What is it about?
The aim is to demonstrate lean primary zone operation in a nonpremixed combustor with stable flame, low emissions, and also low combustion noise. Changes in the exit gas temperature, emissions, and noise level with respect to primary zone equivalence ratio are analyzed. In addition, while approaching towards the LBO condition, the combustor performance parameters are critically monitored, and a remedy is suggested in the form of inclusion of a thermal ring in the primary zone. The effects of a nimonic-alloy-based metal ring kept within the primary zone on the LBO limits and noise level are studied. It is shown that potentially dangerous low-frequency, high-amplitude pressure oscillations that could be detrimental to the structural health of the combustor are suppressed in the presence of the thermal ring, near LBO conditions.
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Why is it important?
Many literature survey indicates that lean combustion at lower combustion temperature may be a good solution to minimize the NOx emissions. However, most of the lean combustion studies correspond to gaseous premixed systems with the aim of avoiding instability and two-phase flow issues. Also most of these studies are carried out in a swirl-stabilized burner. In the present work, the lean primary zone operation concept is attempted in a non-premixed model spray combustor using liquid fuel (jet A). Typical flow features of a gas turbine combustor such as spray injector and primary as well as dilution jet interactions with swirling flow are included in the present combustor geometry. For a fixed fuel flow rate, the overall equivalence ratio is maintained as 0.375 and the operating condition of primary zone is varied from stoichiometric (∼1.04) to leaner condition (0.61), by varying the airflow distribution within the combustor. The primary zone equivalence ratio (Φpri) is modified in steps as 1.04, 0.87, 0.79, 0.72, 0.66, and 0.61 and the optimal equivalence ratio condition is identified.
Perspectives
I N A gas turbine engine, the gas temperature at the inlet of turbine is required to be much less than the adiabatic flame temperature because of thermomechanical limitation of the blade material. Hence, the overall equivalence ratios of gas turbine combustors are maintained at fuel lean conditions (in the range of 0.3–0.5). In the conventional design approach of a gas turbine combustor, the inlet air flow is distributed such that the primary zone is maintained at a near-stoichiometric condition to achieve a higher heat release rate. Balance air is mixed in the dilution zone close to the combustor exit to reduce the gas temperature to the acceptable material limitation of the turbine. However, operating near stoichiometric conditions in the primary combustion zone leads to higher temperature, which gives rise to the production of thermal NOx. In recent times, due to environmental concerns, permissible pollutant emission norms of NOx concentrations have been significantly lowered.
Govindaraj Muthuselvan
Read the Original
This page is a summary of: Effect of Lean Primary-Zone Operation on Emissions and Stability of Non-Premixed Combustors, Journal of Propulsion and Power, October 2020, American Institute of Aeronautics and Astronautics (AIAA),
DOI: 10.2514/1.b37942.
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