Advanced Laser-based Multi-scalar Imaging for Flame Structure Visualization towards a Deepened Understanding of Premixed Turbulent Combustion

Abstract

The work presented in the thesis concerns the developments of laser-based diagnostics and its application to the study of turbulent premixed combustion.

The diagnostic developments, mainly concerned with planar laser-induced fluorescence (PLIF), are intended to provide instantaneous visualization of the key species originating from the combustion processes involved in the burning of hydrocarbons and nitrogen-containing fuels under turbulent conditions. For the burning of hydrocarbons, these species include HCO, hot O2 etc. and for the burning of nitrogen-containing fuels, these include NH3, NH and CN. In connection with this, the potential for instantaneous temperature mapping using two-line atomic LIF (TLIF) with a novel seeding system are also demonstrated.

The study of turbulent premixed combustion involves simultaneous imaging of such scalars as HCO, CH, CH2O and OH as well as temperature. Laboratory-scale premixed CH4/air flames stabilized on the Lund University Pilot Jet burner (LUPJ) and the Low-Swirl Burner (LSB) were investigated over a wide operational range and various combustion regimes. The results from both the LUPJ and the LSB flames provide the first experimental evidences for its being possible to appreciably broaden the reaction zone of premixed flames can be significantly broadened through rapid turbulence mixing, the results being verified by observations of broadened/distributed short-lived radicals, HCO and/or CH. The observations obtained for the two clearly different burner configurations suggest that distributed reactions can be a common combustion mode. For the LUPJ flames, the dependence of the reaction zone broadening on the jet speed and the equivalent ratio was investigated systematically. Spatial correlations between the scalars that were measured were investigated, and the detailed local flame structures with and without the presence of distributed reactions were analyzed and compared. It was found that having a temperature above ~ 1000 K is important for sustaining the distributed reactions. The build-up of radical pools through rapid turbulence transport in regions containing (intermediate) reactants was found to be responsible for the distributed reactions occurring. In addition, a study of Mild combustion that has similarities with the distributed reaction concept was performed using optical diagnostics. Certain insights concerning the reaction zone structure of Mild combustion are discussed.

Abstract (Swedish)

Popular Abstract in English

Nowadays, the majority of energy production comes from combustion which is universally linked to daily human activities. Public awakening of the environmental effects of combustion pollution evokes extensive research activities in improving combustion efficiency and reducing pollutant production. Development of combustion processes also includes the use of renewable biomass-derived fuels has emerged to replace the foreseeable limited energy supply from fossil fuels. However, in spite of the long history over which man has tried to control combustion, our knowledge of combustion processes is still limited. One of the major reasons is the multi-scale nature of combustion process. Practical combustion processes typically consist of thousands of chemical reactions and species interacting with turbulence within a continuous spectrum of time and length scales. Therefore, a comprehensive picture of complex combustion process will typically involve two parts, i.e. the understanding of (1) chemical kinetics of fuels and (2) their interactions with various levels of turbulence. Experimental investigations of both parts can be resorted to the employment of advanced laser-based diagnostic techniques which have proved valuable in providing in-situ, non-intrusive measurements with high spatial and temporal resolution. In the present thesis, such laser-based techniques have been developed for single-shot based visualization of key species in flames of hydrocarbon and nitrogen-containing fuels. The experimental results in laminar flames give insights into chemical kinetics of fuels to better understand the detailed processes how fuels are converted into the final products, release heat, and how pollutants like nitrogen oxides (NOx) are produced. Furthermore, it needs to be answered how combustion chemistry couples with turbulence. In our daily life, one might see a thin chemiluminescent bluish layer in non-sooty laminar flames. This thin layer is called the reaction zone where the primary chemical reactions with major heat release are believed to take place. In the context of low-to-medium turbulent flows, the reaction zone layer has been shown to be wrinkled but remains thin. Contrarily, the existence of a combustion mode with a broadened reaction zone layer has been theoretically predicted under highly turbulent flows as the small length scales of turbulence might start to penetrate into the layer. This could be experimentally verified by the observation of gradually broadened layers of reaction-zone scalars. The present work provides the first experimental evidences that the reaction zone layer can be broadened and even distributed through rapid turbulent mixing. A broadened/distributed reaction zone layer implies a homogeneous heat release during combustion and therefore uniform temperature increase with less maximum temperature. This particular combustion mode has the potential benefits to achieve lower global fuel consumption rate with less NOx emissions. Investigations of the criteria for achieving distributed reactions are also presented in this thesis. The results will provide significant input into the development of numerical combustion models for the high-turbulence regime.

• author: Zhou, Bo
• supervisor: Marcus Aldén; Zhongshan Li
• opponent: Professor Driscoll, James, Department of Aerospace Engineering, University of Michigan, USA
• organization: Combustion Physics
• publishing date: 2015
• type: Thesis
• publication status: published
• subject: Atom and Molecular Physics and Optics
• keywords: Laser Diagnostics, PLIF, High Spatial Resolution, Multi-scalar Instantaneous Visualization, HCO/CH Radicals, Distributed Reaction Zone, Mild Combustion., Combustion Regimes, Turbulence/Flame Interaction, Turbulent Premixed Combustion, Fysicumarkivet A:2015:Zhou
• defense location: Lecture hall Rydbergsalen, Department of Physics, Professorsgatan 1, Lund University Faculty of Engineering
• defense date: 2015-02-13 10:15:00
• ISBN: 978-91-7623-228-6
• language: English
• LU publication?: yes
• id: 551513a4-7002-422f-9402-6c7b0cfe0a56 (old id 4933891)
• date added to LUP: 2016-04-04 09:45:24
• date last changed: 2018-11-21 20:55:25