Investigation On Premixed Combustion Characteristics With Radial-Swirler In Micro Gas Turbine

As key equipment of distributed energy supply system, the micro and small gas turbine have been widely used gradually. In order to reduce the NOx emission of gas turbine, more efforts have been made on developing Low NOx combustion technology in the recent decades, including Lean-Premixed combustion, Rich-burn Quick-quench Lean-premixed combustion (RQL), Catalytic Combustion and Flameless combustion etc. The Lean-Premixed combustion has already been used in industrial heavy gas turbine, but it is prone to thermoacoustic coupled oscillation. Low bands of oscillations can disturb the combustion process and increase the emission of pollution, and in some case may structurally damage the combustor.In this paper, the pollution formation and thermoacoustic coupled oscillation characteristics have been researched with numerical and experimental methods.The radial swirler used in this paper takes tangential rectangular passages as swirl generator, and mount a main fuel spray rod with many tiny holes just upstream of each inlet passage. The fuel and air flow through the swirler inlet passages and mix in the mixing-passage, and then are burned in the mixing-passage and combustor. A pilot nozzle is mounted in the center of radial swirler to stabilize the premixed flame. Three swirl number (S1=0.81, S2=0.93and S3=1.03), two main fuel inject direction (centrifugal and centripetal), two spray holes position (A type: spray holes are away from the swirler wall. B type:the spray holes are near the swirler wall.), two spray rod circumferential position (0°:just upstream of the inlet passage center.10°:rotated10°at the direction of pressure face) and pilot percentage (the percentage of pilot fuel in the total fuel supply) have been researched to reveal all the effects on the flow and combustion characteristics in gas turbine combustor.Firstly, the effect of swirl number, main fuel's inject direction, position of spray hole and spray rod, percentage of pilot fuel on the flow and combustion characteristics of combustor were researched with3-dimentional computation. Our research found that good mixing of fuel could be reached by injecting fuel centripetal, and when fuel is centrifugally injected the fuel will be rich in the center zone of mixing passage. Spray hole positioned with B type could reach good mixing and with A type will form a lean fuel zone at the mixing passage center, which could lead to blow off of the flame. The position of spray rod has strong effect on the distribution of fuel in the mixing passage. When the spray rod is just positioned at0°, a good mixing of fuel will be reached. And if the spray rod is biased to10°, the fuel distribution in the passage will be rich at the center and near the passage wall. When increase the percentage of pilot fuel, the flame will be more stable, but the high temperature zone near the pilot will be extended and thus the NOx formation increased.Secondly, the effects of structural parameters (such as main fuels inject direction, position of spray hole and spray rod) and operating conditions (such as inlet temperature, inlet velocity, equivalence ratio, percentage of pilot fuel and pressure) on pollution emission and thermoacoustic coupled oscillations have been tested. Results show that:1) The nonuniform of fuel\air mixture in the mixing passage formed by the structure will increase the NOx and CO emission, and enhance the thermoacoustic coupled oscillation level. When main fuel is centripetally injected, with main fuel spray rod mounted at0°and B type spray hole position, the NOx emission is the lowest without obvious thermoacoustic coupled oscillation;2) With the increase of inlet temperature and equivalence ratio, the CO will decrease sharply and the NOx increase linearly, the thermoacoustic coupled oscillations'sound pressure level first will increase and then decrease, and hysteresis and bifurcation phenomena will be observed;3) Adiabatic flame temperature Tad is used to combine inlet temperature and equivalence ratio. In the well premixed C₃combustor, when inlet velocity is34⁴6m/s and Tad is1600¹700K, the NOx and CO emission can be less than10ppm@15%O2, and has a good flame stability, NOx is proportional to exp(0.01Tad);4) With the increase of inlet velocity, CO will increase and NOx decrease linearly at the same time, the thermoacoustic coupled oscillations'sound pressure level first will increase and then decrease;5) With the increase of pilot percentage, CO emission will increase quickly and NOx increase gradually, the flame stability will be enhanced, but the thermoacoustic coupled oscillation will appear at some critical pilot percentage, with the increase of pilot percentage, the sound pressure level will increase first and then decrease abruptly;6) The pollution emission of0.11and0.22MPa operating conditions were tested, with the increase of pressure, the effect of pressure on NOx emission is enhanced, when the pilot percentage is0%the NOx emission has no change at the two operating pressure, but when the pilot percentage is15%, the NOx emission is17ppm@15%O2at0.11MPa and25ppm@15%O2at0.22MPa.At last, the best structure of combustor has been tested at modeled operation conditions at the863program's low emission combustor project's request, and the characteristics of ignition, blow-off, total pressure loss, outlet temperature field, wall temperature and pollution emission were tested. Two types of ignition methods were tested, which are pilot ignition and multi-ignition, the multi-ignition has stable ignition characteristics. And the blow-off characteristics is also been tested, with the increase of air mass flow rate the blow-off equivalence ratio will decrease. When air mass flow rate is4.56%of the full-load, the blow-off equivalence ratio is0.038, and when the mass flow rate increased to10%, the blow-off equivalence ratio will decrease to0.019. The total pressure loss is0.063and greater than the aim of0.05. the outlet temperature field is very uniform, the OTDF=0.0327and RTDF=0.0214, and fulfills the aim of OTDF≤0.2and RTDF≤0.08. The combustor wall temperature is no higher than650℃,which is within the allowable limit of the combustor's material. The NOx emission can reach9.5ppm@15%O2at pure premixed combustion, and is much lower than the aim of25ppm@15%O2and CO emission nearly zero.In this paper, structural parameters (the main fuel supply methods and swirl number) and operating conditions (such as inlet temperature, inlet velocity, equivalence ratio, percentage of pilot fuel and pressure) on the pollution emission and thermoacoustic coupled oscillation characteristics of radial swirl lean premix combustor have been researched systematically, and offered technical support for developing of micro and small gas turbines low emission combustor, and have made some complement for the research of thermoacoustic coupled oscillation mechanism in the lean premixed combustion. The radial swirl lean premix combustor can reach ultra low pollution emission and has a stable combustion characteristic, can fulfill the863program's low emission combustor project's request, and thus has a wide application prospect.