Arza Hadad

Arza Hadad



Propulsion and Combustion 219



The research in the field of propulsion is driven by the search for increased energetic

performance and enhanced safety features. Gelled fuels and propellants are promising

candidates for the fulfillment of both requirements.

Gel fuels are prepared by adding a gelling agent to a liquid fuel. The gelling agent

changes the rheological properties of the fuel and as a result the liquid fuel becomes a

yield stress non-Newtonian fluid. During storage, the gel fuel has an extremely high

viscosity and it behaves like a solid. When the fuel flows through the feeding tubes and

the injectors, it undergoes shear stress. Under shear stress, the viscosity of the fuel

decreases, which makes possible the atomization and combustion of the fuel. Gel fuels

can be prepared with use of either an organic or inorganic gelling agent. Organic gelling

agents burn along with the fuel and contribute to the energy content of the fuel. As a

consequence, organic gellants are better than inorganic gellant with regards to energetic


Due to its special state, the gel fuel enjoys the properties of both liquid and solid fuels. A

gel fuel presents safety behavior desirable in fuels: no sensitivity to impact and no

leakage and no spillage during storage. A gel fuel is also advantageous from the

performance point of view: similarly to liquid fuels, a gel fuel allows thrust control and

energy management leading to an optimization of the fuel consumption in a mission,

however, a gel fuel is much more suitable than a liquid fuel to the use of metal additives

thanks to its yield stress behavior and special structure which prevents particle


The currently used hydrocarbon fuels are the result of extensive research and

development and are at the limit of what could be achieved energetically. Thus, in order

to further enhance the performance of fuels, metal particles are added to the fuel. The

tendency of metal particles to sink and accumulate in liquid fuels gives the gel fuel its

advantage over liquid fuels from a performance point of view.

This research focuses on the combustion of a single droplet of gel fuel. Experimental

work showed that a gel fuel droplet has a unique periodical burning mechanism. As the

droplet heats up, the more volatile component evaporates, and as a result, an elastic

gellant layer surrounding the droplet is formed. As the layer thickens, it becomes

impermeable to fuel vapors. This leads to the formation of a fuel vapor bubble inside the

droplet. The accumulation of fuel vapor leads eventually to the bursting of the droplet

and the jetting of the fuel vapor. The elastic shell collapses, the droplet reforms and the

process repeats until all the fuel is depleted.

The proposed work will deal with the mathematical and numerical modeling of the time

dependent combustion of an organic gellant based gel fuel droplet. It is and expansion of

preliminary works on the subject and will include the influence of important processes

that were not treated in previous works, such as effect of finite conduction inside the

droplet, and the interaction between the fuel droplet and the hot gas environment. A

further expansion will include the modeling of metalized gel fuel droplets with a special

attention on gel fuels enriched with boron particles, boron being a metal of a very high

energy density