Gas turbine combustion using Low Swirl Injection - Defining a new swirl number

摘要

Where there is a need for converting existing liquid-fueled jet engines to gas-fueled land based gas turbines with minimal geometric modifications to the combustor geometry (outer case and its liner) Low Swirl Injection (LSI) is a viable option. LSI combustion is a novel method of flame stabilization, achieved solely through a control of the swirl intensity and turbulence of the reactant mixture entering the combustion chamber, and with the capability of ultra-low emissions burning. Yet, such a conversion has not been carried out before. The process of fuel conversion mainly involves changes to the fuel injector and modifications to the primary-zone air flow pattern. This is a challenging undertaking given the empirical design methodology that goes into the development of gas turbine combustors; further complicated by the need to stick to stringent pollutant emissions requirements that are imposed on land-based gas turbine power generation units. The LSI technique, as developed by Lawrence Berkeley National Laboratory, involves using a 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 a lifted flame (detached from the burner exit nozzle) and is stabilized at a bulk 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, thereby exploring the importance of the combination of low swirl and turbulence of a combustible mixture. The investigation method applied was a combined experimental and numerical study of the impact of the length over which this interaction takes place, referred to as the recess distance, the effect of which has not been quantified in detail. The present study used analytical tools to define a new swirl number, incorporating the recess distance, and tested its validity for LSI combustion. A multi-functional, variable-swirl combustion test rig was developed as a part of this study, and basic combustion characteristics were studied for LSI flames. Observations from these studies have led to the formulation of a new swirl number, based on the fundamental governing equations. It is shown that the LSI swirl number (S_(LSI)) presents a more appropriate similarity condition and captures additional physics of the flow in its definition as compared to conventional swirl numbers that have been used for the LSI technique. The CFD simulations captured the effect of the recess distance on the flowfield downstream of the burner exit. Combustion experiments corroborated the results from the simulations, as could be seen through the impact of the recess distance on the emissions. The stable operating range of the burner, in the LSI regime, for different bulk flow velocities (i.e., Reynolds numbers) was mapped. In addition, it could be seen that at a higher equivalence ratio, a lower degree of swirl was required to stabilize the flame. Varying the swirl from high swirl to low swirl resulted in a corresponding variation of emissions levels from a high value to a low value. Low emissions were observed at low swirl conditions, whereas for the same flowrates incomplete burning occurred at the high swirl conditions. The results obtained provide strong evidence that the recess distance does indeed have an effect on the flowfield. This effect has been quantified in the present study, through the effect of the S_(LSI) parameter on the flowfield and on combustion through CFD and experimental studies respectively. Furthermore, low emissions burning (<5ppmvd NO_x and CO at Φ=0.77) of LSI flames were observed in the combustion test, which validates previous studies in this field. The modified non-dimensional swirl number (S_(LSI)) may improve the initial design of any LSI system, by being based on a more physically relevant similarity para