摘要:

Flame flickering of jet diffusion flames reflects the instability of the hot gases ascending column, This phenomenon, which is related to buoyancy, has been characterized in terms of pressure and gravitational force by experiments conducted on the ground and on board of an aircraft. The measurements show that the relationship between the frequency F and the gravity g is very close to F∼ & g2/3when the burners have a sufficiently small outlet diameter. These measurements are entirely in agreement with a theoretical model based on the hydrodynamic instability of the layer of burnt gas under the influence of buoyancy. It is also confirmed, by varying the ambient pressure, that a model of the form F ∼ P1/3closely fits the observed phenomena with gas jet diffusion flames.

ISSN:0010-2202    

DOI:10.1080/00102209708935648

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The objective of this research is to improve understanding of the combustion of binary fuel mixtures in the vicinity of the critical point. Fiber-supported droplets of mixtures of w-heptane and n-hexadecane, initially 1 mm in diameter, were burned in room-temperature air at pressures from 1 M Pa to 6 M Pa under free-fall microgravity conditions. For most mixtures the total burning time was observed to achieve a minimum value at pressures well above the critical pressure of either of the pure fuels. This behavior is explained in terms of critical mixing conditions of a ternary system consisting of the two fuels and nitrogen. The importance of inert-gas dissolution in the liquid fuel near the critical point is thereby re-emphasized, and nonmonotonic dependence of dissolution on initial fuel composition is demonstrated. The results provide information that can be used to estimate high-pressure burning rates of fuel mixtures.

ISSN:0010-2202    

DOI:10.1080/00102209708935649

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

An experimental and numerical study is performed to elucidate the mechanisms by which bromolrif-louoromethane (CF3Br) extinguishes laminar nonpremixed flames. Experiments are conducted on methane flames stabilized in the counterflowing configuration. The fuel and oxidizer streams are diluted with nitrogen and the inhibitor CF3Br is added to one of the reactant streams. The volumetric flow rate of the reactants and their composition at extinction are used to calculate the characteristic strain rate at extinction employing a previously developed theory. The critical conditions of extinction are measured with various amounts of the inhibitor added to either the fuel or the oxidizer stream. Numerical calculations are performed using detailed chemistry and applying boundary conditions similar to those in the experiments. The chemical-kinetic mechanism employed in these calculations includes the detailed description of fuel oxidation and inhibition by CF3Br. In general, the critical conditions of extinction calculated using this detailed chemical-kinetic mechanism are found to agree reasonably well with the measurements. The influence of preferential diffusion is examined. The analysis of the experimental and numerical results show that CF3Br extinguishes the flame by chemical mechanisms. Also, CF3Br is shown to be more effective in extinguishing the methane flame when it is added to the oxidizer stream rather than to the fuel stream.

ISSN:0010-2202    

DOI:10.1080/00102209708935650

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Design and optimization of high performance rocket engines may be improved by detailed studies of the basic combustion mechanisms. Much detailed information exists on elementary processes such as atomization, multiple jet interactions, vaporization of single droplets, structure of spray flames, ignition of nonpremixed systems etc. It is however important to approach the real conditions existing in rocket motors and to this purpose several facilities for cryogenic propellant combustion research have been designed and constructed. One experimental set-up designated as “Mascotte” is operated by ONERA and used for fundamental research as well as technical studies. This article describes results of experiments conducted in this facility by our laboratory. Two series of tests carried out during the last two years have provided a large set of images of combustion in a liquid oxygen/gaseous hydrogen coaxial injection geometry operating at atmospheric pressure and at 5 and 10 bars. The data correspond to laser elastic scattering of the spray, spontaneous emission of OH radicals and planar laser induced fluorescence of these radicals. Fluorescence is obtained by pumping the EQUATION band of OH. Off-resonance light radiation is observed with an intensified CCD camera. The large data base of images collected in these experiments provide the general flame structure in the injector nearfield, and may be used to determine the position of flame stabilization. Effects of global injection parameters (momentum flux ratio, Weber number, mixture ratio) and operating pressure are described. It is shown that the conditions prevailing in the liquid and gaseous jets influence the flame stabilization process, the instantaneous combustion patterns and the mean flame shape.

ISSN:0010-2202    

DOI:10.1080/00102209708935651

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The effect or radiative heat loss from isolated droplet flames is usually assumed to be negligible. For the small droplet sizes studied in most isolated droplet combustion experiments conducted to dale (< 1.0 mm), this assumption has been shown to be reasonable. For example, by neglecting radiation, a detailed numerical model accurately predicts the burning rate, flame position and extinction diameter for 1 millimeter-sized methanol/waler droplets. However, recent space-based 3 to 5 millimeter methanol/water droplet combustion data show an increase in extinction diameter and a decrease in burning rate with increasing initial diameter. These results suggest that, at larger initial droplet diameters, the effect of radiative heat loss cannot be neglected. By including a radiation sub-model, the modified numerical model predicts that at droplet diameters greater than about 1 mm the effect of radiation results in a decrease in burning rate and a non-linear increase in extinction diameter with increasing initial diameter. At very large initial diameters (<6 mm in atmospheric air), the model predicts that the droplet flame will ignite, but quickly self-extinguish due to excessive radiative heat loss.

ISSN:0010-2202    

DOI:10.1080/00102209708935652

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor