Investigation Of Numerical Calculation Of IR Characteristics And Stealth Technology Of Turbofan Engine Exhaust System

The IR signatures of turbofan engine exhaust system with afterburner studied this thesis include the radiations from all the inner structures and hot gases down strem of low-pressure turbine blade of turbofan engine with afterburner. The calculating method of IR characteristics and the stealth technologies on the no afterburning condition were investigated. This thesis consists of four contents.(1) The development and application of IR stealth technology, numerical simulation software for target IR characters were presented. (2) High efficient program were developed to calculate the infrared radiation intensity of turbofan engine exhaust, to analyze the infrared radiation contribution of each component, and to evaluate the effect of IR stealth technology. (3) The effect of low emissivity coating and cooling of the surface on IR signature was studied on scaled experimental model, and the data obtained from experiment were used to validate the calculation method and program. (4) The program developed by this thesis was adopted to calculate the IR characteristics of axisymmetric nozzle with afterburner of a typical turbofan engine, and the effect of low emissivity and cooling on the IR signature is evaluated.The infrared radiation characteristics of exhaust system with afterburner were calculated using reverses Monte Carlo method. Convection-radiation coupled heat transfer calculation method was employed to simulate the flow filed and temperature distribution. The SST k -ωtwo equations turbulence model and weighted-sum-of-gray-gases model was employed to simulate the turbulence and calculate the radiation parameter of gas medium respectively. In order to display the infrared radiation luminance of the target, the detection space is divided into pixels. The radiation contributions of gas, wall, self-radiation or reflected radiation form each component are separated by estimating the ray's terminal. The formula for calculating the detectable range was obtained based on noise equivalent irradiance of detector. The atmospheric transmittance needs an iterative calculation with detectable range. The absorption processes by gas species H₂O, CO₂, CO, CH₄, N₂O and N₂ are considered. The computation consumption has been reduced by involving MPI parallel calculation function bases. And the parallel efficiency reaches to 99%. Comparing with the IR signature of experimental model, the computed results are in reasonable range. Comparing with the data of literature, the detectable range calculated by the method in this thesis agrees with the result of the literature.Under some special conditions, the directional reflection in engine cavity could have an obvious impact on the distribution of IR intensity. The computational ability and applied range are expanded by considering both specular and diffuse reflection. And the calculated error of the experimental model is obviously reduced.The infrared radiation characteristics of axisymmetric exhaust system with afterburner of a typically turbofan engine on the condition of Ma=0 and Ma=1.2 were calculated using the method developed in this thesis. The distributing laws of the IR of the exhaust system by changing temperature and emissivity distributions were opened out. And a low IR scenario was designed. The results show that: The exhaust system's infrared radiation atα= 0°could reduce 47%～49%, and the detectable range could reduce 20% if the emisivity of turbo blade, cone, bypass dividing surface and radial flame holder reduced to 0.2, and the temperature of cone reduces 100K, and the temperature of nozzle reduces 50K.