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Standard plenoptic camera can be used to capture multi-dimensional radiation information of high temperature luminous flame to reconstruct the temperature distribution. In this study, a novel method for reconstructing three-dimensional temperature field is proposed. This method is based on the optical tomography combined with standard plenoptic camera. The flame projection information from different planes is contained in one radiation image. In this model, we introduced the effective concept of the nearest neighbor method in the frequency domain to strip the interference of redundant information in the projection and to realize three-dimensional deconvolution. The flame emission intensity received by the pixels on the charge-coupled device sensor can be obtained according to the optical tomographic model. The temperature distributions of the axisymmetric and non-axisymmetric flames can be reconstructed by solving the mathematical model with the nearest neighbor method. The numerical results show that three-dimensional temperature fields of high temperature luminous flames can be retrieved, proving the validity of the proposed method.

Reactive gaseous oxygen/kerosene spray was visualized by a shadowgraph imaging technique in a lab-scale rocket engine combustor. Dynamic behaviors of inner and outer shear layer were observed morphologically and analyzed quantitatively by image processing. Correlation methods were used to statistically analyze dynamic behaviors of each shear layer and interaction between two shear layers. Relationship between shear layer dynamics and combustion flow-field was investigated by analyzing periodicity of jet-core length, lump-detachment, and luminous flame intensity. The results of correlation and frequency analysis show that dominant frequency of behaviors and interaction of shear layers is 270 Hz and the degree of correlation becomes stronger along the flow direction. The dominant frequency of core length variation and luminous flame formation is around 270 Hz and, which is the same as that of behaviors and interaction of shear layers. This proposes that the surface behaviors and interaction of the inner and outer jets in the upstream affect the downstream combustion flow-field.

Aluminum/cupric oxide (Al/CuO) reactive multilayer films (RMFs) are widely used for micro-chip energetic initiator applications; however, there are only a few examples where they are initiated through the direct input of electrical energy. This study describes the preparation of micro-chip energetic initiators containing Al/CuO RMFs as an interlayer between two Al electrical contact pads that are prepared using an image reversal lift-off process and the magnetron sputtering technology. This design enables the direct transfer of current from the contact pads to Al/CuO RMFs, which results in fast ignition (less than 1.0 μs) and produces luminous flame ball that spread over a wide area (maximum flame area of 13.8 mm2). A metal–interlayer–metal (MIM) structured initiator will not be fired until the external voltage has exceeded the breakdown field strength of CuO layers. These initiators achieved a critical breakdown voltage (VB) of 32–50 V and a complete electrical-explosion voltage (VC) of 72–84 V. The ignition threshold rises with increasing the bilayer thicknesses of the RMFs. These characteristics of this specially structured initiator are well suited for applications in highly efficient and insensitive initiating explosive devices.

Aluminum metal fuel, with combustion heat of 32,000 J/g, is of interest as high energy density material. Aluminum is able to react not only with free oxygen but also with inert combustion gaseous products. This study reports on the impact of ultrafine aluminum particles on combustion characteristics of double-base propellant. Metalized double-base propellant formulations were developed by solventless extrusion process. Combustion characteristics were evaluated using small-scale ballistic evaluation test motor. There was an increase in average operating pressure, burning rate, and characteristic exhaust velocity C∗ with aluminum content. Even though there was no oxidizer available for aluminum oxidation; aluminum alone offered 7.6% increase in average operating pressure, 24.6% increase in burning rate, 7.8% increase in C∗. The main outcome of this study is that this superior performance was achieved at 4 wt% solid loading level. Aluminum particles could alter the combustion wave by increasing the combustion flame temperature, average operating pressure inside the combustion chamber, thermal conductivity of DB propellant. All these combustion criteria could decrease the thickness of the dark zone, allowing the luminous flame to be more adjacent to the burning surface; therefore the reaction could proceed faster. Even though the developed metalized DB formulations were found to be more energetic with an increase in calorific value by 4.6%; they demonstrated good thermal behavior to reference formulation using DSC. These findings confirmed that aluminum is an outstanding energetic ingredient for solid rocket propulsion. It has unique ability to react with inert gases generating substantial heat output.

Radiation is the dominant mode of heat transfer near the burner of coal and biomass-fired boilers. Predicting and measuring heat transfer is critical to the design and operation of new boiler concepts. The individual contributions of gas and particle phases are dependent on gas and particle concentration, particle size, and gas and particle temperature which vary with location relative to the flame. A method for measuring the contributions of both gas and particle radiation capable of being applied in harsh high temperature and pressure environments has been demonstrated using emission from particles and water vapor using an optical fiber probe transmitting a signal to a Fourier Transform Infrared (FTIR) spectrometer. The method was demonstrated in four environments of varying gas and particle loading using natural gas and pulverized wood flames in a down-fired 130 kWth cylindrical reactor. The method generates a gas and particle temperature, gas concentrations (H2O and CO2), total gas and particle intensities, and gas and particle total effective emissivity from line-of-sight emission measurements. For the conditions measured, downstream of the luminous flame zone, water vapor and CO2 radiation were the dominant modes of heat transfer (effective emissivity 0.13–0.19) with particles making a minor contribution (effective emissivity 0.01–0.02). Within a lean natural gas flame, soot emission was low (effective emissivity 0.02) compared to gas (0.14) but within a luminous flame of burning wood particles (500 µm mean diameter) the particles (soot and burning wood) produced a higher effective emissivity (0.17) than the gas (0.12). The measurement technique was therefore found to be effective for several types of combustion environments.

PMMA flames are used as surrogate fuels for fire studies. Horizontal slab PMMA flame is studied here in laminar conditions. In particular, the CH radical field is obtained in a qualitative manner with respect to the luminous flame. CH-PLIF was used to obtain the CH radical field, along with simultaneous imaging of the flame luminosity. This was done at various time instances after ignition, and the change in flame shape and the CH field was observed. It was found that CH field was present in the same zone as the luminous flame. Since the luminosity is due to black body emission from the soot, it can be seen that the CH radicals exist in the same region as the soot region in the PMMA flames. This information along with OH radical field information will be useful for validating computational models for fire prediction.

Methane/coal dust explosions pose a serious threat to the safety of coal mining. Since the flow field in the tunnel is unsteady, the effect of turbulence on flame propagation is essential for the disaster risk assessment and safety protection. In this study, the effect of turbulence intensity (u′) on flame propagation characteristics and extinction limits of methane/coal dust explosions is investigated. The turbulence parameters of the flow field and the turbulence-flame interaction are estimated by Particle Image Velocimetry. Luminous flame (raw image) and reaction front (OH radical image) of methane/coal dust mixture are captured. The results reveal that the flame propagation velocity increases by 78–200%, when the u′ increases from 1.86 to 2.66 m/s. The acceleration of flame propagation velocity is due to the increase of the release of volatile matter and the flame folded regions caused by the turbulence. When the characteristic time of the turbulence disturbance becomes shorter than that of the chemical reactions, the heat sink effect of turbulence will dominate the combustion process. In this case, the extinction limits of CH4/coal flames increase with the u′. When the u′ increases from 1.86 to 2.66 m/s, the extinction limits of CH4/coal flames increase by 14–60%.

Comparative tests were conducted to investigate the flame spread characteristics of a typical hydrocarbon fuel mixture (0# diesel) and a typical pure alcohol fuel (n-butanol) in a reduced-scale (1:10) tunnel model. Experimental data with regard to flame spread appearance, flame spread rate, temperature profile, and subsurface convection flow were obtained and discussed in this study. Results showed that: (1) The diffusion flame of n-butanol propagated in a forward-stop-forward pattern, while diesel fuel pulsated in a forward-back-forward manner with less stability. The flash flame of diesel appeared periodically with significant higher spreading velocity than the enduring flash flame of n-butanol. (2) The average main flame spread rate of n-butanol was faster than that of diesel fuel, whereas the transient flame spread rate showed an opposite trend. (3) No sensitive difference was found in surface temperature profile between two fuels, but n-butanol retained higher vertical temperature due to its higher flame height than diesel. The infrared images discovered that the subsurface convection flow of diesel had a larger size and noticeable temperature gradient. Diesel burned with more luminous flame and denser soot than n-butanol, owing to its more complex fuel components.

The burning rate-pressure relation of solid propellant is an exponential relation; this is means catastrophic combustion process in case of sudden increase in operating pressure. This study reports on novel approach to minimize the dependence of burning rate on combustion pressure using highly energetic nitramines (RDX). Nitramine-based double base propellants with RDX content up to 20 wt% were manufactured by solventless extrusion technology. The impact of RDX content on burning rate, specific impulse, exhaust velocity, and thrust were evaluated using small-scale ballistic evaluation rocket motor. Specific impulse (Is) was enhanced by 20%. The action burning time (tb) was increased by 14%. The total thrust impulse (IFT) was significantly improved by 22%. One of the main outcomes of this study is that burning rate pressure exponent (n) was decreased significantly from 0.35 to 0.05. This novel ballistic performance finding means minimal change in burning rate with pressure variation. These findings confirmed that RDX has a dual function as energetic filler and platonizing agent. RDX blowing effect could alter the combustion mechanism by pushing the luminous flame from the burning surface. Furthermore, RDX decomposition can strengthen the platonization action. This is the first time ever to report on burning rate platonization using RDX. It can be concluded that many advantages have been achieved with one shot.

Autoignited lifted flames of dimethyl ether (DME) in laminar nonpremixed jets with high-temperature coflow air have been studied experimentally. When the initial temperature was elevated to over 860 K, an autoignition occurred without requiring an external ignition source. A planar laser-induced fluorescence (PLIF) technique for formaldehyde (CH2O) visualized qualitatively the zone of low temperature kinetics in a premixed flame. Two flame configurations were investigated; (1) autoignited lifted flames with tribrachial edge having three distinct branches of a lean and a rich premixed flame wings with a trailing diffusion flame and (2) autoignited lifted flames with mild combustion when the fuel was highly diluted. For the autoignited tribrachial edge flames at critical autoignition conditions, exhibiting repetitive extinction and re-ignition phenomena near a blowout condition, the characteristic flow time (liftoff height scaled with jet velocity) was correlated with the square of the ignition delay time of the stoichiometric mixture. The liftoff heights were also correlated as a function of jet velocity times the square of ignition delay time. Formaldehydes were observed between the fuel nozzle and the lifted flame edge, emphasizing a low-temperature kinetics for autoignited lifted flames, while for a non-autoignited lifted flame, formaldehydes were observed near a thin luminous flame zone. For the autoignited lifted flames with mild combustion, especially at a high temperature, a unique non-monotonic liftoff height behavior was observed; decreasing and then increasing liftoff height with jet velocity. This behavior was similar to the binary mixture fuels of CH4/H2 and CO/H2 observed previously. A transient homogeneous autoignition analysis suggested that such decreasing behavior with jet velocity can be attributed to partial oxidation characteristics of DME in producing appreciable amounts of CH4/CO/H2 ahead of the edge flame region.

Planar laser-induced fluorescence (PLIF) imaging of the OH radical was used to study the reaction-zone structure in the near-nozzle region of strongly-pulsed jet diffusion flames in the presence of a swirling coflow. The nominal steady jet velocity at the nozzle exit leads to a Reynolds number of Rejet = 5000. Ethylene-fueled flames were investigated over a wide range of injection conditions and different swirl levels. OH PLIF imaging was successfully employed in these highly sooting flames by careful selection of the laser fluence and camera gate width to minimize soot-related signal interference. The reaction-zone structure, as indicated by OH PLIF, is broadly consistent with the observation of luminous flame structure for these types of flames under the same injection conditions. The overall area of the reaction zones, as indicated by statistics generated from the OH PLIF signals, is generally higher for strongly-pulsed flames compared to steady flames. In addition, the measured reaction-zone a...

Much attention has been directed toward the development of modified double base (MDB) propellant as it can offer high specific impulse as well as wide range of burning rate. One of the main approaches for the development of MDB propellant is the integration of potential oxidizers and metal fuels. This study reports on, the impact of potential oxidizers including potassium perchlorate (KP) and ammonium perchlorate (AP) on combustion characteristics of double base (DB) propellant. The impact of these energetic additives on burning rate and the characteristic exhaust velocity of gaseous product (C∗) were evaluated using small-scale ballistic evaluation test motor. The two potential oxidizers KP and AP exhibited controversy effects; whereas KP positively impact the burning rate, AP positively impact C∗. The partial replacement of AP with aluminum metal fuel demonstrated a positive impact on both the burning rate and C∗. Aluminum metal fuel offered an increase the combustion temperature, and propellant thermal conductivity. Consequently, it could alter the combustion mechanism, by thinning the induction zone, allowing the luminous flame zone to be more adjacent to the burning surface. Accordingly, the combustion reaction could proceed faster. While, MDB based on AP/Al were found to be more energetic with an increase in calorific value to reference formulation using bomb calorimetery; they exhibited good thermal stability in terms of ignition temperature using cook off test, and accepted thermal behavior using DSC.

This paper presents a novel method of identifying coal type based on mechanistic methods. The ratio of the resonance line spectrum of a luminous flame and the continuous spectrum at the same wavelength eliminates the influence of temperature on spectral intensity. The atomic line spectra of Na and K are typical and significant over continuous flame spectra. The concentrations of elemental Na and K in the flame are exclusively relative to coal type and composition. Using an experimental furnace and charge-coupled device (CCD) optical spectrometer apparatus, the continuous spectra and atomic line spectra of Na and K elements were sampled from coal flames in real time. An empirical fitting method was used to simplify the formulas of absorption strength and flame temperature calculation, and rational solutions were obtained by using an iterative algorithm. Due to the change in reaction rate and absorption by soot particles, the relative contents of Na and K in a flame vary with the temperature and absorption strength. Arrhenius’s equation for temperature compensation was adopted. Compensation for soot density in the furnace was also satisfied by an exponential expression. At any one sampling position, the compensation parameters were identical for all coal types. After compensation for temperature and density of soot particles, the relative strength of the Na and K signals and the ratio between them uniquely matched the coal type burnt in various conditions. The results were replicated and verified in various conditions, and the response time of the system was of the order of seconds.

Metal hydrides are regarded as a series of promising hydrogen-supplying fuel for solid rocket propellants. Their effects on the energetic and combustion performances of propellants are closely related to their reaction mechanisms. Here we report a first attempt to determine the reaction mechanism of ZrH2, a high-density metal hydride, in the combustion of a double-base propellant to evaluate its potential as a fuel. ZrH2 is determined to possess good resistance to oxidation by nitrocellulose and nitroglycerine. Thus its combustion starts with dehydrogenation to generate H2 and metallic Zr. Subsequently, the newly formed Zr and H2 participate in the combustion and, especially, Zr melts and then combusts on the burning surface which favors the heat feedback to the propellant. This phenomenon is completely different from the combustion behavior of the traditional fuel Al, where the Al particles are ejected off the burning surface of the propellant to get into the luminous flame zone to burn. The findings in this work validate the potential of ZrH2 as a hydrogen-supplying fuel for double-base propellants.

Due to their natural coupling of low-temperature chemistry and transport, cool flames are a valuable platform for drawing fundamental understandings of complicated phenomena relevant to real engines. In this study, self-sustaining partially premixed cool flames of dimethyl ether are investigated in detail through the use of an ozone-assisted counterflow burner. A double cool flame with distinct diffusion flame and premixed flames sides is visibly observed at increased fuel loading and equivalence ratio. The examination of double cool flames and double hot flames through planar laser-induced fluorescence measurements reveals the stark differences in the role of CH 2 O in each. The results show that while CH 2 O is one of the main product species of the cool flame via low-temperature chemistry, for the hot flame it is only a short-lived intermediate produced in the preheat zone. Comparisons of experimental results with numerical calculations based upon detailed chemical kinetic models show fair agreement on the double cool flame structure but a noticeable discrepancy in the prediction of the second-stage ignition limit, which triggers the transition from cool flames to hot flames. The critical strain rate for second-stage ignition is shown to be much more sensitive to fuel addition on the premixed side of the double flame than on the diffusion side. A mechanism for second-stage ignition in partially premixed cool flames is proposed based upon numerical modeling and experimental observations: H 2 O 2 is primarily formed in the premixed cool flame, diffuses toward the stagnation plane, and then finally decomposes into OH radicals upon approaching the cool diffusion flame.

This paper presents the design, method, and evaluation of a 3-D field reconstruction of a combustion flame. Three images under three different wavelengths were captured together using charge-coupled device cameras, which were specially processed by filters. Three sets of data about the value of radiation under different wavelengths were transformed from the images that were obtained according to the law of thermal radiation and derivation. For a pixel point in flame image, its irradiance can be expressed as the accumulation of the emissive power from luminous flame with different temperatures along the rays through the pixel. Then, through the calculation of the thermal radiation inversion based on the data from pixels in images, the 3-D temperature field of the flame is obtained and displayed. Experimental tests were done using candle flame to evaluate the effectiveness of the system. Ultimately, this method can restore the temperature distribution of the flame temperature field effectively. Compared with other methods, such as the two-color pyrometric method, the device of this method can get synchronous monochromatic images under three different wavelengths, the beams of which were through a beam splitter at the same time to avoid the errors of spatiotemporal matching. Else, we can reproduce the validity and efficiency of temperature field using the reconstruction algorithm that is built on improved simulated annealing algorithm.

This work studied flame spread over polyethylene (PE)-insulated wires at high pressure with an emphasis on the effect of ambient pressure. Laboratory simulated wires were used as a tested wire, which consisted of semi-transparent low-density PE (LDPE) tube as an insulation and copper (Cu) rod or stainless-steel (SS) tube as a core. Increasing ambient pressure produced much soot, leading to an intensively yellow luminous flame. Flame spread rate for Cu rod increased with ambient pressure, whereas that for SS tube decreased. A qualitative analysis via a simplified flame spread model shows that the flame heat flux increases with ambient pressure, but consequently the flame heating decreases due to the decreased flame heating length. Therefore, the flame spread rate for SS tube decreased with increasing ambient pressure. For Cu rod, however, the core heating is dominant enough to compensate the decreased flame heating, and thus the ambient pressure positively affected the flame spread. During the flame spread, melted insulations were often dripped down, but it reduced with increasing ambient pressure. For Cu rod, no dripping occurred when further increasing ambient pressure, which indicates that there is the dripping limit at a high-pressure region. These findings should contribute to electrical fire safety in elevated pressure environments.

This paper presents an experimental investigation on burning rate, flame tilt angle and flame length of small ethanol pool fires under different oblique winds. Square pool fires with dimension of 4 cm and 6 cm with wind attack angles from 0° to 30° and wind speeds from 0 to 3.28 m s−1 for downslope airflow and from 0 to 1.90 m s−1 for upslope airflow are tested. Flame tilt angle and flame length are determined based on luminous flame intermittency from the flame images. The results show that the burning rate under downslope wind is relatively larger than that under upslope wind at the same absolute wind speed and slope angle. The flame tilt angle and flame length generally increase with the increase of wind slope angle for downslope airflow, but decrease with the increase of upslope airflow angle. At a given wind slope angle, the flame tilt angle generally increases with the increase of wind speed until to an asymptotic value when the wind speed exceeds 0.35 m s−1; the flame length first increases and then decreases for downslope wind, but initially decreases then increases before decrease again for upslope wind. In addition, some modified correlations are developed to analyze the flame tilt angle and flame length data under oblique wind conditions.

In this paper, we investigate the effects of nanosecond, repetitively-pulsed, non-equilibrium plasma discharges on laminar, low-pressure, premixed, burner-stabilized hydrogen/O 2 /N 2 and hydrocarbon/O 2 /N 2 flames using CH * chemiluminescence and quantitative OH laser-induced fluorescence (LIF) diagnostics. Two different plasma sources, both of which generate uniform, low-temperature, volumetric, non-equilibrium plasma discharges, are used to study changes in chemiluminescence, temperature, and OH concentration when non-equilibrium plasmas are directly coupled to conventional hydrogen/hydrocarbon oxidation and combustion chemistry. Qualitative imaging of CH * chemiluminescence indicates that during plasma discharge, the luminous flame zone is shifted upstream towards the burner surface with little change in the CH * zone thickness. For the same plasma discharge and flame conditions, quantitative results using spatially-resolved OH LIF and multi-line, OH-LIF thermometry show significant increases in ground-state OH concentrations in the preheating zones of the flame. More specifically, for a particular axial position downstream of the burner surface, the OH concentration increases, which can be viewed as an effective “shift” of the OH profiles towards the burner surface. The increase in OH concentration is conceivably due to an enhancement of the lower-temperature kinetics including O atom, H atom and OH formation kinetics and temperature rise due to the presence of the low-temperature, non-equilibrium plasma.

In the present study, Laser-Induced Breakdown Spectroscopy (LIBS) was applied to the measurement of fuel-rich propane-air luminous flame in a counter-flow configuration to investigate the applicability of LIBS to measure local chemical composition in sooting field. H656nm/O777nm intensity ratio was chosen as the indicator of atomic number ratio H/O which gives direct information on equivalence ratio and is important for analyzing not only blue flame but also sooting flame. Measurement was conducted in the first place on propane-air/air counter-flow flame with the range of local equivalence ratio 0–4, in which the intensity ratio showed a good linear correlation with calculated H/O atomic number ratio as it had been observed in non-sooting conditions. LIBS technique was also applied to uniformly sooting flame supplying the same mixture at equivalence ratio of 2.2 from both nozzles to produce predefined homogeneous atomic composition. The measured intensity ratio was a good agreement with the linear relationship obtained in the propane-air/air diffusion flame experiment. Therefore, robustness of LIBS measurement of atomic/molecular composition in sooting flame was reasonably elucidated.

Thin, cylindrical samples of HMX/HTPB formulations with solids loadings from 85-95% by mass have been heated at 1°C and 10°C minute until a reaction occurred in the new dual window cook-off test vehicle. The test vehicle has captured the response of these formulations, and shown the influence of variables such as confinement, heating rate and sample size. Live imaging of the heated samples revealed that, as with pure nitramine samples, three distinct stages of change take place during heating; phase changes, melting and slow, flameless decomposition with production of gaseous intermediates and finally burning with a luminous flame of the gaseous intermediates. In addition, the binder appears to undergo decomposition before the HMX, darkening along the edge closest to the thermal input before the HMX melts. No apparent volume change has been detected during the beta-delta phase change in HMX, however this could be due to the low-density hand pressed samples, allowing possible occlusions to be present. Prior to violent reaction, flame speeds were measured at approximately 30m/s for high confinement, which reduces by 2-3 orders of magnitude when confinement is lowered.Thin, cylindrical samples of HMX/HTPB formulations with solids loadings from 85-95% by mass have been heated at 1°C and 10°C minute until a reaction occurred in the new dual window cook-off test vehicle. The test vehicle has captured the response of these formulations, and shown the influence of variables such as confinement, heating rate and sample size. Live imaging of the heated samples revealed that, as with pure nitramine samples, three distinct stages of change take place during heating; phase changes, melting and slow, flameless decomposition with production of gaseous intermediates and finally burning with a luminous flame of the gaseous intermediates. In addition, the binder appears to undergo decomposition before the HMX, darkening along the edge closest to the thermal input before the HMX melts. No apparent volume change has been detected during the beta-delta phase change in HMX, however this could be due to the low-density hand pressed samples, allowing possible occlusions to be present. Prio...

The ignition process of a methane diffusion impinging flame was investigated experimentally, using high speed schlieren/stereo imaging and image processing techniques. Two types of flame can be identified during the ignition process: premixed like flame with weak blue colour and diffusion flame with yellow-reddish colour. The 3D structure of blue flame after enhancement and yellow flame has been reconstructed and analysed seperately. The ratio between green and blue colour intensity is used to provide indications of the global fuel/air mixture state of the blue flame, which is found to correlate well with the flow conditions and flame propagation characteristics.

Biomass co-firing with pulverized coal in conventional, large-scale power plants is considered a low-cost strategy to reduce CO2 emissions and fossil-fuel dependence. This work analyzes the effects of biomass co-firing in the physical characteristics and oscillation patterns of the flame, by means of a digital imaging system installed in a 500 kWth semi-industrial scale swirl burner. The system is based on a monochrome CCD (Charged Couple Device) camera of high-speed frame rate (120 frames per second). Processing stage comprises the digital analysis in the spatial and spectral domain of recorded videos. Several co-firing flames with two biomass fuels (Cynara cardunculus and Populus sp.) were investigated, at substitution percentages ranging from 0% to 15%, energy basis. Spatial analysis of swirling flames highlights the effects of biomass addition in terms of luminous fluctuation rates and symmetry properties. With the joint analysis of flame derived parameters and CO emissions, flame flicker has been proved as a potential indicator of combustion performance. Additionally, a deeper investigation of luminous flame signal throughout its resulting histogram, have found new outcomes in the identification of instabilities of combustion process. Kurtosis and skewness parameters have shown an interesting response when CO emissions increase resulting in a promising option to develop monitoring and control systems based in non-intrusive measurements.

To reveal clearly the motion behaviors and velocity distributions of the unburned particles ahead of flame front in hexadecanol dust explosion, the flame propagation process was recorded by a high speed camera combining the particle image velocimetry (PIV) techniques. Flame region was recognized from the direct-light-emission photographs by the color model; Otsu's automatic threshold segmentation model and Fast Fourier Transform were adopted to set the threshold and to measure the flow field, respectively. It was observed that the propagating flame consisted of blue-spots of flame at the leading zone and luminous flame behind them. The flame firstly propagated towards the small particles nearby, when the small particles were completely pyrolyzed, the local pre-mixing flame would continue to heat the larger particles, establishing the local diffusion flame, and then the isolated blue luminous spots. Single dust particle velocity changed with the distance to the flame front. There existed a stagnation region, in which the particle velocities approached zero. Beyond this region, the particles settled down at a constant velocity. Vortex structures appeared in the regions where the settlement particles encountered with the outward movement particles.

The influences of fuel dilution, inlet velocity, and gravity on the shape and structure of laminar coflow CH4-air diffusion flames were investigated computationally and experimentally. A series of nitrogen-diluted flames measured in the Structure and Liftoff in Combustion Experiment (SLICE) on board the International Space Station was assessed numerically under microgravity (mu g) and normal gravity (1g) conditions with CH4 mole fraction ranging from 0.4 to 1.0 and average inlet velocity ranging from 23 to 90 cm/s. Computationally, the MC-Smooth vorticity-velocity formulation was employed to describe the reactive gaseous mixture, and soot evolution was modeled by sectional aerosol equations. The governing equations and boundary conditions were discretized on a two-dimensional computational domain by finite differences, and the resulting set of fully coupled, strongly nonlinear equations was solved simultaneously at all points using a damped, modified Newton's method. Experimentally, flame shape and soot temperature were determined by flame emission images recorded by a digital color camera. Very good agreement between computation and measurement was obtained, and the conclusions were as follows. (1) Buoyant and nonbuoyant luminous flame lengths are proportional to the mass flow rate of the fuel mixture; computed and measured nonbuoyant flames are noticeably longer than their 1g counterparts; the effect of fuel dilution on flame shape (i.e., flame length and flame radius) is negligible when the flame shape is normalized by the methane flow rate. (2) Buoyancy-induced reduction of the flame radius through radially inward convection near the flame front is demonstrated. (3) Buoyant and nonbuoyant flame structure is mainly controlled by the fuel mass flow rate, and the effects from fuel dilution and inlet velocity are secondary.

The decomposition of silane (SiH4), as a model silicon-containing trace compound in renewable natural gas (RNG), has been studied during RNG combustion at ambient pressure conditions, using the opposed-jet, flat-flame experimental configuration. Silane flame concentration profiles were obtained, which indicate that complete SiH4 oxidation occurs in the preflame and luminous flame regions. The laser-extinction technique was used to measure particle volume fractions in the flame, and scanning electron microscopy and energy dispersive analysis by X-ray were used to study the morphology (size and shape) and the elemental composition of the particles formed. It was shown that pure solid silica (SiO2) particles are generated and carried out with the gaseous combustion products into the postflame region. The experimental data were modeled using two detailed SiH4 decomposition models, and the major SiO2 production channels have been identified for both these models via sensitivity and reaction path analyses. It w...

To reveal the characteristics of flame propagation and pressure in organic dust explosions during pneumatic transportation, 30 μm polymethyl methacrylate (PMMA) dust explosions with different airflow velocities were experimentally conducted in a horizontal pipe. The results showed that dust flame transited from the luminous flame with a continuous structure to discrete flames as flame propagating. After the appearance of the discrete flames, flame propagation velocity increased sharply owing to the positive feedback coupling between combustion and expansion. With the increase of dust concentration, the average flame propagation velocity, maximum flame temperature, temperature rising rate, maximum pressure, and maximum rate of pressure rise increased and then decreased. These explosion characteristics (except for the maximum flame temperature) increased significantly with the increase of airflow velocity. When airflow velocity increased from 6.39 m/s to 13.3 m/s, the peaks of average flame propagation velocity increased from 23.9 m/s to 38.2 m/s; the peaks of maximum pressure and maximum rate of pressure rise were increased by 1.9 and 6.1 times respectively. In addition, it was found that the optimum dust concentration decreased as airflow velocity increased. Furthermore, the relationship among flame propagation, temperature, and pressure was discussed in detail.

The combustion characteristics of lean non-premixed Thai synthetic natural gas on improved inverse diffusion flame (IDF) burner were investigated. Air and fuel nozzles were improved from normal diffusion flame burner (NDF) and set as inside and outside exits on burner, respectively. The combustion flames were studied with variations of fuel and air velocity. The luminous flame length was lower and premixed flame length was higher by designed IDF burner due to enhance of mixing between inside air and ambient air. When the equivalence ratio decreased, lower flame length and flame temperature were obtained. Moreover, the highest premixed combustion flames and the maximum temperature were observed equivalence ratio between Φ = 0.76-0.80 on lean non-premixed synthetic Thai natural gas flames with designed IDF burner.

The structure and dynamics of swirled, strongly pulsed, turbulent jet diffusion flames were examined experimentally in a co-flow swirl combustor. The dynamics of the large-scale flame structures, including variations in flame dimensions, the degree of turbulent flame puff interaction, and the turbulent flame puff celerity were determined from high-speed imaging of the luminous flame. All of the tests presented here were conducted with a fixed fuel injection velocity at a Reynolds number of 5000. The flame dimensions were generally found to be more impacted by swirl for the cases of longer injection time and faster co-flow flow rate. Flames with swirl exhibited a flame length up to 34% shorter compared to nonswirled flames. Both the turbulent flame puff separation and the flame puff celerity generally decreased when swirl was imposed. The decreased flame length, flame puff separation, and flame puff celerity are consistent with a greater momentum exchange between the flame and the surrounding co-flow, resu...

A brief review is given of the studies in combustion chemistry and decomposition kinetics of forest fuels (FF). The methods used in the study to investigate the FF pyrolysis kinetics and the combustion of the Siberian forests are described. The experiments on FF pyrolysis were conducted at high heating rates (150 K/s) in a flow reactor by the method of differential mass-spectrometric thermal analysis (DMSTA) in situ using probe molecular-beam mass spectrometry, and at low heating rates (0.17 K/s) by the thermogravimetric method. The kinetic parameters of Siberian FF pyrolysis have been determined for oxidative and inert media and simulation of FF pyrolysis has been conducted using the multi-component devolatilization mechanism. The flame structure of a pine branch has been studied by probe molecular-beam mass spectrometry. Species have been identified in the dark and luminous flame zones; the width of the flame zone has been measured. © 2013 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Asia-Oceania Association for Fire Science and Technology.

Ethylene/air laminar diffusion flames were studied at sub-atmospheric pressures to simulate a non-buoyant environment and at super-atmospheric pressures for comparison, at fuel flow rates of 0.48 mg/s and 1.16 mg/s. Flame properties including flame geometry, soot formation and temperature field of the flames were studied. Overall, luminous flame height decreased with decreasing pressure to the point of visible luminosity disappearance, resulting in blue flames at a near vacuum. Flame width increased with decreasing pressure until the flame was almost spherical. Soot formation was also found to decrease with decreasing pressure and existed at very negligible concentrations in a near vacuum. Subatmospheric peak soot volume fractions ranged from about 0.1 ppm to 0.93 ppm at 0.48 mg/s, whereas at 1.16 mg/s, peak soot volume fractions were substantially higher. At subatmospheric pressures, higher fuel flow rates produced flames with higher soot concentrations. Soot production was restricted to an annular region with this annular region shifting closer to the flame centerline with increasing height from the burner. At locations about halfway between the burner rim and the flame tip, soot volume fraction decreased with increasing radial distance from the flame centerline. These results are consistent for both high and low pressure flames. The annular soot formation region was also located further from the flame centerline for the higher flow rate flames because of the difference in luminous flame shape. At 0.48 mg/s, the fraction of carbon in the fuel converted into soot was between 0.1 % and 1.2 % in the sub-atmospheric pressure range, from 0.2 atm to 1 atm.

This paper describes a ZigBee-based wireless sensor network node for the ultraviolet (UV) detection of flame. The sensor node is composed of a ZnSSe UV photodetector, a current-sensitive front end including a high-gain current-to-voltage amplifier with 120 dB and a logarithm converter, and a transceiver operated at a 2.4-GHz industrial, scientific, and medical band. A passive photodetector is designed to have a cutoff at 360 nm and convert the UV emission of flame into picoamperes. Including mixed signal processing and ZigBee transmission, the speed of flame detection is as fast as 70 ms. The sensor node consumes only an average of 2.3 mW from a 3.3-V supply. The performance of a prototype sensor node was verified when the luminous flame was imaged onto the sensor node with different angles ranging from -30° to 30° and distances of 0.1, 0.2, and 0.3 m enabling effective fire safety applications.

An experimental technique for measuring a temperature field in an axisymmetric non-luminous flame is developed using the flame reaction technique combined with the inverse Abel transformation. Flame visualization is carried out using alkali metal solution of Potassium (K), which is supplied to a premixed methane/air flame in a form of spray mist. The basic principle of this technique is based on the measurement of local emission intensity distribution visualized by the flame reaction, which is a function of temperature according to the Maxwell–Boltzmann statistics of thermodynamics. The relationship between the local intensity and the temperature is obtained from the calibration study, in which the local intensity is evaluated from the line of sight intensity by the inverse Abel transformation, and the temperature is measured by thermocouple. This technique is successfully applied to the measurement of local temperature distribution in steady and flickering premixed methane/air flame. The temperature field in the flickering flame indicates that the local temperature oscillates periodically with the flickering frequency, and the highest temperature is found along the flame front and in the merging region.

Coaxial jet spray flames of kerosene and oxygen are experimentally studied over a pressure range of 0.1–1.0 MPa to determine the relationship between flame structure, droplet behavior, and soot formation region, which varies with changes in pressure. The direct images and chemiluminescence spectra show that the spray flames have three regions: the blue flame region, which has a peak of CH* and C 2 * radical chemiluminescence, luminous flame region caused by soot emission, and blue emission region caused by CO 2 emission. With increase in ambient pressure, the flame length shortens drastically, the luminous flame region envelopes the blue flame region, and the blue emission becomes more intense. The result of phase-Doppler anemometry shows that a large number of small droplets evaporate and disappear near the burner, and the evaporation of large droplets also occurs rapidly under high pressure. The result of temperature measurements shows that high-temperature regions appear near the burner. The flame temperature drastically decreases along the central axis, and a minimum temperature point appears. This point moves upstream with increase in ambient pressure because evaporation of the droplets occurs further upstream. A laser-induced incandescence measurement shows that the soot volume fraction does not monotonously increase or decrease with increase in ambient pressure. The soot volume fraction at the central axis becomes low upstream and high downstream. As pressure increases, the vertical position at which the peak of soot volume fraction appears at the central axis moves upstream.

Ignition and combustion characteristics of a stoichiometric dimethyl ether (DME)/air mixture in a micro flow reactor with a controlled temperature profile which was smoothly ramped from room temperature to ignition temperature were investigated. Special attention was paid to the multi-stage oxidation in low temperature condition. Normal stable flames in a mixture flow in the high velocity region, and non-stationary pulsating flames and/or repetitive extinction and ignition (FREI) in the medium velocity region were experimentally confirmed as expected from our previous study on a methane/air mixture. In addition, stable double weak flames were observed in the low velocity region for the present DME/air mixture case. It is the first observation of stable double flames by the present methodology. Gas sampling was conducted to obtain major species distributions in the flow reactor. The results indicated that existence of low-temperature oxidation was conjectured by the production of CH2O occured in the upstream side of the experimental first luminous flame, while no chemiluminescence from it was seen. One-dimensional computation with detailed chemistry and transport was conducted. At low mixture velocities, three-stage oxidation was confirmed from profiles of the heat release rate and major chemical species, which was broadly in agreement with the experimental results. Since the present micro flow reactor with a controlled temperature profile successfully presented the multi-stage oxidations as spatially separated flames, it is shown that this flow reactor can be utilized as a methodology to separate sets of reactions, even for other practical fuels, at different temperature.

We investigate the radiative heat transfer in a co-flowing turbulent nonpremixed propane-air flame inside a three-dimensional cylindrical combustion chamber. The radiation from the luminous flame, which is due to the appearance of soot particles in the flame, is studied here, through the balance equation of radiative transfer which is solved by the Discrete Ordinates Method (DOM) coupling with a Large Eddy Simulation (LES) of the flow, temperature, combustion species and soot formation. The effect of scattering is ignored as it is found that the absorption dominates the radiating medium. Assessments of the various orders of DOM are also made and we find that the results of the incident radiation predicted by the higher order approximations of the DOM are in good agreement.

Saturation currents and chemiluminescence, especially at the CH ∗ and C 2 ∗ wavelengths, are measured for a range of small, laminar methane flames during progressive addition of air, with the principal objective of distinguishing between pure diffusion flames, premixed flames of compositions falling between the upper and lower flammability limits, and the broad range of aerated flames lying in between these regimes. Flame areas defined by the loci of maximum luminosity and by schlieren contours were recorded, so that saturation current densities, CH ∗ and C 2 ∗ emission per unit flame area, as well as burning velocities could be deduced. For admixtures of less than 70 vol.%, air appears to act, surprisingly, as an inert diluent as regards saturation currents, so that saturation currents are essentially proportional to fuel flow alone. Much the same applies to chemiluminescence. However, schlieren contours, which were recorded both to provide a basis for burning velocity measurements and to explore density changes in the reactants, indicated the presence of a burner – stabilised propagating reaction zone ahead of the luminous flame surface starting at around 50 vol.% and possibly even at lower air admixtures. This evidence of a steep change in refractive index is indicative of a premixed reaction zone involving the added oxygen, which however generates no chemi-ionization and emits no light. Even photographing the flame by radiation emitted at the CH ∗ and C 2 ∗ wavelengths shows no sign of its existence. Its burning velocity is about 10 cm/s, when stabilized by the surrounding diffusion flame. The most plausible rationale for these observations is the formation of syngas by the partial oxidation of methane. The subsequent burning of CO and H 2 is known to occur without chemi-ionization or appreciable light emission.

The common misconception that hydrogen flames are not visible is examined. Examples are presented of clearly visible emissions from typical hydrogen flames. It is shown that while visible emissions from these flames are considerably weaker than those from comparable hydrocarbon flames, they are indeed visible, albeit at reduced light levels in most cases. Detailed flame spectra are presented to characterize flame emission bands in the ultraviolet, visible and infrared regions of the spectrum that result in a visible hydrogen flame. The visible blue emission is emphasized, and recorded spectra indicate that fine spectral structure is superimposed on a broadband continuum extending from the ultraviolet into the visible region. Tests were performed to show that this emission does not arise from carbon or nitrogen chemistry resulting from carbon-containing impurities (hydrocarbons) in the hydrogen fuel or from CO2 or N2 entrainment from the surrounding air. The spectral structure, however, is also observed in methane flames. The magnitude of the broadband emission increases with flame temperature in a highly nonlinear manner while the finer spectral structure is insensitive to temperature. A comparison of diffusion and premixed H2 flames shows that the fine scale structure is comparable in both flames.

Experimental studies of the length of vertical isolated burning flames with hydrogen and methane outflow from cylindrical nozzles of diameter 2, 3, 5 and 10 mm into the atmosphere at Re 0 = 100-6800 have been made. The data obtained have been approximated by simple calculated dependences. The application of various methods for recording the length of a luminous flame (visual, black-and-white photography, video and digital cameras) has made it possible to not only explain the discrepancies between sizes but also record (at short exposures) elementary bending bright cylindrical jets creating, when superimposed on the another, one known spindle-shaped form of the flame.

The present work aims at developing an analytical model for evaluating the thermal radiation of a fire plume formed by burning wildland fuels at a given burning intensity and crosswind. By resolving the equations governing gas continuity, momentum, and energy conservation for a convective (buoyancy) plume, gas velocity and temperature profile along the central line of the plume were determined, which enables a quantification of radiation intensity of flame and buoyancy plume zones. Model features are examined, and the predicted results are compared with the available experimental data. It is found that for a fire with burning intensity exceeding 600 kW m−1, the radiant heat emittable from its plume is not only contributed by the luminous flame zone but also by the buoyancy plume significantly. This indicates, for the first time, a necessity of inclusion of the contribution of the buoyancy plume in evaluating thermal impact of a fire plume on a receiver in the environment. The current work provides an impo...

Investigations of hydrothermal flames were reviewed with three primary objectives: (1) to classify and describe experiments performed to date, (2) to assess the state of the art of experimental instruments and measurement techniques, and (3) to characterize current understanding and outline future research needs of hydrothermal flame processes. For this review, hydrothermal flames are defined as rapid oxidation reactions occurring in a supercritical aqueous environment at sufficient temperature (∼1000 °C and higher) and rate to produce a luminous flame. Under these conditions, chemical kinetics are very fast, with reactions going to completion within 10–100 ms. Hydrothermal flame experiments were divided into two categories: laminar inverse diffusion flames in semi-batch reactor systems and turbulent diffusion flames in continuous reactor systems. Laminar diffusion flame studies have demonstrated the range of temperatures, pressures, and fuel and oxidant types and concentrations from which hydrothermal flames can be produced, while turbulent diffusion flame studies have focused on the application of hydrothermal flames to the remediation of aqueous waste streams. Experimental apparatuses, instrumentation, and techniques used in hydrothermal flame investigations are presented in some detail. Models developed to describe hydrothermal flame processes, especially flow fields in turbulent diffusion flame studies, are presented as well.

Spectral optical techniques, including light extinction and laser induce fluorescence and incandescence measurements, are combined to characterize large-molecule soot precursors and soot in a slightly sooting flame of benzene at atmospheric pressure. Light absorption coupled to in-situ light scattering measurements and ex-situ Atomic Force Microscopy also allowed the evaluation of particle sizes. In the benzene flame high molecular mass structures with typical sizes of 3–4 nm are formed in the main oxidation region of the flame. The radical-rich flame environment in which these compounds are formed promotes their dehydrogenation increasing the level of their aromaticity. As a result, nanoparticles with typical sizes of about 5 nm, absorbing and fluorescing in the visible are formed. These compounds reach a maximum concentration just before the appearance of incandescent soot particles.

This paper presents the results of an experimental investigation of the flame structure of hydrazinium nitroformate (HNF) at pressures of 0.06 and 0.13 MPa. Laser diagnostics, including laser Doppler anemometry (LDA) and coherent anti-Stokes Raman spectroscopy (CARS), has been used to obtain the velocity and temperature profiles above the burning surface of an HNF pellet. Great effort has been made to measure velocity and temperature in the flame regions close to the burning surface (primary reaction zone and nonluminous zone). As a by-product of the LDA and CARS measurements, we found that these regions contain a dispersed phase with aerosols and gaseous reactants being irregularly released from bubbles formed at the foam layer. Aerosols with size larger than approximately 0.1–0.2 μm have been found in the region between the foam layer and the primary reaction zone. At larger distances from the surface, the aerosols shrink to a size below 0.1 μm until they are fully consumed in the beginning of the brightly luminous flame. It appears that at a fixed location above the surface one intermittently finds final products or gaseous reactants with the balance between the two shifting toward the hot products with increasing distance from the surface.

This paper considers experimentally the effect of the oxygen addition to the fuel flow given to the oxidation/ formation characteristics of polycyclic aromatic hydrocarbon (PAH) in a methane-air, counterflow, nonpremixed (diffusion) flame. The amount of oxygen added in the fuel flow is 0.0-15.0% of range in a volumetric flow rate, and a corresponding equivalence ratio is 10 or more. A quartz microsampling probe was inserted into the flame to extract the test gas, and the local amount of lighter hydrocarbons including three or four carbons (C3 and C4, respectively), and PAH was investigated using a gas chromatograph-mass spectrometer. Distribution of temperature, the emissive power (near 650 nm) of a luminous flame, and OH fluorescence intensity were separately measured. On the conditions used in this experiment, it turned out that soot yield and detected PAH mole fraction decreased with the increase in the added amount of oxygen. The depletion degree became weaker gradually and their inhibition by an oxygen addition was reaching the ceiling. Additionally, the peak in the profile of the mole fraction of PAH approached the elevated-temperature zone with an increase in the added amount of oxygen. These results suggest that the promotion of PAH formation is somehow associated with the increase in temperature. The reason for such leveling off is considered as follows. Oxidation of the benzene in the fuel flow is promoted by the increase in the added amount of oxygen, and the components of C3 and C4 are produced. If these arrive at the elevated-temperature zone, oppositely, benzene is produced via C3 and C4. Consequently, a difference will be lost in the total amount of PAH generated.

The effect of non-luminous thermal radiation on the combustion of a suspended methanol droplet in a low temperature (300 K) and low pressure (1 atm) environment is discussed in detail. Numerical results are obtained using a predictive, transient, two-phase, axisymmetric numerical model that includes surface tension effects. Radiation is modeled using the optically thin approximation with the product species CO2 and H2O as the radiating species. Results for combustion in a nearly quiescent atmosphere (initial Reynolds number 0.01) and initial droplet diameters in the range of 0.43 mm to 3 mm are presented. In agreement with the reported literature, it is shown that the effect of flame radiation is negligible when the initial droplet diameter is less than approximately 1 mm and becomes increasingly important for larger droplets. As a result, the average evaporation constant decreases with the initial droplet diameter. Radiation and surface tension combined as well as surface tension alone have a significant...

Ignition and combustion characteristics of diesel spray impinged on an extruded wall, which was a top of cylindrical column, were experimentally investigated. Appearance positions of OH and C2 radical luminosities and luminous flame kernel were stereoscopically observed using a two-way fiber optical system. The extruded column was fixed in a high temperature and high pressure combustion chamber. A diesel spray was impinged vertically to the extruded wall. Effects of cylindrical column diameter (D=5, 10, 15mm, ∞ (flat wall)) on ignition and combustion characteristics were investigated. As the result, appearance position of OH luminosity was observed near the spray tip except for D=∞ (flat wall). The maximum volume of luminous flame impinged on the extruded wall was strongly influenced by the diameter of the column. In a condition of low-ambient temperature, the maximum volume of blue flame on the extruded column of 10 mm diameter was larger than those on other columns.