High Temperature Open-cell Metal Foam for Jet Engine Use

Since the invention of the jet engine and its use in the aerospace industry, the aim has always been to increase turbine operating temperatures since this means better efficiency, more power, less fuel consumption and less pollution for airplanes.

In fact, during the past 30 years turbine airfoil temperature capability has increased on average by about 4°F per year. This created a big challenge since the operating temperatures of turbines were limited by how much their material could handle. Nickel-base superalloys (NBSA) have dominated high temperature jet engine applications in past years but because of their melting point (1455°C for Nickel) the use of NBSA has been limited and other alloys are being considered.

Although NBSA have been alloyed with refractory metals which improved their thermal and mechanical properties, there are still short comings with NBSA mainly because of their relatively low melting temperatures compared to refractory metals. Refractory metals with their high melting point are being increasingly considered as alloying elements and even as base elements as a replacement for NBSA. Refractory elements and their alloys are proving to have better properties than NBSA; not only do they exhibit higher melting points but they also have better thermal and mechanical properties of oxidation, creep and corrosion at high temperature from 1500°C to 1800°C. Researchers investigate the foaming possibilities and processes of the ultra-high temperature alloys. This porous structure will be known as Refractory Open Cell Metal Foam. It is very important to investigate the current turbine vane materials of certain engines and whether these alloys can be made into porous foam. The reason being is that there would be no need to develop new high temperature alloys of current engines if the turbine vane material is already available and is capable of withstanding the temperatures of that specific engine. Another challenge arises from this idea and that is the material the turbine is made from is a bulk or solid structure and not foam so when this material is foamed will the foam ligaments and cells be able to withstand the high temperature, corrosion and oxidation in the same manner as the bulk material would?

Yet regardless of the operating temperatures of turbines or the materials they are manufactured from, the fact remains that turbine blades are always under temperature cycling during operation. Jet engine turbine vanes are hollow for better heat transfer; they are lightweight to reduce centripetal forces; and they are made out high temperature materials to minimize oxidation, creep, thermal fatigue and corrosion. However; current jet engine designs employ the use of outside air to cool or dilute the temperatures exposed to turbine components to prevent them from being damaged or reducing their operating life.

The researchers present a solution that reduces the thermal cycling issue and improve engine efficiency and lifetime while maintaining optimal operational safety. They suggest placing a ring of open cell metal foam directly in front of the turbine blades (after the combustion chambers) for the purpose of mixing the hot and cold gasses. Metal foams are considered porous media and exhibit exceptional heat transfer capabilities since they have increased surface area that allows them to transfer heat at higher rates and quantities compared to bulk materials. Therefore, placing a ring of high porosity metal foam right before the turbine inlet will mix the difference in the temperatures of the gasses entering the turbine and it will create a temperature profile closer to unity; hence, raising the overall average operating temperature. This way, temperature cycling is significantly reduced and as a result, turbine operating life, operating temperatures and efficiency will increase. The foreseeable potential disadvantages are mainly pressure drop and a suitable material for metal foams that can withstand such high temperatures and pressures. The designed material of foam must also be oxidation and corrosion resistant, exhibit good creep and rupture lifetime as well as ductility and strength over a variety of low and high temperature profiles.It must also have good thermal conductivity to ensure maximum heat transfer and sufficient mixing.

Another positive outcome of placing a ring of porous high-temperature metal foam in front of the turbine section is that the metal foam can be coated with a catalytic material to lower harmful NO_x gasses and emissions to the environment. Porous metal foams have been researched in the past few years as a potential replacement for current catalysts and catalytic converters as well as for fuel cell technology. The reason metal foams are attractive for this application is the reduced backpressure resulting from the cell structure (porosity) and the increased surface (area) — volume ratio that also increases catalyst coating per volume compared to conventional catalysts. Current catalysts use a 2-D array of channels that are coated with the chemical; metal foams with their 3-D cell structure not only allow more catalytic material to be coated but it also allows the catalytic reaction to take place in a more rapid and efficient way. This process will not only help the environment but will also contribute to increased turbine life by reducing the Nitrogen and Sulfur (nitrides and sulfides) contents created from combustion. Sulfur and Nitrogen can have long-term devastating effects on turbine components, they contribute to wear, oxidation and corrosion of the turbine especially since the gasses are at temperatures that exceed 1000°C in many cases.

Source:

http://www.google.ru/url?sa=t&rct=j&q=High+Temperature+Open-cell+Metal+Foam+for+Jet+Engine+Use&source=web&cd=1&ved=0CCIQFjAA&url=http%3A%2F%2Frepository.lib.ncsu.edu%2Fir%2Fbitstream%2F1840.16%2F2922%2F1%2Fetd.pdf&ei=n1kqT7eJL4Lu-gbZ_ZX6DQ&usg=AFQjCNETUG4XYHA5vHEJVyoRZicLJBJUxA&cad=rjt