Showing papers on "Laminar flame speed published in 1970"

Flame Spreading Above Liquid Fuels: Surface-Tension-Driven Flows

Abstract: Convective heat transfer through the liquid fuel below a spreading flame is considered as a rate controlling mechanism. Thus, a surface-tension-driven liquid flow, induced by the temperature profile ahead of a spreading flame, is analyzed. Velocities, pressures and surface heights are determined for a two-dimensional flame spreading at a steady rate. It is demonstrated that convection can occur near the suface ahead of the flame and in the direction of propagation and, thus, that liquid-phase convective heat transfer can be a plausible rate-controlling mechanism for flame propagation.

An approach to turbulent flame theory

Abstract: Is it possible to express the problem of calculating turbulent flame speeds as an eigenvalue problem that is analogous to the laminar flame speed problem? It is argued for grid turbulence that the answer is affirmative, and some benefits of pursuing such a calculation are exploited for the limiting case of a first-order reaction with vanishingly small heat release. The streamwise turbulent transport of reactant occupies a central role in the analysis. The equation governing the ensemble average of this quantity assumes different simplified forms in the limits of small-scale and large-scale turbulence. The criterion which is obtained for separating the small-scale and large-scale regimes differs from that of Damkohler and also from that of Kovasznay and Klimov. In the small-scale regime, turbulence produces a spatially varying diffusivity, the form of which can be ascertained only through an investigation of non-linear equations describing the statistical dynamics of production and decay of the velocity–concentration correlation. In the large-scale regime, which is of greater practical importance, the ensemble average of the streamwise turbulent reactant flux satisfies a linear ordinary differential equation whose solution for the growth and decay of the flux contains effects resembling wrinkling of the laminar flame, increasing of the effective diffusivity and augmentation of the effective reaction rate. An exact solution to the linear eigenvalue problem which arises in the large-scale limit reveals that turbulence enhances mean reactant consumption in the upstream portion of the flame and retards reactant consumption downstream. Formulas are given for the increase in flame speed and the increase in flame thickness that are produced by turbulence in the large-scale limit. Since the equations are relatively tractable in the large-scale limit, it is suggested that further study of these equations may yield improved descriptions of realistic turbulent flames.

Effects of Turbulence on Flame Propagation in Closed Vessel

Abstract: The effects of turbulence on the flame propagation in the propane-air mixture in a closed vessel were investigated by analyzing the high speed motion pictures of flame, the ion-currents due to combustion and the change of the gas compositions in combustion zone. In the combustion chamber, the uniform turbulent conditions were produced by moving suddenly a perforated plate which was driven by the spring and the electro-magnetic control system. The main results are as follows: (1) The combustion zone of turbulent flame may be composed of a lot of flame elements which are similar to laminar flame, and the apparent burning velocity is increased by enlargement of combustion zone due to turbulence. (2) during the early stage following ignition, the flame speed is affected by both factors of mixture strength and intensity of turbulence. with further development of the flame, however, the intensity of turbulence becomes a dominant factor determining thee flame speed.

Variation of the Flame Propagatin Time in a Spark Ignition Engine

Abstract: Effects of operating conditions of a CFR engine on variations of flame travel time were studied statistically. Cyclic variations of falme travel time showed some departure form a normal distribution, but those of the "apparent flame propagation velocity", the distance of flame travel divided by the flame travel time, were shown to follow a normal one in general.The mean apparent flame propagation velocity υ^-f was found to have linear correlation with the standard deviation σ. It was shown also that υ^-f/SL (SL : laminar burning velocity) was directly proportional to σ/SL in all the results obtained at different inlet mixture temperatures, and to Harrow's Reynolds number of the mixture before combustion over a wide range.An explanation for actual pattern of cyclic distribution of flame travel was made by incorporating the effects of cyclic variation of mixture stregth and the flame velocity due to the turbulence in the combustion chamber.

Calculation of the stationary zone of the reaction with the flame H2+O2 as an example

Abstract: 1. A method of calculation of the stationary zone of a chemical chain reaction with the boundary conditions set at finite distances was proposed. This method permits a calculation of a unidimensional flowtype chemical reactor. 2. The characteristics of the reaction zone and flame velocity were calculated for a hydrogen-air flame. These characteristics were compared with the experimental data.