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In his previous article, the author wrote about GE’s profitability challenge after the takeover of Alstom’s GT business. In this part, he talks about ABB’s reheat combustor design and proposes a new design wherein fuel would be injected at an angle.
The ABB reheat combustor has a diffuser of a given axial length located just downstream of the first stage impulse turbine blades to reduce the velocity in half and is followed by a series of ramps. The hot gas passes over these ramps, and vortexes are formed at the edges of each ramp. Fuel is injected into these vortexes and thoroughly mixed where auto-ignition takes place. The ramps are followed by a sudden dump extending for another given axial length to create more mixing and an increase in flow area to accommodate the increase in volume of the gas being heated. This reheat combustor has been very successful but does requires a certain critical axial length and rather high pressure loss.
The reheat combustor I have previously proposed to GE a few years ago, through the Houston aero group, with no strings attached, and with sketches and written explanations, would reduce both axial length and pressure drop. I am too old to consider more patents although I have been very successful with them.
An inventor and engineer, versed in steam turbine design, Nathan C. Price, joined Lockheed Aircraft Corporation in early 1940. He was in charge of developing a jet engine and a turboprop gas turbine, and was funded by the US Army Air Force. Price proposed gas turbine reheat between axial-flow turbine stages in one of his designs and got patents on his ideas, but he did not give any details as to how the reheat combustors were to work. He only showed fuel lines running between the turbine stages.
Lockheed had a couple of the simple cycle turbines made and partially tested. They had combustion problems as did both Whittle and Ohain. A book written by James St. Peter titled, ‘The History of Aircraft Gas Turbine Development in the United States … A Tradition of Excellence’ was funded by a group including ASME and IGTI and was published by IGTI/ASME in 1999. This book contains a full chapter on the Lockheed/Price jet engine and presents a number of photos and many details. Several of his ideas are used in today's gas turbines.
Proposed new reheat combustor
Now back to the reheat combustor that I would propose. The turbine blading would exhaust at typically Mach .4 velocity, into a short transition duct to change the flow into a series of pipe-like cross sections to allow fuel lines to enter between the flow sections. A series of airfoil-shaped flow wings, like the first stage stator blades of an axial flow compressor, would be placed around these ducts and so the blunt tips would reach close to the center of each section. They would have a given high lift airfoil, a desired angle of attack and a pre-determined aspect ratio. Fuel would flow through holes in these winglets.
The tips would be cut off sharply to create a strong vortex at the trailing edge of each winglet tip as the gas flows crosswise from the pressure side to the suction side such as what takes place on airplane wings. Fuel would be injected at an angle into the vortexes near the trailing edge tips. The vortexes would rotate together at the center like the meshing of gears so as not to oppose each other and thus form a larger central vortex and the outer portions would oppose rotation to create better mixing for a shorter axial length.
At the winglet exit, the cross sectional flow area would increase to accommodate the extra volume created by the heated gas stream, but not as a sharp dump as is the case for the ABB combustor. Axial length would be saved and the pressure drop would be less.
In the concluding part of this series, the author writes about the use of wing tip dams in commercial airplanes and how the Alstom-ABB reheat gas turbine and GE simple cycle turbine can be integrated to form a ‘super system’ that provides greater 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.)