Compression loaded ceramic turbine disc technology.

Pochari Technologies is reviving long-forgotten turbine engine technology. With advancements in single-crystal nickel alloys, turbine designers have been able to increase turbine inlet temperatures to record levels. The downside of this is the need to utilize large amounts of compressor bleed air, reducing engine efficiency. Conventional engineering ceramics including silicon nitride and carbide have been forgotten in part due to the focus on ceramic matrix composites. A relatively elegant and simple design is the use of a composite (carbon-fiber) hoop holding the centrifugal forces acting on the blade. Conventional engineering ceramics have uneven properties, rendering them unable to tolerate concentrated forces found in conventional mating conditions use in current turbine disc designs. Contrary to popular belief, engineering cermaics do indeed have sufficient tensile strength to be loaded in tensile, such as a beam for example. Silicon carbide has a tensile strength of up to 230,000 psi, more than sufficient to tolerate the tensile loads found in a turbine blade root connection. The primary issue is the lack of uniformity, the material properties are highly heterogeneous, in addition, low flexural strength, and high brittleness further impede its use. The simple (in hindsight obvious), solution is to load the blades purely in compression, (where ceramics shine), in fact, silicon carbide has one of the highest compressive strengths available, 1600 MPa. In order to take advantage of the excellent high-temperature capacity of these materials, a novel architecture is needed, dispensing the existing orthodoxy of turbine disc design. Using this design, turbine inlet temperatures approaching 3000 F are feasible without air cooling. The blades transfer compression loads to hoop loads, the carbon fiber rim performs the same function as a pressure vessel. The carbon fiber parameter containment hoop is not exposed to high gas temperatures. With this technology, it is possible to design small-scale sub-500 hp turboshafts with the efficiency of diesel engines (40%+), enabling jet-pack propulsion with 1 hour plus range.

“Ceramic materials offer a great potential for high-temperature application. This,
however, means it is necessary to live – even in future – with a brittle material with
small critical crack length and high crack growth velocity. Thus it will not be easy to
ensure reliability for highly loaded ceramic components, keeping in mind that for
reaction bonded ceramics the material inherent porosity is in the same order of
magnitude as the critical crack length. A solution to increase the reliability of ceramic
turbines may be a compression loaded rotor design with fiber reinforced hooping”

R. Kochendrfer 1980

“A vaned rotor of the type comprising a central metal hub or rotor body carrying a plurality of rotor blades made of a ceramic material, in which the blades are simply located on the rotor body and held in place by a coil of carbon fibres or ceramic fibres which surrounds the blades. To form a support surface for the coil each blade has a transverse part at the radially outer end thereof, which is partly cylindrical and which together with the transverse parts of the other blades, forms a substantially cylindrical support surface for the coil. Although ceramic materials used for such vanes (silicon nitride, silicon carbide, alumina, etc.) have much better physical properties at high temperatures (i.e., over l,lC than any metal alloy, especially if undergoing compression loads, they are nevertheless very difficult to couple to metal parts because of their relative fragility, lack of ductility, and their low coefficient of expansion.Because of the lack of ductility of ceramic materials, the driving forces exerted during operation of the rotor give rise to a concentration of the load in parts of the coupling areas between the ceramic vanes and the metal body of the rotor. This frequently causes breakages in these parts. The various systems presently in use for attaching a ceramic blade by the root to a metal rotor body for a gas turbine are generally inadequate because these systems, including dovetail fixings having both straight and curved sides, do not take sufficient account of the rigidity and relative fragility of the ceramic vanes.This problem is exacerbated by the fact that present manufacturing techniques for ceramic materials are still not able to provide a complete homogeneity of composition and structure of the material, so that adjacent areas of ceramic material can vary by up to 200% in tensile strength. For this reason the known types of coupling between a support disc forming a rotor body and rotor vanes of ceramic material, which rely on a wedging action, are not satisfactory”

R Cerrato  Fiat SpA, U.S patent 3857650A, 1973

“A Compression Structured Ceramic Turbine looks feasible. A new engine aerodynamic cycle with effective working fins to off set windage loss, a reduced tip speed to enhance aeromechanics and
the possible utilization of leakage gas to augment thrust should be considered. Also, the prospect for more efficient energy extraction offered by inverted taper in the span of the turbine blade should be of prime interest to turbine designers in any future engine utilizing a Compression Structured Ceramic Turbine. Material property data and design refinements based on this data will also have to be seriously considered”

“The “Novel” feature of this ceramic turbine rotor design involves maintaining the ceramic
rotating components in astate of compression at all operating conditions. Many ceramic materials being considered for gas turbine components today display compressive strengths ranging from three to eight times their tensile strengths. Utilizing the high compressive strengths of ceramics in gas turbines for improving ceramic turbine structural integrity has interested engineers in recent years as evidenced by a number of patents and reports issued on Compression Structured Ceramic Turbines with one as early as 1968. Turbine blades designed to be in compression could greatly enhance the reliability of the ceramic hot section components. A design of this nature was accomplished in this contractual effort by using an air-cooled, high strength, lightweight rotating composite containment hoop at the outer diameter of the ceramic turbine tip cooling fins which in turn support the ceramic turbine blades in compression against the turbine wheel. A brief description of the detailed structural and thermal analysis and projected comparable performance between the Compression Structured Ceramic Turbine”.

P.J Coty, 1983

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