Three-color Pyrometry Research Articles

Role of Surface Tension on Heat Feedback and Power from Energetic Composites.

Heat feedback to the unburned reaction interface is an important controlling factor of the velocity of the reaction front and power delivery. In this paper, we investigate the effect of agglomerate surface tension and its relationship to surface residence time and heat feedback on the combustion characteristics by Si addition to an Al/KClO4 composite. Macroscopic imaging demonstrates a significant increase in burn rate with the addition of Si despite the fact that Si/KClO4 has a slightly lower energy density than Al/KClO4. Microscopic imaging coupled with three-color pyrometry reveals that molten liquid forms and evolves into spherical droplets on the burning surface, which are subsequently ejected from the surface. We find that the addition of Si results in a small increase in droplet size and a negligible impact on droplet temperature. However, the droplet formation rate on the surface is slower, leading to a significantly longer surface residence time. This leads to enhanced conductive heat feedback to the unburnt materials, thereby increasing the burn rate and energy release rate. We attribute the decreased droplet growth rate to the lowered surface tension of the liquid mixture with Si addition. This study highlights the crucial role of agglomerate physical property (e.g., surface tension) in influencing the combustion behavior of energetic composites.

Particle-resolved hyperspectral pyrometry of metal particles

We present temperature histories of individual combusting metal particles using hyperspectral pyrometry. This method gives an increase in accuracy over traditionally used two- or three-color pyrometry, while maintaining temporal and spatial resolution. Temperatures can be determined between 1800 to >3000 K with a precision of typically <1%. It is shown that the maximum temperature of the burning iron particles increases from 2760 K to 2840 K with an increasing mean particle size from 32 to 54 μm in air with 21 % oxygen. The relatively high temperatures and its dependence on particle size are possibly related to flow field properties of the current experimental setup. Opportunities for this method, as well as future work, are discussed.Novelty and Significance Statement: In this article, a method to use a CCD camera and spectrograph as a hyperspectral detector, gaining a wavelength dimension while maintaining two spatial dimensions, is demonstrated and validated. This method is applied to measure the temperature of iron particles, a carbon free and circular energy carrier. This method is then used to prove that there is a particle size dependence on the maximum temperature, an open question which is often disputed in literature. This work will also add a dataset that can be used for the validation of numerical models. There are only two such datasets available for iron at this moment. Our analysis suggests that the heat release of iron is dependent on the slip velocity, possibly due to a circulating flow inside of the particle.

Three-color pyrometry study of radiation and gas dynamic processes in a heterogeneous plasma-vortex ring

The paper presents the results of a study of a heterogeneous plasma vortex ring using the three-color image pyrometry technique. The vortex ring is formed during deceleration of a supersonic pulsed aluminum plasma jet in atmospheric air. A technique for calibrating a digital camera and a post-processing process for images are proposed that makes it possible to visualize the internal structure of the vortex. As a result of the study, the effect of vortex separation of condensed and gas flow components was experimentally demonstrated.

Experimental and theoretical analyses on ignition and surface temperature of dispersed coal particles in O2/N2 and O2/CO2 ambients

An experimental and theoretical investigation is conducted on the combustion of pulverized coal particle streams in either conventional or oxy-fuel conditions. The laboratory setup consisted of a Hencken flat-flame burner operated at the same temperatures of 1200, 1500 and 1800K in N2 and CO2 environments. The visible light detection technique and the calibrated three-color pyrometry are separately used to characterize the ignition delay time and the luminous char surface temperature. First, experimental results indicate that the ignition delay time is relatively longer in CO2 environment in heterogeneous mode or even heterogeneous-controlled joint mode. The ignition delay is obviously enhanced in O2/CO2 ambient when the volatile-flame controlled joint mode is prevalent. The model predictions of particle temperature history further reveal that the extra gas flame radiation of CO2 and the declined difference between the thermal conductivity are two main reasons. Then, during char combustion, the measurement shows that the char surface temperature is lower in an O2/CO2 environment than in an O2/N2 environment. It is noted that the temperature difference between N2 and CO2 environments enlarges with the increasing ambient temperature. In contrast to conventional O2/N2 conditions, the lower O2 diffusivity in CO2, the endothermic carbon reaction with CO2 and the higher heat capacity of CO2, which are all temperature dependent, synergistically contribute to the enlargement of char surface temperature differences at high ambient temperature.

Response of Soot Temperature to Unsteady Inlet Airflow Under Modulated Condition and Naturally Occurring Combustion Dynamics

The response of soot temperature to unsteady inlet airflow is characterized using pyrometry. The unsteady inlet airflow is achieved by either modulating inlet air or naturally occurring unstable flame, running on a jet fuel at fuel-rich conditions. The inlet air is modulated by a siren device running at frequencies between 150 and 250 Hz and up to 60% of modulation level (u'/um) is achieved. Also, the combustor can be run naturally unstable at the same inlet operating condition by changing the combustor length. For pyrometry, the emission from whole flame at 660 nm, 730 nm, and 800 nm is recorded and the three-color pyrometry is used to measure soot temperature. The effect of nonisothermal distribution of soot in flame on the measured temperature is also considered. The level of overall temperature fluctuation under inlet flow modulation (Trms/Tmean) is about an order of magnitude lower than that of flame emission fluctuation (Irms/Imean). Under naturally occurring instability, the measured soot temperature is in phase with the pressure measured in the combustor, indicating that the measured soot temperature can be used as a quantity related to combustion dynamics for fuel-rich sooty flames.

The heating and acceleration dynamics of Al2O3 particles in the axisymmetric heterogeneous flow emanating from a plasma torch with inter-electrode inserts

Measured data on the temperature and velocity of Al2O3 particles of size fraction 34±6 μm in the jet emanating from a DC plasma torch with inter-electrode inserts under conditions of axisymmetric heterogeneous flow are reported. The velocity and temperature of individual particles were measured using a laser-optical diagnostic complex, which was a combination of a bifocal laser anemometer and a pyrometer based on a compact spectrometer. For measuring the temperature of individual particles in the particle-laden plasma jet, three-color pyrometry was used. The obtained data on the characteristics of particles in the jet emanating from the plasma spray torch with inter-electrode inserts equipped with a unit for radial-annular injection of powder into the plasma jet show that the implemented conditions for processing powder materials allow reaching a high homogeneity of the aggregate state of particles in the jet flow (∼ 100 % of melted particles).

Single particle ignition and combustion of anthracite, semi-anthracite and bituminous coals in air and simulated oxy-fuel conditions

A fundamental investigation has been conducted on the combustion behavior of single particles (75–150μm) of four coals of different ranks: anthracite, semi-anthracite, medium-volatile bituminous and high-volatile bituminous. A laboratory-scale transparent laminar-flow drop-tube furnace, electrically-heated to 1400K, was used to burn the coals. The experiments were performed in different combustion atmospheres: air (21%O2/79%N2) and four simulated dry oxy-fuel conditions: 21%O2/79%CO2, 30%O2/70%CO2, 35%O2/65%CO2 and 50%O2/50%CO2. The ignition and combustion of single particles was observed by means of three-color pyrometry and high-speed high-resolution cinematography to obtain temperature–time histories and record combustion behaviors. On the basis of the observations made with these techniques, a comprehensive examination of the ignition and combustion behaviors of these fuels was achieved. Higher rank coals (anthracite and semi-anthracite) ignited heterogeneously on the particle surface, whereas the bituminous coal particles ignited homogeneously in the gas phase. Moreover, deduced ignition temperatures increased with increasing coal rank and decreased with increasing oxygen concentrations. Strikingly disparate combustion behaviors were observed depending on the coal rank. The combustion of bituminous coal particles took place in two phases. First, volatiles evolved, ignited and burned in luminous enveloping flames. Upon extinction of these flames, the char residues ignited and burned. In contrast, the higher rank coal particles ignited and burned heterogeneously. The replacement of the background N2 gas of air with CO2 (i.e., changing from air to an oxy-fuel atmosphere) at the same oxygen mole fraction impaired the intensity of combustion. It reduced the combustion temperatures and lengthened the burnout times of the particles. Increasing the oxygen mole fraction in CO2 to 30–35% restored the intensity of combustion to that of air for all the coals studied. Volatile flame burnout times increased linearly with the volatile matter content in the coal in both air and all oxygen mole fractions in CO2. On the other hand, char burnout times increased linearly or quadratically versus carbon content in the coal, depending on the oxygen mole fraction in the background gas.

Combustion behavior in air of single particles from three different coal ranks and from sugarcane bagasse

Comparative combustion studies were performed on particles of pulverized coal samples from three different ranks: a high-volatile bituminous coal, a sub-bituminous coal, and two lignite coals. The study was augmented to include observations on burning pulverized woody biomass residues, in the form of sugarcane bagasse. Fuel particles, in the range of 75–90 μm, were injected in a bench-scale, transparent drop-tube furnace, electrically-heated to 1400 K, where they experienced high-heating rates, ignited and burned. The combustion of individual particles in air was observed with three-color pyrometry and high-speed high-resolution cinematography to obtain temperature–time–size histories. Based on combined observations from these techniques, in conjunction to morphological examinations of particles, a comprehensive understanding of the combustion behaviors of these fuels was developed. Observed differences among the coals have been striking. Upon pyrolysis, the bituminous coal chars experienced the phenomena of softening, melting, swelling and formation of large blowholes through which volatile matter escaped. Combustion of the volatile matter was sooty and very luminous with large co-tails forming in the wake of the particle trajectories. Only after the volatile matter flames extinguished, the char combustion commenced and was also very luminous. In contrast, upon pyrolysis, lignite coals became fragile and experienced extensive fragmentation, immediately followed by ignition of the char fragments (numbering in the order of 10–100, depending on the origin of the lignite coal) spread apart into a relatively large volume. As no separate volatile matter combustion period was evident, it is likely that volatiles burned on the surface of the chars. The combustion of the sub-bituminous coal was also different. Most particles experienced limited fragmentation, upon pyrolysis, to several char fragments, with or without the presence of brief and low-luminosity volatile flames; other particles did not fragment and directly proceeded to char combustion. Finally combustion of bagasse was once again very distinctive. Upon pyrolysis, long-lasting, low-luminosity, nearly-transparent spherical flames formed around slowly-settling devolatilizing particles. They were followed by bright, short-lived combustion of the chars. Both volatiles and chars experienced shrinking core mode of burning. For all fuels, flame and char temperature profiles were deduced from pyrometric data and burnout times were measured. Combustion rates were calculated from luminous carbon disappearance measurements, and were compared with predictions based on published kinetic expressions.

Temperature Measurements of Diesel Fuel Combustion With Multicolor Pyrometry

Optical diagnostics techniques to measure diesel combustion flame temperatures are useful for evaluation and control of combustion processes. In this paper, diesel combustion flame temperatures are measured based on a multicolor pyrometry method respectively adopting an optical fiber spectrometer and a color charge coupled device (CCD). The intensity ratios for various wavelengths/wavebands are utilized as the analytical variables to deduce the temperatures to avoid the need to calibrate each system for the specific geometry conditions. The measured multicolor data can determine the temperature T and the soot factor KL. Extra data collected at many wavelengths is used to reduce the noise and random fluctuations in the measurements. To improve the solving precision, a data-processing method based on the least-squares technique is proposed to fit the data for approximate solutions. Verification experiments using the multicolor pyrometry were conducted in a 54–120 kW test furnace with diesel fuel. Data for 16 wavelengths detected by a fiber optic spectrometer from a diesel flame is analyzed to determine how to choose a suitable combination of three wavelengths for three-color pyrometry. The CCD-based three-color measurements, which would be much more practical in field measurements, are compared with the spectrometer-based results.

Evidence for the transition from the diffusion-limit in aluminum particle combustion

This work presents experimental evidence that the transition from gas-phase diffusion-limited combustion for aluminum particles begins to occur at a particle size of 10 μm at a pressure of 8.5 atm. Measurements of the particle temperature by AlO spectroscopy and three-color pyrometry indicate that the peak temperature surrounding a burning particle approaches the aluminum boiling temperature as particle size is decreased to 10 μm when oxygen is the oxidizer. This reduction indicates that reactions are occurring at or near the particle surface rather than in a detached diffusion flame. When CO 2 is the oxidizer, the combustion temperatures remain near the aluminum boiling temperature for particles as large as 40 μm, indicating that the flame is consistently near the surface throughout this size range. Burn time measurements of 10 and 2.8 μm powders indicate that burn time is roughly proportional to particle diameter to the first power. The burn rates of micron- and nano-particles also show strong pressure dependence. These measurements all indicate that the combustion has deviated from the vapor-phase diffusion limit, and that surface or near-surface processes are beginning to affect the rate of burning. Such processes would have to be included in combustion models in order to accurately predict burning characteristics for aluminum with diameter less than 10 μm.

TEMPERATURE MEASUREMENTS OF ALUMINUM PARTICLES BURNING IN CARBON DIOXIDE

ABSTRACT Aluminum particles are often added to solid propellants and explosives to enhance energy release, where the metal typically burns in mixtures of water vapor and carbon dioxide. In this study, information about the temperature field surrounding 5–10-µm aluminum particles burning at elevated pressure in CO2/Ar and CO2/N2 is obtained via emission and absorption spectroscopy of AlO and three-color pyrometry. Spectra of AlO fit well to Boltzmann distributions and to similar temperatures of 3000 to 3200 K in both excited and ground states, suggesting that both states are in equilibrium. Pyrometry yields slightly lower temperatures, in the range of 3000 to 3100 K. Both temperature ranges are well below the adiabatic flame temperature and the boiling point of alumina but are much closer to the predicted aluminum boiling temperature at elevated pressure. These results are consistent with a flame structure with combustion occurring at or very near the particle surface.

Preparation and characterization of energetic Al-Mg mechanical alloy powders

Metals such as Al and Mg have high combustion enthalpies and they are widely used as additives in energetic materials for propellants, explosives, and pyrotechnics. However, long ignition delays and slow combustion kinetics limit their current applications. An approach suggested in this work is to design new metal-based materials in which pre-determined phase changes will occur and trigger ignition at a desired temperature and also accelerate the rate of heat release during combustion. As a first step, metastable solid solutions of Mg in Al (10–50% of Mg) have been produced by mechanical alloying. The ignition temperatures of the produced alloys in air were determined using digital imaging and three-color pyrometry of the electrically heated filaments coated with different alloy powders. Combustion of mechanical alloys in air was studied using a laminar, premixed flame aerosol burner. The ignition temperatures were around 1,000 K, much lower than the pure aluminum ignition temperature of about 2,300 K. The steady flames of mechanical alloy powders were produced at lower equivalence ratios and had higher propagation velocities than similar pure aluminum powder flames. Phase compositions of the combustion products were determined using X-ray diffraction. In addition to Al 2O 3 and MgO, significant amounts of Al 2MgO 4 were found in experiments.

Observations on the Combustion of Polymers (Plastics): From Single Particles to Groups of Particles

This investigation aimed at examining the combustion behavior of polymer particles in groups and at comparing it to that of single particles. Combustion observations were subsequently related to measurements on emissions of organic pollutants, such as PAHs and soot, which were previously measured in this laboratory under similar conditions. The following plastics (polymers), commonly found in municipal waste streams, were burned in powder form: poly(styrene) (PS), poly(ethylene) (PE), polyvinyl chloride) PVC, and poly(methyl methacry-late) (PMMA). Particles, 125-212um, were introduced to a drop-tube furnace, at steady-flow conditions, and burned in cylindrical streams (clouds), in 1250 K air. The particle mass loading in the furnace was varied to achieve either dilute clouds, where isolated (single) particle combustion occurred, or dense clouds where interactive particle combustion look place. Information on the combustion characteristics and flame temperatures of the particles was obtained by cinematography and three-color pyrometry. Results revealed striking differences in the combustion behavior of the four polymers burning in particle clouds. PVC particles burned with bright individual flames, resisting the formation of group flames, even at high particle number densities. In dense clouds, envelope flame diameters were generally smaller and their durations were shorter than in single particle combustion. Poly(ethylene) particles in very dilute clouds burned individually, surrounded by low-sooting, faint envelope flames. When particles burned in close proximity to each other, they abruptly exhibited localized flash-vaporization and formed highly-luminous, large, group flames, “puffs”. At high particle number densities the entire stream was engulfed in a cooler flame containing finely-dispersed soot. Poly(styrene) particles burned mostly as discrete entities at relatively low flowrates, and the progression to partial group flames was gradual as the particle mass loading increased. PS particles consistently formed flames with long wakes which, upon extinction formed strings of highly-agglomerated soot. Flame temperatures were lower in group combustion than in individual particle combustion. While single particle envelope flames were previously recorded to be in the neighborhood of 2000 K, those of group flames were found to be only a few hundred degrees above the gas temperature, at globally fuel-rich conditions.

Measurements of particle flame temperatures using three-color optical pyrometry

A three-color near-infrared optical pyrometer, with wavelengths centered at 998, 810, and 640 nm, was used to monitor the combustion of polymer particles. Individual spherical poly(styrene) particles, 47–355 μm in diameter, burned in air at 1050–1400 K gas temperatures, surrounded by sooting, diffusion envelope flames. The pyrometric results were interpreted in view of two models for soot radiation: (a) A conventional model, which assumes that the flame is optically thin and isothermal and thus, the spectral emissivity is inversely proportional to the wavelength. With this method the calculated flame temperatures are averages, biased to areas with high temperature and/or soot concentration. (b) An alternative model, in which the envelope flames are assumed to be again optically thin but nonisothermal in the radial direction. The theoretical development of the latter model is included herein. For nonisothermal flames the spectral emissivity was shown to be nearly independent of the wavelength. This model in conjunction with three-color pyrometry may provide a way of estimating the highest temperature of soot in the flame, as well as the temperature gradient across the flame. Temperatures calculated this way were higher by 200–230 K than those calculated using the conventional model. Experimental results suggest that the agreement among the three individual temperatures obtained from three-color pyrometry depends on which of the above models for soot radiation is used. Based on the agreement between temperatures, the degree of isothermality of the flame may be determined and thus, indications about the controlling processes during combustion (oxygen diffusion or volatile combustion) may be obtained. However, additional work is needed, involving simpler, one-dimensional flame configurations to confirm this model.

Combustion Behavior of Poly (Styrene) Particles of Various Degrees of Crosslinking and Styrene Monomer Droplets

Abstract This is a study on the combustion characteristics of poly (styrene) (PS) particles either plain, or with various degrees of crosslinking in the range of 5-25%. Effects of crosslinking were examined by producing and burningindividual particles in the size range of 47—63 /an, while effects of particle size were examined by burning particles of the same degree of crosslinking (8%) in the size range of 47-350 jira. Measurements during combustion of styrene monomer droplets were also conducted. Combustion took place in a laminar, drop tube furnace at thermochemical conditions pertinent to municipal waste incinerators, i.e., in air at high heating rates and gas temperatures ranging from 1050 to 1400 K. Combustion of single particles/drops was monitored by three-color pyrometry and high-speed cinematography, to obtain temporal information on the radiant intensity, size, temperature and soot content of the flame, as well as the total burnout time of the particles. Plain (uncrosslinked) PS particles, in t...

The color temperature of illuminating gas flames

A three-color near-infrared optical pyrometer, with wavelengths centered at 998, 810, and 640 nm, was used to monitor the combustion of polymer particles. Individual spherical poly(styrene) particles, 47–355 μm in diameter, burned in air at 1050–1400 K gas temperatures, surrounded by sooting, diffusion envelope flames. The pyrometric results were interpreted in view of two models for soot radiation: (a) A conventional model, which assumes that the flame is optically thin and isothermal and thus, the spectral emissivity is inversely proportional to the wavelength. With this method the calculated flame temperatures are averages, biased to areas with high temperature and/or soot concentration. (b) An alternative model, in which the envelope flames are assumed to be again optically thin but nonisothermal in the radial direction. The theoretical development of the latter model is included herein. For nonisothermal flames the spectral emissivity was shown to be nearly independent of the wavelength. This model in conjunction with three-color pyrometry may provide a way of estimating the highest temperature of soot in the flame, as well as the temperature gradient across the flame. Temperatures calculated this way were higher by 200–230 K than those calculated using the conventional model. Experimental results suggest that the agreement among the three individual temperatures obtained from three-color pyrometry depends on which of the above models for soot radiation is used. Based on the agreement between temperatures, the degree of isothermality of the flame may be determined and thus, indications about the controlling processes during combustion (oxygen diffusion or volatile combustion) may be obtained. However, additional work is needed, involving simpler, one-dimensional flame configurations to confirm this model.