טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentUzan Segoula
SubjectExperimental Investigation of Blunt Body Film Cooling in
Supersonic High Enthalpy Flows
DepartmentDepartment of Aerospace Engineering
Supervisors Professor Emeritus Alon Gany
Dr. Avishag Pelosi-Atias


Abstract

When an aerospace vehicle returns to the atmosphere at hypersonic speed or flies at very high speed in the atmosphere, extreme conditions are experienced for relatively long times, such as dynamic pressures, aerodynamic drag and thermal loads higher than usual.  Under these conditions, the classical thermal protection materials (TPM) don't survive, and need to be replaced by lightweight and actively cooled components. One way of alleviating the problem is the injection of a cooling fluid into the flow layer at the vehicle surface. The coolant operates as a thin, cool insulating layer, yielding a reduction of the heat flux to the surface. This method, also known as “film cooling”, creates a thickening of the boundary layer, thus reducing the temperature gradient and the resulting heat flux to the surface. An optimum flow injection rate provides a minimum heat flux to the wall, whereas an excessive amount of cooling gas injection causes the detachment of the boundary layer from the wall and an increase in the heat flux to the surface.

The purpose of the research is to test experimentally the efficiency of the film cooling method in cooling spherical envelopes of crafts that fly at supersonic and at high enthalpy flow. Film Cooling has proven to be effective in propulsion systems, but its use for external components in hypersonic flows is still under research and development. Usually gases are used as coolant when the gas is selected according to its physical properties. In addition to the fluid properties, the quality of cooling varies with flight conditions, geometrical configuration, the form of injection and the flow rate of the coolant.

Eleven successful experiments were performed in the Aerothermodynamics Laboratory at the Faculty of Aerospace Engineering of the Technion, where a 5-MW arc plasma tunnel enables simulation of high enthalpy flows. Two models (with and without injection) were exposed to a Mach 2.2 freestream with a total enthalpy of 1.7 MJ/kg. Nitrogen and helium were used as coolant gases. A parametric study shows the influence of coolant physical properties and mass flow rate on the characteristics of the flow surrounding the model and on the resulting temperature reduction at different locations on the test model.

The results, obtained from temperature measurements and video recording of the tests, show clearly the effectiveness of film cooling, demonstrating temperature reduction of up to 400°C when the coolant is injected. The use of helium yields the best cooling performance with the lowest coolant mass flow rate due to its thermodynamic properties. Low coolant mass flow rates result in better cooling performance. When the gas mass flow rate is increased over a critical point, the bow shock bulges out and the main airflow separates ahead of the model, resulting in poorer cooling. Variation of the coolant mass flow rate yields cooling optimization.