Advanced materials for next-gen turbines

This is a two-part series in which the author explores the CMC technology with respect to industrial gas turbines. In this article, he talks about the use of CMC blades in jet engines.

Has the industrial gas turbine reached a limit in output and efficiency in both the simple cycle mode of operation and the combined cycle configuration? Some engineers think so because the TITs have reached 2800 to 3000

^o

F which is the threshold of the drastic increase in NOx above this temperature, even if better cooling techniques and materials become available. However, General Electric engineers do not think so because there is now its new technology of CMCs for the near future in the offering.

What is CMC technology? Where did it come from? What does it mean regarding industrial gas turbines? CMC is an abbreviation standing for “Ceramic Matrix Composites” which have been developed for the next generation of fan engines such as the GEnx, the GE9x and the Leap. GE, over the past two decades, has spent over a billion dollars on developing this technology and has one manufacturing plant already built, and another on the way to make both CMC high temperature engine parts and huge carbon fiber fan blades for these new fan engines.

Solar, several decades ago, spent a lot of time and money trying to develop ceramics for gas turbine but could not make ceramic parts for gas turbines that were not too brittle to use. Solar wrote a book on this activity. This research work was dropped as it was not worth any further effort. However, GE, about 15 years ago, took over where Solar left off, in developing ceramics for jet engines. At last they have been successful.

Use of CMCs for jet engines

Stationary parts for combustion liners, shrouds and other hot section parts are now a reality for the new fan engines. However, rotating blades have only recently been tested to be used in the future. Rotating blades weigh only a third as much as high temperature metal blades. The result is that no cooling air is required for the blades themselves and the discs can be much lighter to carry the load of the CMC blades with a reduction of cooling air required.

GE recently announced that it has successfully completed a 500-hour life cycle running of a jet F414 development engine equipped with last stage CMC blades under its ADVENT (adaptive jet engine for sixth-generation fighter jets) program. A picture by GE shows these yellow and bronze colored blades from the back of the engine after the test runs. This is considered a breakthrough by GE and points in the direction of using CMCs for high temperature stationary vanes and rotating blades requiring considerably less cooling air and reduced sizes of the discs for jets and industrial GTs. The result will be lighter and more durable engines, having a greater output at a higher efficiency. The cost of industrial GTs could be reduced.

In his next article, the author discusses about the future of CMCs and how their use in machinery can affect output and efficiency.

(Ivan G. Rice was past chairman of the South Texas Section of ASME (1974 - 75), past chairman of the ASME Gas Turbine Division (now IGTI) (1975 - 76). A Life Fellow Member of ASME and Life Member of NSPE/TSPE, he has authored many articles and ASME papers on gas turbines, inter-cooling, reheat, HRSGs, steam cooling and steam injection.)