טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentGershon Kats
SubjectInvestigation of Flame Ignition in Fuel Spray Systems
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Emeritus Greenberg Jerrold


Abstract

In the aerospace engineering context, the importance of the ignition problem stems from the fact that modern combustors in aircraft need to be (re)-ignited under a wide range of (sometimes extreme) operating conditions. The presence of liquid fuel in the form of a multi-sized spray of droplets that must be ignited only serves to increase the difficulty of ignition. Our focus will be on forced ignition which involves an external source of energy, e.g. an electrical spark, as it is the most common and popular way of igniting fuel-oxidant mixtures.

The main aim of the current research was to achieve a basic understanding of the underlying mechanisms at work in the thermal ignition of combustible fuel spray-oxidant mixtures. The mode of investigation was mathematical modeling, that exploits asymptotic methods, supplemented with numerical computations. Thermal ignition will occur only if enough energy is added to the gas to heat the combustible to overcome the rate of thermal energy dissipation. When the opposite condition exists, ignition is impossible. The effect of spray presence on the ignition process is twofold. On the one hand, droplet evaporation produces the vapor that will fuel any subsequent combustion. On the other hand, the droplets absorb heat to enable evaporation to occur.

In contrast to the abundance of experimental and numerical treatments of spray ignition in the literature theoretical treatments of ignition considering a two-phase gas-liquid mixture are rather sparse. The current research examined for the first time theoretically the influence on ignition of spray-related parameters, such as droplet size, initial spray composition, initial droplet size distribution, evaporation rate and system-related parameters such as minimal ignition energy, pulse duration etc. Also, volumetric heat loss resulting from heat conduction was examined in the context of its effect on spray ignition.

This new analysis demonstrated the important role that build-up of vapor from the fuel droplets plays during the spray ignition process as well as heat loss to the droplets for evaporation. In addition, a full picture of both successful and failed ignition scenarios of the spray-air mixture clearly demonstrates the critical role of the minimal ignition energy and pulse duration. Finally, it is noted that the orders of magnitude of the time for ignition, when achieved, match those reported in the literature for more realistic systems indicating that the model does indeed manage to capture essentials of the ignition process.

Three problems were tackled in the thesis. One uses a steady state analysis of the ignition of a combustible mixture of liquid fuel droplets and air in stagnation point flow. The second and third problems use a time-dependent analysis for the problem of initial thermal inhomogeneity and a pulsed heat energy flux, respectively, applied to a fuel droplets-air mixture.

Once a propagating flame is established the possibility of extinction also exists. The role of both radiative heat loss and that of heat absorbed by a spray of evaporating droplets in determining premixed spray flame propagation and extinction was also investigated analytically.