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Constant Volume Combustion: the Ultimate Gas Turbine Cycle

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Constant Volume Combustion: the Ultimate Gas Turbine Cycle INFRASTRUCTURE MINING & METALS NUCLEAR, SECURITY & ENVIRONMENTAL OIL, GAS & CHEMICALS Constant volume combustion: the ultimate gas turbine cycle About Bechtel Bechtel is among the most respected engineering, project management, and construction companies in the world. We stand apart for our ability to get the job done right—no matter how big, how complex, or how remote. Bechtel operates through four global business units that specialize in infrastructure; mining and metals; nuclear, security and environmental; and oil, gas, and chemicals. Since its founding in 1898, Bechtel has worked on more than 25,000 projects in 160 countries on all seven continents. Today, our 58,000 colleagues team with customers, partners, and suppliers on diverse projects in nearly 40 countries. Guest Feature Also in this section Constant volume combustion: 00 DARPA-funded CVC projects the ultimate gas turbine cycle 00 Power cycle thermodynamics 00 History of CVC engineering By S. C. Gülen, PhD, PE; Principal Engineer, Bechtel Power Pulse detonation combustion holds the key to 45% simple cycle and close to 65% combined cycle efficiencies at today’s 1400-1500°C gas turbine firing temperatures. The Kelvin-Planck statement of the Second Law of Thermo- Why constant volume combustion? dynamics leaves no room for doubt: the maximum efficiency In a modern gas turbine with an approximately constant pres- of a heat engine operating in a thermodynamic cycle cannot sure combustor, the compressor section consumes close to exceed the efficiency of a Carnot cycle operating between 50% of gas turbine power output. the same hot and cold temperature reservoirs. Assume one could devise a combustion system where All practical heat engine cycles are attempts to approxi- energy added to the working fluid (i.e. air) via chemical re- mate the ideal Carnot cycle within limits imposed by mate- action with fuel would simultaneously increase the enthalpy rials, mechanical considerations, size and cost. The operat- (temperature) and pressure of the product gas. For that same ing principles of such engines are described by idealized amount of air and fuel, the resultant saving in power spent “closed” cycles with a pure “working fluid” as detailed in the for air compression would result in an increase in net cycle Thermodynamics section of this article (page 00), output and efficiency. Combustion engine cycles Ultimate Gas Turbine Cycle Acronyms Based on combustion process, the engine cycles are clas- sified as internal combustion engine cycles such as Otto, CPC constant pressure combustion Diesel, Atkinson and Brayton (vehicular engines and gas (current GT designs) turbines) or external combustion engine cycles such as CPHA constant pressure heat addition Ericsson, Stirling and Rankine (Stirling engines and steam CVC constant volume combustion (pressure gain) turbines). The internal combustion engine cycles are further clas- CVC-GT constant volume combustion gas turbine (the sified according to their heat addition process based on ultimate) constant pressure heat addition (such as Brayton gas turbine CVHA constant volume heat addition cycle) or constant volume heat addition (Otto and Diesel car GT gas turbine and truck engines). In the remainder of the main body of this article, the GTCC gas turbine combined cycle academic term “heat addition” will be replaced by the techni- METH mean-effective temperature (high) cal term for its practical implementation in actual engines, METL mean-effective temperature (low) namely fuel-air combustion. While constant volume combustion is presently limited PDC pulse detonation combustion to reciprocating (piston-cylinder) engines, the “first economi- PDE pulse detonation engine cally practical” gas turbine – as declared by none other than PR pressure ratio Aurel Stodola – was in fact a constant volume bona fide combustion (CVC) gas turbine invented and developed by PR' precompression pressure ratio Hans Holzwarth around the turn of the 19th century. RF realization factor Subsequently however, starting in the early 1930s, gas R-G Reynst-Gülen cycle turbine development has centered almost exclusively on con- stant pressure combustion (CPC) technology for aircraft SSSF steady state steady flow propulsion and land-based power generation, This article is TIT turbine inlet temperature intended to provide a glimpse into the potential benefits of a USUF uniform state uniform flow return “back to the future” that CVC technology has to offer. 1 GAS TURBINE WORLD: November - December 2013 In layman’s terms, this is the simplest explanation of the Figure 1. Computed state points (1-2-3-4) apply to a conceptual Chapman-Jouguet benefit afforded by what is fre- detonation taking place in a semi-closed detonation tube fed with a stoichiometric CH4 air mixture at 118 psia (P ) and 480°F (T ). Exit conditions: pressure ratio (P /P ) = 2.4 quently referred to as pressure- 1 1 4 1 and T = 3205°F. Not shown is state 5, where combustion products mix with purge air gain combustion vis-à-vis the 4 steady-flow combustion pro- before entering the turbine. cess in gas turbine combustors. (It must be pointed out that in practice, just as the former is x = 0 Tube x = L not exactly a constant volume process, the latter is not a con- stant pressure process.) von Neumann spike – 15.8 Constant volume heat ad- CH4 LHV 21, 515 Btu/lb dition is the ideal cycle proxy Ma = 3.72 fuel / air - 0.0415 Qin for pressure gain combustion 5617 fps whereas constant pressure heat 1713 m/s addition is the proxy for steady- Wave Shock Reaction Zone plus flow combustion. – 8.8 Products, 4,396°F Rarefaction Wave 2 The ideal air-standard cycle Constant State analysis provides the thermody- namic explanation for the fun- Blowdown Pressure Ratio damental difference between the – 3.5 3,559°F two processes without resort- 3 3,205°F ing to a highly complex ther- – 2.4 4 1 mochemical treatment of the – 1 actual combustion phenomena CH4 – air mixture in their practical embodiments 118 psia, 480°F Time (i.e., combustion chambers or combustors). A very brief introduction of Carnot cycle. The difficulty is in designing (conceptually as the thermodynamic principle behind the superiority of con- well as physically) a steady-flow device that can accomplish stant volume combustion vis-à-vis its constant pressure combustion with simultaneous temperature and pressure rise. counterpart appears in the Thermodynamic section of this Constant volume or pressure-gain combustion in an oth- article. The reader not satisfied by the simplified explanation erwise steady flow system is characterized by intermittency (above) is encouraged to examine the more rigorous albeit or pulsation (in other words, unsteadiness). In fact, unsteady brief thermodynamic explanation. or intermittent flow approximating the ideal constant-volume The cited references and any textbook can be consulted combustion has been the main characteristic of historical for a more in-depth understanding; the remainder of this machines and their modern-day descendants, gas-fired recip- article is focused on the more practical aspects of constant rocating engines. volume combustion. Why detonation? From a purely theoretical perspective, constant volume com- bustion is clearly the superior process. By the same token, its practical, non-ideal embodiment (pressure-gain combustion) is superior to steady-flow quasi constant pressure combus- tion. Indirect proof for this assertion can be found in effi- ciency comparison of reciprocating piston-cylinder engines (over 45% efficiency) and heavy duty industrial gas turbines (37-40% efficiency). Recip efficiencies are only matched by aeroderivative gas turbines with extremely high pressure ra- tios of 40 to 1 and higher. Theoretically, comparison with other air-standard cycles Figure 2. General Electric R&D 3-tube pulse detonation com- (see Thermodynamics section) shows CVC to be the ulti- bustion test rig used to gather data on performance, operation mate, i.e. highest efficiency approach to reaching the ideal and noise levels. GAS TURBINE WORLD: November - December 2013 2 With precise control of pressure, temperature and mixture Turbine composition, ignition and energy release occurs behind the Multi-Tube PDC Exhaust shock wave. Almost all practical manifestations of detona- tion combustion in laboratory experiments (as well as in test engines) have been achieved in semi-closed tubes as succes- Primary/Secondary sive detonations with a given frequency Air Inlets Chapman-Jouguet (Figure 1), hence the name pulse(d) detonation. Thus, just like turbo-compound and pulse jet engines, the pulse detonation combustion gas turbine is also an approxi- mation of the ultimate steady-state, steady flow constant vol- ume combustion/heat addition cycle. Single-Stage Nevertheless, pulse detonation combustion gas turbines Axial Turbine or, more concisely pulse detonation engines (PDE), have (inside) the potential to lead to high power density designs compa- Fuel/Spark rable to modern frame gas turbine units, but with higher ef- ficiency and (possibly) lower emissions. Figure 3. GE-NASA multi-tube pulse detonation combustion with 1000 hp single-stage power turbine. A brief history of pulse detonation The idea of using intermittent or pulsed detonation combus- ....From a practical perspective, reciprocating (piston-cylin- tion for aircraft gas turbine engines goes back before the der) engines or gas turbines with piston-cylinder engines as 1950s. Research engineers in Germany reportedly considered combustors are rather poor candidates
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