This article discusses benefits and challenges of Pressure-Gain Combustion Systems for Gas Turbines. The article also highlights that one approach to substantially improve gas turbine thermal efficiency is to replace the nearly constant pressure combustion process with some form of pressure-gain heat release such as either a constant volume or detonative mode of combustion. These systems commonly possess some form of rotating inlet valve design to control the filling process for an annular array of combustors and maintain the appropriate amount of inlet isolation. Although evaluation of turbine life and performance needs to continue, turbine efficiencies approaching values comparable to those of steady-state operation have been reported. The article concludes that the collaborative efforts, such as listed in the article, are ultimately required in times of reduced funding for continued technology development. Even with the risks and challenges associated with this technology, a high payoff potential exists with hybrid gas turbine architectures.
Gas turbine efficiencies continue to improve incrementally by increased operating pressure ratios, higher turbine inlet temperatures, and better aerodynamic efficiency of compressors and turbines. However there has been little real change in the fundamental thermodynamic cycle employed in these machines since most gas turbines have utilized a simple Brayton or nearly constant-pressure combustion cycle. While more complex cycles, utilizing intercooling, reheat, and regeneration/recuperation, have been used and can offer advantages, the simple Brayton cycle is by far the most used in both power and propulsion applications. For modern cycles,further improvements in turbine inlet temperatureand aerodynamic component efficiencies, while still useful, will have diminishing returns.
A schematic of how an existing core engine could be retrofitted with a PDE combustion system is shown in Figure 2.
One approach to substantially improving gas turbine thermal efficiency is to replace the nearly constant pressure combustion process (in fact there is generally a 4 to 5 percent reduction in total pressure during combustion in a gas turbine) with some form of pressure-gain heat release such as either a constant volume or detonative mode of combustion. Constant volume combustion inherently possesses an associated pressure rise, larger temperature increase, and a lower entropy generation for the working fluid during the combustion process, resulting in an increase in the available enthalpy for work extraction when compared to a constant pressure combustion event at the same initial conditions. Brayton cycles have nearly steady-flow conditions, whereas the challenge for utilizing near constant volume combustion involves dealing with the highly transient processes and the associated time-varying flow conditions throughout the cycle. Although challenges exist with these combustors, hybrid gas turbine systems that utilize pressure gain combustion strategies are being explored. Two examples of suchtechnologies are the wave rotor and Pulse Detonation Combustor (PDC) enginecores. Such a modified core is shown in Figure 1.
Wave rotor systems involve the transfer of energy through gas dynamic processes rather than the motion of solid surfaces. They have been utilized in a variety of applications and typically involve an assembly of channels that rotate, fixed ports at both ends, which are designed to vary circumferentially to accommodate expansion or compression waves that are moving through the rotating channel assembly. A convenient characteristic of the wave rotor concept is that the unsteady processes are confined within the channel passages, not the exiting outlet manifold, thereby allowing the wave rotor "combustor" to interfacemore easily with more conventional components.
A PDC system involves the repeated filling, detonating, and purging of multiple combustors. These systems commonly posses some form of rotating inlet valve design to control the filling process for an annular array of combustors and maintain the appropriate amount of inlet isolation. PDCs have not been investigated as long as the wave rotor concepts, and many of the associated technologies have been under limited development during the past 10-15 years only. One of the largest challenges for utilizing PDCs in a gas turbine hybrid configuration is that power extraction under unsteady turbine inlet flow conditions is a relatively new area of research. During the past five years, many industry representatives no longer view unsteady turbine operation as a majorimpediment for system development. Although evaluation of turbine life and performance needs to continue, turbine efficiencies approaching values comparable to those of steady state operation have been reported.
The direct thermal efficiency advantage of a hybrid gas turbine system is a relatively straight forward argument, but the practicality and method of implementation remain to be demonstrated. Since the overall pressure ratio of a Brayton cycle is one of the primary parameters determining thermal efficiency, the need for highcompression ratios is often mandatory for high cycle efficiency. This requirement comes at a price due to the complexity and cost of the high-pressure spool of most modern gas turbine systems. If the mechanical compression ratio requirement could be kept at a moderate level, the constant volume or detonative combustion process would be able to inherently deliver an additional pressure gain and increase the final overall operating pressure ratio of the device at a potentially lower systemcost. Conversely, if the compression ratio itself was maintained at state-of-the-art levels and the combustion process modified to be near-constant volume or detonative, an increase in thermodynamic efficiency and overall operating pressure ratio would likely be realized resulting in lower SFC and higher thrust density.
General Electric, Rolls-Royce, and United Technologies Corporation are some of the companies investing in this area and have an interest in continuing to explore the advantages of these systems. Government agencies such as NASA, Air Force Research Lab (AFRL), and the Office of Naval Research (ONR) are also interested in this technology and are often working in collaborative efforts with universities and industry on these new devices. Fundamental PDC research at the Naval Postgraduate School is supporting both ONR and industry efforts.
Due to the long legacy and developmental history of gas turbine engines,funding for a quantum leap in performance will not be a low-cost initiative. The collaborative efforts, such as those listed above, are ultimately required in times of reduced funding for continued technology development. Even with the risks andchallenges associated with this technology, a high payoff potential exists with hybrid gas turbines architectures.