OH Radical Emission Research Articles

Diagnosis of OH Radical by Optical Emission Spectroscopy in Atmospheric Pressure Unsaturated Humid Air Corona Discharge and its Implication to Desulphurization of Flue Gas

OH radical in the corona discharge with pipe–nozzle–plate electrode has been diagnosed by optical emission spectroscopy. Spatial variations of OH radical emission in discharge gap have been measured. Relative intensity of OH radical emission spectroscopy increases with increasing water vapor flux injected into the reactor or intensity of electric field supported. In positive pulsed corona discharge, relative intensity is higher than that in positive DC corona discharge and lower than that in negative DC corona discharge. Strongest intensity of OH radical spectrum appears within the range of 5 mm near the discharge nozzle- electrode. In addition, it is proved that the efficiency of desulphurization from flue gas by pulsed corona discharge plasma processes can be improved when OH radical is produced in the reactor.

Symptoms of self-excited combustion oscillation and their detection

Monitoring of OH chemiluminescence through an optical fiber was demonstrated to be a useful method in detecting self-excited combustion oscillations. OH chemiluminescence intensity detected by the optical fiber showed mostly excellent agreement with those obtained by high speed CCD camera measurements when combustion oscillations were strong. Symptoms of self-excited combustion oscillation were also studied in order to predict the onset of combustion oscillation before it proceeded to a catastrophic failure. For the purpose, we have found and proposed unique measures to tell the onset of self-excited combustion oscillations based on the careful statistics of fluctuating properties in flames, such as pressure or emission of OH radicals.

연소공기의 산소부화농도에 따른 난류확산 평면화염의 연소특성

Combustion using oxygen enriched air is an energy saving technology that can increase thermal efficiency by improving the burning rate and by increasing the flame temperature. Flame figures, OH radical intensities, temperature distributions and emissions concentration were examined according to oxygen enriched concentration(OEC) in a turbulent diffusion flat flame. As long as the oxygen enriched concentration was increased, the length and volume of the flat flame was decreased while OH radical intensity was raised and the flame temperature was increased. However, RMS of the fluctuating temperature was decreased, and more homogeneous temperature field was formed. Thermal NO also was increased with increase of oxygen enriched concentration, but CO was decreased due to the increase of chemical reaction rate.

An Experimental Study of Counterflow Diffusion Flames Subject to Flame Stretch Gradients.

The flame stretch effect on counterflow diffusion flames has been experimentally investigated in the configuration of Tsuji burner in terms of the OH radical emission intensity and temperature. This is motivated by our previous study about flame base structures of turbulent lifted diffusion flames which showed lower quenching scalar dissipation rates at tips of the bases than those of the counterflow diffusion flames. Counterflow diffusion flames subject to flame stretch gradients, called edge flames, are examined to model flamelets at the lifted flame bases. Fuels are methane and propane. It is found that the increase in the scalar dissipation rate leads to the enhancement of reaction in the fully burning condition, because the OH radical emission intensity reflecting the reaction rate increases with the flame stretch until the extinction. This reveals that the effect of the finite reaction is limited to a narrow range near the quenching scalar dissipation and the Damkohler number dose not decrease monotonously. Extinctions of edge flames take place at lower flame stretches than those of the counterflow diffusion flames under the uniform flame stretch. The flame stretch gradient weekends the edge flame and quenches at a lower flame stretch. It is also found that the flame edge reacts strongly by the enhancement of the diffusion of reactants and radicals at the edge.

Microgravity experiments on flame spread of an n-decanedroplet array in a high-pressure environment

The flame spread phenomena of an n -decane droplet array in the supercritical pressure range were experimentally investigated in microgravity. Experiments were conducted at presure up to 5.0 MPa, which is over the critical pressure of n -decane. Observations of the flame-spread phenomenon were conducted using OH-radical emission, Schlieren, and back-klit images recorded by a high-speed video camera. The flame-spread rates were calculated on the basis of the time history of the OH-emission images. Im microgravity, the flame-spread rate decreased with increasing pressure, had a minimum at a pressure around half of the critical pressure, and then increased again. It had a maximum at the pressure near the critical pressure and then decreased gradually with pressure. In normal gravity, the flame-spread rate monotonously decreased and there was a pressure limit beyond which flame spread did not occur. Around the critical pressure, a jet-like flow of fuel vapor from an unburned droplet heated by the flame of a burning droplet was observed. The fuel-vapor jet from theside opposite the heating region of the unburned droplet reached another adjoining unburned droplet and then flame propagated along the jet, leading to heating pior to ignition of the unburned droplet. The mechanism of the fuel-vapor jet was examined based on the shear flow near the droplet surface induced by Marangoni convection of the unburned droplet heated non-niformly by the burning deoplet. The droplet internal flow rate was measured in normal gravity and confirmed the existence of Marangoni convection. The internal flow rate increased with pressure and had a maximum near the critical pressure. It was expected that the mechanism responsible for the maximum flame-spread rate near the critical pressure was the enhanced heat and mass transfer caused by the fuel-vapor jet and flame propagation along that jet.

Spectroscopic measurements of discharges inside bubbles in water

In the present research, a new method for a water treatment utilizing the discharge of bubbles in water is proposed. The radicals with short lifetime can be effective for water purification because the discharge is expected to occur inside the bubbles in water. Magnetic-pulse-compression pulsed-power generator was used as a repetitive power supply. The bubbles generated by a bubbling tube were passed through a discharge region between the electrodes. In this phase of the experiment, argon was used as a bubbling gas. The experimental results showed that a discharge was generated inside the bubbles in water by the mentioned pulsed-power generator. Simultaneously, measurements of the excited OH radical emission were carried out using a spectroscopic technique. The repetition rate of pulsed-power generator, the peak value of applied voltage and the gap length were 100 pps (pulse per second), 20 kV and 7.5 mm, respectively.

A NO x diagnostic system based on a spectral ultraviolet/visible imaging device

The present investigation is aimed to develop and test a NO x diagnostic system based on a spectral ultraviolet/visible imaging device. The study was performed in a small-scale laboratory furnace fired by a propane or ethylene swirl burner. The data reported include flue-gas concentrations of O 2, CO 2, CO, unburnt hydrocarbons and NO x , obtained with conventional techniques, and photometric data of spontaneous emission of OH and CH radicals, obtained with the imaging device, from various regions of the flame. All these measurements have been obtained for twenty three furnace operating conditions which quantify the effects of the gaseous fuel (propane and ethylene), flue-gas recirculation, heat losses through the furnace walls to the surroundings and excess air. Against this background, it was possible to obtain a wide range of NO x emissions. In addition, the analysis of detailed near burner in-flame data of local mean major gas-phase species and gas temperatures collected for four furnace operating conditions has indicated that NO x formation was strongly dominated by the prompt (or Fenimore) mechanism. The results reveal that there is an excellent correlation between the NO x emissions from propane or ethylene flames and the OH+CH photometric data, which can be mathematically expressed as a logarithmic function, provided that the radicals images are collected from a flame zone close to the burner exit.

Ignition and combustion characteristics of two-stage injection diesel spray

Ignition and combustion characteristics of a two-stage injection diesel spray were experimentally investigated. A constant volume combustion chamber was filled with air which was controlled at 3.0 MPa and 743–923 K. In order to measure the ignition delay and the ignition position, a high speed video system was used. A 306 nm interference filter and an image intensifier system were attached to the camera for detecting the OH radical emission. The results show that the ignition delay of a two-stage injection spray becomes shorter compared with that of a single injection spray. The ignition positions of two-stage injection spray are observed nearer to the nozzle than that of single injection spray. Also, the temperature limit of complete combustion on a two-stage injection spray becomes lower than that of a single injection spray.

Analysis of OH Radical Emission Intensity During Autoignition in a 2-Stroke SI Engine.

This research focused on the light emission behavior of the OH radical (characteristic spectrum of 306.4 [nm]) that plays a key role in combustion reactions, in order to investigate the influence of the residual gas on autoignition. The test engine used was a 2 stroke, air cooled engine fitted with an exhaust pressure control valve in the exhaust manifold. Raising the exhaust pressure forcibly recirculated more exhaust gas internally. Emission measurements were made under three conditions : normal combustion without any forced application of internal EGR, forced application of light internal EGR and forced application of heavy EGR. When a certain level of internal EGR is forcibly applied, the temperature of the unburned end gas is raised on account of heat transfer from the hot residual gas and also due to compression by piston motion. As a result, the unburned end gas becomes active and autoignition tends to occur.

Radical Emission of Hydrocarbon Combustion Induced by Flash Photolysis.

Hydrocarbon/oxygen mixture seeded with nitrogen dioxide were ignited by xenon flash photolysis. Combustion appeared uniformly throughout the entire volume. C2, CH and OH radical emissions were measured under the several initial pressures and equivalence ratios. Two different emission peaks of C2 and CH were found depending on initial pressure and equivalence ratio. OH emission intensity varies smoothly with time, and increased linearly with pressure. The second emission peaks of C2 and CH could count among the thermally exited emission because of the matched OH emission peaks. The first ones were considered to he the chemically exited emissions. An attempt is made to explain that the two emission peaks appear by the reaction : CH2→C*2, CH* (chemical excltation)→C*2, CH* (thermal excitation) associated with polymerization/destruction.

Spectroscopic Investigation of Nitromethane Flames

AbstractNitromethane (CH3NO2) Pool Fire Flames have been investigated analysing both, the radiation emitted from intermediate combustion radicals (diatomic molecules) in the ultraviolet (UV) and visible (VIS) spectral range, and the near infrared spectra dominated by broad water bands. Comparing the UV/VIS spectra to calculated band profiles, rotational and vibrational temperatures (Trot, Tvib) of OH, NH and CN have been determined. At local thermal equilibrium Trot and Tvib must be equal. The determined temperatures show that the OH and NH emissions originate from pure thermal excitation (Trot = Tvib = 2300 K–2380 K) but the CN emissions show additionally a chemical excitation indicated by different vibrational and rotational temperatures in the Boltzman factor (Trot = 2100 K, Tvib = 4300 K). For a more detailed study of the flame, the emission of OH radicals has been investigated more extensively by monitoring a two‐dimensional rotational temperature and emissivity profile. In cooler flame regions, the observed spectra are dominated by the near infrared radiation emitted from stable combustion products like water and carbon dioxide. Therefore, NIR spectra (1000 nm to 2500 nm) have been compared to broad band profiles calculated with a self‐developed code basing on the data of the ‘Handbook of Infrared Radiation of Combustion Gases’. The flame temperatures obtained by this method range from 1800 K to 1900 K. The results are correlated to flames of methane and nitrogen oxide which emit similar spectra indicating similar reaction mechanisms in the gaseous phase.

Analysis of Flame Patterns in Cryogenic Propellant Combustion

Design and optimization of high performance rocket engines may be improved by detailed studies of the basic combustion mechanisms. Much detailed information exists on elementary processes such as atomization, multiple jet interactions, vaporization of single droplets, structure of spray flames, ignition of nonpremixed systems etc. It is however important to approach the real conditions existing in rocket motors and to this purpose several facilities for cryogenic propellant combustion research have been designed and constructed. One experimental set-up designated as “Mascotte” is operated by ONERA and used for fundamental research as well as technical studies. This article describes results of experiments conducted in this facility by our laboratory. Two series of tests carried out during the last two years have provided a large set of images of combustion in a liquid oxygen/gaseous hydrogen coaxial injection geometry operating at atmospheric pressure and at 5 and 10 bars. The data correspond to laser elastic scattering of the spray, spontaneous emission of OH radicals and planar laser induced fluorescence of these radicals. Fluorescence is obtained by pumping the EQUATION band of OH. Off-resonance light radiation is observed with an intensified CCD camera. The large data base of images collected in these experiments provide the general flame structure in the injector nearfield, and may be used to determine the position of flame stabilization. Effects of global injection parameters (momentum flux ratio, Weber number, mixture ratio) and operating pressure are described. It is shown that the conditions prevailing in the liquid and gaseous jets influence the flame stabilization process, the instantaneous combustion patterns and the mean flame shape.

Fuel-rich hydrogen-air combustion for a gas-turbine system without CO 2 emission

We propose a new and innovative gas-turbine system using fuel-rich hydrogen combustion, which we call a chemical gas-turbine system. It involves a fuel-rich hydrogen-air combustor as a major component. We have focused on a coaxial diffusion flame under normal pressure. The effects of equivalence ratio and swirl number have been investigated by measuring temperature profiles, gas compositions, and flame structures using direct observations of OH radical emissions. The flames were shortened and NO χ emission decreased with swirling under fuel-rich conditions.

Flame stabilization in cryogenic propellant combustion

Cryogenic combustion is of considerable technological interest in propulsion applications. Cryogenic propellants used in rocket engines provide the high performance needed for spacecraft launching and have operated safely for a number of years, but the processes that control combustion in such devices are still not well understood. Among the many important issues, flame stabilization constitutes one basic problem. This question is investigated in this article by imaging the flame originating from a single, coaxial injector fed with liquid oxygen and gaseous hydrogen. Results of experiments carried out on a facility for cryogenic propellant combustion research operated by ONERA are used to characterize the mechanisms that control the flame-holding process at atmospheric pressure, 5, and 10 bars. Data acquired correspond to elastic scattering by the spray, emission of OH radicals, and planar laser-induced fluorescence of these radicals. Fluorescence is obtained by pumping the X2II (v″=0)→A2Σ(v′=1) band of OH, and off-resonance light radiation is observed. This database provides the general structure of the flame in the injector near-field, and may be used to determine the position of the flame stabilization region. Simultaneous acquisition of laser-induced fluorescence and elastic scattering was used to locate the flame in respect to the liquid. It is shown that in all cases investigated the flame is initiated at a close distance from the injector exhaust plane.

Chemiluminescence Emission of C2, CH and OH Radicals from Opposed Jet Burner Flames.

Chemiluminescence emission from opposed jet burner flames was measured by a charge coupled device (CCD) camera with an image intensifier. Radial profiles of the emission intensity of OH, CH and C2 radicals were obtained by the Abel inversion of the side-on observations. The profiles of emission intensity of these radicals are similar, and each maximum is located at the center of the flame zone. The emission of C2 and CH radicals produced in the early stage of the combustion reaction was observed even on the burned-gas side of the flame zone where the mean temperature is significantly high, suggesting that unburned mixture exists on the burned-gas side of the flame zone. It was found that the profiles of OH emission intensity and NO concentration are similar. This fact suggests that NO originating from the prompt NO mechanism is predominant in the distributed reaction zone.

Experimental and numerical study of transient laminar counterflow diffusion flames

In the present article, we analyze the nonsteady behavior of counterflow diffusion flames subjected to a time-dependent injection velocity in the case of hydrogen—air flames. The numerical study, using a finite-difference implicit linear multistep method and employing complex kinetics, is done for sinusoidal injection velocity variations, for moderate values and also for values near the extinction limit. The frequency response is obtained in these different cases. The experimental setup comprises two axisymmetric nozzles. In order to change the inlet velocities periodically, we use a vibrating mechanism in each burner. The amplitude and the frequency of injection velocities of the opposed nozzles are synchronously modified. The OH radical emission intensity is used to measure the flame response to velocity fluctuations. The experimental results are compared to numerical calculations. Good agreement is obtained in this comparison.

Synthesis of Diamond in Combustion Flame under Low Pressures

The oxygen-acetylene combustion flame method has been investigated as to low pressure, gas ratio and substrate temperature effects, and flame emission spectral analysis. Non-diffusion of oxygen from ambient atmosphere enhanced expansion of the area and the amount of deposited diamond. H and OH radical emission have not been observed on combustion flames in a chamber. The reactions relating C2 and CH radicals have been revealed to have important roles in diamond synthesis.

Chemiluminescence Emission of C2, CH and OH Radicals from Opposed Jet Burner Flames.

Chemiluminescence emission from opposed jet burner flames was measured by a CCD (Charge Coupled Device) camera with an image intensifier. Radial profiles of the emission intensity were obtaine by the Abel transform of results of the side-on observation. The profiles of emission intensity of these radicals are similar and each peak is located at the center of the flame zone. The emission of C2 and CH radicals produced in the early stage of the combustion reaction was observed on the burned gas side of the flame zone where the mean temperature is significantly high, suggesting that unburned mixture exists even on the burned gas side of the flame zone. From the emission profiles of OH radical which can be used as a measure of the reaction zone, it can be seen that the outer diameter of the flame zone increases with the mixture supply velocity, while the inner diameter remains constant. It was found that the profiles of OH emission intensity and NO concentration are similar. This fact suggests that NO originating from prompt NO is predominant in the distributed reaction zone.

Emission Characteristics of OH and C2 with Knocking.

Under the knocking condition, luminous emission from the autoignition portion in actual S.I. engine is observed using optical fibers. The difference in emission intensity between the knocking and no-knocking conditions is mainly caused by the intensities of OH and C2 radical emission. The emission intensity from these two species increases steeply at the occurrence of autoignition. In addition, emission intensity from C2 radical in some cycles has two peaks. As the first peak is due to the autoignition reaction, the second peak is understood to be caused by the soot formation in some cycles under the knocking condition. This second peak corresponded to the shadowlike object under the expansion stroke which was taken by our high-speed shadowgraphy.

Determination of nitrogen and oxygen and species containing nitrogen by molecular non-thermal excitation spectrometry (MONES) using microwave-induced plasma (MIP) and furnace atomization non-thermal excitation spectrometry (FANES) sources

A microwave-induced plasma (MIP) source produced through a surface-wave propagation process (surfatron) and a furnace atomization non-thermal excitation spectrometry source have been used to determine N and O in gaseous and aqueous samples, respectively, through the formation and emission of NH and OH radicals. Water is removed through liquid nitrogen cold trapping and hydrogen is added to the He plasma gas. The spectral characteristics of the two sources have been compared.