Effect Of Radiation Reabsorption Research Articles

Effects of radiation reabsorption on the flame speed and NO emission of NH3/H2/air flames at various hydrogen ratios

Ammonia has attracted extensive attention in the direct combustion applications due to its carbon–neutral properties. In this study, the numerical simulations of NH3/H2/air one-dimensional premixed laminar flame under the stoichiometric condition with different H2 ratios (η = XH2/(XNH3 + XH2) = 0.2–0.7) were conducted to explore the influence of radiation reabsorption on laminar flame speed and NO emission. Also the effect of radiation reabsorption on NO generation and emission in NH3/H2/air mixture when η = 0.6 with different equivalence ratios was discussed. The results showed that the radiation reabsorption effect on flame speed decreased monotonically with the H2 ratio increasing from 0.2 to 0.7, and the maximum effect was 17%. The effect on flame speed was regulated by the combination of direct radiation effect and chemical effect, which was influenced by reaction R257 NH2 + NO = NNH + OH via OH radical when η = 0.2–0.7 and ϕ = 1.0. The promotion effect of radiation reabsorption on NO generation and emission decreased monotonically with the increasing H2 proportion. The enhancement of NO generation caused by radiation reabsorption was mainly controlled by R222 N + OH<=>NO + H. On the other hand, R220 N + NO<=>N2 + O, R240 NH + NO = N2O + H and R242 NH + NO = N2 + OH affected the NO emission in the post-flame zone. At η = 0.6 and ϕ = 0.65–1.5, the promotion of NO generation by radiation reabsorption was non-linear, which was mainly affected by R257, R241 NH + NO = N2O + H and R222 reactions. The NO emission was mainly influenced by R241 and R233 in the post-flame zone at ϕ = 0.65–1.05. And R241, R242, R256 NH2 + NO = N2 + H2O and R257 in the post-flame zone dominated the NO emission when ϕ > 1.05.

C-shaped extinction curves and lean fuel limits of methane oxy-fuel diffusion flames at different oxygen concentrations

The extinction characteristics and lean fuel limits of methane oxy-fuel diffusion flames with oxygen mole fractions of 0.35 and 0.4 were numerically studied by using two different radiation models, namely optically thin radiation model (OTM) and discrete ordinates method (DOM). Experimental studies on extinction limits of the oxy-fuel diffusion flames under normal- and micro- gravity conditions were conducted. The results of methane air diffusion flame from present normal-gravity and previous micro-gravity experiments were used for comparison and analysis. C-shaped extinction curves, which consist of the stretch and the radiation extinction branches, were obtained by numerical computations using OTM and DOM for the oxy-fuel diffusion flame. The computational results by using the two different radiation models were compared with the experimental results. The computational lean fuel limit by using the DOM agrees well with the experimental result, while the result by OTM is slightly higher. The primary reason is due the effect of radiation reabsorption, which is not considered in the OTM. The computational and experimental results show that the effects of oxygen concentration of the oxidizer on the combustion region and lean fuel limit of the oxy-fuel diffusion flame are significant. The lean fuel limit of oxy-fuel diffusion flame increases with increasing oxygen concentration. Both computational and experimental results suggest that the lean fuel limit of the methane oxy-fuel diffusion flame with oxygen mole fraction of 0.4 is higher than that of the methane air diffusion flame.

Effect of radiation emission and reabsorption on flame temperature and NO formation in H2/CO/air counterflow diffusion flames

The radiation effect on flame temperature and NO emission of H2-lean (0.2H2+0.8CO) and H2-rich (0.8H2+0.2CO) syngas/air counterflow diffusion flames was numerically investigated using OPPDIF code incorporated with the optical thin model, statistical narrow band model and adiabatic condition. Firstly, the coupled effect of strain rate and radiation was studied. Disparate tendencies of NO emission with an increasing strain rate between H2-lean and H2-rich syngas flames were found at very small strain rate, and the effect of radiation reabsorption on NO formation can be neglected when the strain rate was greater than 100 s−1 for both H2-lean and H2-rich syngas flames. Because the radiation effect is vital to flames with small strain rate, its impact on flame temperature and NO emission was investigated in detail at a strain rate of 10 s−1. The results indicated that NO formation is more sensitive to radiation reabsorption than flame temperature, especially for the H2-rich syngas flame. The underlying mechanism was discovered by using reaction pathway analysis. Furthermore, the radiation effect under CO2 dilution of the syngas fuel was examined. It was demonstrated that the radiation effect on flame temperature became more prominent with the increase of CO2 concentration for both H2-lean and H2-rich syngas. The radiation effect on NO emission increased first and then decreased with an increasing CO2 content for H2-lean syngas, whereas for H2-rich syngas the radiation effect is monotonic.

Perturber Dependence of Disalignment Cross Sections of the Argon 2p2Atoms Measured at Temperatures between 77 and 295 K

In a temperature controlled glow gaseous discharge of pure argon, or a mixture of argon and helium or neon at 77, 135, 190, 240, and 295 K, a linearly polarized laser pulse produces polarized argon atoms on the 2p 2 (in Paschen notation) level from the 1s 3 level. We measure the direct fluorescence from the 2p 2 level to the 1s 2 level with its polarization components resolved. From the temporal evolution of the intensities for these components, we determine the disalignment rate. We also determine the disalignment rate coefficients due to helium, neon and argon atom collisions by separating the effect of radiation reabsorption, and then determine the effective cross section of disalignment from the rate coefficient divided by the mean relative velocity of colliding atoms. It is found that perturber rare gas atoms with larger atomic number have larger depolarization cross section. This effect can be attributed to larger values of the dipole polarizability for perturbers with larger atomic number.

Effect of Radiation Reabsorption on Laminar Burning Velocity of Methane Premixed Flame Containing with Steam and Carbon Dioxide

Flame properties in the ambience of steam and carbon dioxide are complicated because of the third-body effect and radiation reabsorption. Thus, we performed detailed chemical kinetic calculations that include the effect of radiation reabsorption to clarify the premixed laminar flame speed of such a mixture, being one of the most important properties for controlling combustion. Pressure was varied up to 5.0MPa to simulate the 1700°C class combined gas turbine system. The results show a marked increase in laminar burning velocity by considering radiation reabsorption. Laminar burning velocity was increased up to 150% in cases of methane-oxygen and steam or carbon dioxide mixtures. It was found that the preheating of the upstream, unburned mixture caused this increase. The influence of radiation reabsorption was much larger in the case of lower pressures.

The effect of radiation reabsorption on NO formation in CH4/air counterflow diffusion flames

The effects of radiative heat loss on temperature and NO formation of CH4/air and that of CH4/CO2/air counterflow diffusion flames were numerically investigated by using detailed chemistry and transport properties with emphasis on assessing the importance of radiation reabsorption. Radiative heat transfer was calculated using the discrete ordinate method, and the radiative properties of the main emitting species, H2O, CO2 and CO, in these flames were represented using a statistical narrow-band model. Results show that radiation reabsorption has little influence on the NO formation of CH4/air flame. However, radiation reabsorption does have a significant effect on the NO formation and temperature of CH4/CO2/air flames, especially for low stretch rates.

Heat loss in the channel of a plasmatron

A mathematical model is constructed and a computational experiment is carried out to determine the characteristics of a longitudinally blown electric arc in the channel of a plasmatron under conditions of local thermal equilibrium. A feature of the model is that it makes allowance for radiation transfer in the real spectrum of an electric- arc plasma. An algorithm for solving the problem is presented and the effect of radiation reabsorption on the radiation and conductive heat loss in the plasmatron channel is shown.

Radiation reabsorption in a laser-produced plasma

Taking into account the emission and absorption of resonance radiation in a recombining laser-produced plasma of intermediate density, the system of rate equations for the population densities coupled with the radiative transfer equation is approximately treated. In the case of spatially varying absorption we derive an approximate form of the rate equation determining the population density of the upper resonance level. By applying this relation to an axially symmetric plasma a simple formula that describes the effect of radiation reabsorption on the spatial behaviour of the population density is obtained.

Effect of radiation reabsorption on thermal characteristics of electric arcs in an air flow

The thermal characteristics of electrical arcs calculated with assumption of volume scintillation and in the presence of radiation reabsorption are compared. It is shown that consideration of radiation reabsorption significantly decreases the axial temperature and the thermal flux to the plasmotron wall.

Theoretical model of visible radiation from rare gas flashlamps

A simple theoretical model of visible light emission from xenon flashlamps is presented. The continuum light emission is calculated from the rate of electron-ion recombination in the xenon plasma, which is treated as a greybody in thermal equilibrium. The effect of radiation reabsorption is calculated in terms of the temperature-dependent greybody emissivity. The model predictions of radiated power and energy are compared to measured data. Reasonable agreement is obtained over a wide range of parameters of practical interest. Thus the model provides a useful analytical tool for first-order engineering design of xenon flash-lamp illumination systems.