Computational Investigation of an Advanced Gas Turbine Combustor Test Rig Laminar Counterflow Diffusion Flames

Abstract

The main objective of this thesis is to explore methods of controlling the properties of hydrogen combustion to align it more closely with the combustion characteristics of the existing infrastructure. This was done through the study of the flame structure of H2/O2 counterflow diffusion flames and compare it with CH4/O2 counterflow diffusion flames for three degrees of strained flames, namely; practically strainless, mildly strained, and strongly strained flames while suppressing the inertial effects to the extent possible. This study further extends to examine the effect of different dilution agents on H2 flames in Air, such as H2 – N2/Air compared against CH4 – N2/Air as well as a comparison between H2 – N2/Air and H2 – H2O2/Air as it was found in the literature that hydrogen peroxide (H2O2) has a significant effect on the ignition time as well as other advantages. To provide a comprehensive comparison on these fuels, ANSYS Fluent, a commercially available computational tool as well as fuels chemistries were validated against published experimental and computational data.

Hydrogen combustion in strained diffusion flames was found to have a wider and more aggressive thermal expansion compared to methane combustion. This results in methane flames having narrower high-temperature regions. Consequently, hydrogen flames achieve higher temperatures and exhibit greater stretching.

Dilution studies aimed to achieve similar temperature distributions for diluted hydrogen flames as observed in methane flames, particularly for their application as an alternative fuel in gas turbines. By adding methane and hydrogen in equivalent mole fractions, comparable temperature distributions and axial velocity profiles were obtained. The proximity of the fuel mole fraction to the rich flammability limit of methane minimized the temperature distribution difference between the two flames. Substitution methods of hydrogen into combustion hardware that employs natural gas diffusion flames is certainly doable but it requires control in terms of mixture dilution and strain rate in the flow field.

A comparison between hydrogen dilution with nitrogen and hydrogen peroxide showed that nitrogen dilution led to slower flame propagation rates and lower flame temperatures in localized flame zones. However, the addition of hydrogen peroxide acted as an oxidizer, promoting a distributed reaction zone. The molar percentage of hydrogen peroxide in the fuel influenced the concentration of hydroxyl radicals (OH) and the maximum flame temperature along the centerline. Hydrogen peroxide decomposition facilitated the formation of hydroxyl radicals, enhancing hydrogen oxidation and ultimately increasing the maximum temperature of the flame.

Date of Award	Aug 2023
Original language	American English
Supervisor	Dimitrios Kyritsis (Supervisor)