Infrared Signature Prediction And Optimization Design Method For Serpentine Nozzle

Low observable signature is one of the particular features for the modern stealth combat aircrafts.Advancements in technology have significantly improved the sensitivity and reliability of infrared detector,thus making the aircraft more susceptible.Therefore,it is of great important to reduce the infrared signature of the aircraft for improving the infrared stealth ability.Due to the potential in suppressing the infrared signature of exhaust system,the serpentine nozzle is of strategic and tactical interest to the military forces and researchers.In this paper,the mechanism of the infrared signature for the serpentine nozzles was investigated as well as the effects of the geometry parameters,the inlet conditions and the multi-objective optimization of the serpentine nozzle.Firstly,the numerical method of infrared signature calculation for exhaust system has been invsetigated.The line-by-line method has been used in the infrared radiative heat transfer calculation,and the applications of spectroscopic databanks HITEMP2010 and CDSD-4000 in calculating the gas radiative property parameters have been compared.Moreover,in order to meet the needs of the calculation in engineering,the statistic narrow band model has been studied as well as three schemes for calculating the model parameters.The results shown that the CDSD-4000 spectroscopic databank is more accurate and reliable in calculating the spectral transmissivity for CO2 in high temperature conditions as compared to the HITEMP2010 spectroscopic databank.Compared to the mean method,the least-square method is more suitable for calculation the statistic narrow band model parameters.Based on the above researches,a code for calculating the infrared signature of exhaust system has been built using the C++programming language,and the validation study shown that the code is reliable to meet the needs of engineering applications.Secondly,this paper has carried out a systematic study on the infrared signature of the serpentine nozzle.The mechanism of the infrared signature for the serpentine nozzles has been investigated as well as the effects of the geometric parameters,the inlet conditions,and the low infrared signature design criteria.The results shown that small length ratio would increases the infrared intensity on the lower side of the vertical plane;for the serpentine nozzle,the centerline curve with modest turning has a lower infrared intensity;the geometric parameters of the first S section have marked influences on the aerodynamic performance,but limited on the infrared signature;the aspect ratio has a significant influences on the infrared signature of the serpentine nozzle,the gas infrared radiative intensity in the horizontal plane decreases with the increment of aspect ratio;for the turbojet,the total infrared radiative intensity of the serpentine nozzle is dominated by the wall infrared radiative intensity,but the gas infrared radiative intensity plays a more important role in the total infrared radiative intensity for the turbofan;the engine swirl could induce the hot streak on the wall of the serpentine nozzle,but the effects of the hot streak on the total infrared radiative intensity is limited;the inhibition of engine swirl to the total infrared radiative intensity is mainly due to the suppression of the gas infrared radiative intensity,and with the increment of the swirl angle,the infrared radiative intensity of the serpentine nozzle decreases;from the take-off condition to the cruise condition,the wall temperature and the core plume of the serpentine nozzle decrease,and the infrared radiative intensity of serpentine nozzle drops down;the shield ratio and the visible area ratio can be used to evaluate the inhibition effect of the serpentine nozzle on the infrared signature,for the turbojet,the visible area ratio should be controlled under 0.15 for the single serpentine nozzle and under 0.35 for the double serpentine nozzle,and for the turbofan,the visible area ratio is recommended to be less than 0.04.Lastly,in order to resolve the contradiction between the demands of the well aerodynamic performance and the low infrared signature,the method of optimization design for the serpentine nozzle has been studied.The surrogate model based on the radial basis function and the multi-objective particle swarm optimization have been investigated,and three geometric parameters of the serpentine nozzle were selected for multi-objective optimization which is designed to achieve high aerodynamic performance and low infrared signature.The results shown that the aspect ratio has the least impact on the aerodynamic performance,but greatest impact on the infrared signature;the length ratio has the greatest impact on the aerodynamic performance,and the shield ratio has the least impact on the infrared signature.This paper has carried out a systematic study on the infrared signature and the optimization design of the serpentine nozzle,and provides theoretical support for the establishment of the low observable signature serpentine nozzle design system,moreover gives some suggestions to the designers.