Combustion Science and Technology

Kim, Ju Pyo; Schnell, Uwe; Scheffknecht, Günter

doi: 10.1080/00102200701838735pmid: N/A

Emissions of nitrogen oxides from fossil fuel combustion are a major environmental problem because they have been shown to contribute to the formation of acid rain and photochemical smog. MILD (Moderate and Intensive Low oxygen Dilution) combustion is a promising technology to decrease pollutant emissions and to improve combustion efficiency. A combination of air preheating and fuel dilution with combustion products of low oxygen concentration are the main features of this technique. In MILD combustion mode, preheated air and fuel are gradually mixed with large amounts of recirculated exhaust gas. The objective of the present work is to investigate a global reaction mechanism for natural gas combustion to predict the observed nitrogen oxide and carbon monoxide levels in MILD combustion mode. For this purpose, the comparison of several global reaction mechanisms during the combustion process is paramount. The interaction between turbulence and chemistry is modeled by an advanced Eddy Dissipation Concept (EDC) in order to compare some global reaction mechanisms. For validation purposes, this model is applied to a non-premixed turbulent jet flame of a pilot methane-air mixture (Sandia flame D), and to the semi-industrial scale experiments at the IFRF Research Station. The modeling results are discussed and compared with measurements.

Tachajapong, Watcharapong; Lozano, Jesse; Mahalingam, Shankar; Zhou, Xiangyang; Weise, David R.

doi: 10.1080/00102200701838800pmid: N/A

Crown fire initiation is studied by using a simple experimental and detailed physical modeling based on Large Eddy Simulation (LES). Experiments conducted thus far reveal that crown fuel ignition via surface fire occurs when the crown base is within the continuous flame region and does not occur when the crown base is located in the hot plume gas region of the surface fire. Accordingly, the focus in this article is on crown fuel ignition when the crown base is situated within the intermittent flame region. In this region, the flame shape and height changes with time over the course of pulsation. This causes the flame to impinge on the crown fuel base and the hot gas is forced through the crown fuel matrix. Under certain conditions, it is observed that the crown fuel bulk density affects the impingement of flame and the ignition of crown fire. The crown fuel properties used were estimated for live chamise (Adenostoma fasciculatum) with a fuel moisture content of 44% (dry basis). As the crown fuel bulk density is increased from 0.75 kg·m−3 to 1.75 kg·m−3, it is observed that the average hot gas velocity inside the crown matrix decreases from 0.70 m·s−1 to 0.52 m·s−1, thus, resulting in less entrained air passing through the crown fuel and more energy accumulation inside the crown fuel matrix. Higher bulk density also influences the surface fire. As the hot gas flows into the crown fuel matrix is retarded, the average hot gas temperature at the crown fuel base increases from 768 K to 1,205 K. This is because the mixing rate of air and combustible gas around the base of crown fuel increases. Although higher fuel bulk density means more fuel must be heated, the increase in accumulated energy per unit volume within the crown fuel matrix is higher than the additional heat needed by the fuel. Thus, the average crown fuel temperature increases and ignition occurs at higher bulk density.

Wakatsuki, Kaoru; Jackson, Gregory S.; Kim, Jungho; Hamins, Anthony; Nyden, Marc R.; Fuss, Stephen P.

doi: 10.1080/00102200701838941pmid: N/A

Infrared transmission of propane, n-heptane and propylene samples was measured using Fourier transform infrared spectroscopy (FTIR) for temperatures ≤1000 K to facilitate calculation of absorption coefficients of fuel molecules at temperatures representative of non-premixed flame interiors. Spectrally resolved fits of the absorption coefficient data using a semi-empirical quantum-based expression provide a basis for calculating infrared spectra at any temperature. Experimentally-derived Planck mean absorption coefficients (κp) of these fuels are compared with that of methane calculated from the HITRAN database since methane absorption has been used to model fuel absorption in fires. κP's for propane and heptane have similar characteristics over the entire temperature range. Methane and propylene with their higher proportion of absorption in low frequency bands have peak κP at lower temperatures where blackbody radiation intensity peaks near the spectral range of these bands. Propylene with low frequency absorption bands associated with the C=C bond has the highest κP at temperatures <600 K. N-heptane has the largest κP at temperatures ≥800 K where blackbody emission intensity peaks near or above the spectral range of the C–H stretching bands. Implications of these results on fuel absorption of radiative heat transfer in flames are discussed.

Park, Seul-Hyun; Choi, Seuk-Cheun; Choi, Mun Young; Yozgatligil, Ahmet

doi: 10.1080/00102200701839022pmid: N/A

Spherically symmetric ethanol droplet combustion experiments were performed to investigate the influence of initial droplet diameter, ambient pressure and inert substitution on the burning and sooting behaviors of ethanol droplet flames. Experiments were performed using the 2.2 sec reduced-gravity droptower facilities at the NASA Glenn Research Center. Noting the importance of transport characteristics of heat and species and their attendant effects on flame temperature and residence time on the sooting mechanism of diffusion flames, parameter adjustments were made to vary the sooting over a wide range of conditions. In these experiments, the residence times for fuel vapor transport were varied using changes in initial droplet diameters (from 1.6 mm to 2.2 mm) and ambient pressure (from 0.10 MPa to 0.24 MPa) and inert substitutions (He, Ar, and N2). The flame temperatures and flame standoff ratios were varied using different inert substitutions. For each experiment, the soot volume fraction, droplet burning rate, sootshell and flame dynamics, flame temperatures, and flame radiative emission were measured. These measurements enabled calculation of the fuel vapor transport residence times (from droplet surface to the flame front) which provides a measure of the duration for pyrolysis reactions, soot nucleation, and soot growth. The experimental measurements demonstrated that ethanol droplets burning in Ar inert environments produced the highest soot volume fraction, followed by N2 inert environments, and He inert environments, which produced the lowest soot volume fraction. For the various inert environments, the flame temperature distribution and the flame standoff ratio were only weakly affected by changes in both initial droplet diameters and ambient pressures. However, significant increases in soot volume fraction were observed as the initial droplet diameter and ambient pressure were increased. The coupled analysis of the flame temperature and the residence time for fuel vapor transport provided good correlation with the observed variations in sooting in microgravity droplet flames.

Yamashita, Kiyotaka; Imamura, Osamu; Osaka, Jun; Tsue, Mitsuhiro; Kono, Michikata

doi: 10.1080/00102200701839097pmid: N/A

The burning rate constants of ethanol droplet flame under uniform electrical fields were investigated experimentally and numerically. The droplet was burned between two flat electrodes and 1–7 kV DC Volts were applied. The combustion chamber was dropped from the top of a drop tower and the experiment was carried out under microgravity environment to eliminate the effect of buoyancy. Direct photographs of the droplet flame were captured and the change in droplet diameter was measured to obtain burning rate constants. A two-dimensional numerical simulation was also conducted on the droplet combustion in a uniform electrical field. A chemical reaction model with some elementary reactions of ion species was applied. The deformation of the electrical field was predicted by solving the Poisson equation. The predictions show that the burning rate constant increases and the flame deformation becomes large as the applied voltage increases, which is in qualitatively agreement with the experimental results. The relationship between the burning rate constant and the induced flow velocity by Coulomb force is discussed in the analogy of the burning behavior in a convective flow.

Ouf, François-Xavier; Vendel, Jacques; Coppalle, Alexis; Weill, Marc; Yon, Jérôme

doi: 10.1080/00102200701839154pmid: N/A

Fire is one of the greatest industrial risks. In nuclear facilities this is complicated by the need to ensure the containment of airborne contamination at all times, in both normal and accidental situations. In the event of a fire, the soot particles emitted in the smoke may have a double impact, first on containment (clogging of filtration barriers in the ventilation network) and secondly on fire propagation, through the radiative properties of these particles. Consequently, a better understanding of their properties is needed, not only in the fire zones, but also in the smoke of diffusion flames encountered during a fire. Here, we present a study of the physical and optical parameters of soot particles sampled in the plumes of over-ventilated diffusion flames of acetylene, toluene and polymethyl methacrylate. For these three fuels, and relative to several global equivalence ratios, we have established the size distribution for the primary particles and soot aggregates, along with morphological parameters (prefactor and fractal dimension), soot emission factors and the mass specific extinction coefficient.

Henriksen, Tara L.; Ring, Terry A.; Eddings, Eric G.; Nathan, G. J.

doi: 10.1080/00102200701845524pmid: N/A

A new approach for characterizing puffing frequency was established by performing total extinction measurements on pool fires of JP-8 (Jet Propulsion Fuel 8) and heptane using a multiple beam extinction experiment. A maximum entropy method (MEM) was applied to extract a characteristic extinction frequency that was found to correlate well with puffing frequency. The measured extinction frequency for both flames was found to have some variation with height, though this is small. The amplitude of the frequency of the measured oscillations was found to be higher for JP-8 than for heptane, and became constant one diameter above the fuel pan for both flames. The variance of total extinction in the JP-8 and heptane pool fires was approximately 20% and 17%, respectively. Correlation statistics between the various extinguished beams reveal an increase in axi-symmetry of the instantaneous oscillations with height above the pool.

Halter, F.; Chauveau, C.; Gökalp, I.

doi: 10.1080/00102200701851266pmid: N/A

Planar Rayleigh scattering measurements were used to investigate the inner structure of flamelets in premixed turbulent Bunsen flames. Measurements were performed in a high pressure chamber, where pressure is varied from 0.3 to 0.9 MPa. Two lean equivalence ratios (0.6 and 0.7) of methane/air mixtures were studied. Local information regarding the thickness and the local curvature of the flamelets were obtained. It is observed that an increase of the equivalence ratio induces a decrease of the mean local flamelet thickness. When pressure is increased from 0.3 to 0.9 MPa, the mean flamelet thickness does not decrease as for laminar case. The different external phenomena (turbulence and stretch (curvature and strain rate)) which could explain this trend are investigated.