymptotic analysis by Garcia-Ybarra and Clavin [1]. Namely, the Soret diffusion moderately influences the flame speed and structure (even for hydrogen/air systems), but shifts significantly diffusional-thermal stability limits. Secondly, this study quantifies this phenomenon for reliable chemistry model and opens perspectives to use it as additional source of the detailed model validation. The structure of the diffusional-thermal stability limit as well as the structure of pulsating flame is thoroughly discussed.

Experimental study on initial temperature influence on flame spread characteristics of diesel and gasoline–diesel blends

A set of experiments were conducted to investigate the initial temperature effect on the flame spread characteristics of diesel and gasoline–diesel blend (dieseline), including the flame shapes, main flame spread velocity, subsurface flow velocity, surface temperature, flash flame velocity, pulsation frequency and pulsation amplitude. The flame spread process was recorded visually while the fuel temperatures were measured during the experiments. The results showed that the mean spread velocity of main flame of both fuels increased with initial fuel temperature while the dieseline main flame spreads almost twice that of diesel. During flame spread, the front temperature and velocity of subsurface flow increased with initial fuel temperature and the velocity of dieseline increased faster due to longer subsurface flow. Furthermore, it was found that the pulsation amplitude of diesel flash flame increased with initial fuel temperature while the one of dieseline decreased initially and then increased with increasing initial fuel temperature. The pulsation frequency and velocity of flash flame were found to be independent to the initial fuel temperature.

Investigation of self-sustained beating oscillations in a Rijke burner

Thermoacoustic oscillations occur in a Rijke burner with a porous plug stabilized premixed flame. In addition to regular limit cycle oscillations, the thermoacoustic system also presents self-sustained periodic amplitude modulations, or beating oscillations, near the flame equivalence ratio limits of thermoacoustic instability. The beating oscillations exhibit approximately 1Hz or even lower frequency amplitude modulations for the acoustic pressure and the heat release rate. It is also found that low frequency flame pulsations arise with the amplitude modulations. The different characteristics of the beating oscillations near the upper and lower equivalence ratio limits are distinguished and explained by the coupling between the low frequency flame pulsations and the thermoacoustic instability. To simulate the beating oscillations, this paper proposes a flame model that incorporates the flame dynamics at the amplitude modulation time scale (∼s) and the acoustic time scale (∼ms). The simulation results of the thermoacoustic system suggest that this flame model can qualitatively predict the amplitude modulation caused by the slow pulsating flame temperature and heat release rate. Flame heat loss plays an important role in low frequency flame pulsations, and its effects on beating oscillations are discussed. Several simulation cases at different equivalence ratios are performed to retrieve the amplitude modulations near the upper and lower equivalence ratio limits. The results are in keeping with the experimental observations.

Freely-propagating flames in aluminum dust clouds

The free propagation of isobaric flames through aluminum dust clouds is investigated in an extensive series of experiments using two facilities with different scales. In small-scale laboratory experiments, spherical flame propagation occurs in aluminum dust clouds contained within 30-cm-diameter latex balloons, whereas in large-scale tests, flames propagate vertically through unconfined aluminum dust clouds with a vertical scale of about 4 m. The balloon experiments are performed with suspensions of aluminum powder in oxygen mixed with nitrogen, argon, or helium with various concentrations of oxygen and aluminum. It is found that stable flame propagation is only observed for aluminum concentrations near stoichiometric to rich conditions. Pulsating and spiral-like flames are discovered in fuel-lean mixtures, and flames with cellular patterns occur in very-fuel-rich suspensions. The burning velocities in the stable propagation regime derived from the balloon experiments correlate well with data previously obtained with stabilized Bunsen-type flames. The flame speed of stable flames is found to be a strong function of the heat conductivity of the gas mixture. In addition, the oxygen concentration has a strong influence on the flame speed for fuel-rich mixtures but dependence is reduced for fuel-lean mixtures. In the large-scale experiments, the burning velocity is estimated to be about two times larger than for the small-scale experiments. The increase in burning velocity is attributed to preheating of the unburned mixture by radiation from the condensed-phase combustion products. The degree of preheating, determined with an array of fine thermocouples, is found to be in the range of 150–200 K. The propagation of stable flames is discussed in light of existing qualitative dust flame models, whereas the pulsating flame propagation regime observed is interpreted in terms of the thermo-diffusive instability theory developed for high Lewis number flames in gases and solid reactive powder mixtures.

The large-amplitude combustion oscillation in a single-side expansion scramjet combustor

The combustion oscillation in scramjet combustor is believed not existing and ignored for a long time. Compared with the flame pulsation, the large-amplitude combustion oscillation in scramjet combustor is indeed unfamiliar and difficult to be observed. In this study, the specifically designed experiments are carried out to investigate this unusual phenomenon in a single-side expansion scramjet combustor. The entrance parameter of combustor corresponds to scramjet flight Mach number 4.0 with a total temperature of 947K. The obtained results show that the large-amplitude combustion oscillation can exist in scramjet combustor, which is not occasional and can be reproduced. Under the given conditions of this study, moreover, the large-amplitude combustion oscillation is regular and periodic, whose principal frequency is about 126Hz. The proceeding of the combustion oscillation is accompanied by the transformation of the flame-holding pattern and combustion mode transition between scramjet mode combustion and ramjet mode combustion.