Research On Predicting Lean Blow-off In Aero-engine Combustor Based On Damk?hler Number

Aero-engine combustors need to maintain stable under wide operating conditions.The stable operation of aero-engine combustor is limited by combustion efficiency,combustor outlet temperature,lean blow-off?LBO?and total pressure loss,etc.LBO performance is one of the most important combustor performances.Both military aircrafts that often need to make large maneuvering flights and civil aircrafts that have strict requirements on pollution emissions,it is necessary to repeatedly perform LBO predictions and combustor structure optimizations.Hence,an accurate LBO prediction method with low computational cost is of great importance for shorten the design cycle of the combustor with high LBO performance.Numerical and experimental studies are conducted for the LBO of aero-engine combustors.The impacts of the inlet temperature?T₃?,inlet pressure?P₃?and inlet mass-flow rate?m_a?on the LBO performance of a combustor are quantitively analyzed.The LBO experiments of a three-dome fan-shaped model combustor show that the increase of T₃ or P₃ will lead to the improvement of the LBO performance,and the increase of m_a will lead to the deterioration of the LBO performance.Combining the experiment and simulation results,it could be observed that the inlet condtions mainly affect the LBO performance by varying the fuel evaporation rate,the energy balance between fresh mixture and hot products,and the requirements of flame propagation speed for fame stabilization.Meanwhile,it could be observed from the simulated flow fields that the reaction zone mainly concentrates upstream of the primary holes,and the flow and chemical characteristics of the reaction zone have crucial effects on the LBO performance.In the current study,different definitions of the critical reaction zone are compared,including defining the zone by velocity,temperature,fuel concentration,and the distribution of different combustion products.Three LBO prediction methods and a flame strength analysis method are proposed based on Damk?hler model and the characterizing of the reaction zone.1)The single reactor prediction method based on Damk?hler model simplifies the reaction zone as a single Perfectly Stirred Reactor.The flow time scale is obtained based on the residence time of the inert particles in the reaction zone.The chemical time scale is calculated by Perfectly Stirred Reactor model.The single reactor prediction method is applied to swirl-stabilized combustor and bluffbody-stabilized combustor.The Da number under different conditions near LBO fluctuates near 1,and increases rapidly when the fuel-air ratio is increased,which indicates the combustion becomes stable for the increase of fuel-air ratio.2)Considering that the atomization quality may vary a lot among different operating conditions.Atomization experiment is conducted to obtain the performance of the fuel nozzle under different conditions.The atomization quality under different conditions is described by Rosin-Rammler model and integrated into the numerical simulation.The definitions of flow and chemical time scale are the same with those in single reactor prediction method,while the inlet surface of the reaction zone is separated to make the calculation of chemical time scale more accurate.This prediction method is applied to swirl-stablized combustor,the Da numbers near LBO conditions fluctuate near 1,while under design condition the Da number reaches 20.3,which proves that the method could characterize the combustion state accurately.3)Although drastic mixing occurs with the reaction zone,it is still difficult to avoid the nonuniform distribution of the temperature and species,hence the author proposes a multiple-reactor LBO prediction method on the basis of reaction zone division.The reaction zone is splitted into sub-regions of same size,and PSR model is applied to each sub-region for the calculation of local chemical time scale.The global flow time scale is still obtained by tracking the inert particle in numerical simulation,while the global chemical time scale is obtained by taking the average of the local chemical time scale.The Da number is obtained based on global flow and chemical time scale.The errors brought by the nonuniform distribution of temperature and species can thus be minimized.Near LBO conditions the Da number is about 1,and under design condition it is about 11,which prove that the method could recognize different combustion state accurately.Local Da numbers can also be generated specify to each sub-region,and the distribution of local Da numbers could describe the flame strength.4)In this paper,a Da number field generation method is proposed.Da numbers are defined as the ratio of local flame strain rate and extinction flame strain rate.The extinction flame strain rates are calculated by PSR model,which reduces the computational cost effectively.The Da fields near LBO conditions show that the flames near the outlet of the Venturi tube is strongly stretched and the reactions are far from equilibrium.Improving the flame strength in this region may be the key to improving the LBO performance of the combustor.