Research On The Off-desgin Performance Of An Adjustable Fuel Feeding Combustor Of Micro-gas Turbine

Off-design operation is a common practice for a micro-gas turbine(MGT)installed in a combined cooling,heating and power system,which is used to satisfy the system requirements in terms of the different atmospheric environment,and cooling,heating and power loads and their ratio.However,the MGT combustor would have some performance problems at off-desgin operations,such as increasing pollutant emissions,decreasing combustion efficiency(?)and lean blow-out.At present,the traditional MGT has few adjustable methods to respond to the off-design operation problems.The applications based on the compressor inlet guide vanes and the combustor multi-fuel nozzles have been utilized widely in heavy-duty gas turbines to change the combustor inlet air flow and switch the combustion mode,respectively.But the MGT is primarily limited by its budget cost,structure size and weight,so that it has not yet reached a level of technological maturity to possess the relatively complex devices.Aiming at the off-design performance problems of the MGT combustors mentioned above,this paper considers the active variations of spatial fuel premixing in the combustor and proposes an MGT combustor with an adjustable fuel feeding(AFF)method.Different from the traditional MGT combustors,the AFF combustor possesses a main fuel distribution device.Through the main fuel distribution device at the cost of adding two control parameters,the AFF combustor can switch various groups of working main fuel tubes(MFTs)to realize the circumferential and radial stagings of the fuel premixing,ultimately achieving the AFF.Each group has a different symmetrical distribution feature of working MFTs.Based on these features,a characteristic value(U)of the AFF method is propsesd to be a representative of the AFF method at each group.In this paper,four different working MFT groups are presented,and the values of U are calculated to be 0.143,0.333,0.429 and 1 for the AFF method at the groups of 4,6,8,and 12 MFTs,respectively.Hereby,this paper has proposed a control logic relationship of the AFF method in practical engineering applications.To research the effect of the operation conditions and the fuel premixing characteristics on the off-design performance of the AFF combustor,the low-pressure modeling experiments of the AFF combustor prototype in terms of the ignition combustion performance are first conducted at the ultralow equivalence ratio(?).The experimental results indicate that at the relatively high pilot fuel ratio,the AFF combustor can ignite successfully at the relatively wide interval of ?,and have a lowest temperature-rise of combustor mean outlet temperature(T4)for the quick start of gas turbines at the relatively low ?,but pollutant emissions would increase and ? would decrease slightly at the same time.Thus,when the lowest temperature-rise of T4 is satisfied,the MGT should improve ? to increase ? and control pollutant emissions at the MGT start process.Then,the experiments of the off-design dynamic performance are employed at the starter-driving process by a MGT prototype including the AFF combustor prototype.The experimental results show that,to prevent the over-temperature of the MGT,? and its trend should be first focused.Among the MGT operations and the non-dimensional structure sizes of the AFF combustor mentioned in the current paper,the temperature-rise of T4 that is suitable for the MGT operations to decide indirectly the successful combustor ignition is revised to 40 K,and the flow resistance coefficient of 35 is satisfied for the design requirement of the AFF combustor.Finally,this paper employes the numerical simulations of the off-design static performance of the AFF combustor at the single variaiton of the combustor inlet air temperature(T3),inlet air total pressure(P3)and ? and at the double variaitons of T3 and ?.Moreover,the applicability of the numerical calculation method has been verified by the thermal state experimental data.The simulation results show that,the premixing of the fuel and air in the primary air passage and the vane cavity of the primary air swirler is affected by the fuel mass flow and jet velocity of the working MFTs,which causes that NO emission has a different trend as U varies at the single variation of ? and the double variaitons of T3 and ?,respectively.Only at the single variation of T3 to 600 K,CO emission is greatly affected by the AFF method.At the others operations,the AFF method has almost no influences on CO and CH4 emissions that have a very low emission level,and ? is very close to 100%.The flow field distribution,outlet temperature distribution and total pressure loss of the AFF combustor are less affected by the AFF method,but the outlet temperature distribution factor(OTDF)increases as U rises.Based on the variation trends of NO emission and OTDF,the selection method of the optimal value of U is proposed,and the optimal values of U are decided at the corresponding operations.