Next generation aeroderivative -V

May 14 2013 - Ivan G Rice

Now let us examine the PW1100 geared fan engine if industrialized to generate electric power or to drive a mechanical load. The fan would be eliminated and the planetary gear would be sized with the proper speed ratio for driving a forward 3 to 4 stage front axial flow optimized RPM compressor. The second compressor would drive the planetary gear and a three element compressor would be formed. Water spray could be applied to cool the exit air of the forward compressor. The overall pressure ratio would rise to perhaps 50 and the core air flow would be increased to develop more power output. The new unit would fill the power gap of the old LM 6000 and would not compete with the new P&W 4000 under development now to be carried forward by HI.

Two new power turbines would be designed by HI, one for 50 Hz and one for 60 Hz. The new unit could possibly have a manufacturing cost advantage and also higher efficiency. The planetary gear loss would only amount to about 1/2 percent because the gear would only be driving about 1/3 of the compressor load and no fan. HI together with P&W can take a look at the new geared unit to see exactly how an advantageous arrangement could be put together.

History of TITs, PRs and bypass ratios of fan engines

The TITs of airplane gas turbine engines have risen over the years from 1200o F in the 1940s to 1400o F by 1950. Then the TITs took a jump to about l600o F and then on to 2000o F year by year, and today the TITs stand at about 3000o F. By 1960, the TIT of the GE FT 39 fan engine for the C5A cargo military plane had reached 2400o F using film air cooling for the blading. With advanced cooling, single crystal blades and today’s TBC coatings the TITs have risen to over 3000o F for military engines. This last jump has truly been a game change for both aircraft and industrial GTs.   

The aero engines have increased their PRs steadily to obtain optimum GT cycle efficiencies and output. The aero engines are designed to obtain maximum efficiency for a given TIT and power output and therefore the PRs have risen from about l4 in 1940 to 30 by 1960 for the TF 39. Today, for the 3000o F TIT level, the CRs have climbed to 52 for the new R-R Trent 1000.  

Likewise, the fan bypass ratios have risen from 0 to 1 to 1 when the first P&W bypass engine was introduced on the 707. Then it rose to 5.5 for the GE TF 39 engine and now it has risen to 10 to 1 and on to 12 to 1 for the new geared fan engine. These high bypass ratios better fit the sub sonic speeds of the airplanes to offer better fuel economy.

Fan blading

The first fan blades were made of solid steel and were of a narrow cord design; and weight was a factor. They were first only extensions of the first stage compressor rotating blades. The TF 39 fan engine and the P&W equivalent for the first 747s had a bypass ratio of about 5.5 using the narrow cord design. The GE engine actually used a two stage fan for the C5 A plane.  

R-R was the first to apply composite wide cord fan blades using fiber glass and an epoxy binder. These blades were developed for the Lockheed L 1011 airliner using the RB 211 engine. Tests soon showed that hale and rain from severe storms eroded the fan blade leading edges and caused the blades to fray. Metal cladding of the leading edges did not help. As a result, the L 1011 plane was delayed about 2 years until R-R had fixed the problem by going back to an all-metal design.

Then, at the turn of the century, along came the far more durable and lighter carbon fiber composites which could then be used to make wide cord light weight but durable fan blades. At last, this material worked out very well for making wide cord fan blades which were more aerodynamic. This development came along too late to help R-R fix their RB 211 blade problem on the L 1011.

Combined cycle considerations

The situation for TITs and CRs is different when considering the industrial HD GTs and combined cycles. It was shown by Dick Fosterpeg of Westinghouse in 1960 and later by GE that for the HD combined cycle, the exhaust gas of the gas turbine should not be supplementary fired to raise the gas temperature to obtain maximum combined cycle efficiency and that the exhaust temperature should be at least l050o F to obtain 2400 psig, 1000/1000o F steam temperatures for the bottoming cycle. Therefore, the TITs and PRs had to line up to get this exhaust temperature. If the PR was too high then the exhaust temperature would be too low and the CC efficiency would suffer – vice versa for the TITs, but in general, the designer chooses the highest TIT that can be justified for maximum output and efficiency and then chooses a PR to give the desired exhaust temperature.

The steam conditions for the recent HRSGs and steam turbines have recently risen to 1100/1100o F and the HD exhaust temperatures have likewise risen upward to about 1200o F regardless of the higher TITs. In order to obtain these exhaust temperatures, the PRs of HD units at present day TITs of 2800 to 3000o F have values of 20 to 24 to obtain maximum GT output and maximum CC efficiency.

The Alstom reheat GT 24s and 26s with their present day 34 PRs are limited to somewhat lower TITs because of the double heating and expansion, and the cooling challenges they face. The exhaust temperature of these units is also around l200o F brought about by the second TIT.

The author suggests that Alstom restudy CLSC for the stationary GT 24 and 26 hot parts such as being done by MHI today. CLSC has been widely used by MHI and field proven to be cost effective and reliable for CCs. Although R-R does not have direct experience with CLSC such as GE or MHI, R-R does have valuable heat transfer computer programs and test facilities, and could help Alstom through their existing technical exchange program. The GT 24 and 26 TITs could be increased to 2800 to 3000o F and thus Alstom could leap frog the other simple cycle HD gas turbine manufacturers for GT output and CC efficiency, particularly at part load.

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.

Next generation aeroderivative-I

Next generation aeroderivative-II

Next generation aeroderivative-III

Next generation aeroderivative-IV