Search

To investigate dynamics of vortex clusters and large-scale structures in the outer layer of wall turbulence, direct numerical simulations of turbulent channel flows have been conducted up to Reτ = 1270. In the outer layer, the vortex clusters are composed of coherent fine-scale eddies (CFSEs) of which diameter and maximum azimuthal velocity are scaled by the Kolmogorov length and velocity. The large-scale structure in the outer layer is composed of these clusters of the CFSEs, which contributes to the streamwise velocity deficit (i.e.low-momentum region). The CFSE clusters are observed in the low-momentum regions of the outer layer, and the scale of those clusters tends to be enlarged with the increase of a distance from the wall. The dynamics of large-scale structures reveals that the cluster structure generated in the bottom of the logarithmic region moves downstream and its scale increases with the increase of the low-momentum region. The CFSE clusters in the low-momentum regions of u' ≤ -urms consist of the relatively strong CFSEs, which play an important role in the production of the Reynolds shear stress and the dissipation rate of the turbulent kinetic energy. The process of destruction of the CFSE cluster is also clarified in the outer layer. [ABSTRACT FROM AUTHOR]

Hydrogen-fueled gas turbines play one of the key roles in future carbon-neutral energy structure. Due to hydrogen's high flame speed and extreme flame temperature, design of burners applying hydrogen as fuel is of challenge. In this work, we conduct direct numerical simulations to investigate a non-premixed steam-diluted oxygen/hydrogen combustion, which is formed by inclined impinging fuel and oxidizer jets injected by sub-millimeter nozzles. This configuration inherently prevent flashback and has a higher mixing efficiency enhanced by jet impingement, thereby leveraging advantages of both conventional premixed and non-premixed configurations. The flame is lifted flame held at the jet impinging position far away from the wall boundary. The most intensive mixing process occurs at the outer sides of the two branches of hydrogen jet after impinging, forming the flame base and resulting in an edge-flame configuration. A larger jet inclination angle leads to more effective bulk turbulence mixing because of the increased mixing volume, thereby enhancing combustion completeness. Simultaneously, it results in lower wall heat flux due to gentler upstream recirculation of burnt products. Chemical explosive mode analysis is conducted to analyze the combustion procedure from ignition to burnt out. The flame is held by the recirculation of high-temperature burnt products upstream of the impingement position, and its structure is primarily formed by the simultaneous ignition of a widespread explosive mixture enhanced by intense turbulence. • DNSs are conducted to investigate the combustion of impinging fuel and oxidizer jets. • Edge-flame configuration is formed by impinging fuel and oxidizer jets. • Fuel and Oxidizer are intensively mixed near the oxidizer jet branches. • Inclination angle of jets affects the statistical thermophysical performance. • Flame is held by hot product recirculation. [ABSTRACT FROM AUTHOR]

Abstract Nitrogen oxide (NOx) emission regulations are strengthening to reduce fine particles (particulate matter) in the East Asian, countries including Korea. In this paper, empirical evidence of exhaust gas (NOx) reductions considering the pilot fuel split ratio of a combustion-tuning methodology is provided using the full-scale gas turbine combustion test facility of the KEPCO (Korea Electric Power Cooperation) research institute. The combustion-tuning methodology confirmed in the laboratory was verified by load tests (50%, 70%, and 100% loads) for the Gunsan CCPP (combined cycle power plant) at Korea Western Power. Particularly, empirical tests of the NOx reduction effect of around 20% without the combustion instability phenomenon under a partial load were successfully carried out. The change in the fuel ratio through the tophat and pilot nozzle and the change in the combustion air volume through a bypass valve were analyzed to assess the sensitivity to the amount of NOx emission generated under actual gas turbine (Mitsubishi Hitachi Power System's GT model for power generation: 501G) operating conditions. It was also confirmed qualitatively that the trends of the NOx emission due to a change in the pilot fuel split ratio from full-scale atmospheric test results in a laboratory and actual operation data of the actual gas turbine under a 50% load were similar. Based on the reduced amount of NOx by the combustion-tuning methodology, the basic deductible savings levels for each gas turbine unit due to the exhaust gas regulation were introduced. [ABSTRACT FROM AUTHOR]

The quenching mode of local flame–wall interaction (FWI) is investigated for its response to different levels of turbulence intensity as well as its effect on quenching distance, wall heat flux, and near-wall reaction. For that, direct numerical simulations of turbulent premixed methane combustion in a constant volume vessel are carried with initial Karlovitz numbers (Ka) of 1.0, 10.0, and 30.0. Local flame–wall quenching positions are identified based on the local fuel consumption speed during the turbulent combustion process, and the local FWI events have been classified into four quenching modes according to the flame–wall geometric relationships of quenching positions, namely head-on quenching (HOQ), oblique-wall quenching, side-wall quenching (SWQ), and back-on quenching (BOQ). The results show that in the case with higher initial Ka, the flame surface shows a more complicated wrinkled structure due to the flame–turbulence interaction. Meanwhile, the local quenching distance defined based on the identified quenching position is strongly influenced by the near-wall flow, and the range of the local quenching mode extends further to BOQ. However, for all three cases, HOQ and near-HOQ modes account for the majority of local FWI. Wall heat flux and heat release rate (HRR) of near-wall reaction yield high values for the FWI under HOQ or BOQ and are low for SWQ. In addition, there is a discrepancy in the near-wall transportation of some species under different quenching modes, which further leads to the difference in FWI-induced near-wall reaction regarding its total and elementary HRR. [ABSTRACT FROM AUTHOR]

To facilitate fuel interchangeability for industrial gas turbines, various kinds of fossil fuels including syngas from coal gasification and steel mill gases including Blast Furnace Gas (BFG) have been investigated by previous researchers to determine their viability as alternative fuels for power generation. In the present study, various syngas components produced from coal gasification were combined to produce a simulated mixed gas, which was fed into a pressurized gas turbine combustion test rig to determine the combustion characteristics. A scaled model of the 7EA combustor from GE was used to conduct a basic comparative study on the emission trend (NO x ), exhaust gas temperature distribution according to the combustor pressure, syngas composition and dilution ratio at the fuel side. The major findings were that modification of the empirical formula for pressure affects NO x emission and nitrogen dilution ratio affects the temperature profile uniformity on the fuel side. [ABSTRACT FROM AUTHOR]

Direct numerical simulations of turbulent premixed planar flames in the corrugated flamelets and thin reaction zones regimes are analysed to investigate the effect of turbulence on the flame structure and morphology. A tool based on topological invariants called shapefinders, consisting of the planarity and filamentarity, is applied to assess the flame morphology. Several statistics show that the filamentarity, which represents lumped effects of the turbulence on the flame morphology, is closely correlated with the Damköhler number, but not with the Karlovitz number. To investigate which scale of turbulent fluctuations is responsible for the flame morphology evolution, the conditional averages of the Kolmogorov length scale and the Taylor microscale are studied. The conditional averages show strong correlation between the Taylor microscale and the filamentarity, while similar strong correlation is not observed for the Kolmogorov length scale. These results suggest that the turbulence–flame interaction relevant to the flame morphology occurs at the length scale greater than the Taylor microscale for relatively large Damköhler number conditions. The fractal dimension is computed for the DNS and filtered reaction progress variable fields with different filter sizes. The computed fractal dimensions between the resolved and the Taylor-microscale filtered fields are almost identical. Also, it was shown that 93–97% of flame surface area is recovered when the filter size of the Taylor microscale is used. However, this fraction rapidly decreases when the integral length scale is used for the filter size. A similar trend was observed for the flame wrinkling factor. [ABSTRACT FROM AUTHOR]

Direct numerical simulation is employed to investigate the turbulent flow characteristics and their effect on local flames for mean reaction rate modelling in turbulent swirling premixed flames. Two swirl numbers having significant effects on the formation of a central recirculation zone in the combustor are considered. The large velocity gradients in the higher swirl number case produce high turbulence intensity in a relatively upstream region compared to the lower swirl number case. The conditional Probability Density Functions (PDFs) of the reaction rate and dissipation rates of turbulent kinetic energy and scalar fluctuations are also examined. The PDFs show correlations between the turbulence energy dissipation and reaction rates and between the scalar dissipation and reaction rates, suggesting that the heat and radicals from the hot products trapped in the recirculation zones are mixed with the reactants, not only through scalar dissipation rate (i.e. scalar gradient) but also by small-scale processes of turbulence relevant to turbulent kinetic energy dissipation rate. Therefore, both scalar and velocity gradients have a strong influence on the chemical reactions through mixing of cold reactant and hot products. A conventional flamelet and EDC models are used to estimate the mean reaction rate, and to study the balance between these two mixing mechanisms. Although both models show a qualitative agreement with the DNS results, these models compensate their limitations each other, depending on the local turbulence and thermochemical conditions. A simple approach is proposed to exploit the advantages of these two models by considering the balance of two mixing mechanisms based on the chemical and turbulence time scales. The estimated mean reaction rate using the proposed model is significantly improved for the higher swirl number case, although the estimated value slightly shifts away from the DNS results for the lower swirl number case. The improved modelling estimate and the balance of turbulence and chemical time scales suggest that the locations of intense reaction zones are strongly related to the dissipation rates of both scalar and turbulent kinetic energy. [ABSTRACT FROM AUTHOR]

In this study, with the aim of deriving a temperature-based energy equation and evaluating the fractal dynamic SGS (FDSGS) combustion model under an isochoric condition, modelling of the filtered energy conservation equation in the temperature form is considered and is tested in the contexts of a priori and a posteriori, comparing with a corresponding DNS of turbulent combustion in a constant volume vessel. Conditional averages of various terms in the filtered energy conservation equation in the temperature form show that three unclosed terms, the SGS scalar flux, the SGS pressure-dilatation and the heat release rate, are leading terms. New SGS models for the SGS pressure-dilatation and the heat release rate are proposed and are tested in a priori manner. The zeroth-order models are also compared. The heat release rate models are proposed for reaction zones and post-flame zones separately, since the present analysis revealed a substantial contribution of heat generated in the post-flame zones. In addition to the tests of the SGS models for the energy conservation equation, isochoric treatments for various flame quantities under varying mean pressure and unburnt temperature due to the isochoric condition are also proposed and are tested. LES using the FDSGS combustion model with the new SGS models and isochoric treatments is performed considering the same combustion conditions and configuration as the DNS. The LES with several other SGS combustion models are also performed and are compared. The performed LES show reasonable prediction of flame propagation for any of the SGS combustion models tested, although the prediction by the FDSGS combustion model shows closest to the DNS. Regardless of the SGS combustion models, the peak values of the mean pressure are close to the DNS value, which suggests that the proposed SGS models and isochoric treatments for the energy conservation equation work well. [ABSTRACT FROM AUTHOR]

Direct numerical simulation (DNS) data of moderate or intense low-oxygen dilution (MILD) combustion and a planar flame are analysed to identify quantities influencing the unique features of co-existing combustion modes and develop a model to identify them in MILD combustion. The results show the existence of direct relationship between the scalar dissipation and reaction rates in MILD combustion, whereas the correlation is weaker than the planar flame. Also, rotational turbulent motions identified by the enstrophy-strain balance also show a substantial influence on the MILD combustion field, via the principal component analysis, suggesting the mechanism by which the non-flamelet part of reaction zones is controlled. A neural network (NN)-based model was proposed to identify the filtered local combustion mode for MILD combustion fields in LES context. The NN is trained based on DNS data of MILD combustion at two different conditions for different filter sizes. The model assessment was performed using the third MILD condition having a higher Ka and dilution level than those of the two training data sets. Several filter sizes ranging from 0.5 to 2.0 times of a corresponding laminar flame thickness were considered, and also prediction performance of a "zeroth-order model" is compared with that of the NN-based prediction. The assessment shows very high correlation between the NN-based prediction and the mode directly obtained from DNS. The predicted local combustion mode could be used to exploit advantages of both flamelet and non-flamelet-type combustion models to predict co-existing MILD reaction zones. [ABSTRACT FROM AUTHOR]

Effects of H 2 -enrichment on structures of CH 4 /air turbulent swirling premixed flames affected by high intensity turbulence in a gas turbine model combsutor are investigated by conducting direct numerical simulations. Two stoichiometric mixture conditions, of which volume ratio of CH 4 :H 2 = 50:50 and 80:20, are simulated by considering a reduced chemistry (25 species and 111 reactions). Results showed qualitatively different flame shapes and reaction zone characteristics between the cases. For the higher H 2 -ratio case, the flame is stabilized both in the inner and outer shear layers. For the lower H 2 -ratio case, the flame is stabilized only in the inner shear layer and extinction occurs in the outer shear layer. Comparison of the reaction zone characteristics with unstrained and strained laminar flames in phase space showed that H 2 mass fraction for the lower H 2 -ratio case and reaction rate profiles for both cases deviate from the corresponding laminar values. Analysis of fuel species conservation equation suggests that the turbulent transports are substantially influential to determine local and global flame structures. These findings would be useful for designing practical H 2 -enriched gas turbine combustor in the aspect of flame structures under high intensity turbulence. Image 1 • Effects of mixture conditions on CH 4 /H 2 /Air turbulent swirling premixed flames are investigated. • Qualitatively different global flame features between the mixture conditions are observed. • The role of H 2 in each mixture condition remains unchanged even under intense turbulence. • Reaction zone characteristics deviate from unstrained and strained laminar flame profiles. • Contributions by convection and diffusion are influential on the local and global flame structures. [ABSTRACT FROM AUTHOR]

This study is aimed at verifying the reliability and reproducibility of combustion tests, including ignition, load change and fuel changeover, conducted at a well-resourced full-scale gas turbine syngas combustion test facility. The 10 MWth, single-can, syngas-fired combustion test facility was equipped with analytical equipment to measure air and fuel flow rates to the combustor, the metal/gas temperature in the combustor, and exhaust gas composition and temperature distribution at the combustor's outlet. To confirm the test facility's reliability, the repeatability of the fuel changeover test from natural gas to syngas was evaluated. Reliability was also verified by cross-validating the theoretical and measured values for fuel/air (F/A) ratio and Turbine Inlet Temperature (TIT). In this study, the deviation between the averaged F/A ratio based on O 2 and CO emission data and the F/A ratio based on the mass flow rate was under 2% at most, when the F/A ratio exceeded 20%. And, the calculated TIT for syngas, taking thermal dissociation and heat loss into consideration, correlates well with the experimental result which is the corrected TIT value based on heat balance at the temperature sensor tip. Image 1 • Full scale gas turbine combustor test rig on syngas was evaluated for reliability. • F/A ratio & Turbine Inlet Temp.(TIT) were studied as key indices for verification. • F/A ratios based on the flow rate and emission data were cross-validated. • Measured TIT was compared with the estimation based on enthalpy conservation rule. • Estimated TIT using air, fuel flow rate based on enthalpy conservation is reasonable. [ABSTRACT FROM AUTHOR]

Clustering of coherent fine-scale eddies in a turbulent mixing layer has been analyzed by using direct numerical simulation (DNS) data at Re λ  ≃ 250. The coherent fine-scale eddies are defined based on the second invariant of the velocity gradient tensor and the vorticity vector. The clustering is evaluated by the number density of coherent fine-scale eddies, and the large-scale structures are extracted by low-pass filtered velocity fields. Conditional averaging shows that the large-scale enstrophy increases with the number density, whereas the large-scale strain rate stays around the average in the high number density region. The alignments of the vorticity vector and the eigenvectors of the large-scale strain rate tensor are conditioned by the number density or the strain rate magnitude. The eigenvectors and the vorticity vector indicate strong preferential alignments under the intense large-scale strain rate condition. On the other hand, those alignments become weak in the high number density regions. The inter-scale energy transfer between grid and subgrid scales is significantly correlated with the magnitude of the large-scale strain rate while there is no apparent correlation with the number density. [ABSTRACT FROM AUTHOR]

Direct numerical simulation (DNS) of laminar lean methane–air and n -heptane–air premixed flames with high exhaust gas recirculation (EGR) ratios propagating towards inert walls in a head-on quenching configuration is conducted to investigate flame–wall interactions at relatively high initial pressure conditions. This study considers the flame propagation under isochoric process after ignition while the piston is at top dead center (TDC). The effects of EGR ratio, equivalence ratio, initial pressure and wall temperature on heat loss and quenching distance are investigated. The results showed that change of EGR ratio in fuel mixture significantly affects the maximum wall heat flux and the heat flux induced by the burned gas temperature. The normalized flame–wall interaction time is not influenced over a range of EGR ratios, equivalence ratios and initial chamber pressures for methane–air and n -heptane–air flames at fixed wall temperature conditions. The dimensional flame–wall interaction time is almost constant when the chamber pressure is doubled. The influence of low temperature oxidation in n -heptane flames on wall heat flux induced by temperature differences between the preheated fuel mixture and the wall is found to be insignificant. Moreover, both thermal conductivity near the wall and quenching distance are sensitive to the wall temperature, and have a substantial influence on the wall heat flux. [ABSTRACT FROM AUTHOR]

Direct numerical simulation of a stoichiometric hydrogen–air turbulent premixed flame in a rectangular constant volume vessel has been conducted to gain fundamental insights into turbulence–flame interactions and heat loss characteristics under a pressure rising condition. The turbulent vortices are significantly weakened in the burnt side due to expansion and viscosity increase. The conditionally averaged turbulent kinetic energy is suppressed near the wall and it takes mostly constant in the rest of the domain which suggests that there is no significant turbulence production near the wall, and the wall just damps the turbulence in the present simulation's set up. It is found that the turbulent kinetic energy still takes a significant value in the burnt side due to the velocity induced by the expansion, not much inherent to the turbulent eddies. Temporal evolution of the wall heat loss characteristics is studied. Although the heat loss induced by the burnt gas dominates the total heat loss at the end of combustion, the heat loss induced by flame impingements contributes to substantial portion of total heat loss. Finally, turbulence–flame interactions modeling is studied using a conventional flamelet model for reaction rate closure. The result shows that turbulence–flame interaction mechanism does not change significantly under pressure rising conditions, suggesting that this model could be also used in a pressure-evolving combustion system without further modifications. [ABSTRACT FROM AUTHOR]

Corrigendum to "Data driven analysis and prediction of MILD combustion mode" [Combust. [Extracted from the article]

Direct numerical simulation (DNS) results are analysed to investigate the effect of flame–flame interaction on the scalar distribution in a large eddy simulation context. The DNS data consist of turbulent premixed planar flame, V-flame and swirl flame cases with different turbulence and equivalence ratio, and relatively large Damköhler number conditions are considered for the three DNS cases. The volumetric fraction of non-flamelet type reaction zones Φ ¯ , which are caused by flame–flame interaction (FFI), is identified by the trained neural network. The quantification of scalar PDF mode is carried out by means of the bimodality coefficient. Various scalar PDFs are constructed with different sample volumes of the filter size. The PDFs with high bimodality coefficient samples show clear bimodal distributions, whereas low bimodality coefficient samples show unimodal or plateau distributions. The conditional PDF and conditional average of the bimodality coefficient conditioned based on Φ ¯ clearly show negative correlation between these two quantities. These results suggest that the presence of FFI events leads to bimodality loss of the scalar field, even when the Damköhler number is large. Reaction zones with non-bimodal scalar distribution are also found to have very small scalar gradient. Thus, the reaction rate of such reaction zones could be underestimated in flamelet-type modelling. However, such deficit may be straightforwardly complemented additively, for example, by considering a zero-dimensional canonical reactor with the fraction Φ ¯. [ABSTRACT FROM AUTHOR]

Deep neural network (DNN) is applied to mean reaction rate modelling. Two DNN structures, species-dependent (SD) and species-independent (SI), are considered. 1 1 Code and data to use the trained deep neural network models in this work are available at https://github.com/minamoto-group/PGDNN%5fRANS%5fmodel%5fNakazawa%5fetal. Due to the explicit inclusion of all species variables in the input layer for SD-DNN, this model may consider relationships of different chemical species. However, the prediction can be performed only for simulations with chemical mechanisms considering the same set of species as the one used in training data. SI-DNN circumvents this constraint, and can be used for any set of species appearing in the combustion. For the efficient learning and better prediction performance, two physics-guided loss functions are proposed and employed, which consider mass conservation of the mixture and elemental species in a specific formulation that yields a larger number of constraint conditions. These DNNs are trained and validated using direct numerical simulation (DNS) data of three different turbulent premixed planar flames, and tested using DNS results of a fourth turbulent premixed planar flame and turbulent premixed V-flame to assess the robustness of the present models for an unknown combustion configuration as well as unknown turbulent combustion conditions. [ABSTRACT FROM AUTHOR]

• DNS of premixed flames propagating and impinging on the wall were performed. • The flame propagation speed evolution has a local minimum value before quenching. • The local minimum value is almost identical to a corresponding laminar flame speed. • The trend is consistent for a range of mixtures, ignition and wall reactivity. • This behaviour could be used as a robust laminar flame speed measurements. Direct numerical simulations of premixed flames propagating and impinging on the wall surface are performed to investigate the near-wall flame behavior. The evolution of the flame front, which is established from an ignition kernel initially to mimic a standard experimental procedure, shows that the near-wall flame propagation has unique characteristics and following results are obtained. Displacement speed of the flame propagating normal to the wall yields a local minimum value near the wall before temporarily increases and quenches. This local minimum value becomes almost identical to a corresponding laminar flame speed when the wall temperature is close to the unburnt gas temperature initially. These trends are consistently observed for various ignition positions, equivalence ratios and fuels. Effects of radical removal by the surface reaction on the near-wall flame behaviour are also investigated. It is found that the adsorption of H-atom makes a substantial contribution for the suppression of the near-wall heat release rate under the conditions of present study. However, since the region where the radical removal affects heat release rate is much closer to the wall surface than the quenching distance, there is negligible effect on the near-wall flame displacement speed behaviour. These findings suggest that the present configuration could be used as a robust flame speed measurement technique, similar to the conventional double-kernel method, but without adverse effect of thermo-diffusive instability. [ABSTRACT FROM AUTHOR]