|M.Sc Student||Diskin David|
|Subject||Theoretical Investigation of a Boron Loaded Gel Fuel Ramjet|
|Department||Department of Aerospace Engineering||Supervisor||Professor Benveniste Natan|
A numerical simulation of a ramjet combustor with gel fuel and boron has been conducted. In the frame of this study, a parametric investigation was performed, and the ramjet engine performance was evaluated.
The ramjet is an air-breathing propulsion engine, which relies on the ram effect for compressing the entering air, thus, eliminating the need for a compressor and a turbine; however, it requires a high speed flight for good efficiency.
In the present research, the ramjet engine fuel is gelled kerosene in which boron particles have been added. Boron has 140% energy per unit mass in comparison to hydrocarbon fuels and 300% energy per unit volume. Gelled kerosene is produced by addition of a gellant allowing the addition of the boron powder preventing particle sedimentation. Various studies on boron containing ramjets have been performed, however, the potential of boron energy has not been yet utilized because of complicated ignition and combustion processes.
An earlier study by Hadad et al. showed the high performance potential of a boron loaded ramjet engine with specific impulse higher than 2,000 s. The scope of the present research was to evaluate the performance of a ramjet loaded with boron and to investigate the effect of the various parameters, such as bypass ratio and boron content. A commercial numerical CFD code (FLUENT) was used to simulate a 3D combustion chamber of a ramjet with an aft burner in which bypass air is injected.
Boron ignition and combustion have been modelled using King’s model. The model divides the combustion of boron into two stages. Initially, the boron particles are covered by a boron-oxide thin layer that prevents from ambient oxygen to reach the boron core and react. After the removal of the boron oxide layer, combustion begins releasing the chemical energy of boron.
The simulation of boron combustion was assumed as time-dependent and dealt with the particle phase separately from the CFD simulation. In each time step the diameter of the boron particles and the heat release were calculated. Particle diameter and temperature change rate depended on the flow properties. The distribution of the particles was polydisperse and they were injected into the flowfield upstream of the fuel injection. In order to simulate the process of evaporation of the gel fuel and the separating of boron particles from the gel, an iterative process was used to calculate the separating point and evaporation behavior.
The results include the temperature, velocity and particle distribution flowfield and eventually the performance of the ramjet is derived. The main conclusion from the results is that significant increase in the specific impulse can be obtained under certain conditions. The increase in the specific impulse was found to depend strongly on the time of boron particle combustion completion, and the time varies with the amount of oxygen and temperature in the combustor.