Gas Turbine Engine
Shock Wave Based Ramgen Engine
Ramgen initiated development of its unique shock wave based engine in 1998. At a Design Review organized by DOE/NETL in 2002, researchers from DOE, NASA, and the Air Force Research Laboratory conducted an in-depth review of the Ramgen engine project and concluded that the technology was feasible and important. It was recognized that shock wave based compression offered an absolutely unique opportunity to significantly improve energy efficiency.
Because of the complexity of the technology, the Design Review recommended that the most effective development approach was a two-prong program. Since the compressor and the combustor each had individual applications it was recommended they be developed independently and, when sufficient progress was made with both, they be brought back together and combined into the engine. Ramgen accepted and has followed this recommendation.
In 2004 Ramgen was asked by DOE to analyze whether the advanced compression system had applications with CO2. It was determined that the Ramgen shock based compressor had significant advantages over conventional technologies when used with this heavier gas. Because of the importance to the nation and world of reducing the cost of CCS, developing a working demonstration of a commercial scale CO2 compressor became the focus of the company. Currently a 13,000 hp CO2 compressor is being constructed for testing.
Significant progress has been made in designing and testing the Advanced Vortex Combustor (AVC) system including tests at NETL funded by the California Energy Commission. Now with the construction of the 13,000 hp compressor the foundation is set for returning to the recommendation of the DOE/NETL Design Review to combine the two highly innovative systems and complete the Ramgen engine. The breakthroughs with the compressor and the combustor are based on using CFD analysis that has been steadily advanced over the past ten years to produce ever increasing accuracy. These advanced CFD tools, backed up with tests to validate projections, can now be applied to the full Ramgen engine.
The Ramgen Integrated Supersonic Component Engine (ISCE) consolidates the compressor, combustor and turbine of a conventional gas turbine into a single wheel. This paper outlines how the knowledge gained in the development of the advanced Ramgen compressor has resulted in important modifications in the original engine design. The lessons learned from the work on advanced compression and from the successful AVC tests provide an in-depth understanding of how to complete the Ramgen engine.
Description of Technology
While the ISCE operates based on the same Brayton thermodynamic Cycle as a conventional gas turbine, the mechanical implementation of the process is quite different. One important advantage is that because the compression, combustion and expansion processes are all integrated into a single constant speed rotor, there is no physical acceleration of the rotating components required as the system transitions from idle to full power. The output torque and power are modulated from the full-speed no load condition to the full-speed, full power condition by adjusting the fuel flow. As a result, the system can transition from idle to full power as quickly as the fuel flow can be adjusted.
Testing performed by Ramgen with support from DARPA at the Air Force Research Laboratory has demonstrated a transition from combustor heat release levels consistent with a power variation from idle (pilot fuel only) to full power (full fuel/air premix) in periods as short as 150-200 ms. This is possible in significant part due to the dramatic stability of the Advanced Vortex Combustor (AVC) system that Ramgen has developed and demonstrated in a number of full scale tests supported by past DOE contracts.
It is this combination of the constant speed operating mode of the ISCE power wheel architecture and the stability of the AVC combustor that result in this unique ability to load follow from idle to full power in time scales as short as a few hundred mili-seconds for the ISCE compared with a response rate of 7-10 seconds for most intermediate sized gas turbine electric power generating systems.
This ability to very rapidly load follow is particularly well suited to a number of power generation applications. One such application is power quality conditioning or smoothing. This is of particular interest as power distribution grids are being required to absorb increasing amounts of relatively time varying and transient power from a range of renewable power generation sources. Wind power in particular produces an integrated output which has a high degree of time based fluctuation as the wind speed levels vary in time.
Capability of the Ramgen Engine to Use Dilute Fuels to Generate Electricity
Methane is the second largest anthropogenic greenhouse gases contributor,
after carbon dioxide, to global warming. Methane, per volume, traps 21
times more heat in the atmosphere than carbon dioxide. (Eliminating 1
ton of methane equals 21 tons of CO2.) Engineers from the Jim
Walter Resources mining company and Ramgen staff developed an approach
that can use up to 75% of the methane worldwide now being emitted into
the atmosphere as fuel to generate electricity. Currently approximately
90% of this methane is vented.
ISCE Technology Feasibility
Advances in Ramgen Technology Support Feasibility of the ISCE
The DOE/NETL Design Review concluded that the Ramgen shockwave based engine was feasible. There has been strong additional evidence since that review supporting this conclusion, including a proof-of-concept demonstration of an early version of the system. Working from what has been learned from developing shock wave based air and CO2 compressors, and from the successful demonstration of Advanced Vortex Combustion (AVC), Ramgen is now positioned to combine supersonic shock compression and AVC to produce a working design of the ISCE.
Next Generation System Configuration
The initial proof of concept Ramgen engine used an un-shrouded rotor configuration mounted on a single high speed shaft driving a generator/starter motor through a speed reducing gearbox. The ISCE engine system will incorporate a fully shrouded flow path power wheel configuration. The power-wheel now proposed will be directly supported by a magnetic bearing system and will incorporate permanent magnets into the inner diameter of the rotor. This rotor mounted magnet system will rotate around a central stator winding to form an integrated high speed permanent magnet motor/starter system on the inner hub of the power-wheel. This configuration will eliminate the need for a speed reducing gearbox and the discrete separate motor/generator. This consolidation will result in a significantly more compact, light weight, low cost generation system compared to any other conventional turbo-generator system.
This integrated power-wheel system is illustrated in Figure 1. Also shown in figure 1 is the engine feature of a propulsive flowpath that is fully shrouded and formed by a series of nested rim segments supported by a metal-matrix or polyimide composite outside diameter support ring.
The ability to incorporate a magnetic bearing rotor support system and
integrated permanent magnet motor/power wheel system is possible today
because of significant technology advances over the past decade. The magnetic
field strength, performance and capabilities of these systems have dramatically
improved in recent years.
Improved Material Selection
The propulsive ramjet flowpath configuration represented in Figure 1 rotor/power-wheel structure is formed by a series of interlocking or nested rim segments that would be made from either nickel based alloys or a modern engineered ceramic such as Si3N4 (Silicon Nitride) that has a good combination of strength and toughness. This ring of rim segments is then attached to the inner working disc that in turn supports the permanent magnets for both the bearing system and high speed motor rotor. The ring of rim segments is further supported on its external diameter by the high strength metal matrix or polyimide support ring.
One of the key lessons learned in the previous integrated rotor system work was the cooling and material selections required to deliver adequate structural integrity to the hot combustor section components on the rim of the wheel. The combination of light weight engineered ceramic rim segment components and a high temperature/high strength capability support ring represent an improved material selection and structural configuration that are capable of delivering an operable system with sufficient structural margins over the entire operating speed and temperature ranges for the rotor.
Advances in Proprietary Supersonic Compression
The majority of work performed at Ramgen over the past ten years has focused on the use of supersonic shock compression to deliver compact, high pressure ratio, high efficiency compression stages for a wide range of process gasses. Ramgen has developed exceptional and, to the best of our knowledge, unique design and analysis tools which are necessary to apply this proprietary technology to a wide range of working gasses and compression ratios. The first step in the ramjet propulsive process as employed by the power-wheel is a supersonic compressor section. This section will utilize all the design features and tools developed and optimized by Ramgen over the past decade of R&D. Based on a CFD capability that is one of the most advanced in the world, and tests that support it, Ramgen is now positioned to incorporate its supersonic compression process into the power wheel and have it directly integrated with the combustion and expansion of the working fluid required for a highly efficient power generation cycle.
Design and Test Experience with AVC Combustor
It is the combination of supersonic compression and a high velocity combustion process that enables the use of premixed fuel in the intake air flow. Without these two processes, the premixed fuel and air would certainly pre-ignite and/or flash-back.
In addition to the significant advances made in the area of supersonic compression, Ramgen has developed a high velocity combustor design uniquely suited for direct integration with the supersonic compression process. In addition to delivering very stable combustion, low pressure loss levels and simultaneous low NOx and CO2 emissions levels, the high flow velocities employed in Ramgen’s Advanced Vortex Combustor (AVC) are sufficient to prevent flashback in methane-air mixtures.
Ramgen has performed multiple full scale combustor demonstration tests
at private and government labs, including NETL, demonstrating various
capabilities and features of the AVC design and it is now well demonstrated