|M.Sc Student||Shtauber Itzhak|
|Subject||A study of Polydisperse Spray Diffusion Flames and their|
Extinction in Co-flow
|Department||Department of Aerospace Engineering||Supervisor||Professor Emeritus Jerrold Greenberg|
|Full Thesis text - in Hebrew|
The study of the behavior of flames is a central issue in planning and improving processes of combustion in many applications. In many cases the flame is turbulent. A popular method for modeling turbulent flame structure is by describing it as a collection of laminar 'flamelets'. Understanding the behavior of these 'flamelets' can be significant for the overall behavior of the turbulent flame. In many combustion engineering applications the fuel is supplied as a spray of droplets. Thus, an extended modeling of turbulent spray flames needs to consider laminar spray 'flamelets' and their behavior. In this context it is very important to examine the conditions for flame extinction which impacts on the efficiency of the combustion, sooting of the structure of the engine thereby damaging it and increasing pollution to the environment.
In their classical article from 1928, entitled "Diffusion Flames", Burke and Schumann discussed the behavior of a diffusion flame in a configuration in which fuel vapor flows in an inner duct and air flows in an outer duct. They suggested an analytic solution for the shape of the flame.
In 1989, Greenberg reexamined the Burke-Schumann diffusion flame assuming the fuel is introduced in the form of liquid droplets suspended in an inert gas stream, as is common in jet engines. The author suggested an analytic solution for the flame shape and revealed that use of the spray can quite markedly affect the flame characteristics. Although a realistic spray contains various sizes of droplets, for mathematical simplification the latter work assumes a uniform size distribution of droplets.
In the present work, a model of a polydisperse spray diffusion flame in co-flow is suggested and an investigation of extinction at the tip of the flame is conducted in order to examine how the non-uniform distribution of the spray droplets influences the flame characteristics; in particular, tip extinction. This study was conducted in order to understand better this dependence and to try to provide tools to control the efficiency of the combustion and to prevent sooting and environment pollution.
Based on new analytical solutions, new results are presented for the flame characteristics and tip extinction sensitivity versus the spray characteristics. The results show that the flame characteristics, and, particularly, extinction at the tip, are strongly affected by the droplets distribution. Furthermore, a polysectional model is indeed necessary in order to examine spray diffusion flames.