Chemical reactions and NOx emissions in bluff-body stabilized flames

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Chemical reactions and NOx emissions in bluff-body stabilized flames

 

Author: Shen, Hongmei
Title: Chemical reactions and NOx emissions in bluff-body stabilized flames
Degree: Ph.D.
Year: 2001
Subject: Flame
Nitrogen oxides
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Mechanical Engineering
Pages: 1 v. (various pagings) : ill. (some col.) ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1563078
URI: http://theses.lib.polyu.edu.hk/handle/200/4336
Abstract: The objectives of this thesis are the investigation of the characteristics of partially premixed disk-stabilized flames. Among them, chemical reaction and NO/NOx emission in partially premixed disk-stabilized flame are the main topics. The present work includes experimental studies and numerical modelling. An intensive experimental investigation had been carried out to study the characteristics of chemical reaction, NO/NOx emissions and visual characteristics of a partially premixed bluff-body stabilized flame. A circular disk was used as the bluff-body to stabilize a LPG flame. Effects of diameter of the disk and equivalence ratio of the air/fuel mixture on the chemical reactions and NOx emissions were fully determined. Temperature distributions, and CO, CO2, O2 and NO/NOx concentration distributions in the partially premixed disk-stabilized flame, for four different diameters of the disk and four equivalence ratios, were measured. Continuous flame images were recorded under various testing conditions, and analyzed using an image-processing technique. Four image feature vectors of the flame (i.e. flame area, flame perimeter, flame complexity and Y-center) have been used to describe its visual characteristics. Based on these characteristics, quantitative results were obtained by using Fractal theory to produce the Fractal behaviours of the flame. In order to understand better the mechanism of chemical reaction and the NOx formation of the flame, two factors, the Spatial Reaction Index (SRI) and the Modified Mixture Fraction (MMF), have been established respectively. The SRI was provided to give additional information about the structure of a flame, whereas the MMF was related to the NO/NOx concentration and temperature distribution in the flame. A numerical model for the disk-stabilized flame, which provided the information about its detailed chemical reactions and NOx formation by burning butane in air, has been developed. Predictions obtained by using the model were compared with the experimental results. By carrying out the present experimental and numerical studies, I have made the following contributions towards a better understanding of the partially premixed bluff-body stabilized flame. 1. A non-traditional approach, the image-processing technique, has been applied to analyze the visual characteristics of the partially premixed disk-stabilized flame. The very complex flame characteristics are described by the four image feature vectors (i.e. flame area, flame perimeter, flame complexity, Y-center), values for which was deduced in the present investigation. Based on Fractal theory, a geometrical factor has been developed to produce quantitative results to describe the fractal behaviours of the flame, and which are rarely reported in the literature for disk-stabilized flames. 2. The present experimental results provide an excellent database for predicting the NO/NOx formation mechanisms in the disk-stabilized flame. The variations of temperature and NOx concentration distributions with time have been measured. These provide a good foundation for developing turbulence models of the partially premixed disk-stabilized flame. 3. A Spatial Reaction Index (SRI), which is the ratio of O2 chemical reaction rate to O2 mass transfer rate, based on the present experimental results has been proposed. The SRI distribution in the partially premixed disk-stabilized flame provides a good description of the local chemical reaction and combustion rates in the flame. 4. A Modified Mixture Fraction (MMF), which is the mass ratio of the carbon containing species to all gaseous species, has also been proposed based on the present experimental findings. Its relationships between temperature distribution and NO/NOx formation of the partially premixed disk-stabilized flame have been determined, these give a good description of the NO/NOx formation mechanism in the flame. 5. A numerical model of the disk-stabilized flame, which is based on the calculation codes of a Perfect Stirred Reactor, has been developed. It provides valuable information about the detailed chemical reactions of the flame produced by burning butane in air. The predictions obtained by using this model have been validated against the experimental findings, and good agreement was achieved.

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