| Aero-derivative gas turbines are widely used as ship propulsion,ground-based power generation,driving pipelines and for many other facilities.China has purchased a large number of certain military aircraft engines from abroad.These engines are to be retired after several rounds of overhaul.It is a good idea to make these retired aero engines into gas turbines,either for power plant,or for ship propulsion.Aero-derivative gas turbines must comply with increasingly stringent pollutant emission regulations as commercial products.As NOx emission is particularly limited by governments,reducing NOx emission is a major problem to modify aero engines into gas turbines.However,due to the small axial size of the combustion chamber of the prototype,it is not conducive for the combustor to consume fuel completely with significant low NOx emission.In this work,advanced combustion technologies are adopted to achieve low NOx emission without changing the casing.First,the conventional diffusive combustion with dilution air and humidified fuel is adopted as a measure to lower NOx emission.With humidified fuel combustion,the concentration of NOx and CO emissions will be simultaneously reduced.On the basis of redesigning the fuel nozzle,the three-dimensional model is meshed and numerically simulated.The qualitative analysis of the simulation results shows that under a certain fuel humidification ratio,this combustion technology can greatly reduce the NOx emission concentration.At the same time,the CO emission concentration is also at a low level;though the concentration of NOx is relatively high and hard to reach related national standard.To this end,after investigation and comparison,the liner structure of the combustion chamber was modified,and the method of high-speed jet and geometric structure guidance was used to acquire MILD combustion in the limited casing space of the prototype.However,the numerical simulation results show that in the limited space of the prototype,only redesigning the flame tube without changing the casing cannot lead to MILD combustion scheme of uniform temperature field and low peak temperature.This shows obvious difficulties to achieve MILD combustion under the influence of adverse factors,such as high pressure,high mass flow rate,lifted injection speed,and small volume of the liner.Finally,the micro-mixing combustion technology is applied to the prototype for its potential of further reducing pollutant emissions.A micro tubes configuration is proposed within the fixed casing of the prototype.It is determined that the inner diameter of the modified flame tube is 5 millimeters.Totally there are 2 352 tubes,and 4 fuel holes are scattered uniformly around the tube wall upstream.The fuel and air are injected into fuel tubes and meet vertically.The numerical simulation results show that the global maximum temperature is 2069k when the equivalence ratio of head tubes is 0.64,which is significantly lower than that of combustion with humidified fuel.Due to the low global combustion temperature,the NOx at the outlet is only 7.5ppmvd@15%O2,while CO is equal to 9.4 ppmvd@15%O2.Although a uniform temperature field is obtained in micro mixing combustion mode,it is necessary to further optimize the mixing holes to improve the outlet temperature distribution.For the short annular combustion chamber studied in this work,lower NOx emission level is obtained through fuel humidification combustion without changing the casing structure.In the casing space,MILD combustion under high pressure and thermal intensity can not be realized.With micro mixing combustion,stable combustion,uniform temperature field and low NOx emission can be achieved,which make it a promising combustion technology to adapt aero-engine short annular combustors into low NOx combustors.The results also show that the outlet temperature distribution needs to be further improved through optimization of mixing holes in liner walls. |
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