Spray penetration and cloud combustion

In the analysis considered in the preceding section, it was assumed that a uniform spray had been established initially and that once ignited, each droplet burned with an envelope flame around it. These conditions have been achieved reasonably well in the laboratory for various fuel-lean sprays [65]. However, in practical systems the sprays are not uniform, the manner in which the spray penetrates the oxidizing gas is important, and a cloudburning mode of combustion (in which diffusion flames surround groups of droplets, see the last paragraph of Section 3.3.6) may occur [2], [79]. These realities motivate studies of spray penetration and cloud combustion. 

Literature on theories of collective effects of droplet interactions was cited in Section 3.3.6. An approximate criterion for the occurrence of a cloud-burning mode with diffusion flames surrounding droplet clouds is readily stated. The overall fuel-gas ratio / v (where v is the stoichiometric mass ratio and / is the equivalence ratio) is (/ v From 

For typical hydrocarbon sprays in air, v and by using B 10 and Pilpf 10, we find that equation (115) becomes (/ 0.7. Although there are a number of inaccuracies here, for example, associated with the use of equation (3-63), it may be concluded that stoichiometric hydrocarbon sprays in air may be expected to experience at least some cloud-burning effects. Analyses of spray deflagrations with droplet-group burning have not been pursued. 

Williams, Combustion of Sprays of Liquid Fuels, London Elek Science, 1976. 

Williams, General Description of Combustion and Flow Processes, in 

Williams, General Description of Combustion and Flow Processes, in Liquid Propellant Rocket Combustion Instability, D. T. Harrje and F. H. Reardon, eds., NASA SP-194, Washington, D.C. U.S. Government Printing Office, 1972, 37-45. 

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The presentation of the subject of spray combustion in Chapter 11 is not greatly different from that in the first edition. An updated outlook on the subject has been provided, and the formulation has been generalized to admit time dependences in the conservation equations. The analysis of spray deflagration has been abbreviated, and qualitative aspects of the results therefrom have been anticipated on the basis of simplified physical reasoning. In addition, brief discussions of the topics of spray penetration and of cloud combustion have been added. 

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