M.Sc Thesis


M.Sc StudentAsaf Farage
SubjectInvestigation of the Dynamics of an Edge Flame with Fuel
Spray
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Emeritus Greenberg Jerrold
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

Diffusive-thermal instabilities are well known features of premixed and diffusion flames, and have been extensively studied in recent years. The disparity between the thermal diffusivity of the mixture and the molecular diffusivities of the fuel and oxidizer is responsible for the development of these instabilities. The practical ramification of understanding such a phenomenon in liquid fueled rockets is the strong linkage between the undesirable occurrence of flame oscillations and flame extinction.

In this study, configuration of two co-flowing streams, one of fuel vapor and droplets and the other of oxidizer vapor and droplets, separated initially by a semi-infinite splitter plate was investigated. Downstream, beyond the tip of the plate, the fuel vapor and oxidizer vapor interdiffuse and a mixing layer that grows downstream is formed. A gas-phase oxidation chemical reaction takes place in the mixing layer, and what is known as an edge flame is created, standing at some distance from the tip of the plate, and followed by a trailing diffusion flame downstream.


An investigation of the way in which the initial presence of a spray of fuel droplets and/or spray of oxidizer droplets, the droplets characteristics like the evaporation coefficient and the entrance temperature of the species affects the combustion in the mixing layer is carried out. It was found that oscillations occur mainly when the diffusion rate is sufficiently low (i.e. large values of Lewis numbers or low value of the representative diffusion Damköhler number) or when the volumetric heat losses are large enough, for instance due to radiation or heat required for evaporation of liquid droplets. A large initial droplet load and/or small vaporization Damköhler number lead to a delay in availability of fuel and oxidizer vapor to sustain the flame, thereby causing the flame to be located far downstream of the splitter plate and enhancing the possibility of oscillations near extinction. It was also found that for some conditions of initial droplet loads for the fuel and oxidizer, stability can be achieved even when equivalent gaseous flames are unstable. This is done by changing the energy balance between heat generated by reaction and heat consumed by evaporation and convection to the plate surface.