Carbon Monoxide Flame Research Articles

Flame bands: CO + O chemiluminescence as a measure of gas temperature

Carbon monoxide flame band emission (CO + O → CO2 + hν) in CO2 microwave plasma is quantified by obtaining absolute calibrated emission spectra at various locations in the plasma afterglow while simultaneously measuring gas temperatures using rotational Raman scattering. Comparison of our results to literature reveals a contribution of O2 Schumann–Runge UV emission at T > 1500 K. This UV component likely results from the collisional exchange of energy between CO2(1B) and O2. Limiting further analysis to T < 1500 K, we demonstrate the utility of CO flame band emission by analyzing afterglows at different plasma conditions. We show that the highest energy efficiency for CO production coincides with an operating condition where very little heat has been lost to the environment prior to ∼3 cm downstream, while simultaneously, T ends up below the level required to effectively freeze in CO. This observation demonstrates that, in CO2 plasma conversion, optimizing for energy efficiency does not require a sophisticated downstream cooling method.

Manipulation of Large-Scale Vortical Structures and Mixing in a Coaxial Jet with Miniature Jet Actuators Toward Active Combustion Control

Manipulation of large-scale vortical structures and associated mixing in a methane-air coaxial jet is carried out by using miniature jet actuators installed on the inner surface of the annular nozzle. The periodic radial miniature jet injections are achieved with a rapid-response servo-valve. The spatio-temporal primary jet structures are investigated through phase-locked 2C-PIV (2 Component Particle Image Velocimetry) and stereoscopic-PIV. It is found that intense ring-like vortices are produced perfectly in phase with the periodic miniature jet injections regardless of the valve-driven frequency fv examined. When the Strouhal number Stv, which is defined with fv, is larger than unity, the ring-like vortices are densely formed and thus methane/air mixing is prompted with low periodic fluctuation. The diameter of the vortices becomes small as Stv is increased, so that the transport range of the inner methane and outer air fluids can be controlled by changing Stv. In addition, the evolution of counter-rotating vortex pair is also observed in the cross-sectional plane. These streamwise vortices are directly formed as a result of the radial miniature jet injection, which leads to entrainment of the ambient fluid near the primary jet shear layer, and they also contribute to the mixing enhancement. Moreover, it is demonstrated that coaxial jet flame characteristics such as carbon monoxide (CO) emission and flame holding can be drastically improved under different equivalence ratios by the present jet control scheme.

Measurements and statistics of mixture fraction and scalar dissipation rates in turbulent non-premixed jet flames

The present study adds to the limited database of mixture fraction and scalar dissipation rate measurements within turbulent flames. One noteworthy aspect is that the current measurements are two-dimensional images, which provide the physical structure of the scalar dissipation rate field. Two turbulent carbon monoxide flames are studied at two different jet-exit velocities, where the latter flame is near blowoff. The effects of increasing levels of local extinction (by increasing the jet velocities) on the structure and statistics of the mixture fraction fluctuation and scalar dissipation rate fields are investigated in detail. In general, the topology of the scalar dissipation rate layers is similar to that found previously in turbulent non-reacting flows, with some notable exceptions. For the lower jet-velocity case, the dissipation layers were somewhat smooth, isolated, and preferentially aligned in the axial direction, while for the higher jet-velocity case (near global extinction), the scalar dissipation rate field was characterized by an increased number of shorter, thinner, and highly wrinkled dissipation layers. It appears that as the flame nears blowoff, the scalar dissipation rate field exhibits an increasing level of isotropy, resembling turbulent non-reacting flows. In addition to providing new insights on the physical nature of the dissipation fields, the two-dimensional statistics of the mixture fraction and scalar dissipation rate are examined in detail, in the form of conditional averaging and through the construction of probability density functions. It is found, that although the two flames have apparent differences in physical structure, the corresponding statistical properties of the scalar dissipation rate were similar. Probability density functions (pdfs) of the scalar dissipation rate exhibited large departures from strict log-normality and the pdfs of the scalar dissipation rate were found to be significantly different if only the values of the scalar dissipation conditioned on the stoichiometric contour were considered compared to the entire flowfield. This has implications for modeling assumptions that seek to describe the entire turbulent flowfield by a single pdf. The nature of the scalar dissipation rate in the current lower-Reynolds number turbulent flames, as revealed through the statistics, is compared to previous, well-known data sets obtained in the Sandia series of piloted jet flames as well as other piloted non-premixed jet flames at higher Reynolds numbers. The present flame conditions are significantly different from previous work, yet the general structure, statistical trends, and general effects of increasing levels of local extinction agree favorably with the previous experimental data.

Nonthermal Ignition of Premixed Hydrocarbon-Air Flows by Nonequilibrium Radio Frequency Plasma

Results are presented of nonequilibrium rf plasma-assisted combustion experiments in premixed air-fuel flows. The experiments have been conducted in methane-air, ethylene-air, and CO-air mixtures. The results show that large volume ignition by the uniform and diffuse rf plasma can be achieved at significantly higher flow velocities (up to u = 25 m/s) and lower pressures (P = 60-130 torr) compared to both a spark discharge and a dc arc discharge. The experiments also demonstrated flame stabilization by the rf plasma, without the use of any physical obstacle flameholders. Fourier transform infrared (FTIR) absorption spectra of combustion products show that a significant fraction of the fuel (up to 80%) burns in the test section. Temperature measurements in the diffuse rf discharge using FTIR emission spectra show that the flow temperature in the plasma before ignition (T = 250-550°C at P = 60-120 torr) is considerably lower than the autoignition temperatures for ethylene-air mixtures at these pressures (T = 600-700°C). Visible emission spectroscopy measurements in C 2 H 4 -air flows in the rf discharge detected presence of radical species such as CH, C 2 , and OH, as well as O atoms. In CO-air flows, O and H atoms have been detected in the rf plasma region and CO 2 emission (carbon monoxide flame bands) in the flame downstream of the rf plasma.

Flame heat transfer in storage geometries

This paper presents measurements of the heat flux distribution to the surface of four square towers exposed to buoyant turbulent flames.The steel towers represent an idealisation of a rack storage configuration at reduced scale. Each tower was 1.8 m high and 0.3 m×0.3 m wide. The fuel was supplied from a circular gas burner at the floor. Three different gaseous fuels were used: carbon monoxide (CO), propane (C3H8), and propylene (C3H6). These fuels cover a wide range of flame sootiness resulting in distinctly different flame heat fluxes. At the same overall heat release rates the peak heat fluxes from C3H8 flames were twice those from CO flames, whereas the peak heat fluxes from C3H8 flames were 2.8 times those from CO flames. Heat fluxes were measured by thermocouples spot-welded to the back of the steel sheets. They were measured at 52 different locations. This measurement method turns out to be simple, accurate and robust in addition to being inexpensive. Formulas are provided for the flame heat flux distribution in terms of the overall fire heat release rate, fuel sootiness and separation distance between the towers. The formulas are suitable for direct use by engineering models of fire growth in storage geometries. The paper also provides additional data needed for the development of more general CFD models capable of predicting fire growth of other geometries.

Fuel Mixing Effects on Propagation of Premixed Flames. I. Hydrogen + Carbon Monoxide Flames

Abstract The combustion mechanisms in premixed flames having a mixture of hydrogen and carbon monoxide as the fuel and the stoichiometric amount of air as the oxidizer were investigated by numerical simulation to elucidate the fuel-mixing effects on the burning velocity. Hydrogen and carbon monoxide have similar physical properties and their stoichiometric flames with air are known to propagate through the same type of mechanism, governed by active species diffusion. The flames of the mixed fuel hydrogen+carbon monoxide were found here to have their heat-release rate distributions changing with conformity in shape when the fuel composition is changed. The burning velocity was found to vary almost linearly with the hydrogen content in the fuel. Furthermore, the flux of chemical energy which is carried into low-temperature regions by hydrogen atoms diffusing from the flame front was also found to be roughly proportional to the burning velocity.

Elimination of water peak in optical fibre taper components

A water peak of typically 0.4 dB at 1380 nm is apparent in the loss spectra of single-mode fibre taper components made with conventional water-producing flames. The loss peak is avoided entirely by using a water-free heat source, such as a carbon monoxide flame.

Further study of continuous-wave CO flame chemical laser of the CS2/O2/N2O type

This report describes an experimental examination of the output characteristics of the continuous-wave (cw) carbon monoxide flame chemical laser (FCL) of the CS2/O2/N2O type in case of small CS2/O2 reactants ratios (tipically CS2/O2≦1/10). A linear burner which gives a homogeneous and stable flame was used during the experimental study. The measurements of temperature distribution in CS2/O2 as well as CS2/O2/N2O flames show maximum temperatures of 1040 and 890 K, respectively. The addition of nitrous oxide (N2O) leads to dramatically enhanced output laser power caused primarily by V−V transfer processes. A chemical efficiency, based on the reaction O+CS→CO*+S, of 3% was achieved. The spectral composition of the CO FCL of the CS2/O2/N2O type shows lasing in the region from 5.130 to 5.586 μm. Experimental results were obtained with a nondispersive optical cavity.

Radiofrequency resonance study of alkali metal ionization in a carbon monoxide flame. Part 1.—General design of the radiofrequency system

In Part 1 an analysis is given of the radiofrequency resonance technique. This technique is used for investigating alkali metal ionization relaxation in a carbon monoxide flame (see Part 2). The r.f. resonance measurements yield direct information about the densities of the electrical-charge carriers in the flame plasma. These densities are determined as a function of rise-time of the flame gases. In this context the advantages and disadvantages of the method in comparison with probe technique and microwave methods are discussed. However, we would emphasize that it is not necessary to restrict the application of the r.f. resonance method to the analysis of ionization phenomena in flames.Since we are interested in small differences in electron density we would note that our r.f. resonance method which operates on the h.f. band (3–30 MHz), is more sensitive in the detection of electrons than microwave methods. Under our experimental conditions the minimum detectable electron density (variation) was 0.6 × 108 cm–3. A linear response of the r.f. system exists only for electron densities < 1 × 1010 cm–3. The temporal resolution in our kinetic studies is ca. 1 ms, depending on condenser dimensions as well as flame rise velocity.

Kinetic modelling of the oxidation of carbon monoxide in flames

Implicit solutions of the time-dependent flame equations have been used to calculate, for assumed reaction mechanisms, the properties of a series of hydrogen/carbon monoxide/oxygen/nitrogen flames. Comparison of the predictions with experiment, over a wide range of composition and conditions, has established the probable kinetic mechanism in the flames. The major part of the observed changes in burning velocity from those of hydrogen/air flames can be satisfactorily explained by the addition of the single reaction (xxi): OH + CO ⇄ C O 2 + H ( xxi ) with k21=1.5×107T1.3 exp (+385/T) cm3 mole−1 sec−1, to the mechanism already established for the hydrogen/oxygen/nitrogen system. This applies particularly to fuel-rich mixtures not too far from stoichiometric. For reasonable values of its rate coefficient, reaction (xxii): O + CO + M ⇄ C O 2 + M ( xxii ) never exerts more than a minor influence on the burning velocity. However, for fuel-rich flames far from stoichiometric, in which the ratio [OH]/[H] becomes small, agreement between predicted and measured burning velocities is improved by adding to the mechanism a series of chain terminating steps involving the formation and subsequent reactions of the formyl radical. These reactions were investigated by measuring the influence of carbon monoxide addition on the burning velocity of a low temperature, fuel-rich hydrogen/oxygen/nitrogen flame. On the assumption that k23.H2=5×1014 exp (−755/T) cm6 mole−2 sec−1, the analysis of the results favours the low heat of dissociation of the formyl radical to H and CO and, on the further assumption of zero activation energies for reactions (xxiv) and (xxv), leads to k25=(4±1)×1013 and k24=3×10(12±1) cm3 mole−1 sec−1. H + C O + M ⇄ H C O + M ( x x i i i ) H C O + O 2 ⇄ H O 2 + C O ( x x i v ) H C O + H ⇄ H 2 + C O ( x x v ) With additional reactions to provide for (a) attack of OH and O on the formyl radical, and (b) conversion of methane (via CH3 and CH2O) to formyl, this same mechanism and rate coefficients are capable of predicting the burning velocities of both lean and slightly rich methane/air flames at atmospheric pressure. A mechanistic interpretation which covers both the carbon monoxide and methane flames gives an optimum value of k24=(3.5±0.5)×1012. Most of the flame simulations have been carried out by means of an adiabatic flame model. In order to match the experimental conditions in one of the carbon monoxide flames studied, the boundary conditions were modified in two calculations to take account of truncation of the flame profiles at the cold end due to heat losses at the burner. For simulations on the basis of the same final temperature in the flame, the results from the two models showed a small reduction in the burning velocity in the truncated cases.

Recombination of carbon monoxide and oxygen atoms

AbstractThe recombination of carbon monoxide and oxygen atoms was studied in reflected shock waves in H2:O2:CO:Ar = 0.1:1:24:75 with 1300 &lt; T5 2200 K and 2 &lt; P5 &lt; 4 atm. Reaction progress was monitored by observations of the carbon monoxide flame spectrum near 435 nm and carbon dioxide thermal emission near 4.2 μm. Data analysis was accomplished with the aid of computer modeling using a 27‐reaction mechanism. Computer modeling experiments also showed that these measurements were sensitive primarily to the rate of the reaction CO + O + M = CO2 + M and only slightly sensitive to the rates of other reactions. The best fit to the data was achieved with a rate constant for this reaction of 7.7 × 10−35 exp[19 kJ/RT] cm6 s for the temperature range of these experiments. Correlation of this result and previous data covering the temperature range 250 &lt; T &lt; 11,000 K confirms that this recombination reaction is governed by a nonadiabatic curve crossing with an activation barrier of about 20 kJ and subsequent deactivation of a singlet CO2 molecule.

Some photometric observations of trace additives in dry carbon monoxide flames

Nitric oxide, iodine, cyanogen, and lithium bromide have been added to CO/N 2 /O 2 flames containing less than .007% of hydrogen-containing impurities. Relative oxygen atom concentrations, derived from measured intensities of emission bands of CO 2 , NO 2 , and IO, are compared. Addition of NO produced a significant reduction in the O-atom concentration, the effect being greatest in O 2 -rich flames. Measurements of the resonance absorption by lithium atoms indicate that LiO is the major compound formed by lithium in O 2 -rich flames, but they do not rule out the possibility that significant amounts of LiO 2 are also formed. A recalculation of earlier data yields D o Na-O 2 = 234 ± 13 kJ mol −1 ; we estimate D o Li-O 2 = 222 ± 25 kJ mol − .

Decomposition of phosgene in air.

Thermal decomposition of 10 ppm of phosgene in air was studied under simulated environmental conditions. It was shown that temperature and time of contact with hot surfaces affect the amount and rate of decomposition. A clean iron surface enhances decomposition and combines with the decomposition products at relatively low temperatures. “Corroded” iron surfaces enhance decomposition, and chloride is “liberated” from those surfaces on contact with phosgene at relatively low temperatures. Humidity causes only slight increase in decomposition. The amount of phosgene decomposition depends on the temperature of the open flame; phosgene was essentially completely decomposed on contact with a carbon monoxide flame. The presence of carbon tetrachloride and/or its decomposition products markedly decreases decomposition of phosgene. Phosgene is decomposed to chloride which is subsequently converted to chlorine by further heating.

LiO formation in dry carbon monoxide flames

The concentration of uncombined lithium atoms has been measured as a function of time from primary combustion in isothermal pairs of CO-rich and O 2-rich flames. The lower concentration of lithium atoms in the O 2-rich flames is accounted for by the balanced reaction

Formation of nitric oxide in premixed hydrocarbon flames

The formation was followed by probing flames of ethylene, methane, or propane with N2-O2 mixtures. The known kinetic laws of the well-understood N2-O2-NO system (limited by the rate of the reaction O+N2→N+NO) described the growth of nitric oxide in the post-flame gas, after hydrocarbons were consumed, but could not describe a faster, transient formation of nitric oxide in the primary reaction zone. The transient formation did not occur in hydrogen or carbon monoxide flames, and therefore, may involve an attack of carbon or hydrocarbon radicals on nitrogen molecules (e.g., CH+N2→HCN+N). At atmospheric pressure, the transient formation gave its greatest yield in flames containing 1.3 to 1.4 times the stoichiometric amount of fuel. Its yield increased somewhat with pressure, particularly in fuel-lean flames, so that at 3 atmos pressure the nitric oxide formed quickly in the primary reaction zone was about the same in flames of 0.8 as of 1.4 mixture strength. The transient formation possessed a relatively weak temperature dependence. Extrapolating the measurements, one can suggest that the new mechanism may contribute significantly to the emission of nitrogen oxides from a gas turbine, but the major contribution occurs by the known mechanism of the N2-O2 system.

Calculations of CO2 Energy Levels: The Ã1B2 State

The Ã1B2 state of CO2, identified by Dixon (1963) as the upper level of the ‘carbon monoxide flame bands’, must be of importance in the upper atmospheres of Venus and Mars. New calculations of the high vibrational levels of the ground state, which lead to improved fits of the observed vibration-rotation bands, confirm Dixon's analysis, except that the numbering must be lowered by two, and fix the energy of the v = 0, K = 0 level of 1B2 at 45210 ± 10 cm−1 = 5.605 eV.

Studies of opposed jet diffusion flames I. Temperature distribution

Opposed jet diffusion flames using ethylene, methane, and carbon monoxide as fuels were studied. The effects of mass fluxes and fuel and oxygen concentrations on the thermal structures of these flames are discussed. Ethylene and methane flames behave similarly. Isotherms are essentially horizontal, with a slight inclination slanting upward toward the periphery. Maximum temperatures occur near the periphery. The combustion zone lies slightly in the fuel side of the geometrical center plane. It lowers toward the fuel side with an increase in mass flux or oxygen concentration, and rises toward the oxidant side with an increase in fuel concentration. As mass flux, fuel concentration, or oxygen concentration increases, the combustion zone temperature rises and, furthermore, a high-temperature region bounded by a specific isotherm expands or shifts. Carbon monoxide flames are difficult to stabilize: once stabilized, they behave differently from ethylene and methane flames. Maximum temperatures occur at a point well within the periphery. The combustion zone lies well below the geometrical center plane, toward the fuel side.

Formation and decomposition of sulfur trioxide in flames and burned gases

The oxidation of sulfur dioxide has been studied in preheated and premixed carbon monoxide-nitrogen-oxygen and methane-nitrogen-oxygen flames. S O 2 + O + M → k 1 S O 3 The subsequent decomposition of sulfur trioxide in the burned gases from these flames has also been studied. S O 3 + M → k 2 S O 2 + O + M In a complementary series of experiments, sulfur dioxide-argon and sulfur trioxide-argon mixtures have been shock-heated and the nature of the consequent light emission examined. A comparison of k 1 derived, assuming that no dissociation occurred in the flame itself, with the results of previous workers 2–4 for the oxidation at room temperature showed that significant amounts of decomposition occurred in the flame. A fast decomposition process via the mechanism suggested by Fenimore and Jones (Ref. 9) with M in Eq. (2) atomic oxygen and with a velocity constant k 2a =1×10 12 cm 3 mole −1 sec −1 led to reasonable agreement between the presently determined value of k 1 =2×10 15 cm 6 mole −2 sec −1 at 2150°K and previous results, whereas dissociation involving nitrogen as M was too slow to account for significant decomposition in the flame. The velocity constant for the decomposition of sulfur trioxide, derived from the steady level of sulfur trioxide observed in the burned gases from the carbon monoxide flame is k 2b =8×10 8 cm 3 mole −1 sec −1 at·1740°K with M as nitrogen. The common features of the emission from shock-heated mixtures of sulfur dioxide-argon and sulfur trioxide-argon indicated that sulfur trioxide decomposes behind the incident shock at temperatures of 1740°K. This observation leads to a velocity constant k 2c =1×10 8 cm 3 mole −1 sec −1 at 1740°K with M as argon.

Chemi-ionization of nitric oxide in flames containing hydrocarbon additives

Observations are reported of the ionization of nitric oxide additive in premixed carbon monoxide/oxygen and hydrogen/oxygen flames containing small (≈0.5%) amounts of acetylene. Mechanisms of NO + formation in these flames are proposed from measurements of the concentration profiles of NO + , O 2 + , CHO + , and H 3 O + ions obtained using a flame-ion mass spectrometer. Carbon monoxide flames show two regions of NO + formation, a primary one in the reaction zone and a secondary one downstream of it. Formation of NO + in hydrogen flames is restricted, however, solely to the primary region. The concentration of secondary NO + decreases with increasing nitric oxide, while primary NO + increases both with increasing nitric oxide and acetylene. Secondary NO + formation is linked with that of the O 2 + ion, which has been previously proposed to occur through a cascading sequence of exothermic transfer reactions terminated by the balanced reaction O + +CO 2 |O 2 + +CO. Nitric oxide addition results in the rapid replacement of O 2 + by NO + , and is shown to be consistent with the lower termination of this sequence according to the more exothermic step O 2 + +NO→O 2 +NO + . Formation of NO + in the primary region of both flame systems is shown to be consistent with charge exchange with the CHO + ion, and the rate constant for the process CHO + +NOCHO+NO + is evaluated as 1.2×10 −10 cm 3 molecule −1 sec −1

Energy transfer from carbon monoxide flames

Heat fluxes in the vicinity of the reaction zone of carbon monoxide—air flames were measured with a conductivity-type heat-flow meter. It was found that, near the flamefront, the heat flux is much greater than that predicted from ordinary forced convection. Calculations were made by means of a correlation which includes terms for the diffusion and recombination of various labile species present in the flames. The concentrations of atoms and radicals necessary to produce the experimentally determined heat fluxes were estimated and found to be considerably higher than expected from equilibrium considerations. The catalytic effect of different surfaces was examined by coating the probe tip with either platinum or an oxide mixture. The effect of gaseous additives on the heat flux was also studied. Water vapor has a marked effect on the combustion mechanism of carbon monoxide, and drying of the combustible gases reduced the heat flux because of the decrease in concentration of hydrogen atoms. Traces of bromine added to the moist flame produced a similar effect.