Showing papers on "Laminar flame speed published in 1973"

Flame spread through porous fuels

Abstract: A quantitative analysis of the steady-state propagation of a quasi-one-dimensional flame through a thermally thin, porous layer of fuel is presented. The rate of energy transfer from the combustion zone to the fuel is assumed to control the rate of flame propagation. Energy-transfer mechanisms considered are: flame and ember radiation, surface and internal convection, turbulent diffusion of flame eddies, and gas-phase conduction. The effects of ambient flow, fuel moisture, fuel-bed slope, and endothermic pyrolysis are included. A nondimensional flame-spread velocity is obtained as a function of nondimensional fuel, flame, and ambient flow properties. It is found that, in the case of ambient flow in the direction of flame propagation, the flame-spread velocity increases rapidly as the ambient flow velocity increases. From model predictions for a moderately porous fuel, typical of pine-needle beds, the following conclusions are drawn: (1) with no ambient flow, the dominant preheating mechanism is flame radiation, with contributions from ember radiation and gas-phase conduction; (2) for most monzero ambient flow velocities, the primary mechanism is convection with a significant contribution from flame radiation; (3) energy transferred by turbulent flame eddies appears to be generally negligible; and (4) energy absorbed by pyrolysis, prior to ignition, is negligible. Excellent quantitative agreement with experiment has been obtained. More-rigorous analytical models may be formulated, based on the relative import of the energy-transfer mechanisms indicated here. Applications are discussed.

Experimental studies on the flame structure in the wake of a burning droplet

Abstract: Droplets burn in envelope flames at low approach velocities and in wake flames at high velocities. Detailed measurements on pentane burning on a porous sphere have shown that the wake portion of the envelope flame is quite different from the wake flame. The former has a near wake of a few diameters, which resembles a gaseous laminar diffusion flame, followed by a long far wake in which soot burns. The latter has a short near wake which is similar to the flame zone behind a flame holder, except for the lack of a recirculation zone, followed by a short zone of soot combustion. The burning rate for the envelope flame is about three times greater than the wake flame at a slightly higher approach velocity. Peak temperatures are about 1400°–1500°K, located near the droplet in the wake flame, but in the far wake in the envelope flame. Radiation from envelope flames is far greater than from wake flames; peak emittances are in the ratio of about 3.5 to 1. The contribution of soot to droplet flame radiation is negligible in the near wake, and approaches gas radiation in the far wake. Peak soot concentrations in the envelope flame are more than three times greater than in the wake flame.

Aerodynamic and thermal structures of the laminarboundary layer over a flat plate with a diffusion flame

Abstract: The influence of the flame reaction on the aerodynamic and thermal structures of the laminar, two-dimensional boundary layer over a flat plate has been studied experimentally The diffusion flame was produced by injecting fuel gas (methane or propane) from a porous plate surface into a uniform air stream flowing along the plate The results show that acceleration of the gas stream is observed, even at the station closeto the leading flame edge located just above the edge of the porous plate The temperature in the blue-flame zone is found to increase with the distance from this leading edge Changes in the velocity and temperature profiles are also observed in the boundary layer upstream of the leading flame edge The pressure distribution has been calculated using the measured velocity and temperatureprofiles The pressure distortion on the air-stream side of the flame zone can be atributed to the rapid increase in the thickness of the laminar boundary layer with a diffusion flame The velocity overshoot near the flame zone may be caused by this pressure distortion It is shown that the aerodynamic structure of the boundary layer changes considerably in the presence of a diffusion flame, and that even in the laminar boundary layer over a flat plate, the pressure cannot be considered to be uniform if a diffusion flame is established in it

A Theory of Turbulent Flame Development and Nitric Oxide Formation in Stratified Charge Internal Combustion Engines

Abstract: A theoretical model of turbulent flame propagation in a reciprocating stratified charge engine is developed and calculations of temperature and species concentrations as functions of space and time are made. Also pressure is calculated as a function of time. A spacial variation of mixture ratio is considered such that there is a fuel-rich region in the center of the cylinder and a fuel-lean region near the cylinder walls. The spark in the fuel-rich region results in a premixed-type of flame propagating toward the walls. A diffusion-type flame results in the wake of the other flame due to the mixing of the excess air and fuel.

Diffusion Flame Stability

Abstract: (1973). Diffusion Flame Stability. Combustion Science and Technology: Vol. 7, No. 6, pp. 241-243.

Effect of Acoustic Fields on Flames

Abstract: Experiments performed to investigate the effect of ultrasonic fields on gas diffusion flames show that they can either increase or decrease the amount of flame noise generated. The studies described show that a reduction in flame noise can only be obtained with a single frequency field. The presence of a field composed of more than one frequency results in an increase in flame noise at frequencies related to the beat frequency of the applied signals. Schlieren photographs of the flame indicate a relationship between the position of its transition to turbulence and the character of the applied field.

“Pretreatment” of a homogeneous fuel-air mixture with an electric field

Abstract: Results are shown of a study concerning the effect which “treating” a homogeneous fuel-air mixture with an electric field has on the velocity V of the flame travel through it Data are presented pertaining to the flame front velocity V and the flame front shape as functions of the electric field intensity E, the field application time t, and the time intervalτ between field removal and subsequent ignition

Mathematical theory of the stationary propagation velocity of a fine-scale turbulent flame

Abstract: A boundary-value problem in the theory of propagation of a fine-scale turbulent flame is investigated, taking into account the influence of temperature and concentration pulsations on the magnitude of the heat liberation rate. In contrast to [1], the case when a second-order reaction proceeds in the flame is examined in detail. Conditions are found for the existence of a turbulent flame; the structure of the flame front is studied by a computational method. A change in the progress of the reaction is disclosed near the propagation limits.