Moderate or Intense Low-oxygen Dilution (MILD) combustion is one of combustion technologies which can improve efficiency and reduce emissions simultaneously. This combustion type is characterised by the highly preheated reactant temperature and the relatively small temperature rise during combustion due to the intense dilution of the reactant mixture. These unique combustion conditions give MILD combustion very attractive features such as high combustion efficiency, reduction of pollutant emissions, attenuation of combustion instabilities and flexibility of the flow field. However, our understanding of MILD combustion is not enough to employ the MILD combustion technology further for modern combustion devices. In this thesis, Direct Numerical Simulation (DNS) has been carried out for turbulent MILD combustion under four MILD and classical premixed conditions. A two-phase strategy is employed in the DNS to include the effect of imperfect mixing between fresh and exhaust gases before intense chemical reactions start. In the simulated instantaneous MILD reaction rate fields, both thin and distributed reaction zones are observed. Thin reaction zones having flamelet like characteristics propagate until colliding with other thin reaction zones to produce distributed reaction zones. Also, the effect of such interacting reaction zones on scalar gradient has to be taken into account in flamelet approaches. Morphological features of MILD reaction zones are investigated by employing Minkowski functionals and shapefinders. Although a few local reaction zones are classified as thin shape, the majority of local reaction zones have pancake or tube-like shapes. The representative scales computed by the shapefinders also show a typical volume where intense reactions appear. Given high temperature and existence of radicals in the diluted reactants, both reaction dominated and flame-propagation dominated regions are locally observed. These two phenomena are closely entangled under a high dilution condition. The favourable conditions for these phenomena are investigated by focusing on scalar fluxes and reaction rate. A conditional Probability Density Function (PDF) is proposed to investigate flamelet/non-flamelet characteristics of MILD combustion. The PDF can be obtained by both numerically and experimentally. The PDF shows that MILD combustion still has the direct relationship between reaction rate and scalar gradient, although the tendency is statistically weak due to the distributed nature of MILD reaction zones. Finally, based on the physical aspects of MILD combustion explained in this work, a representative model reactor for MILD combustion is developed. The model reactor is also used in conjunction with the presumed PDF for a mean and filtered reaction rate closure. The results show a good agreement between the modelled reaction rate and the DNS results.

Fourier pseudo-spectral method on equispaced grid is one of the approaches in turbulence simulation, to compute derivative of discrete data, using fast Fourier Transform (FFT) and gives low dispersion and dissipation errors. In many turbulent flows the dynamically important scales of motion are concentrated in certain regions which requires a coarser grid for higher accuracy. A coarser grid in other regions minimizes the memory requirement. This requires the use of Non-equispaced Fast Fourier Transform (NFFT) to compute the Fourier transform, by solving a system of linear equations. To achieve similar accuracy, the NFFT needs to return more Fourier coefficients than the number of non-equispaced gridpoints, making it an under-determined system. The minimum L2 norm solution of the system is refined using an iterative reconstruction algorithm, FOCUSS. The NFFT and FOCUSS algorithms yield accurate results with smaller test case of a Direct Numerical Simulation on a grid of 64 gridpoints in each dimension, using Taylor green initial condition. The computational speed for this case was found to be unacceptably slow and few methods to improve the performance have been discussed. The approach of NFFT and FOCUSS was tested on a line extracted from 3-dimensional turbulent flow field. Fourier transform of the extracted line, sampled on 1024 non-equispaced gridpoints, computed for 2048 coefficients and the corresponding numerical derivative are found to be inaccurate. It can be observed that the NFFT and FOCUSS approach works for sparse Fourier transform, but not for turbulent fields having a wideband Fourier transform.

The utilization of ammonia as a fuel is a pragmatic approach to pave the way towards a low-carbon economy. Ammonia compromises almost 18 % of hydrogen by mass and accepted as one of the hydrogen combustion enablers with existing infrastructure for transportation and storage. From an environmental and sustainability standpoint, ammonia combustion is an attractive energy source with zero carbon dioxide emissions. However, from a practical point of view, the direct combustion of ammonia is not feasible due to the low reactive nature of ammonia. Due to the low combustion intensity, and the higher nitrogen oxide emission, ammonia was not fully investigated and there is still a lack of fundamental knowledge of ammonia combustion. In this thesis, the computational study of ammonia premixed flame characteristics under various hydrogen addition ratios and moderate or intense low oxygen dilution (MILD) conditions were investigated. Particularly, the heat release characteristics and dominant reaction pathways were analyzed. The analysis revealed that the peak of heat release for ammonia flame occurs near burned gas, which raises a question regarding the physics of this. Further analysis identified the dominant reaction pathways and the intermediate species (NH2 and OH), which are mainly produced in the downstream and back diffused to the leading edge and produce some heat in the low-temperature zone. To overcome low reactivity and poor combustion performance of pure ammonia mixture, the onboard ammonia decomposition to hydrogen and nitrogen followed by blending ammonia with hydrogen is a feasible approach to improve ammonia combustion intensity. With increasing hydrogen amount in the mixture, the enhancement of heat release occurs due to both transport and chemical effect of hydrogen. Another approach to mitigate the low reactive nature of ammonia may be eliminated by applying the promising combustion concept known as MILD combustion. The heat release characteristics and flame marker of ammonia turbulent premixed MILD combustion were investigated. The high fidelity numerical simulation was performed to answer fundamental questions of ammonia turbulent premixed combustion characteristics.

A bluff-body has been employed as the flame stabilization scheme for many combustion devices such as gas turbines and aviation engines. Although the bluff-body flame holder has a key advantage of generating a hot gas recirculation zone behind it and assist in stable combustion, it also induces flow field and combustion instabilities such as unstable vortex shedding, which can adversely affect the flame stability and lead to blow-off. The understanding of the physical mechanism of flame stabilization and blow-off processes has been one of the critical subjects in premixed combustion systems under highly turbulent conditions. As considering this, the present dissertation presents insight of flame stabilization and blow-off mechanisms using several series of computational studies and detailed analysis using diagnostic approaches. Two-dimensional direct numerical simulations are conducted to examine flame/flow and blow-off dynamics in lean premixed hydrogen-air and syngas-air flames stabilized on a meso-scale bluff-body in a square channel. Several distinct effects on flame stabilization and blow-off dynamics are investigated, such as reduced confinement, hydrodynamic instability, flame time scale, and differential diffusion effects. For the analysis, a proper time scale analysis is attempted to characterize the flame blow-off mechanism, which turns out to be consistent with the classic blow-off theory of Zukoski and Marble. The combined approach of computational singular perturbation and tangential stretch rate is applied to examine chemical characteristics in blow-off dynamics. As an extension from Eulerian to Lagrangian viewpoint, Lagrangian particle tracking analysis of post-processing the pre-computed results is performed to examine the local characteristics during the critical transient event of local extinction and recovery.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18306号 / 工博第3898号 / 新制||工||1598(附属図書館) / 31164 / 京都大学大学院工学研究科化学工学専攻 / (主査)教授 山本 量一, 教授 松坂 修二, 教授 古賀 毅 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

京都大学 / 新制・課程博士 / 博士(工学) / 甲第23503号 / 工博第4915号 / 新制||工||1768(附属図書館) / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 黒瀬 良一, 教授 花崎 秀史, 教授 岩井 裕 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The implementation of a hybrid spectral/finite-element discretization on the unsteady, incompressible, Navier-Stokes equations with a semi-implicit time-stepping method, an explicit treatment of the advective terms, and an implicit treatment of the pressure and viscous terms leads to an algorithm capable of calculating 3D flows over complex 2D geometries. This also results in multiple Fourier mode linear systems which must be solved at every timestep, which naturally leads to two parallelization approaches: Fourier space partitioning, where each processor individually and simultaneously solves a linear system, and physical space partitioning, where all processors collectively solve each linear system, sequentially advancing through Fourier modes. These two parallelization approaches are compared based upon computational cost using multiple solvers: direct sparse LU, smoothed aggregation AMG, and single-level ILUT preconditioned GMRES; and on two supercomputers of different memory architecture(distributed and shared memory). This study revealed Fourier space partitioning outperforms physical space partitioning in all problems analyzed, and scales more efficiently as well. These differences were more dramatic on the distributed memory platform than the shared memory platform. Another study compares the previously mentioned solvers along with one additional solver, pointwise AMG, in Fourier space partitioning without parallelization to better understand computational scaling for problems with large meshes. It was found that the direct sparse LU solver performed well in terms of computational time, scaled linearly, but had very high memory usage which scaled in a super-linear manner. The single-level ILUT preconditioned GMRES solver required the least amount of memory, which also scaled linearly, but only had acceptable performance in terms of computational time for coarse meshes. Both AMG methods scaled linearly in both memory usage and time, and were comparable to the direct sparse LU solver in terms of computational time. The results of these studies are particularly useful for implementation of this algorithm on challenging and complex flows, especially direct numerical and large-eddy simulations. Reducing computational cost allows the analysis and understanding of more flows of practical interest.

Wall-dominated turbulent dispersed multiphase flows occur in a variety of industrial, biological and environmental applications. The complex nature of the  arrier and the dispersed phase is elevated to a higher level introducing geometrical complexities such as curved walls. Realising such flows and particulate phases poses challenging problems both from computational and also physical point of view. The present thesis tries to address some of these issues Lagrangian computational frame. In the first step, turbulent flow in straight pipes is simulated by means ofdirect numerical simulation with a spectrally accurate code nek5000 to examine the Reynolds number effect on turbulent statistics. Adding the effect of the curvature to these canonical turbulent pipe flows generates Prandtl’s secondary motion of first kind. These configurations, as primary complex geometries in this study, are examined by means of statistical analysis to unfold the evolutionof turbulent characteristics from a straight pipe configuration. A fundamentally different Prandtl’s secondary motion of second kind is also put to test by means of adding the side-walls to a canonical turbulent channel flow and the evolution of various statistical quantities with varying the duct aspect ratios is discussed. After having obtained a characterisation of the turbulent flow in the geometries of bent pipes and ducts, the dispersion of small heavy particles is modelled in the bent pipe by means of point particles which are one-way coupled to the flow. For this purpose a parallel Lagrangian Particle Tracking (LPT) scheme is implemented in the spectral-element code nek5000. Its numerical accuracy, parallel scalability and general performance in realistic situations are scrutinised in various situations. Also, the resulting particle fields are analysed, showing that even a small degree of geometrical curvature has a profound impact on the particle concentration maps. For each of the aforementioned turbulent flow cases new and challenging questions have arisen to be addressed in the present and upcoming research works. Along with an improved understanding of the particle dispersion in the considered complex geometries, the current project is particularly intended to improve the numerical aspects of the current LPT module suitable for largescale computations. / <p>QC 20140226</p>

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