Earlier in 1979, the author had suggested in a dual ASME paper (79-GT-7 and 8) applying a GE LM 5000 gas generator and reheating the gas before the power turbine. This scheme had three drawbacks. The first was the cost involved. Secondly, the power turbine had a low expansion ratio of only about 2.5. Thirdly, there was the problem of cooling the RH combustor and the power turbine. The author turned to a closed loop steam cooling system to solve the cooling problem as was outlined in another dual ASME paper (79-JPGC-2 and 3). Nothing came of this study, but the closed loop steam cooling (CLSC) scheme was thus born and later used by GE in the 9-H DOE-funded combined cycle project.
GE 9 H CLSC gas turbine combined cycle
GE took to the CLSC for the basis of the DOE 9 H GT CC after considerable DOE money was spent studying a scheme whereby a portion of the compressor discharge air was first cooled and then further compressed and cooled, and then used as cooling air for the hot gas parts. The GE 9 H GT has had limited success due to the complications associated with steam cooling the discs and rotating blades. Steam shaft seals were also a problem. GE has recently returned to all air cooling its 9 FB design for the European 50 Hz market. However, the first company to use CLSC has had very good success with such cooling of the stationary parts and is using such cooling on its latest and largest 350 MW 60 Hz “J” GTs that fire at about 3000o F.
New advancements in aero fan engines
Now let us return to the link between the new advanced fan engines for commercial airplanes and future prospects for industrial power generation. There are three new “kids on the block”, so to speak, that are like hungry hound dogs scrapping over two large bones (Boeing and Airbus). These three kids are: the R-R Trent 1000, the GEnx and the P&W geared PW-1100 fan engine with half the thrust of the other two. The first two have a thrust rating of about 70,000 lbs and are tailored to power the all new Boeing 787 Dreamliner just now entering service. The GEnx is also suitable for the new and larger Boeing 747-8 now being developed by Boeing. Both engines have been certified for the 787 and each will burn about 15 percent less fuel than existing equivalent fan engines.
The R-R Trent 1000, being made by a group of six companies, has a very high pressure ratio of 52 whereas the GEnx unit has a PR of 43. The Trent 1000 is an improvement of the existing Trent 60 and the GEnx is based on the successful GE 90. A group of seven companies joined together to fund and manufacture the GEnx engine. The GE engine is expected to replace the GE CF6 engine. R-R took the lead for the 787. The Trent 1000 powered the first 787 and the first commercial flight of the All Nippon Airline 787 a year or so ago. R-R has captured some 40 percent of the market so far, but GE with strong ties with Boeing and US airlines has 60 percent.
Boeing, several years ago, decided to build a new commercial plane to supplement and/or replace the older 757 and 767 planes. Boeing made two studies and two offerings to the airlines. The first was for a Mach 2 plane to replace the Concord SST. This plane would burn a lot of fuel. The second offering was for a Mach .9 plane, slightly faster than the 757 and 767s, but would have a 20 percent lower fuel burn, 15 percent coming from new engines and 5 percent coming from the aerodynamics and lower weight of the plane itself through the use of carbon fibers. The airlines opted for the latter to save fuel and not have to deal with the sonic boom issues.
Thus, the design of the 787 Dreamliner was started. Many worldwide orders were placed for the new plane. Boeing ran into problems with the light weight carbon fiber parts and the plane was delayed several years. At last the first commercial flight took place on October 26, 2011 by the All Nippon Airlines. Another problem developed several months ago after being flown by several airlines. The new lithium batteries started shorting out causing fires in flight. The plane was grounded worldwide. Boeing says they have the problem solved and have asked to be granted test flights using a modified lithium battery arrangement.
P&W PW1100 geared fan engine
The third kid on the block is the all new 30,000 lb thrust P&W PW 1100 that has a planetary reduction gear located between the LP shaft and a near optimum high bypass ratio fan (12 to 1) which allows both the fan and the LP turbine to operate at their optimum aerodynamic speeds. In effect, the PAW 1100 has a three-element fan/compressor system coupled to a two-element turbine arrangement. The high bypass ratio better matches the plane speed to provide a lower fuel burn. The other two larger engines for the new 787 have a 20 to 30 percent lower bypass ratio which is slightly less than optimum. The P&W 1100 compressor has a 3-stage LP compressor and an 8-stage HP compressor making 12 stages counting the fan. The PR for the 1100 is not given but is believed to be about 40.
P&W has spent some 14 years developing the new engine through the help from the German company MTU. The engine is now certified for the latest Airbus 320 as an option and hopefully to be accepted for the Boeing 737s. The engine will have a 15 percent better fuel burn over existing equivalent engines. The design makes it possible to greatly reduce the number of turbine stages to a total of only 5, 2 for the HP and 3 for the LP turbines, and to thus reduce the number of power turbine stages and to reduce the diameter of the power turbine section to save weight and parts count. On the other hand, there is an increase in weight due to the speed reduction gear and also a power loss of about two percent because of the gear loss to partially off-set the other gains. Gears are very reliable and have been used for years for engines such as the Allison/RR FT-56 turboprop for the C-130 freighter.
The geared engine has a competitor for the 737 plane and that fan engine is the new GE/CFM Snecma LEAP X, a replacement for the older GE/Snecma 56. This new engine will have a competitive fuel burn. There could be a manufacturing cost advantage in favor of the geared unit. The scrapping for this 737 business is now going on.
Since Pratt & Whitney introduced the geared turbofan aircraft engine, the question has been what is the next generation aeroderivative? Ivan Rice explores that question in this series.
