Research On Multi-objective Optimization And Design Of The Lobed Mixer Exhaust System

The lobed mixer is widely applied in the exhaust system of turbofan engine with low and intermediate bypass ratio,for its advantages in reducing the weight of the exhaust engine,increasing the net thrust,and suppressing exhausting noise and infrared radiation.The researchers had concluded some design criteria for the lobed mixer on the basis of a comprehensive study of its mixing mechanism and the parametric effect(e.g.aerodynamic parameters,geometric parameters).However,as the coordination and integrated design of the components of the exhaust system is becoming more and more important in the design of the exhaust system,the application scope of these design criteria is limited.So it is necessary to develop a new method for the design and optimization of the lobed mixer.This paper employed a numerical optimization method based on a sequential approximate optimization framework to construct the multiobjective optimization process of the lobed exhaust system.The rise angle(?)and the ratio of lobe wavelength to height(?)was chosen as the design parameters of the optimization problem,which aimed to improve the mixing efficiency and thrust and control the total pressure loss.The BP neural network and the genetic algorithm(or NSGA-II algorithm)were employed as approximation model to fit the objective function and solve the optimization problem,respectively.Based on the optimization process,a multi-objective optimization of a lobed mixer exhaust system was conducted in normalized and non-normalized ways,respectively.The aerodynamic performance of the optimized lobed mixer was evaluated under both design and off-design conditions.The de-swirling ability of the optimized lobed mixers and the mechanism that the optimized lobed mixers enhance the aerodynamic performance of the exhausting system were analyzed and the feasibility of the multi-objective optimization method of the lobed mixer was verified.Under the off-design(inlet swirl)condition,the intensified leakage swirling flow between the central body and the trough of the optimized lobed mixer led to the reduction of the thrust.In reaction of this phenomenon,an integrated lobed mixer exhaust system was designed and optimized.The leakage swirling flow was restrained by the integrated Out Guide Van(OGV),and the purpose of increasing the thrust and reducing total pressure loss of the exhaust system was achieved.The effect of ? and ? on the aerodynamic performance of the lobed mixer was coupled.The simultaneous increase or decrease of the lobed mixer should be avoided,a proper increase of ? and a decrease of ? can achieve more thrust and less total pressure loss at the less cost of mixing efficiency.For the both optimized lobed mixers,their rise angle,fall angle and lobe height was decreased,suppressing the radial scale of the streamwise vortices and leading to a mixing efficiency reduction,a total pressure loss reduction and a thrust augmentation.Under inlet swirl conditions,for the optimized lobed mixers,the reduction of their fall angle intensified the leakage swirling flow between the trough and the central body,and their aerodynamic performance was more easily influenced by the leakage swirling flow.Compared with the normalized optimization method,the non-normalized optimization method is more suitable for the lobed mixer.Compared with the reference lobed mixer,the non-normalized optimized lobed mixer achieved a reduction of mixing efficiency by 4.46%,a thrust augmentation by 2.29% and a decrease of total pressure loss by 0.64%.Its thrust augmentation was 0.78% under the inlet swirl of 20° conditions,which is higher than that of the normalized optimized lobed mixer.The integrated OGV of the integrated lobed mixer was installed near the trailing of the trough.According to the lobe number,there were 15 OGVs in a while circle.The integrated OGV eliminated the leakage swirling flow between the trough and central body,which was beneficial to the increase of the thrust.The abilities that the OGV eliminates the swirling flow and enhances the thrust was greatly suppressed by the flow separation on the surface of the OGV and further limited its ability to enhance the thrust.