M.Sc Thesis


M.Sc StudentJoseph Kalyan Raj Isac
SubjectOn the conversion of an aviation liquid-fuelled gas turbine
combustor to natural gas operation
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Emeritus Levy Yeshayahou


Abstract

There exists a need for converting existing liquid-fuelled jet engines to gas-fueled land based gas turbines with minimal geometric modifications to the combustor (with liner) geometry. The preliminary literature review study led to the conclusion that LSIs can serve as a combustion method for converting existing liquid-fuelled jet engine combustors, as this has not been attempted before. The process of fuel conversion mainly involves changes to the fuel injector and modifications to the primary-zone air flow pattern. The LSI technique, as developed by LBNL (Berkeley Labs), involves using the combination of a swirling peripheral flow and a turbulent core flow of air and fuel to stabilize a flame, while inhibiting the formation of a central recirculation zone. A stable LSI flame is lifted, i.e., detached from the burner exit, and stabilized at a bulk flow velocity much higher than the laminar flame speed.

The objective of the current work was to study the flame stabilization method of LSIs while focusing on the interaction between the swirling and non-swirling streams. The investigation method applied was a combined experimental and numerical study of the distance, called recess distance (Rc), over which this interaction took place. The research employed analytical tools to define a new swirl number, which incorporates Rc, and tested its validity using computational fluid dynamic (CFD) simulations. A multi-functional, variable-swirl combustion test rig was developed as a part of this study, and basic combustion characteristics were studied for flame flowfields quantified using the LSI swirl number (SLSI). The burner was also equipped with capability to vary turbulence levels in the central non-swirling flow using grids of different hole sizes.

Observations from these studies have led to the formulation of a new swirl number, based on the fundamental equations. It is shown that the LSI swirl number (SLSI) is a more appropriate similarity condition capturing additional flow physics in its definition compared to conventional swirl numbers. CFD simulations captured the effect of the recess distance on the flowfield, upstream of the flame. Combustion experiments corroborated the results from the simulations, witnessed in the impact on emissions for recess distance variations. The stable operation range of the burner, in the LSI regime, for different bulk flow velocities (i.e., Reynolds numbers) was mapped. Experiments showed that at a higher equivalence ratio, a lower degree of swirl was required for stable flames. Varying the swirl from high swirl to low swirl number resulted in a corresponding variation, from very high to ultra-low, of emissions levels. The numerical and experimental results obtained provide strong evidence that the effects of the Recess Distance (Xc) on the LSI combustion flowfield, quantified using the SLSI, is pronounced. Further, the low emissions burning (<5ppmvd NOx and CO at φ=0.77) of LSI flames, as observed in the combustion tests, supports the implementation of the LSI method as a low NOx combustion method suitable for conversion of the GT combustor. The development of the SLSI provides a more physically relevant similarity parameter for improving the initial design of any LSI system.