Howard Levinsky

Temperature dependence of laser-induced fluorescence of nitric oxide in laminar premixed atmospheric-pressure flames

The temperature dependence of laser-induced NO A (2)?(+)-X (2)? fluorescence in the hot gases of natural gas-air flames, seeded with known quantities of NO, has been determined experimentally by means of a difference method. The flame temperature at three fixed equivalence ratios was changed when the mixture velocity was varied through a water-cooled, flat-flame burner and was measured by coherent anti-Stokes Raman spectroscopy. When the possible reburning of part of the seeded NO is allowed for, the results in the range 1700-2150 K are best described by the temperature dependence obtained from a model in which quenching corrections are neglected, as in the case of a saturated two-level system, when millijoule pulse energies are used. Measurements of the fluorescence intensity at constant seed concentration as a function of equivalence ratio between 0.75 and 1.3 also indicate that quenching corrections are unnecessary under these excitation conditions. Using the measured intensities of the seeded flame as a calibration factor, we determined the absolute NO concentrations as functions of the equivalence ratio at 1 cm above the burner. The results indicate that, with the calibration method presented here, a relative accuracy of 5% should be obtainable.

A LIF and CARS investigation of upstream heat loss and flue-gas recirculation as NOx control strategies for laminar, premixed natural-gas/air flames

Measurements of temperature and NO mole fraction are reported in laminar premixed natural-gas/air and methane/air flames stabilized above water-cooled sinter and ceramic radiant surface burners. The temperature and NO mole fraction were measured by coherent anti-Stokes Raman scattering (CARS) and laser-induced fluorescence (LIF), respectively. At equivalence ratios of 1 and 1.3, the flame temperature was changed either by varying the degree of heat transfer to the burner surface, through varying the exit velocity of the fuel-air mixture, or by flue-gas recirculation (FGR). At φ=1, the variation in flame temperature with exit velocity observed with the hot radiant burner is indistinguishable from that obtained from the water-cooled sinter, indicating a unique relationship between heat transfer to the burner and actual burning velocity. The variation in NO mole fraction with exit velocity shows marginal differences between the two burners. Plotting the measured NO mole fraction as a function of flame temperature for the surface burners and FGR shows that both control strategies have the same effectiveness in lowering the NO concentration at this equivalence ratio. Consideration of the consequences of these strategies on burning velocity shows that FGR can yield a higher thermal input than upstream heat loss. At =1.3, greater differences are observed between upstream heat loss and FGR as NOx abatement techniques than at φ=1. However, uncertainty in the concentrations of HCN and NH3 in the fuel-rich hot gases, and how they are affected by upstream heat loss and FGR, precludes drawing conclusions concerning the relative efficacy of these techniques. Calculations using GRI-Mech 3.0 show excellent agreement with the results at φ=1 but inadequately predict both the absolute magnitude of the mole fractions and the observed trends with abatement technique at φ=1.3.

A lif and cars study of the effects of upstream heat loss on no formation from laminar premixed burner-stabilized natural-gas/air flames

Measurements of NO concentration and temperature in laminar, premixed, natural-gas/air flames stabilized on a water-cooled sinter burner at atmospheric pressure are reported. The NO concentration and temperature, determined by laser-induced fluorescence and coherent anti-Stokes Raman scattering, respectively, are seen to vary with exit velocity of the fuel-air mixture and thereby with the degree of heat transfer upstream to the burner. For lean and stoichiometric flames, the NO concentration decreases rapidly with increased stabilization, as the contribution from the zeldovich mechanism is diminished. Further, near extinction, these flames exhibit a finite NO concentration (12±2 ppm at =1 and 2±2 ppm at =0.85), ascribed to the Fenimore mechanism that form a lower limit to NO x control strategies based on upstream heat loss, such as radiant surface combustion. Measurements on fuel-rich flames at equivalence ratios of 1.3 and 1.4 also show a strong dependence on exit velocity, allowing for the possibility of operating low-NO x radiant surface burners under fuel-rich conditions. The results are compared with those from one-dimensional flame calculations to gain more insight into the processes involved. Whereas the fuel-lean and stoichiometric flames appear to be well understood, the origin of the decrease in NO concentration with increasing stabilization in the fuel-rich flames is unclear. In addition, the practical implications of the results are discussed.

A laser-induced fluorescence and coherent anti-Stokes Raman scattering study of NO formation in preheated, laminar, rich premixed, methane/air flames

Measurements of temperature and NO mole fraction are reported in flat, laminar, stoichiometric and fuel-rich premixed methane/air flames subjected to varying degrees of preheating, up to 400 K. The flames are stabilized above a perforated ceramic tile burner. By varying the exist velocity of the preheated mixture, the flame temperature could be varied over a wide range. The temperature and NO mole fraction were measured by coherent anti-Stokes Raman scattering and laser-induced fluorescence, respectively. At φ=1, plotting the measured NO mole fraction as a function of flame temperature shows the exponential growth characteristic for the Zeldovich mechanism. At φ=1.3, the plot of NO versus flame temperature shows a region of more than 400 K in which the NO mole fraction grows only by a factor of 2, reflecting the behavior of the Fenimore mechanism. At temperatures above ∼2250 K, the NO formation initiated by O+N2 becomes important, and the mole fraction begins to rise more steeply. At φ=1.5, the NO mole fraction is less than 10 ppm between 1750 and 1950 K, but increases by a factor of 9 within the next 200 K. Calculations using GRI-Mech 3.0 capture the essence of this behavior well, but show quantitative shortcomings for the fuel-rich flames. Comparison with the calculated equilibrium mole fractions as a function of temperature shows the observed NO mole fractions under fuel-rich conditions to be generally in excess of their equilibrium values. The experimental profiles and the GRI-Mech calculations indicate that relaxation to equilibrium is very slow, even at high temperatures. The results at φ=1.5 suggest that preheated fuel-rich combustion may be fruitful for practical use.

The Influence of Ambient Air Entrainment on Partially Premixed Burner Flames: LIF Imaging of CO and OH

Abstract The influence of the presence and availability of ambient, “secondary” air on the oxidation of CO in partially premixed burner flames is considered, using laser-induced fluorescence. Composite two-dimensional images of the distributions of OH-radicals and CO-molecules in partially premixed bunsen flames, burning in the open air, arc compared with expectations based on the assumption of a closed system. The results indicate that many features of the structure of these flames arc intrinsically due to the interaction of the flame with the ambient environment. The imaging technique is further applied to elements from a burner system from a household appliance. In these experiments, the availability or secondary air is varied by changing the distance between two of the component burners. The images show a declining OH density between the burners as the distance decreases, with a concomitant increase in CO.

CO behavior in laminar boundary layer of combustion product flow

Abstract Results are reported from the experimental investigation and numerical simulation of the laminar boundary layer formed on the flat plate in flowing combustion products of propane in air. During the experiments such parameters as the temperature of the plate surface and the composition of the oncoming flow (due to a change of the fuel to air equivalence ratio φ) have been varied. In addition, when performing numerical simulations, the catalytic properties of the surface have been changed. The comparison of the measured profiles of velocity, temperature, OH and CO concentrations with the results of the calculation based on the chemically reacting boundary layer model has revealed that the model adequately described both distributions of parameters in the boundary layer and their dependence on the boundary conditions and parameters of the free stream flow.

NO formation in the burnout region of a partially premixed methane-air flame with upstream heat loss

Abstract Measurements of temperature and NO concentration in laminar, partially premixed methane–air flames stabilized on a ceramic burner in coflow are reported. The NO concentration and temperature were determined by laser-induced fluorescence (LIF) and coherent anti-Stokes Raman scattering (CARS), respectively. Upstream heat loss to the burner was varied by changing the exit velocity of the fuel–air mixture at a constant equivalence ratio of 1.3; this alters the structure of the flame from an axisymmetric Bunsen-type to a strongly stabilized flat flame. To facilitate analysis of the results, a method is derived for separating the effects of dilution from those of chemical reaction based on the relation between the measured temperature and the local mixture fraction, including the effects of upstream heat loss. Using this method, the amount of NO formed during burnout of the hot, fuel-rich combustion products can be ascertained. In the Bunsen-type flame, it is seen that ∼40 ppm of NO are produced in this burnout region, at temperatures between ∼2100 K and ∼1900 K, probably via the Zeldovich mechanism. Reducing the exit velocity to 12 cm/s reduces the flame temperature substantially, and effectively eliminates this contribution. At velocities of 12 and 8 cm/s, ∼10 ppm of NO are formed in the burnout region, even though the gas temperatures are too low for “Zeldovich” NO to be significant. Although the mechanism responsible for these observations is as yet unclear, the results are consistent with the idea that the low temperatures in the fuel-rich gases caused by upstream heat loss retard the conversion of HCN (formed via the Fenimore mechanism) to NO, with this “residual” HCN then being converted to NO during burnout.

Lean Premixed Burners

Publisher Summary This chapter discusses the lean premixed burners. The challenges associated with lean combustion in burners are often under- appreciated because these ubiquitous combustion devices have been operating lean and with low emission reliably, efficiently, and economically for decades. However, as emission requirements tighten, the demands on lean burners increase. Because these systems are usually fan or blower driven, they are susceptible to acoustic feedback; and as the mixtures approach their lean limits, the heat release becomes sensitive to fluctuations. In addition, relatively small changes in fuel composition can produce substantial changes in local stoichiometric conditions because the airflow is rarely controlled relative to the instantaneous fuel content. This combination of challenges means that turndown windows narrow if traditional high-swirl injection is used to provide flame stabilization. This chapter also discusses a reconsideration of the need for recirculation-stabilized flames by presenting a relatively new concept for lean premixed combustion, the low-swirl burner. Many of the factors discussed in this chapter, including sensitivity to fuel composition, the role of high swirl in flame stabilization, and the potential for acoustic instability growth, cut across several lean combustion technologies.

On the effects of fuel leakage on CO production from household burners as revealed by lif and cars

Measurements of the distributions of CO, OH, and temperature in flames from two commonly used, commercially available household burners are presented. The local mole fractions of CO and relative distribution of OH have been obtained using laser-induced fluorescence, while the local temperatures have been determined by coherent anti-Stokes Raman scattering (CARS). For both burners, burning in the open air, CO formation outside the main flames has been observed and attributed to the leakage of fuel-air mixture at the edges of the flame, where the fuel is subsequently converted to CO in the boundary layer between the flame and the surroundings. For a rich-premixed, multiblade burner, which gives Bunsen-like flames, the CO produced by the leaking fuel appears to be oxidized by OH arising from the outer cones of adjacent flames, and burns out to low concentrations. In the case of a lean-premixed burner, the CO produced by fuel leakage remains in the cool boundary layer without adequate burnout. Possible consequences for appliance behavior are discussed.

Spontaneous site reorientation in a mixed molecular crystal: Tetracene in benzoic acid

Absorption and fluorescence spectra of tetracene in a benzoic acid host crystal at 1.5 K are presented. The fluorescence zero‐phonon line is shifted by more than 800 cm−1 to the red of the maximum of the 120 cm−1 broad absorption origin. This shift is attributed to a lateral site reorientation of the guest upon excitation, permitted by the difference in size between the tetracene and the benzoic acid dimer it replaces. In addition, other features in the fluorescence spectrum are ascribed to proton tautomerization occurring in the host dimers in the vicinity of the guest. These features disappear upon deuteration of the host acid protons, while the magnitude of the red shift is virtually unchanged.