F. Liu

The chemical effect of CO2 replacement of N2 in air on the burning velocity of CH4 and H2 premixed flames

When CO2 is added to air or is used to replace N2 in air, it is anticipated that the burning velocity of fresh fuel mixtures may be affected through the following three mechanisms: i) the variation of the transport and thermal properties of the mixture, ii) the possible direct chemical effect of CO2, and iii) the enhanced radiation transfer by CO2. Experimental measurements of the burning velocity of CH4/O2/ CO2 mixtures at various equivalence ratios and pressures have been conducted by Zhu et al. [1] using double flames in the counterflow configuration. The radiative effect of CO2 on the burning velocity of CH4/O2/N2/CO2 mixtures has been recently studied by Ju et al. [2] and Ruan et al. [3]. Moreover, it has also been pointed out in several studies that CO2 is not inert but directly participates in chemical reactions primarily through CO OH 7 CO2 H [4-6]. The objective of this study is to numerically investigate the chemical effects of CO2 replacement of N2 in air on the burning velocity of lean to stoichiometric CH4/O2/N2/ CO2 and H2/O2/N2/CO2 mixtures at 1 atm.

Implementation of an advanced fixed sectional aerosol dynamics model with soot aggregate formation in a laminar methane/air coflow diffusion flame

An advanced fixed sectional aerosol dynamics model describing the evolution of soot particles under simultaneous nucleation, coagulation, surface growth and oxidation processes is successfully implemented to model soot formation in a two-dimensional laminar axisymmetric coflow methane/air diffusion flame. This fixed sectional model takes into account soot aggregate formation and is able to provide soot aggregate and primary particle size distributions. Soot nucleation, surface growth and oxidation steps are based on the model of Fairweather et al. Soot equations are solved simultaneously to ensure convergence. The numerically calculated flame temperature, species concentrations and soot volume fraction are in good agreement with the experimental data in the literature. The structures of soot aggregates are determined by the nucleation, coagulation, surface growth and oxidation processes. The result of the soot aggregate size distribution function shows that the aggregate number density is dominated by small aggregates while the aggregate mass density is generally dominated by aggregates of intermediate size. Parallel computation with the domain decomposition method is employed to speed up the calculation. Three different domain decomposition schemes are discussed and compared. Using 12 processors, a speed-up of almost 10 is achieved which makes it feasible to model soot formation in laminar coflow diffusion flames with detailed chemistry and detailed aerosol dynamics.

Investigation of Thermal Accommodation Coefficients in Time-Resolved Laser-Induced Incandescence

Accurate particle sizing through time-resolved laser-induced incandescence (TR-LII) requires knowledge of the thermal accommodation coefficient, but the underlying physics of this parameter is poorly understood. If the particle size is known a priori, however, TR-LII data can instead be used to infer the thermal accommodation coefficient. Thermal accommodation coefficients measured between soot and different monatomic and polyatomic gases show that the accommodation coefficient increases with molecular mass for monatomic gases and is lower for polyatomic gases. This latter result indicates that surface energy is accommodated preferentially into translational modes over internal modes for these gases.

Asymptotic analysis of radiative extinction in counterflow diffusion flames of nonunity Lewis numbers

Abstract The effects of radiation heat loss and variation in near-unity Lewis numbers on the structure and extinction of counterflow diffusion flame established near the stagnation plane of two opposed free streams of fuel and oxidizer are analyzed using the asymptotic method of large activation energy. Radiation heat loss from the reaction zone is accounted for using the optically thin assumption. The main concern of this study is the thermal effects of radiation heat loss and Lewis numbers on diffusion flame extinction, particularly at small stretch rates. The existence of two extinction limits, the radiation extinction limit at a small stretch rate and the conventional quenching limit at a large stretch rate, is theoretically reproduced. This simplified analysis is able to predict the existence of inflammable limits of counterflow diffusion flames in terms of the near-unity Lewis numbers and concentrations of the fuel and oxidizer streams.

Modeling of Oxidation-Driven Soot Aggregate Fragmentation in a Laminar Coflow Diffusion Flame

In this study, three different oxidation-driven soot aggregate fragmentation models with 1:1, 2:1, and 10:1 fragmentation patterns are developed and implemented into a laminar coflow ethylene/air diffusion flame, together with a pyrene-based soot model and a sectional aerosol dynamics model. It is found that the average degree of particle aggregation (n p ) in the soot oxidation region is not correctly predicted if oxidation-driven aggregate fragmentation is neglected; whereas the incorporation of aggregate fragmentation significantly improves the n p prediction in the soot oxidation region. Similar results are obtained using the 1:1 and 2:1 fragmentation patterns. However, as the pattern ratio increases to 10:1, appreciable difference in the predicted n p is observed. As the pattern ratio becomes larger, the fragmentation effect diminishes and the predicted n p approaches that of the original model neglecting fragmentation.

A robust and accurate algorithm of the β-pdf integration and its application to turbulent methane–air diffusion combustion in a gas turbine combustor simulator

The β-pdf has been widely assumed for the probability distribution of the mixture fraction in many turbulent mixing and turbulent nonpremixed combustion models in the literature. The numerical integration of the β-pdf often encounters the singularity difficulties and only few publications have addressed this issue. An efficient, accurate and robust numerical treatment of the β-pdf integration was proposed. The present treatment of the β-pdf integration was implemented into a flamelet model to calculate turbulent methane–air combustion in a model gas turbine combustor. Numerical results obtained using the present β-pdf integration method and those based on the properties of the beta and gamma functions were compared to illustrate the accuracy of the present method. Effect of assuming the β-pdf to the mass-weighted pdf and unweighted pdf of the mixture fraction on the calculated density field was also investigated. uf6d9 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Modeling DME Addition Effects to Fuel on PAH and Soot in Laminar Coflow Ethylene/Air Diffusion Flames Using Two PAH Mechanisms

Effects of dimethyl ether (DME) addition to fuel on polycyclic aromatic hydrocarbons (PAH) and soot formation in laminar coflow ethylene/air diffusion flames were revisited numerically. Calculations were conducted using two gas-phase reaction mechanisms with PAH formation and growth: one is the C2 chemistry of the Appel, Bockhorn, and Frenklach (ABF) mechanism with PAH growth up to A4 (pyrene); the other is also a C2 chemistrymechanism newly developed at the German Space Center (DLR) with PAH growth up to A5 (corannulene). Soot was modeled based on the assumptions that soot inception is due to the collision of two pyrene molecules, and soot surface growth and oxidation follow a hydrogen abstraction carbon addition (HACA) sequence. The DLR mechanism predicted much higher concentrations of pyrene than the ABF mechanism. A much smaller value of α in the surface growth model associated with the DLR mechanism must be used to predict the correct peak soot volume fraction. Both reaction mechanisms are capable of predicting the synergistic effect of DME addition to fuel on PAH formation. The locations of high PAH concentrations predicted by the DLR mechanism are in much better agreement with available experimental observations. A weak synergistic effect of DME addition on soot formation was predicted by the ABF mechanism. The DLR mechanism failed to predict the synergistic effect on soot. The likely causes for such a failure and the implications for future research on soot inception and surface growth were discussed.

The Oxygen Index on Soot Production in Propane Diffusion Flames

An experimental study of the effect of oxygen index (OI) on soot formation in laminar coflow propane diffusion flames is presented. The OI was defined as the oxygen volumetric concentration in the oxidizer flow, O2/(O2+N2), which was varied from 21% to 37%. The influence of the OI was quantified by means of three variables: the flame height, the soot volume fraction, and the vertical distribution of radiative heat flux. The flame height was based on CH* spontaneous emission and found to vary inversely with the OI, following the classical theory of Roper. As the OI increases, the rates of soot growth and soot oxidation are enhanced, and the maximum soot volume fraction and the peak of integrated soot volume fraction also increase. Moreover, the evolution of the peak of radiative heat flux and the maximum soot volume fraction are found to follow the same evolution with the OI.

Simulation of Laser-Induced Incandescence Measurements in an Anisotropically Scattering Aerosol Through Backward Monte Carlo

Laser-induced incandescence (LII) measurements carried out in aerosols having a large particle volume fraction must be corrected to account for extinction between the energized aerosol particles and the detector, called signal trapping. While standard correction techniques have been developed for signal trapping by absorption, the effect of scattering on LII measurements requires further investigation, particularly the case of highly anisotropic scattering and along a path of relatively large optical thickness. This paper examines this phenomenon in an aerosol containing highly aggregated soot particles by simulating LII signals using a backward Monte Carlo analysis; these signals are then used to recover the soot particle temperature and soot volume fraction. The results show that inscattered radiation is a substantial component of the LII signal under high soot loading conditions, which can strongly influence properties derived from these measurements. Correction techniques based on Bouguer’s law are shown to be effective in mitigating the effect of scatter on the LII signals.

Radiative Heat Transfer Through the Fuel-Rich Core of Laboratory-Scale Pool Fires

Radiative heat transfer calculations are conducted along the axis of six axisymmetric pool fires by using the “exact” line-by-line (LBL) method, the narrow band correlated k (NBCK) model, the full-spectrum correlated k (FSCK) model, the multi-scale full-spectrum k-distribution (MSFSK) model, and the wide-band model implemented in the fire dynamic simulator (FDS). The two baseline cases correspond to 34 kW and 176 kW methane pool fires generated on a burner of 0.38 m diameter. For each heat release rate, two other moderately and heavily sooting pool fires were generated by considering higher soot volume fractions while keeping temperature and gaseous species concentrations unaltered. For each radiative model, the corresponding absorption coefficients for carbon dioxide, water vapor, carbon monoxide, and methane were generated from the same high-resolution spectroscopic databases. Model results show that the contribution of carbon monoxide to the radiative intensity can be neglected, whereas that of methane increases with the heat release rate (HRR) and decreases as the soot loading increases. It is also found that the gray approximation for soot holds for the 34 kW pool fires and the weakly and moderately sooting 176 kW pool fires but ceases to be valid for the heavily sooting 176 kW pool fire. Concerning the accuracy of the different approximate radiative models, comparisons with the LBL solutions show that the NBCK model can be used as a reference if LBL solutions are not available. On the other hand, the FDS wide-band model fails in predicting accurately the radiative intensity through the fuel-rich core of pool fires. Finally, the FSCK provide predictions within 10% of LBL solutions with the exception of the heavily sooting 176 kW pool fire where the strong attenuation of radiation by methane invalidates the “correlated” assumption of the absorption coefficient. In this case, the MSFSK model must be considered, improving substantially the predictions of the FSCK.