Two-colour Pyrometry Research Articles

On the sudden reversal of soot formation by oxygen addition in DME flames

Dimethyl ether (DME) is a non-toxic and renewable fuel known for its soot emissions reduction tendencies. In laminar co-flow DME diffusion flames, adding oxygen to the fuel stream increases the sooting tendency until a critical point is reached, at which point the trend suddenly reverses. This work unravels the mechanisms behind this reversal process, and characterizes their contribution to controlling soot production. A series of experimental measurements using diffuse-light line-of-sight attenuation and two-colour pyrometry were performed to measure soot volume fraction and soot temperature considering a fixed mass flow rate of DME and variable addition of oxygen. Soot volume fraction increases from 0.095 ppm in the pure DME flame to 0.32 ppm when the added oxygen concentration reaches 33%. When the oxygen concentration is slightly increased to 35%, soot volume fraction is reduced by 60%. To explain the reasons behind the reversal, a series of numerical simulations were performed, which successfully demonstrated the same trend. Results show that the chemical effects of adding oxygen to the fuel stream are exceedingly more important than the thermal and dilution effects. It was found that the reversal occurred when nearly all DME disassociated before exiting the fuel tube, indicating a sudden transition from a partially premixed DME flame, to one which primarily burns C1 fuel fragments. An analysis of soot formation and oxidation rates showed that near the reversal, soot inception is the least affected process; furthermore, soot precursor availability is not significantly affected in magnitude, rather they appear further upstream. It is concluded that the favourable conditions for rapid DME decomposition into soot precursors enhances soot inception while depleting the necessary species for further soot mass growth, dramatically reducing soot concentration.

Particle temperature and composition measurements in the ignition phase of single coal particles and particle groups under conventional and oxy-fuel atmospheres

The transition from single to group particle combustion is experimentally investigated in a laminar flow reactor. Optical techniques including two-colour pyrometry and a sampling probe provide data on surface temperature and composition of high-volatile bituminous coal particles during ignition, volatile combustion, and char burnout. Adjustable particle number densities (PND) permit studying the transition from single to group particle combustion. An optical imaging pyrometer provides simultaneously acquired data on the soot particles during the volatile combustion and the particle temperature, particle size and position in the 3D particle streak. These parameters are investigated as a function of the PND, which is used to differentiate between single and group particle combustion. Additionally, particle sampling provides particle composition data, which together with the particle temperatures extends a previously published data set from the same reactor. The results show a direct relation between the particle temperature during char burnout and the PND. As particles in group combustion absorb a significant amount of thermal energy from the surrounding atmosphere and reduce the oxygen content in the group, the measurements prove a lower temperature for particles that burn in group compared to single particle. Furthermore, sampling at several residence times indicates a notable difference in the burnout process between single particle (SP) and group particle (GP) combustion. While the effect of different atmospheres impacts the results of SP combustion, GP combustion is significantly less affected. The results of the current study are jointly discussed with previously acquired data of the same burner for a thorough interpretation.

Robust two-colour pyrometry uncertainty analysis to acquire spatially-resolved measurements

Two-colour pyrometry (2CP) has been used over several decades to study engine-relevant combustion processes, but results are generally regarded as qualitative or semi-quantitative. In many current 2CP systems, large measurement errors are introduced by parallax because the two measured wavelengths are not from the same line of sight. This work presents a spatially-resolved 2CP system with the objective of quantifying and reducing measurement uncertainty. An optical setup that eliminates parallax in 2CP is used together with pixel-by-pixel calibration of the camera sensor to increase measurement accuracy. Primary uncertainty terms are identified, and an error propagation analysis is performed to compute uncertainties in the final results of soot temperature, soot concentration parameter, KL, and soot mass. These methodologies are applied to investigate an auto-igniting fuel spray in a constant pressure flow rig at diesel-like conditions of high ambient pressure and temperature. Results show bias uncertainty of around 200 K (≈10%) for temperature and about 40%–60% for KL. High uncertainty was found to occur on the diffusion flame front where both optical thickness and soot concentrations are small. However, these uncertain measurement zones with relatively low soot concentrations contribute minimally to the total soot mass present in the reacting jet during the temporal evolution of the flame.

Evaluation of in-cylinder endoscopic two-colour soot pyrometry of diesel combustion

Flame temperature and soot concentration imaging was performed using endoscopic two-colour (2C) soot pyrometry to investigate the characteristics of in-cylinder diesel engine combustion processes and provide validation data for engine simulation and design. To appropriately interpret the 2C image results, this paper focuses on the uncertainty and challenges of the technique, the line-of-sight nature of the measurement and presents comparable information for validation exercises. A line-of-sight flame light intensity model was created to explore how the temperature T and soot concentration KL measured by the 2C technique can relate to non-uniform flame temperature and soot distributions. It was found that T and KL measured from the 2C technique were likely to relate differently to the actual distribution depending on where in the flame the measurement was taken and on assumptions made about the flame spatial structure. Assessment has been made of the range of the maximum and minimum flame temperatures (assumed to correspond to reaction zone temperature and flame centreline respectively) that are consistent with measured temperature T and soot concentration KL. The analysis of uncertainties, flame temperature and soot distribution along the line-of-sight, and image averaging allows for better quantitative comparison of 2C soot pyrometry images to CFD simulation, which increases confidence in simulation-driven engine development.

The improvement design of two-colour pyrometry for pulsed laser deposition system

In this work, we develop a digital pyrometry with an automatic control system. A two-colour pyrometer is applied to measure the substrate surface temperature in the pulsed laser dep-osition (PLD) system. A digital computer and two-colour photodetector are coupled to obtain the substrate surface temperature and the output signal from the respective detectors with high accuracy. These sets enable us to determine the substrate temperature during the process of deposition remotely and potentially allow us to develop control routines for maintaining a constant temperature throughout the process. The designed system has a broad spectral response range from 0.32 to 1.7μm and achieves a deficient full-scale temperature error of <1.8°C. Factors affecting the temperature measurements are studied to identify the sensor limitations, such as possible damage on the two-colour photodetectors, spectral losses, responsivity, and the distance between the photodetector end to the substrate. Finally, this pyrometric system is applied in the PLD process to maintain a stable substrate surface temperature.

Experimental study of the effect of C8 oxygenates on sooting processes in high pressure spray flames

Oxygenated compounds have the ability to reduce soot emissions and to improve the combustion efficiency in engines. Most studies have focused on the soot reduction potential of shorter carbon chain oxygenates, whilst longer carbon chain oxygenates are still relatively unexplored. In this work, the soot reduction potential of long carbon chain oxygenates having similar thermo-physical and chemical properties to those of diesel have been studied, viz., 2-octanone (ketone), 1-octanol (alcohol), hexyl acetate (ester) and octanal (aldehyde). These oxygenates were injected at high pressure into a constant volume chamber maintained at high ambient temperature conditions, and their spray flames were investigated using a high-speed, two-colour pyrometry system. It was found that for the same injected fuel mass, the oxygenates reduced the overall soot when compared to diesel. Small differences in sooting tendencies were observed between different oxygenated moieties but these were smaller than those relative to diesel. The absence of aromatic groups and the presence of oxygen directly bonded to carbon atoms seemed to have a larger effect on soot reduction than the oxygenated functional group. The oxygenates altered the local oxygen equivalence ratio in the spray, influencing the soot formation and its distribution in the flame. Under the high pressure conditions studied, the average sooting tendency of the long carbon-chain oxygenates studied increased in the order of: ester< aldehyde ~ alcohol<ketone.

Study of the anode energy in gas metal arc welding

Recent research of gas metal arc welding (GMAW) has proved that the sheath voltage dominates the total voltage fall in the current circuit and provides the most energy to the wire and the weld pool. In this paper, the energy delivered to the wire anode is studied experimentally for a typical pulsed GMAW process in the one drop per pulse mode to provide a more complete picture of the energy balance of the process. Welding of mild steel under Ar with 2.5% CO2 with different peak currents from 350 to 650 A and constant wire feed speed of 4 m min−1 as well as constant arc height are considered. The processes were studied with non-intrusive optical methods, i.e. without strong modifications. The droplet energy was determined by high-speed two-colour pyrometry and an analysis of the droplet surface temperature and geometry. In addition, Joule heating in the wire anode was also estimated from surface temperature measurements. It was found that the averaged droplet energy is almost independent of the peak current and approximately one-third of the total electric energy. A voltage equivalent of the energy delivered to the wire anode from the plasma was defined. Substracting the work function of iron, the value of this voltage is in the range of 3.25± 0.43 V and increases by 0.5 V when the current increases from 350 to 650 A. The sum of the cathode and anode sheath voltages have been estimated to be in the range between 18 and 23 V in preceding works.

Pyrometric imaging of soot processes in a pilot ignited direct injected natural gas engine

This work assesses the soot formation and oxidation processes in a high-pressure direct injection natural gas engine using simultaneous high-speed two-colour pyrometry and OH* chemiluminescence imaging. A parametric investigation considers the effects of fuel injection pressure, injection duration and relative pilot and natural gas injection timings. A typical combustion event consists of natural gas being ignited by diesel pilot combustion products, followed by partially premixed combustion of natural gas near the piston bowl wall. Soot is formed as natural gas is injected into this reaction zone. A toroidal soot cloud forms and grows towards the centre of the piston bowl. For the range of operating conditions tested, the peak apparent heat release rate and the onset of detectable soot were correlated, as were the timing of the peak soot fraction and end of natural gas injection. The latter indicates that the soot formation and transition to a net oxidation process are strongly influenced by the injection process, similar to diesel engines. Changes in the relative timing of the diesel pilot and natural gas injections influenced natural gas premixing times, with increased premixing leading to a higher peak apparent heat release rate and lower peak soot fraction. The injection pressure affected the peak soot fraction though enhanced oxidation was expected to ultimately reduce the engine-out soot for higher injection pressures. Based on this parametric investigation, an existing conceptual model of pilot ignited direct injected natural gas combustion is extended to also describe the soot formation and oxidation processes.

The impact of cyclic fuels on the formation and structure of soot

This paper investigates the impact of cyclic fuels on the nanostructure, nucleation and overall production of soot in an n-heptane (C7H16) laminar coflow diffusion flame. The fuels selected to dope the n-heptane flame are cyclopentene (C5H8), cyclohexene (C6H10) and methylcyclohexane (C7H14). These fuels were chosen for their differences in their structure and sooting tendency. The flame structure was studied with Differential Mobility Spectrometry (DMS) for particle size distribution determination, two-colour ratio pyrometry to calculate the soot volume fraction and soot temperature. The soot nanostructure was investigated using Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM). The addition of cyclic fuels was found to promote the formation of soot nanoparticles earlier in flames. In addition, the soot volume fraction was increased significantly by the addition of the cyclic fuels, especially by the addition of cyclopentene. The addition of 20% of cyclopentene increased the soot volume fraction by a factor of 2. HRTEM results suggest a significant influence of cyclopentene on the soot nanostructure; cyclopentene addition promotes the incorporation of five-membered rings (pentagonal rings) leading to highly curved fringes. This suggests cyclopentene could be used as a fuel to promote curvature in different carbonaceous materials to modify their properties.

Effect of n-butanol addition on the burning rate and soot characteristics during combustion of single droplets of diesel–biodiesel blends

The effect of n-butanol addition on the burning rate and soot characteristics of single droplets of diesel–biodiesel blends was systematically studied. A single droplet of a diesel–biodiesel-n-butanol mixture of approximately 1 mm in diameter was suspended on a silicon carbide fibre and burned in air at 973 K in an electrically heated tube furnace. The droplet size during combustion was continuously recorded using a colour CCD camera and the burning rate based on the d2 law was determined. The soot intensity as indicated by theKLfactor and flame temperature were estimated from the CCD camera spectrum data using a two-colour pyrometry technique. Soot particles were also sampled using a thermophoreric deposition probe and characterised using a high resolution transmission electron microscope (TEM) for particle size and morphology. The results showed that n-butanol addition increased the burning rate of diesel–biodiesel blends, reduced the KL factor, being more profound with increasing n-butanol addition. The temperatures of the flames surrounding the fuel droplets with and without n-butanol addition showed little differences. The mean diameter of primary soot particles decreased and the particle size distribution shifted towards a smaller size range with n-butanol addition. At low levels of n-butanol addition the soot particles consisted of predominantly amorphous carbon, however, became progressively more graphitic in the inner structure with increasing n-butanol addition.

Experimental study on the combustion characteristics of high-pressure octanal spray flames

In view of the stringent emission regulations, oxygenated fuels are growing in popularity owing to their potential to reduce emissions in diesel vehicles and off-road applications. It is important to explore the combustion characteristics of oxygenates that can match the energy content and density of diesel whilst reducing the formation of pollutants. Furthermore, having similar thermo-physical properties to diesel could yield them as drop-in fuel candidates. In this work, the combustion characteristics of a long carbon chain oxygenate, octanal (C8H16O), and diesel, were investigated using the two-colour pyrometry method to study the temperature and soot distributions in the spray flames. This investigation was done by injecting the fuels at high pressures in an optically accessible constant volume chamber under a diesel engine-like environment, at high ambient pressures and temperatures. The soot lift-off length for octanal was longer than for diesel, which reduced the amount of time available for soot to grow. For the same injected mass of diesel and octanal, the combustion of octanal was faster due to the presence of fuel-bound oxygen. The two-colour pyrometry results revealed that the spatial distribution of soot for octanal was slightly different from that of diesel. The chemical structure of octanal, consisting of a long chain alkane with an aldehyde functional group at the end and without the presence of any aromatic structures, hindered the soot formation significantly throughout the combustion process when compared to diesel.

Emittance of typical ash minerals: Influence of particle size and mixtures of carbonates and sulfates

The current paper presents measurements of the normal spectral and total emittances of mixtures of different powdery material, all typical constituents of coal ashes. Measurement of sample surface temperature, a key parameter for the calculation of emittance, is carried out by thermocouples and by two-colour pyrometry. Two size fractions of 0 to 32 µm and 125 to 160 µm were examined. The influence of particle size on emittance is investigated by measurement of mixtures of different size fractions of SiO2. In addition, mixtures of CaCO3, CaSO4, K2SO4, Fe2O3 are examined. Measurements were made from 500 up to 1000 °C. The spectra were recorded from 1.6 µm to 12 µm, the major wavelength range for radiative heat transfer in boilers. The results show that the larger the particle size, the higher the emittance. In carbonate/sulfate mixtures absorption bands are visible in the spectra. Fe2O3 shows no absorption bands and seems to dominate the spectra when mixed with carbonates or sulfates.

Effect of ambient temperature on the ignition and combustion process of single aluminium particles

This experimental work aims to examine the effects of ambient temperature on the ignition and combustion process of single aluminium particles (40–170 μm). Ambient temperatures considerably influence the particle ignition delay time, but the influence on the combustion time is limited. Particle ignition probability is very sensitive to the ambient temperature. The particle ignition probability can be improved by approximately 6.7 times by increasing the ambient temperature by approximately 300 K. As the diameter increases, the ignition probability increases firstly and then decreases in the experimental conditions of Cases 02–06. The diameter ranges for the particle ignition probability of >90% in the experimental conditions of Cases 03–06 can be extended by the high ambient temperature. Moreover, the aluminium particles with high unevenness level can be ignited easily, which should be resulted from the local flame near the raised part. The characteristic particle temperature is measured using the method of two-colour pyrometry. Experimental results show that the aluminium particle in these experimental conditions can barely burn in a pure diffusion-limited regime. The structure and components of the oxide film on the unignited particle show that the oxide film fracture is a key process for particle ignition.

Investigations of the long-term effects of LII on soot and bath gas

A combination of high-repetition rate imaging, laser extinction measurements, two-colour soot pyrometry imaging, and high-resolution transmission electron microscopy of thermophoretically sampled soot is used to investigate the long-term and permanent effects of rapid heating of in-flame soot during laser-induced incandescence (LII). Experiments are carried out on a laminar non-premixed co-annular ethylene/air flame with various laser fluences. The high-repetition rate images clearly show that the heated and the neighbouring laser-border zones undergo a permanent transformation after the laser pulse, and advect vertically with the flow while the permanent marking is preserved. The soot volume fraction at the heated zone reduces due to the sublimation of soot and the subsequent enhanced oxidation. At the laser-border zones, however, optical thickness increases that may be due to thermophoretic forces drawing hot particles towards relatively cooler zones and the rapid compression of the bath gas induced by ...

Study of thermodynamic properties of carbon nanoparticles by the laser heating method

A new experimental approach to the analysis of thermodynamic properties of amorphous carbon nanoparticles synthesized via hydrocarbon pyrolysis behind shock waves is discussed. The proposed approach is based on the analysis of thermal radiation of nanoparticles heated by a laser pulse. The sublimation temperature of the carbon nanoparticles might be determined by the two-colour pyrometry; their sizes, by laserinduced incandescence; and the volume fraction of the sublimated substance, by the laser extinction method. The sublimation temperature depends on both the particle size and the temperature conditions of their formation. The value of surface energy for amorphous carbon nanoparticles was estimated.

Formation of different arc-anode attachment modes and their effect on temperature fluctuation for carbon nanomaterial production in DC arc discharge

Diffuse and multiple arc-anode attachment modes were observed when using DC arc discharge to prepare carbon nanomaterials. The effects of arc current and electrode gap distance on the formation of different attachment modes were investigated. The formation of different attachment modes is explained by the competition of the cathode jet and the anode jet. During the arc discharge, the surface temperature of the carbon electrode was measured by two-colour pyrometry combined with a high-speed camera employing appropriate band-pass filters. The relationship between the arc-anode attachment mode and the temperature fluctuation of the electrode surface was investigated. Due to the unbalanced Lorenz force, the rotation of arc spot was severe in diffuse arc-anode attachment mode, which led to relatively large temperature fluctuations on the anode surface, resulting in low purity of synthesised multi-wall carbon nanotubes on the anode deposit.

Analysis of the effects of diesel/methane dual fuel combustion on nitrogen oxides and particle formation through optical investigation in a real engine

Great attention was paid to nitrogen oxides and particulate matter emitted by the compression ignition engines. The adoption of methane dual fuelled with diesel could contribute to the reduction of these pollutants.This paper aims to investigate the combustion phenomena occurring when a premixed methane/air charge is ignited by the direct injection of diesel fuel. The research activity was performed on a production compression ignition engine, three-cylinder, 1.0L, equipped with a common rail injection system. In order to operate in diesel/methane dual fuel mode, the intake manifold of the engine was modified to set an electronic port fuel injector suitable for gaseous fuels. Different engine speeds and loads were tested. For each engine condition, a small part of the total energy was provided by the direct injected diesel fuel while the remaining by the methane. Thermodynamics analysis of the combustion process was performed through conventional measurement involving in-cylinder pressure acquisition. Endoscope based optical techniques were carried out for the combustion visualization with high spatial distribution and temporal evolution. Two-colour pyrometry method was applied to the flame images to evaluate the temperature and the soot concentration. This method allows to achieve a better insight about the pollutant formation. Experimental results revealed that DF combustion occurs with lower temperature and soot formation than diesel operation thus leading to lower nitrogen oxides and particle emissions at exhaust.

Analysis of combustion phenomena and pollutant formation in a small compression ignition engine fuelled with blended and pure rapeseed methyl ester

This paper deals with an investigation on the combustion process and pollutant formation of a small diesel engine fuelled with blended and pure biodiesel. The engine is a three-cylinder, 1028 cc, equipped with a common rail injection system. Endoscope based optical setup was used to observe in the cylinder without significant interference to the combustion process. Combustion images were post-processed by two-colour pyrometry method to evaluate the flame temperature and the in-cylinder soot concentration. Optical data were correlated to the nitrogen oxides and the particulate matter emissions measured at exhaust. Experiments were carried out at different operating conditions. It was found out that without EGR (exhaust gas recirculation), blended and pure RME (rapeseed methyl ester) were characterized by higher flame temperature and also by higher NOx emissions. In presence of EGR, lower flame temperature was detected for biodiesel; in this case higher NOx emissions were measured with biodiesel but the difference with NOx emissions from diesel fuel were reduced. Moreover, blended and pure biodiesel combustion was characterized by lower in-cylinder soot formation and also lower PM (particulate matters) at exhaust.

Effect of oxygenates addition on the flame characteristics and soot formation during combustion of single droplets of a petroleum diesel in air

The effect of methyl oleate and ethanol addition on the combustion characteristics and soot emission of single droplets of diesel was studied by means of experimentation and kinetic modelling. The experiments were performed using a high-temperature tube furnace operating at 973K and the soot intensity indicated by the soot KL factor, and flame temperature were simultaneously measured using a two-colour pyrometry technique and a colour CCD camera. A detailed chemical kinetic calculation was performed using CHEMKIN-Pro. The experimental results showed that adding methyl oleate to diesel significantly suppressed the soot formation. The burning rate and flame temperature of the droplets decreased with increasing methyl oleate addition up to 20vol% and slowly increased again at methyl oleate addition exceeding 20vol%. Ethanol addition up to 10vol% in the diesel also led to less soot formation, lower flame temperature and burning rate. Kinetic modelling showed satisfactory agreement with the experimental results: the effectiveness of ethanol and methyl oleate in soot abatement was linear with the oxygen content in the fuel mixtures with ethanol being more effective than methyl oleate. The kinetic analysis also showed that ethanol utilised its oxygen atoms more efficiently than methyl oleate in suppressing the formation of soot precursors.

Effect of arc behaviour on the temperature fluctuation of carbon electrode in DC arc discharge

Diffuse and multiple arc-anode attachment modes were observed in a DC arc discharge with a carbon electrode. During the arc discharge, the surface temperature of the electrode was successfully measured by two-colour pyrometry combined with a high-speed camera which employs appropriate band-pass filters. The relationship between the arc-anode attachment mode and the temperature fluctuation of electrode surface was investigated. The diffuse arc-anode attachment mode leads to relatively large temperature fluctuation on anode surface due to the rotation of the arc spot. In the case of diffuse mode, the purity of synthesized multi-wall carbon nanotube was deteriorated with temperature fluctuation.