M.Sc Thesis

M.Sc StudentRosentsvit Leonid
SubjectCombustion Instability Reduction in 'Dry Low NOx Premixed
Lean Combustion' Gas Turbine Combustors
DepartmentDepartment of Aerospace Engineering
Supervisor PROFESSOR EMERITUS Yeshayahou Levy
Full Thesis textFull thesis text - English Version


The current work presents the results of experimental and numerical (Chemical Reactors Network - CRN model) research of lean premixed flame stabilization proposed method in gas turbines and jet engines. The rising stress over the past twenty years on reducing NOX emission, created by gas turbines, drives gas turbine manufacturers to develop new combustion methods that would reduce NOX emissions. One of the most promising methods to reduce NOX emission in gas turbines, without the addition of any liquid coolant, is the Lean Premixed (LP) combustion. It is well known that the LP method suffers from severe combustion instability problems caused by the very fuel lean combustion regime it employs. It was hypothesized in this research that this combustion instability is directly linked to local extinctions of the flame (fuel lean), and therefore exists a strong correlation of the limiting conditions to combustion instability with the lean blowout (LBO) limit of the flame. The main objectives of this work were to explore a novel method to broaden the combustion stability limits of LP combustors in low NOX gas turbines, to determine whether it is feasible and to suggest future directions. The proposed method of increasing combustion stability was the injection of reactive free radicals into the fuel lean flame, employing a relatively small, annular, premixed pilot burner that produces a restricted, fuel rich mass flow output. The combustion system consisted of an LP main conical combustor (without any intended flame holding device), an annular, small premixed pilot burner and a cylindrical flame retaining port. The experimental results showed the obvious positive correlation of the equivalence ratio with the temperature along the combustor's axis, but more importantly it demonstrated the delicate dependence of the combustion chamber’s LBO limit on ϕpilot (stabilization) and the moderate increase in NOX emission which accompanied the use of a relatively hot pilot burner. The CRN model design process was assisted by a simplified CFD (Fluent 6.3) combustion model. The CRN model was constructed in CHEMKIN-PRO. This model was investigated with a focus on exploring the LBO limit of the main combustor and its combination with the pilot burner, while monitoring emissions. The final model, which was mainly based on PSRs (perfectly stirred reactors), demonstrated the effectiveness of the pilot burner at stabilizing the LP flame, while keeping a moderate NOX emission level.