M.Sc Thesis

M.Sc StudentBalas Shachar
SubjectInvesrtigation of the Condensation of Boron Oxide in a
HC-Boron Gel-Fuel Ramjet
DepartmentDepartment of Aerospace Engineering
Supervisor PROF. Benveniste Natan
Full Thesis textFull thesis text - English Version


The ramjet engine is a novel propulsion system for a variety of missions. It is very attractive because of its high specific impulse compared to rockets and its relative simplicity compared to other air breathing devices. The ramjet operates at high Mach numbers required for the compression of air and taking advantage of the ram effect. 

The use of gel fuels increases safety and improves the energetic performance of the engine since it allows the addition of certain metals to the kerosene-based.

Boron exhibits remarkable theoretical energetic performance with the highest energy density of all elements, i.e., about three times that of hydrocarbon fuels.  One of the main combustion products is boron oxide, B2O3, which has a boiling point of 2300 K and a latent heat of vaporization of 366.5 kJ/mol.

The research examines the option of condensing boron oxide, thus releasing the heat stored in the gaseous phase before they exit the engine; therefore, increasing the combustion efficiency.

A two-stage combustor can allow better utilization of boron energy. The air flow coming through the inlets is split at the diffuser exit. The first part is burned with boron loaded gel fuel at a higher than stoichiometric fuel-to-air ratio.  In the second stage, the bypass air is mixed with the combustion products. Using an adequate bypass ratio, the addition of cold bypass air to the combustion products cools the mixture below the boron oxide boiling point, leading to condensation of boron oxide and consequently to the release of the latent heat of vaporization. For this setup to be advantageous, the overall fuel-to-air ratio should be less than the stoichiometric fuel-to-air ratio.

The mixing chamber geometry, bypass air injection characteristics define the properties and the behavior of the mixing process, which affects the time required to complete the boron oxide condensation. This time should be lower than the residence time in the after-mixing chamber in order to achieve high combustion efficiency and exploit the energetic potential of boron.

Three theoretical models (diffusion controlled, kinetically controlled and a combined one) were used to describe the condensation process. Using CFD tools, the behavior and the properties of the flow were simulated in the after-mixing chamber, producing the required input for the condensation models. Eventually, the time required for condensation of boron oxide, based on the condensation models, was estimated.

The results indicate that specific impulse increases as a result of the condensation.