How CMCs can help gas turbines realize their full potential

In the previous article, the author wrote about the benefits of CMCs and its rapid progress. Here, he talks about the reheat gas turbine and other

 advanced developments.

The reheat gas turbine was an early dream of an inventor by the name of  Nathan C. Price. He was awarded a patent on the reheat aero gas turbine at the close of  WW II, but he did not show just how the reheat combustor was to work. He only showed a fuel line placed between the stages of the expansion. Many of Price's innovations are used today on our gas turbines. He was far before his time but a couple of his engines were actually built and partially run. 

A full chapter on Price and his work with Lockheed Aircraft is devoted to him in the ASME/IGTI published book “The History of Gas Turbine Engine Development in the United  States… A Tradition of Excellence'' by James St. Peter, 1999. A number of photos and cross sectional drawings of the L1000 (XJ37-1) are given in this book.  

The United States decided to separate the engine development from the air frame and the project was turned over to the other gas turbine manufactures to use as they so chose. The L1000 was dropped, but many of Price's innovations were retained and others showed up 10 years later.

The extra cooling required for the second combustor and second set of hot nozzles and rotating blades has always been a stumbling block for the reheat gas turbine. Nevertheless, ABB did successfully develop the GT24 and GT 26 gas turbines using reheat (sequential combustion). Later Alstom advanced the design to a level of “F” as they now stand whereas the single shaft units have advance to higher firing temperatures and higher air flows of the “H” and “J” models. This appears to be only a “me too” or “catch up” machine (GT 36) to those of GE, MHI and Siemens. 

It is reasoned with the take over of the GT 24 and 26 gas turbines by the Italians (Ansaldo Energia Company) that the 60 Hz 24 will be dropped and the cost of advancing the 26, 50 Hz, to” H” or “J” levels will be too high for the Italian company to bare considering the cost of continuing the development of the new GT 36 50 Hz “H” class large single shaft simple cycle GT now underway.  

However, the Chinese owns part of Ansaldo (40 %)  and could pour in the needed  money. The development of shockproof CMCs for hot gas path blading, combustors and other stationary parts could change this present assessment if CMCs comes along soon enough. If so, the GT 26 could be 

made into a new ”K” class machine and again leap frog the simple cycle machines.

New GE X turbines

There are other advanced material and process developments that are already here besides high temperature CMCs and TiAls such as carbon fibers for the large new GE X turbines wherein the large front end fans will have 25% fewer fan blades that will have a thinner and wider cord for a more optimum and higher by pass ratio of about 10. These blades will have stainless steel leading edges to give them the needed extra strength and resistance to inlet hale and water wear. 

This new carbon fiber blade technology could invade the front stages of the HD compressors and the last stages of the CC steam turbines. The advent of CMCs, requiring little or no cooling air, the use of CMCs for combustion liners (already being developed), and for other fixed parts sheds new light on the subject of the reheat gas turbine to be sure. Could Nathan Price's dream come true for the reheat  aero engine? How about the large heavy duty gas turbines and all the smaller ones? Could this bring the simple cycle efficiency up to about 40% with higher durability of a small 8,000 HP size to compete favorably with the diesels, converted to burn 80% natural gas, and make it economically advantageous to drive railroad locomotives for passenger and freight trains –  and even switchers?

A step in the right direction is the new 3000 HP GE3000 helicopter engine with 25% better efficiency, 50% higher output, 20% longer life and 20% lower in maintenance cost than the Black Hawk engine it replaces. This new engine has CMC hot section and 3 D printed parts. Under this program, GE plans call for future mechanical drive units from 5000 to 10,000 HP with 35% higher 

efficiency and 45% lower production and maintenance costs. 

The United States HDs started with the locomotive and what goes around comes around. Where do we go with gas turbines after full use of CMCs become a reality? Keep up with the gas turbine news and we will find out sooner than we think.

(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.)