Sub-grid models for Large Eddy Simulation of non-conventional combustion regimes

Novel combustion technologies ensuring low emissions, high efficiency and fuel flexibility are essential to meet the future challenges associated to air pollution, climate change and energy source shortage, as well as to cope with the increasingly stricter environmental regulation. Among them, Moderate or Intense Low oxygen Dilution (MILD) combustion has recently drawn increasing attention. MILD combustion is achieved through the recirculation of flue gases within the reaction region, with the effect of diluting the reactant streams. As a result, the reactivity of the system is reduced, a more uniform reaction zone is obtained, thus leading to decreased NO x and soot emissions. As a consequence of the dilution and enhanced mixing, the ratio between the mixing and chemical time scale is strongly reduced in MILD combustion, indicating the existence of very strong interactions between chemistry and fluid dynamics. In such a context, the use of combustion models that can accurately account for turbulent mixing and detailed chemical kinetics becomes mandatory. Combustion models for conventional flames usually rely on the assumption of time-scale separation (i.e., flamelets and related models), which constrain the thermochemical space accessible in the numerical simulation. Whilst the use of transported PDF methods appears still computationally prohibitive, especially for practical combustion systems, there are a number of closures showing promise for the inclusion of detailed kinetic mechanisms with affordable computational cost. They include the Partially Stirred Reactor (PaSR) approach and the Eddy Dissipation Concept (EDC) model. In order to assess these models under non-conventional MILD combustion conditions, several prototype burners were selected. They include the Adelaide and Delft jet-in-hot-coflow (JHC) burners, and the Cabra lifted flames in vitiated coflow. Both Reynolds Averaged Navier Stokes (RANS) and Large Eddy Simulations (LES) were carried out on these burners under various operating conditions and with different fuels. The results indicate the need to explicitly account for both the mixing and chemical time scales in the combustion model formulation. The generalised models developed currently show excellent predictive capabilities when compared with the available, high-fidelity experimental data, especially in their LES formulations. The advanced approaches for the evaluation of the mixing and chemical time scale were compared to several conventional estimation methods, showing their superior performances and wider range of applications. Moreover, the PaSR approach was compared with the steady Flamelet Progress Variable (FPV) model on predicting the lifted Cabra flame, proving that the unsteady behaviours associated to flame extinction and re-ignition should be appropriately considered for such kind of flame. Because of the distributed reaction area, the reacting structures in MILD combustion can be potentially resolved on a Large Eddy Simulation (LES) grid. To investigate that, a comparative study benchmarking the LES predictions for the JHC burner obtained with the PaSR closure and two implicit combustion models was carried out, with the implicit models having filtered source terms coming directly from the Arrhenius expression. The results showed that the implicit models are very similar with the conventional PaSR model on predicting the flame properties, for what concerns the mean and root-mean-square of the temperature and species mass fraction fields. To alleviate the cost associated to the use of large kinetic mechanisms, chemistry reduction and tabulation methods to dynamically reduce their size were tested and benchmarked, allowing to allocate the computational resources only where needed. Finally, advanced post-processing tools based on the theory of Computational Singular Perturbation (CSP) were employed to improve the current understanding of flame-turbulence interactions under MILD conditions, confirming the important role of both autoignition and self propagation in these flames

In situ adaptive tabulation (ISAT) for combustion chemistry in a network of perfectly stirred reactors (PSRs) for the investigation of soot formation and growth

This paper presents an efficient computational implementation of the in situ adaptive tabulation (ISAT) approach (Pope, 1997) for combustion chemistry in a network of perfectly stirred reactors (PSRs) for the investigation of soot formation and growth. This study, for the first time, extends the thermochemical composition vector to contain the soot moments, using the method of moments with interpolative closure (MOMIC) as a soot model with six concentration moments. A series of PSR calculations is carried out using the direct integration (DI) and ISAT approaches. Assessment of the accuracy of ISAT approach is conducted through direct comparisons with DI calculations. Moreover, complimentary cumulative distribution function (CCDF), sensitivity of ISAT calculations with respect to the absolute error tolerance values and speedup are analyzed for two different test cases of ethylene-air using two different chemical kinetic mechanisms. A maximum speedup of 50 x was achieved for an error tolerance of 10(-4). (C) 2018 Elsevier Ltd. All rights reserved

Soot Evolution in Acoustically Forced Laminar Non-premixed Jet Flames

With current concerns for sustainability, the ability to control pollutant emissions is a crucial factor in the design of modern combustion systems. Among pollutants of concern is soot, a type of particulate matter that is emitted from flames and fires due to incomplete hydrocarbon combustion. Interest in soot study spans a diverse range of combustion systems from simple laminar jet flames on a stove, to large-scale boilers and furnaces and complex aircraft turbine operating under high pressure. This thesis reports on work performed to advance understanding of soot evolution in a series of time-varying laminar non-premixed ethylene–nitrogen flames. Time-varying laminar flames offer diverse combinations of residence time, temperature histories, local stoichiometries and strain rates, that are inaccessible under steady conditions. These parameters are not only interdependent but also significantly influential to soot evolution. The thesis aims to provide a deeper understanding of interactions between flame chemistry, time-dependent flow field and soot evolution through a combined experimental and computational study. In the experiments, optical-based techniques are utilised to study the effects of acoustically induced flow modulations on the macroscopic aspects of time-varying laminar flames. The computational study focuses on the chemistry of soot evolution under unsteady combustion. This thesis consists of a compilation of four journal articles, presenting results and findings from a combination of experimental and computational studies. The experiments examine three operating parameters, namely, the forcing frequency, the burner diameter and the fuel flow modulation.Various optical-based techniques were used, including Two-Line Atomic Fluorescence (TLAF), Laser- Induced Incandescence (LII), Time-Resolved LII (TiRe-LII), Planar Laser-Induced Fluorescence of OH (OH-PLIF) and Particle Imaging Velocimetry (PIV). The experimental data set comprises simultaneous two-dimensional measurements of gas temperature, soot volume fraction, primary particle diameter and OH radical with a subsequent measurement of velocity field. All measurements, except OH-PLIF, are quantitative. Nevertheless, the qualitative measurement of OH serves as a reliable indicator for flame location. Furthermore, the experimental data spans a wide range of flame conditions, which is not only valuable for advancing current knowledge of soot evolution but also particularly useful for model development and validation. The results reveal that the coupling between the flame structure and time-varying flow field is most effective at frequencies near to the flame natural flickering frequency, which is approximately 10 Hz for low-velocity flows, independent of the burner diameter. The peak soot concentrations in 10-Hz time-varying flames are 120% larger than that measured in a steady flame burning with the same averaged fuel volumetric flow rate, whereas the enhancement for the 20- and 40-Hz time-varying laminar flames is only 87% and 10%, respectively. The same trend evident in the relative peak soot concentrations between steady and forced flames can be found in the primary soot particle diameter as well. Under the same flow and forcing conditions (a frequency of 10 Hz at a 50% amplitude of fuel flow modulation), time-varying flames with different diameters of fuel nozzles (4.0, 5.6 and 8.0 mm inner diameters) exhibit different structures of the soot field. Time-varying flames with smaller burner diameters, 4.0 and 5.6 mm, exhibit a low-soot region on the centreline, encompassed by highly sooting region. In contrast, such a structure is not observed in the time-varying flame with an 8.0-mm-diameter burner. Consequently, the peak volume-integrated soot volume fraction in the time-varying flame with a 4.0-mm-diameter burner is 50% larger than that with an 8.0-mm-diameter burner. The experimental study also examined the response of flame structure and the soot behaviour at different amplitudes of fuel flow modulation. The results show that the soot production increases as the amplitude of fuel flow modulation increases. This trend is found for amplitudes up to 50% where soot production is enhanced in these ethylene–nitrogen flames. However, further increasing the fuel flow modulation from 50% to 75% does not increase the soot production. On the contrary, the time-varying flame with a 75% fuel flow modulation exhibits a lower peak soot concentration compared with the 50% case. A numerical study simulating the steady and time-varying flames with a 5.6-mmdiameter burner predicts the temperature, mixture fraction, species concentrations and kinetic rate of reaction as a function of the spatial coordinates and time. The computational results show that the prolonged ethylene pyrolysis in the neck of the time-varying laminar flame leads to the increase in the spatial extent of high acetylene concentration, which in turn contributes to the higher peak soot concentrations and larger maximum primary particle diameter measured in the time-varying flame relative to its steady counterpart. Furthermore, the residence time analysis shows that the soot growth rate in the early stages of timevarying flame is comparable or even slower than that in the steady equivalent. Additionally, in the later stages of the time-varying flame, the soot growth rate on the centreline is quicker than those in the annular region of the flame. The different combinations of time–temperature–mixture fraction histories not only help deepen understanding of soot evolution in unsteady combustion but also potentially useful to the development of soot models by serving as validation cases.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 201