| Multi-nozzle combustion is normally used in actual heavy duty gas turbine combustors in which burners are typically arrayed in an annular,can or can-annular configuration.The multi-nozzle burner can increase the power output and turndown of the gas turbine,as well as ensure that if one flame is extinguished,the other nozzles will reignite it.In addition,multi-nozzle combustion has the advantage of combustion noise inhibition.This dissertation takes a multi-nozzle burner as the main research object.Experiments and numerical simulations are combined to investigate the characteristics of premixed multi-nozzle combustion from the aspects of flow,flame structure,performance and combustion oscillation.Firstly,experimental and numerical investigations were performed on premixed multi-nozzle combustion from the points of view of flow and flame structure.Planar laser-induced fluorescence was employed to detect the structure of premixed multi-nozzle flame.Large Eddy Simulations(LES)of premixed multi-nozzle combustion were conducted with flamelet combustion model.Equivalence ratio was varied from 0.51 to 0.8 with burner outlet velocity being 6.2 m/s.Results show that neighboring flows interact with each other,generating a highly turbulent interacting zone where intensive reactions take place.As equivalence ratio is decreased,flame liftoff height increases gradually and the interaction between neighboring flames gets weaken.The interaction disappears when equivalence ratio is less than 0.53.The center flame is extinguished when equivalence ratio is equal to 0.51.The position of flow merging point is not affected by equivalence ratio.The location of flow combining point moves downstream with equivalence ratio being increased.The combined velocity increases significantly as equivalence ratio is increased.The center main recirculation zone is smaller than outer ones.As equivalence ratio is increased,the outer main recirculation zones shrink while the size of the center main recirculation zone increases first and then decreases.The rotation direction of Precessing Vortex Core(PVC)is opposite to the flow swirling direction.As equivalence ratio is increased,PVC is suppressed.The center PVC disappears when equivalence ratio is larger than 0.7.Numerical results based on the flamelet model are in good agreement with the experimental data,which validates the accuracy of LES on predicting premixed multi-nozzle combustion.Secondly,systematic study on the performance of co-swirl and counter-swirl multi-nozzle burner was carried out.The effects of equivalence ratio and burner outlet velocity on the performance of multi-nozzle burner were discussed.The burner outlet velocity was varied from 4 m/s to 15.5 m/s with equivalence ratio being changed from 0.5 to 0.85.Four operation regions: stable combustion region,unstable combustion region,flashback region and extinguishing region are observed for both burners.Experimental results show the multi-nozzle flame tends to become unstable at low velocity with leaner condition.Increase of burner outlet velocity or equivalence ratio contributes to inhibition of combustion instability.As equivalence ratio is increased,the pressure drop tends to decrease.The pressure has a minimum value when outlet velocity is 9.3 m/s.NOx emission shows log-linear dependency on the adiabatic flame temperature,while NOx emission for multi-nozzle flame is less sensitive to the flame temperature than that for single nozzle.Combustion instability results in increase of NOx emission.Finally,experiment and LES of multi-nozzle combustion instability were carried out.Firstly,the acoustic mode of the multi-nozzle combustion system was analyzed using finite element method.Secondly,phase-locked OH planar laser-induced fluorescence was employed to measure flame shape.The fluctuations of co-swirl and counter-swirl flame were analyzed.The triggering mechanisms of combustion instability were revealed using flame Rayleigh index.At last,LES of combustion instabilities of partially premixed single flame and premixed multi-nozzle flame were conducted.The fluctuation of flame and vortex/flame interaction were discussed.Results show the counter-swirl flame triggers the 2L mode of the combustion system at 41 Hz while co-swirl flame incites three longitudinal modes(42 Hz,98 Hz and 196 Hz)with the highest amplitude near 3L.Combustion instability of multi-nozzle flame is driven by a variety of different triggering mechanisms.For counter-swirl flame,thermo-acoustic couplings in flame interaction region,flame base and tail regions,as well as Kelvin–Helmholtz instability mechanism induced by vortex shedding in shear region are the main reasons for inducing self-excited combustion instability.For co-swirl flame,the couplings of pressure fluctuation and heat release oscillation at interaction region,lip region and the tail of center flame are the main reasons for combustion instabilities.In addition,combustion characteristics of co-swirl and counter-swirl multi-nozzle burner are compared.Results show there are significant differences in the flowfiled of the two burners.For co-swirl multi-nozzle burner,the flow is distorted in clockwise direction and eventually develops into a unified swirling motion similar to a single swirling flow.For counter-swirl array,the flow keeps the individual swirling structure.The pressure fluctuation amplitude for co-swirl burner is always larger than the counter-swirl array at the same operating condition.The stable operation window for counter-swirl array is wider than that for co-swirl arrangement.The pressure drop,exhaust temperature and NOx emission for counter-swirl multi-nozzle burner are all less than the co-swirl arrangement.The counter-swirl multi-nozzle burner is preferred to the co-swirl array under the consideration of pressure fluctuation,exhaust temperature,emission,stable operating window,etc. |
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