The transition in the energy market, from large fossil fuel consumption to the broad diffusion of renewable energies, involves an intermediate phase where more efficient techniques are developed for the existing power generation technologies. Among new combustion techniques, MILD (Moderate or Intense Low-oxygen Dilution) combustion is particularly attractive because of its potential characteristic to enhance thermal efficiency and reduce emissions like nitrogen oxides (NOx). The successful application of MILD combustion requires a significant entrainment of hot combustion products into the fuel and/or oxidizer stream(s), yielding an increase of the reacting mixture temperature over its auto-ignition value. Such intense dilution causes a reduction in peak temperature levels, with a consequent reduction of NOx emissions, and a homogeneous temperature field followed by enhanced flame stability. Also the overall thermal efficiency is improved because of the recuperated heat. The relative ease of obtaining reactant dilution in a full scale burner makes the MILD combustion regime interesting also from a technological point of view. Despite some interesting applications of MILD technique in industrial cases, its broad adoption is prevented by gaps in the knowledge of this combustion regime. Particularly, the development of simple and reliable numerical models is required to allow testing of full scale industrial burners with reasonable computational expense. The present dissertation is focused on how the oxidiser temperature, oxidiser concentration and fuel concentration affect the complex interaction among molecular transport, chemical kinetics and turbulence that leads to self-ignition in MILD combustion. The diffusion-chemistry contribution to ignition is investigated by means of a one dimensional (1D) zero velocity Direct Numerical Simulations (DNS) of two mixing layers representing a cold fuel mixture and a hot diluted oxidiser. Different oxidiser mixtures as well as different fuel blends are considered. Each case studied showed a different ignition behaviour. An in deep investigation of physical and chemical changes observed for each case along the ignition period is provided. A temporal and a spatial scaling methods are proposed to account for ignition behaviour differences and compare cases. The comparison revealed different aspects of the self-ignition process. The differential diffusion effect plays an important role in the early stages of ignition, for cases presenting methane/hydrogen fuel mixture. If high is the level of hydrogen in the fuel blend, major stages of methane (CH<sub>4</sub>) consumption pathway,from the CH<sub>4</sub> dehydrogenation to the carbon dioxide (CO<sub>2</sub>) release, are significantly affected by hydrogen (H<sub>2</sub>) chemistry. In the latest stages of ignition, the methane pathway is also affected by the drop in oxygen level. The influence of turbulence on the diffusion-chemistry interaction is studied by means of three-dimensional (3D) Direct Numerical Simulations modelling a methane/hydrogen circular jet mixing with a diluted oxidiser co-flow. The effects of different fuel and oxidiser blends is also considered in the 3D study. In cases where large is the H2 presence in the fuel jet, the presence of turbulent mixing has a minimal effects on early stages of self-ignition, where instead differential diffusion still plays a major role. As turbulence develops, more marked difference between 1D and 3D studies are observed. The role of turbulent mixing dominates over chemistry where the fuel blend includes a low amount hydrogen. For this configuration the temperature increment is strongly limited compared to corresponding 1D study. The outcome of this study is expected to be of use to other researches in MILD combustion, particularly those adopting existing RANS and LES models to MILD combustion cases.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:740066 |
| Date | January 2016 |
| Creators | Panebianco, Vincenzo |
| Contributors | Buick, James Maxwell ; Aspden, Andrew |
| Publisher | University of Portsmouth |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://researchportal.port.ac.uk/portal/en/theses/a-numerical-study-of-turbulenceflame-interaction-in-mild-combustion(0d9aaac8-b620-498a-b6e4-26ff5f45d4d0).html |
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