Analysis of high pressure H2/O2, H2/air and kerosene/air reacting shear flows
Başlık çevirisi mevcut değil.
- Tez No: 509954
- Danışmanlar: Dr. RICHARD S. MILLER
- Tez Türü: Doktora
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2016
- Dil: İngilizce
- Üniversite: Clemson University
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 126
Özet
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Özet (Çeviri)
Direct Numerical Simulation (DNS) data for high pressure H2/O2 and H2/Air flames using the compressible flow formulation, detailed kinetics, a real fluid equation of state, and generalized diffusion are analyzed. The DNS is filtered over a range of filter widths to provide exact terms in the Large Eddy Simulation (LES) governing equations, including unclosed terms. The filtered heat flux vector is extensively compared with the heat flux vector calculated as a function of the filtered primitive variables (i.e. the exact LES term is compared with its form available within an actual LES). The difference between these forms defines the subgrid heat flux vector. The analyses are done both globally across the entire flame, as well as by conditionally averaging over specific regions of the flame; including regions of large subgrid kinetic energy, subgrid scalar dissipation, subgrid temperature variance, flame temperature, etc. In this work, both the subgrid heat flux vector and its divergence are found to be substantially larger in reacting flows in comparison with mixing due to the associated larger temperature gradients. However, the divergence of the subgrid heat flux vector tends to be significantly smaller than other unclosed terms in the energy equation with decreasing significance with increasing Reynolds number. Then a reduced (29 step, 10 species) Kerosene/Air mechanism including a semi-global soot formation/oxidation model associated with an optically thin medium radiative heat flux model has been added to the same code to investigate soot formation/oxidation processes in a temporarily developing hydrocarbon flame operating at both atmospheric and elevated pressures for a both real gas law (RGL) and the ideal gas law (IGL) equation of states (EOS). Good agreement with the limited literature of atmospheric pressure flames [97, 46, 45] has been achieved for both 3D the RGL and the IGL EOS predictions for the soot formation/oxidation process. High values of the soot volume fraction have been shown to be independent from high temperature flame regions by occupying the flame volumes whose temperature varies from 1300 K to 1800 K. Additionally, the soot number density has been shown to be highly dependent on the temperature, while the soot volume fraction is dominated by local flow characteristics which is also in good agreement with Ref. [97]. Lignell at el. [46, 45] have reported two distinct behaviors of soot mass farction I- the slow soot nucleation process has caused the soot mass fraction to be widely scattered in the flame, and II - turbulent transportation has carried the soot to the fuel rich region by emphasizing the importance of the turbulence transportation in sooting flames. Similar behavior has been also observed in the current work. The soot generation rate has been shown to have a similar trend with soot mass fraction, while Lignell at el. [46, 45] have observed a high dependency on flame temperature for the soot generation rate. Furthermore, a slight difference has been observed between the RGL and the IGL EOS model predictions of soot quantities in atmospheric pressure flames. This implies employing the IGL EOS might be reasonable to eliminate the complexity of mathematical models of real gas effects. However, at 35 atm, the IGL EOS model has been shown to extremely over-predict not only the flame temperature but also the soot quantities by 25% to 100% in comparison with the RGL EOS model predictions. In elevated pressure flames, a similar trend has been observed for the soot volume fractions and the soot number density, while high values of the soot mass fraction exhibits a less scattered profile in the mixture fraction coordinate which is limited in the range of =0.4 to 0.9. The soot generation rate has been observed to a have smaller standard deviation than its means in elevated pressure flames, while they have been observed to be larger than its means in atmospheric pressure flames. Additionally, like Lignell at el. [45] have reported due to the short simulation time, and small soot load in comparison to the domain size, radiative heat transfer is found to be insignificant. The radiative heat flux effect is needed to be analyzed artificially by increasing the Planck mean absorption coefficient of the optically thin medium model. The unity Lewis (Le) number assumption on the soot formation/oxidation process has been studied in 2D DNS of atmospheric pressure flames for both the RGL and the IGL EOS models. The results have shown that the unity Le number assumption results under-predicting the soot quantities and the flame temperature. The known effect of unity Le number on the enthalpy has been clearly seen in these atmospheric pressure flames. Ignoring Le number effects has been shown to under-predict the soot quantities by at least an order of magnitude. A further investigation of the unity Le number assumpton is required for 3D DNS at elevated pressures for both RGL and IGL EOS models. After testing the validity of the current model with past literature, and revealing the importance of real gas effects on the soot formation/oxidation process, 2D DNS have been conducted to investigate pressure effects on the process in a much deeper manner. It has been known for decades that in hydrocarbon flames soot production has been increased by increased ambient pressure. Such a behavior has been noted in the current work by investigating flames of 1, 5, 10 and 35 atm with the RGL and the IGL EOS models. These predictions show the clear effects of pressure on the soot production/oxidation processes. For the first three pressures the IGL EOS model has not deviated from the RGL EOS model significantly. However, for the flames of 35 atm the differences becomes highly significant. In order to gain insight into effects of pressure on the process 3D DNS flames need to be studied with an artificially enhanced radiative heat flux model.
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