טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentMatsibeker Evgeny
SubjectNumerical Solution of the Flowfield in an Aluminized Gel
Fuel Ramjet
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Benveniste Natan
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

The Ramjet Engine is an efficient propulsion system for high speed vehicles. The advantages of ramjets are high range and simplicity. The ramjet is air breathing; therefore it carries only the fuel, since the ambient air serves as the oxidizer. The main disadvantage of the ramjet is the requirement to be accelerated by a rocket engine to the point where air compression is sufficient for the ram effect to take place. Gels are liquids, whose rheological properties have been altered by the addition of gelling agents. The existence of yield strength in gels, in combination with their high viscosity, allows the addition of high-energy metal particulates without sedimentation. Previous studies on the burning mechanism of organic-gellant-based gel fuels found that due to differences in the boiling temperature of the gelant, solvent and fuel, an elastic gellant layer forms around the droplet, which prevents evaporation from the outer surface. Bubbles are created inside the droplet until the layer ruptures and the bubble vapor breaks through. The layer collapses back to the droplet and the process repeats itself. This can be described as pulsatile combustion of the droplet.

A theoretical model was developed and solved numerically using the CFD code FLUENT. The governing equations were the conservation of continuity, momentum, energy, chemical species and turbulence. Themodel with the addition of a wall function turbulence model described the turbulent flow. The kerosene droplets and the aluminum particles were injected discretely to the flowfield. Solution was achieved using a multi stage approach. Introduction of a periodic change in the vapor pressure allowed adequate description of the droplet evaporation. The reaction of the kerosene vapor with oxygen was assumed to be single stage, one way, mixing-limited at infinite chemical kinetic rate. The aluminum particles were added separately from the fuel, downstream from the fuel injection. The combustion of aluminum was modeled as a surface reaction, limited by oxygen diffusion to the particle.

Conclusions: (a) The pulsatile evaporation mechanism of the organic gellant-based gel kerosene droplets creates a rather smooth temperature field in the combustor for different values of fuel/air ratio, droplet diameters and droplet evaporation frequency; (b) The periodic burning of the gel droplets slightly lowers the combustor temperature; (c) The addition of aluminum particles to the engine lowers the combustor temperature, and most probably, the energetic performance of the engine per unit mass decreases; yet, the energetic performance per unit volume is improved.