טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentDvorjetski Ariel
SubjectA Theoretical Investigation of the Behaviour of Counterflow
Spray Diffusion Flames
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Emeritus Jerrold Greenberg
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

In many aeronautical applications such as jet-engine combustion chambers, the combustion is carried out using a liquid fuel spray.

Combustion processes in two phase systems are very complicated, depending on many coupled processes, including mass, momentum and energy exchanges between the liquid spray phase and the gaseous environment. Since the interactions between physical phenomena that occur in such two phase flows are highly complex, there is an important need to develop analytical tools for the engineer, which will enable prediction of the influence on the burning process of fuel spray behavior, droplet trajectories, the polydisperse evaporation process and droplet loading. Basic research is an essential step in delineating the complexities of the two phase flow mechanism, and in formulating models of fairly simple subsystems.

In this thesis laminar planar flow of a spray of fuel droplets and vapor impinging on a laminar counter flow of oxidant is considered. The work is unique in that it permits an extensive parametric analytical investigation of the behavior and extinction of counter flow spray diffusion flames to be performed. The research focuses on phenomena that have not been analyzed in depth in the literature: combined effects of transport (via the Lewis numbers) and droplet loading on the characteristics of spray flames, endothermicity and finite rate of evaporation, as well as spray polydispersity. In particular, a new analytical investigation of the influence of spray dynamics on the flame characteristics and extinction conditions is carried out by adopting a novel approach. The analysis of the dynamics considers two limits: mild slip velocity between the phases for which the Stokes number (corresponding to the ratio of droplet response time to the flow time) is much less than unity, and large slip velocity between phases for which the Stokes number is much greater than unity. The large slip analysis is a significant innovation and the analytical tools enable a description of the mechanism responsible for independently observed oscillatory motion of droplets.

The research also includes an application of the model to the problem of suppression of flames using water droplets. The analytical tools developed allow effective and ineffective regimes of flame suppression to be mapped in terms of the (suppressant) water droplets characteristics. The results are consistent with independent numerical and experimental work, suggesting 20 [mm] as an optimum droplet diameter. In addition, the model demonstrates that the optimal water droplet diameter for suppression is not only based on the dynamics between the two phases, but also on the spray polydispersity.