Transforming Combustion Research through Cyberinfrastructure (2011) / Chapter Skim
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3 Combustion and Cyberinfrastructure
3 Combustion and Cyberinfrastructure
Pages 41-67
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From page 41... ...
and other emissions into the atmosphere while at the same time significantly increasing combustion efficiency so as to make fuels last longer. Most combustion systems are based on technologies that are very old; gasoline and diesel engines were invented more than 100 years ago.
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From page 42... ...
For example, it is now possible to probe turbulent flames experimentally in ways that elucidate turbulent-flame structure in detail both spatially and temporally. In addition, time-resolved velocity fields and two-dimensional planar images of flame markers to capture the interac tion of a flame with turbulent flows can now be measured.
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From page 43... ...
Among the features of a cyberinfrastructure that would immediately be very helpful would be common repositories and/or registries for experimental and computational data, which would make data sets available to any researcher. The establishment of collaborative groups working to produce, improve, and maintain these common data sets would lead to more accurate and robust models for chemical kinetics, molecular transport, radiation parameters, and thermochemical databases.
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From page 44... ...
For more realistic combustion systems, it is necessary to introduce additional complexity to the model. Most practical combustion devices operate in a turbulent environment; consequently, even for gaseous combustion, one must deal with the complexities of turbulent flows.
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From page 45... ...
Over the past decade, the advent of HPC and advances in numerical algorithms have changed this, and it is now feasible to simulate realistic turbulent flames with detailed kinetics for simple fuels (Chen, 2011; Lu and Law, 2009)
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From page 46... ...
Direct injection can also be an enabler for homogeneous charge compression ignition (HCCI) engine technology, a new combustion strategy under investigation that approaches the high fuel efficiency of a diesel engine while producing very low NOx and soot emissions -- so low that there is the potential to meet the 2010 emissions stan dards without exhaust aftertreatment.
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From page 47... ...
Reciprocating engines, found in most internal combustion engines, operate as repetitive time-dependent systems in a cyclically changing environment, whereas turbines used in aircraft and for power generation and industrial burners are statistically stationary turbulent flames (see Box 3.2) in which oscillation instabilities related to acoustic flame interactions are important phenomena.
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From page 48... ...
Combustor modeling has assisted significantly in the design of recent generation engines and can be expected to be even more critical at the extreme conditions in cycles of high-efficiency engines. To enable such modeling, accurate simulations of phenomena such as fuel injec tion, spray and vaporization, fuel and air mixing, mixing and combustion in swirling and jets in-cross flows, heat transfer, fuel chemistry, and heat release rates in flows with Reynolds numbers on the order of 106 are required.
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From page 49... ...
Thus, a full-system model for a combustor is usually based on an underlying computational fluid dynamics (CFD) framework for turbulent flow that provides sufficient spatial resolution to describe the overall combustion process.
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From page 50... ...
The physical and chemical parameters in a typical combustion model cover wide ranges, and the characteristics of these data are one of the primary reasons for the development of a CI for combustion. In particular, combustion systems cover temperature ranges from room temperature to levels of 3000 K in the products of fuel and oxygen flames, and from very low pressures of a few torr in laboratory flame studies to hundreds of atmospheres of pressure in diesel engines and some types of detonations.
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From page 51... ...
Lifted flames are found in practical applications such as industrial burners for power generation, in which a lifted jet flame is utilized to reduce damaging thermal stresses to the nozzle material by minimizing contact between the flame and the nozzle. Lifted flames are also found in stratified combustion -- for example, in direct injection gasoline engines, diesel engines, and gas turbines -- where the fuel and oxidizer streams are partially premixed prior to combustion.
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From page 52... ...
There is, in fact, an entire discipline of mechanism reduction for chemical kinetics. Some of these approaches use an elementary approach by just removing chemical species and elementary chemical reactions that are relatively unimportant, whereas other, more complex approaches take advantage of the fact that strong coupling exists between the evolution equations for groups of chemical species (Prager et al., 2009; Liang et al., 2009; Hughes et al., 2009; Bykov and Maas, 2009)
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From page 53... ...
In addition to chemical kinetics and turbulent flow, other areas amenable to study using a combustion CI include, but are not limited to, surface chemistry and catalysis and condensed-phase phenomena. In conducting full-scale practical combustion simulations, however, it is not possible to resolve all of the scales needed to represent turbulence.
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Each of these simulations requires as inputs chemistry data relevant to the fuel of interest and some reacting-flow submodels (e.g., for turbulent mixing and radiative heat transfer)
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A generic name for these large-scale features is an "eddy," which qualitatively describes a swirling motion that is characteristic of turbulent flows. Thus, LES models offer enhanced representations of turbulent eddies and therefore bet ter predictive capabilities.
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The proposed combustion CI can build on Thermochemistry, Chemistry rates, community reaction mechanisms Engineers Central creating new database engines and shared and simulation alternative tools fuels Validated simulation methods and Reacting flow and submodels turbulent flame community FIGURE 3.1 The cyberinfrastructure will collect the information and submodels needed by engineers to simulate novel combustion systems and alternative fuels Figure 3-1.eps under all operating conditions.
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From page 57... ...
These include specific heats, entropies, and enthalpies of formation; molecular transport coefficients; heats of fusion and vaporization; and other quantities that are specific for each individual chemical species involved in kinetic mechanisms, which are used to evaluate macroscopic variables such as the equations of state. Other species-specific quantities of importance include ionization potentials, radiative opacities and cross sections, and other spectroscopic parameters.
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From page 58... ...
For coupled chemical kinetic models, one differential equation must be solved for each chemical species in the reaction mechanism, and these differential equations are coupled through their mutual chemical reactions. While a hydrogen oxidation mechanism may be as small as 8 to10 species and is relatively easy to solve numerically, a mechanism for the important diesel reference fuel n-hexadecane (n-C16H34)
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From page 59... ...
The CI must provide not only comprehensive mechanisms for a given fuel but also a hierarchy of reduced mechanisms for which the trade-off of fidelity versus computational complexity is well characterized. The data needed to develop the high-fidelity chemical kinetics mod els represent a synthesis of data from a broad range of disparate sources.
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From page 60... ...
The second of these subcommunities consists of researchers who develop and apply simulation methodology to study flame behavior at the continuum scale using a "first principles" approach; namely, solving the reacting Navier-Stokes equations. At the forefront of this subcom munity is a small number of research groups whose work focuses on harnessing the most powerful supercomputers to perform large-scale direct numerical simulations of turbulent flames.
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From page 61... ...
For example, mixed combustion modes can include partially premixed flame propagation into low-temperature autoigniting mixtures for which traditional, purely premixed or purely non-premixed combustion models do not apply. Experimentation in these adverse environments is limited, and high-fidelity simulation is providing complementary data required to understand and model these complex combustion regimes.
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From page 62... ...
Time-dependent direct numerical simulations of turbulent flames being performed today are able to model billions of spatial zones for relatively simple gaseous fuels using either detailed or reduced chemical mechanisms. Typically, detailed mechanisms for hydrogen, syngas, or methane can be incorporated in DNS, whereas reduced mechanisms for larger hydrocarbon and oxygenated fuels -- for example, n-heptane, di-methyl ether, butanol -- accurately rep resenting low-, intermediate-, and high-temperature kinetics at pressure (i.e., transporting approximately 30 to 80 reactive species)
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From page 63... ...
The addition of high-fidelity simulation benchmark data and their comparison with experimental and RANS and LES models will require additional CI to accommodate the sheer volume of the simulated data. Data Flow All of the subcommunities involved in developing and verifying the molecular properties needed by the combustion community are in the chemical sciences, so they all communicate in a common scientific "language," and they all could be served by similar molecule-oriented user interfaces.
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From page 64... ...
Today, problems such as soot inception, growth, and oxidation are being pursued by many independent, individual groups, with minimal interconnection. Radiation transport in combustion devices, turbulence modeling, and many other data-rich submodels have the same characteristics as those outlined above.
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and international (the biennial International Combustion Symposium) , and by helping manage the premier journal publications in combustion science.
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From page 66... ...
2011. "Petascale Direct Numerical Simulation of Turbulent Combustion -- Fun damental Insights Towards Predictive Models." Proceedings of the Combustion Institute, Vol.
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From page 67... ...
2009. " A Comprehensive Detailed Chemical Kinetic Mechanism for Combustion of n-Alkane Hydrocarbons from n-Octane to n-Hexadecane." Combustion and Flame, Vol.
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