Droplet combustion

Most theories of droplet combustion assume a spherical, symmetrical droplet surrounded by a spherical flame, for which the radii of the droplet and the flame are denoted by and respectively. The flame is supported by the fuel diffusing from the droplet surface and the oxidant from the outside. The heat produced in the combustion zone ensures evaporation of the droplet and consequently the fuel supply. Other assumptions that further restrict the model include (/) the rate of chemical reaction is much higher than the rate of diffusion and hence the reaction is completed in a flame front of infinitesimal thickness (2) the droplet is made up of pure Hquid fuel (J) the composition of the ambient atmosphere far away from the droplet is constant and does not depend on the combustion process (4) combustion occurs under steady-state conditions (5) the surface temperature of the droplet is close or equal to the boiling point of the Hquid and (6) the effects of radiation, thermodiffusion, and radial pressure changes are negligible. 

The study of the combustion of sprays of Hquid fuels can be divided into two primary areas for research purposes single-droplet combustion mechanisms and the interaction between different droplets in the spray during combustion with regard to droplet size and distribution in space (91—94). The wide variety of atomization methods used and the interaction of various physical parameters have made it difficult to give general expressions for the prediction of droplet size and distribution in sprays. The main fuel parameters affecting the quaHty of a spray are surface tension, viscosity, and density, with fuel viscosity being by far the most influential parameter (95). 

Theoretical modeling of single-droplet combustion has provided expressions for evaporation and burning times of the droplets and the subsequent coke particles. A more thorough treatment of this topic is available (88,91—93,98). 

At the initiation of combustion, the heat-up (second) term of Eq. (6.136) can be substantially larger than the vaporization (first) term. Throughout combustion the third term is fixed. Thus, some [27, 28] have postulated that droplet combustion can be considered to consist of two periods namely, an initial droplet heating period of slow vaporization with... 

The value of G was shown to have a profound effect upon the flame location and distribution of temperature, fuel vapor, and oxygen. Four types of behaviors were found for large G numbers. External sheath combustion occurs for the largest value and as G is decreased, there is external group combustion, internal group combustion, and isolated droplet combustion. 

Isolated droplet combustion obviously is the condition for a separate flame envelope for each droplet. Typically, a group number less that 10 2 is required. Internal group combustion involves a core with a cloud where vaporization exists such that the core is totally surrounded by flame. This condition occurs... 

Other applications of microparticles include spray drying, stack gas scrubbing, particle and droplet combustion, catalytic conversion of gases, fog formation, and nucleation. The removal of SO2 formed in the combustion of high-sulfur coal can be accomplished by adding limestone to coal in a fluidized bed combustor. The formation of CaO leads to the reaction... 

Methylated PCU Alkene Dimer — C24H28, is a low-melting point (about 55 °C) mixture of up to 64 isomers. In contrast to the base PCU alkene dimer, the methylated PCU alkene dimer dissolved in a large proportion in JP-10, which was the selected fuel system used in this study and stable solutions in proportion of up to 18% have been obtained. The addition of methylated PCU alkene dimers to JP-10 induced droplet boiling and increased the fuel heat output. These mixtures have been studied by means of suspended droplet combustion and spectrometric analyses. 

Stable mixtures to 18% of methylated PCU alkene dimers in JP-10 have been obtained. This concentration is sufficiently large to increase the density of the fuel to a significant degree and also to augment the exothermicity of the droplet combustion. 

The time to achieve effervescent droplet combustion is reduced as the concentration of the HED component in the mixture increases. However, this... 

Takahashi, F., I. J. Heilweil, and F.L. Dryer. 1989. Disruptive burning mechanism of free slurry droplets. Combustion Science Technology 65 151-65. 

Card, J. M., and F. A. Williams. 1992. Asymptotic analysis with reduced chemistry for the burning of n-heptane droplets. Combustion Flame 91 187-99. 

Godsave examined the burning of drops in unconfincd air. Ignition w as accomplished by momentarily exposing the droplet to a small gas flame. Kumagai and Isoda carried out their tests in essentially the same manner. In some of their later tests they introduced a vibrating air field for investigation of its effect on droplet combustion. Kobayasi and Nishiwaki both studied the combustion of individual drops in a horizontal,... 

Each of the above experiments possesses features which facilitate study of particular aspects of the complex process of droplet combustion. However, all suffer from the shoiftcoming that the drops or model diameters investigated are appreciably greater than the sizes most commonly experienced in modern propulsion systems and industrial furnace applications. On the basis of the results obtained, only extrapolations and... 

EFFECT OF GAS TEMPERATURE ON BURNING RATE. It has been common practice in certain industrial applications to preheat the air before it enters the combustion region. The theoretical analysis of the droplet combustion process indicates that such an increased air temperature does not change materially the mass burning rate,... 

Rosner, D. E. (1967). On liquid droplet combustion at high pressure. American Inst of Aeronautics Astronautics J., 5 163-166. 

Breen, B.P., Beltran, M.R., Steady-State Droplet Combustion with Decomposition Hydrazine-NTO. Dynamic Science Corp. Monrovia, Calif., 1966. 

Measurements from synthetic fuel spray flames and laboratory droplet reactors indicated the extent to which fuel properties and combustion conditions influenced particulate yields. A series of seven fuels were tested in a 21 kW spray combustor for total particulate by gravimetric sampling and soot by Bacharach smoke number. Variations in total particulate were dominated by the tendency of the fuel to form ceno-spheres while smoke number correlated with the C H ratio of the fuel. The laboratory droplet studies were performed in a gas flame supported reaction environment. These results confirmed the correlation between soot yield and C H ratio. In addition, two distinct forms of disruptive droplet combustion were observed. 

Experimental. To further understand the process of droplet combustion and particulate formation, a more fundamental study of the effects of droplet size, local stoichiometry and gas-droplet relative velocity has been carried out. This work made use of a controlled flow variable slip reactor in which the combustion of droplet streams can be examined under well defined conditions. 

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