Combustion Science and Technology

MElBURG, EC KART

doi: 10.1080/00102209008951621pmid: N/A

Abstract Abstract-We develop a new Lagrangian numerical technique to investigate the interaction between fluid mechanics and chemical reaction in diffusion flames through volumetric expansion and baroclinic vorticity production. We base our numerical method on the mixture-fraction variable formulation and the Burke-Schumann limit of an infinitely thin flame sheet, In this way. we do not attempt to resolve the flame structure in detail, but instead are in a position to simulate combustion in more complex flow fields. The method employs Lagrangian blobs that carry gradients of the mixture-fraction variable. Other blobs act as sources to account for thermal expansion, while vortex blobs are introduced to represent the baroclinic vorticity production. These blobs deform as a result of diffusion, strain, and thermal expansion. In its Lagrangian nature, our approach to diffusion flames corresponds to the technique recently developed by Ghoniem for premixed flames. In the incompressible flow limit, we demonstrate accuracy and convergence of our technique by comparison with available finite-difference results for the case of a flame sheet wrapping around a vortex. We then proceed to include the effects of volumetric expansion and baroclinic vorticity production. We find that the effect of thermal expansion is very strong in regions of temperature gradients in the circumferential direction, where fluid particles undergo rapid temperature changes. We furthermore observe the production of both co- and counterrotating vorticity that significantly affects the dynamics of the flow field. Two vorticity layers of opposite sign adjacent to the flame sheet transport additional fluid towards the burnt core, thus causing it to grow in an accelerated fashion.

ARAI, M.; SAITO, K.; ALTENKIRCH, R. A.

doi: 10.1080/00102209008951622pmid: N/A

Abstract Under certain circumstances, the water on which a burning pool of liquid fuel is supported may begin to boil. The water vapor that is released and escapes through the fuel surface tends to atomize the oil, which results in an emulsive-droplet flame above the fuel surface. This phenomenon, called boilover, has been observed for large scale pool fires, but the mechanism causing it to occur has not been fully investigated yet. This paper describes fundamental aspects of the effect of a boiling water sublayer on the behavior of pool fires. A burner system in which the burning surface of the fuel can be fed into the flame so that the fuel/water interface with respect to the edge of the container remains fixed was used. Ten different single-component and six different multicomponent fuels were tested. Conventional flow visualization techniques were applied to study the liquid motion, and results for an ethylbcnzene pool in a 4.8cm diameter pan are presented. Data obtained include temperatures and mass loss history of the liquid fuel and water, flame height, and irradiance from the flame. Results show that when the water sublayer starts to boil, the temperature gradient across the fuel layer vanishes, the temperature level of the fuel decreases, and the burning rate of the fuel decreases. Contrary, the large pool fires show an opposite result that when the boilover occurs, the burning rate of the fuel increases. Thus it was suggested that for the small pool fires, additional external radiant heat is required to simulate quantitatively the boilover phenomenon for the large pool fires.

L ELLZEY, J.; BERBE, J. G.; TAY, E. Z. F.; FOSTER, D. E.

doi: 10.1080/00102209008951623pmid: N/A

Abstract In this paper, we examine the soot yield from a propane diffusion flame in a cross-flowing air stream. Soot yield, defined as the net amount of soot per gram of fuel, from a diffusion Hame in cross-flow was compared to that from a diffusion flame in co-flowing air. We observed that the orientation of the fuel jet relative to the air stream has a significant effect on the flame shape and on the soot yield. We varied the air velocity, fuel velocity, and jet diameter to determine The effect of each of these on the soot yield from the flame in cross-flow. At low air velocities, increasing the air velocity decreases the soot yield dramatically but further increases do not have a significant effect unless the fuel velocity is increased also. The jet diameter does not affect the soot yield at a constant velocity. Finally, we propose a correlation of our data to a "mixing index" defined as the product of the air and fuel velocities.

SITARSKI, MAREK A.

doi: 10.1080/00102209008951624pmid: N/A

Abstract A two-dimensional model and an algorithm for numerical studies of vaporization dynamics of small multiphase fuel droplets suspended in the superheated vapor and irradiated from one side by a high-temperature thermal radiation is presented. Theoretical estimates of distributions of radiant heal sources inside the droplets are obtained by an application of light scattering theory and the extended effective medium theory. The plotted three-dimensional absorption patterns show the hot spots within the irradiated fuel droplets. The two-dimensional model of transient heat dissipation, inside the rapidly vaporizing coal-water slurry droplet, with kinetic boundary conditions is simulated by an application of the Monte-Carlo method. The effect or temperature of the incident thermal radiation, as well as the effects of temperature and pressure of the ambient gas on the feasibility of explosive boiling of the fuel droplets arc studied in detail. The results of compulations predict conditions at which the radiation induced secondary atomization of the slurry droplets in combustors at atmospheric pressure might be possible. In combustors of heat engines operating under high pressures the described mechanism of the radiation induced internal boiling is predicted not to lead to the secondary atomization of coal-water slurry droplets.

CHOW, W. K.; LEUNG, W. M.

doi: 10.1080/00102209008951625pmid: N/A

Abstract The solid-wall boundary effect on a building fire field model is discussed. In this model, the k - e model is used to predict the fire-induced flow (in primitive variables) and temperature field. The low Reynolds number model is proposed to substitute the traditional wall-function approach for describing solid-wall boundary effects. This is better because both the velocity and temperature distribution can be predicted within the buffer layer. Numerical experiments are performed on a physical model studying compartmental fire and an experiment on unconfined ceiling jet, both performed at the National Bureau of standards, U.S.A. For these simulations, better results arc achieved using this modified solid-wall boundary approach.

MEGARIDIS, CONSTANTINE M.; DOBBINS, RICHARD A.

doi: 10.1080/00102209008951626pmid: N/A

Abstract The soot formed in a coannular ethene diffusion flame was extracted by a thermophoretic sampling technique for examination by transmission electron microscopy. A detailed analysis of the particle statistics of the samples is presented. Primary particle diameters range from 10 to 40 nm and their spatial variation within the flame affords quantitative information on the specific soot surface growth and oxidation rates. The moment ratio of the aggregate volume-equivalent diameters (D63D30,) is found to be close to the values predicted for the self-preserving size distribution in either the continuum or free molecular limit. The fractal dimensions (1 62 and 1.74) for Iwo samples examined are in the range reported in other combustion related experiments as well as in computational simulations of particle growth when cluster-cluster aggregation is an important growth mechanism The primary particle diameters that have been reported by various investigators in a wide variety of flame environments are reviewed. It is concluded that, while aggregate size may vary over many orders of magnitude, the values of both fractal dimensions and primary sizes of flame-gcneraled carbonaceous soot and silica fume are narrowly confined.

DAGAUT, PHILIPPE; CATHONNET, MICHEL; BOETTNER, JEAN-CLAUDE

doi: 10.1080/00102209008951627pmid: N/A

Abstract Propyne oxidation was modeled using a comprehensive kinetic reaction mechanism including the most recent findings concerning the kinetics of the reactions involved in the oxidation of propyne. The proposed mechanism is able to reproduce experimental data obtained in our high-pressure jet stirred reactor and in shock tube in the pressure range 1-13 atm, for temperatures extending from 950 to 2000 K and equivalence ratios 0.5 to 2. The proposed propyne oxidation mechanism is able to correctly reproduce ignition delay times measured in shock tube and molecular species concentrations measured in our jet stirred reactor.

PETTERSSON, LARS; SJO¨STRO¨M, KRISTER

doi: 10.1080/00102209008951628pmid: N/A

Abstract The possibility of obtaining high efficiency and low emissions of hazardous compounds by using decomposed methanol, i.e., hydrogen and carbon monoxide, as a fuel for spark-ignition engines has been investigated theoretically and experimentally. A cycle simulation predicts that the most important advantage with the system is the opportunity to run the engine on lean air/fuel mixtures. In the experiments, bottled gas was used instead of gas from a decomposition reactor. The results indicate that the efficiency gain, relative to neat methanol, is 15-20% in the torque interval 10-60 Nm. Detected NO, emissions are very low. A control system for the onboard decomposed methanol engine has been developed, which uses engine speed as the governing parameter.

ISHIDA, HIROKI; WATANABE, HISAYOSHI

doi: 10.1080/00102209008951629pmid: N/A

Abstract The suppression of spread of flame over A-Heavy oil pool by W/O (water-in-oil) type emulsification was studied experimentally in detail. W/O emulsified A-Heavy oil, in which the water content is 2-10 vol %, was prepared by mixing A-Heavy oil with water without surface active agent in the gear pump type emulsification apparatus. The effects of W/O emulsification on the flash point, the velocity of flame spread over the pool and on the development of preceding surface flow ahead of flame leading edge were studied. The results were discussed comparing the W/O type emulsified A-Heavy oil with the original one, The flash point of the W/O emulsion rises 2 ∼ 4°C from that of the original A-Heavy oil with increase in the water content up to 2 vol %. The velocity of flame spread can be suppressed owing to the emulsification to about ½ ˜ ⅔of that over the original A-Heavy oil. Preceding surface flow ahead of flame is promoted owing to the emulsification. From these results, the present study concludes that the suppression of flame spread by the emulsification is mainly due to both the rise in flash point and the increase in convective heat loss caused by elongated surface flow.