With the increase of flight speed, infrared (IR) seeker may be seriously affected by high temperature shock-layer in front of dome. Shock-layer radiance is produced by molecular processes and has implications for sensor waveband selection. This paper examines the impacts of high temperature shock-layer IR radiance from a molecular radiometry point of view. We derived the mathematical expressions of rotational quantum number of maximum intensity and the corresponding wavenumber of P and R branch. The theoretical calculation and the simulation based on HITRAN database are conducted. Results show that with the increasing of temperature, the rotational quantum number of maximum intensity will be larger. Ultimately, the wavenumber of P and R branch moves towards long wavelength and short wavelength direction respectively. The conclusions subserve to select sensor waveband.
In order to study the optical transmission and radiation characteristics in a hypersonic shock layers over infrared optical window, an approximation calculation approach is proposed. With this method, the formal solution for shock layer radiative transport equation is derived considering both target signature and windows thermal radiance. Consequently, the transmissivity of heating shock layer can be expressed quantitatively, which shows that the shock layer is optically thick in the 4~4.5μm band mainly. Besides, the radiance flux density of shock layer is calculated when temperature is 800K, pressure is 7atm and thickness is 1cm. Shock layer radiance is found to be larger in the mid-wave infrared band compared with long-wave infrared band and short-wave infrared band.
Since the dome experiences the convective heat loading, thermal stress will be generated in the thickness direction. Thus, estimation of the thermal shock and analysis of the thermal shock resistance of the dome are the key to the design of the dome. In this paper, thermal shock resistance of CVD ZnS dome is analysed based on the flight condition of 6000m altitude and 3.0 Mach. We obtained the critical Reynolds number through a rockets pry experiment, which deduced that there exists a transition from laminar flow to turbulent flow at somewhere over the dome. We calculated the heat transfer coefficient over dome through heat transfer coefficient engineering formula of high-speed sphere with turbulent boundary layer near the stagnation point. The largest heat transfer coefficient is 2590W/(m2.K). Then, we calculated the transient thermal stress of dome by using the finite element method. Then we obtained the temperature and thermal stress distribution of different time through the direction of thickness. In order to obtain the mechanical properties of CVD ZnS at high temperatures, the 3-point bending method was used to test the flexure strength of CVD ZnS at different temperature. When compared the maximum thermal stress with flexure strength at different temperature, we find that the safety factors were not less than 1.75. The result implied that the dome has good safety margin under the proposed application condition. Through the above test and analysis, we can get the conclusion that the thermal shock resistance of the CVD ZnS dome satisfied the requirements of flight conditions.
For the optical guidance system flying at low altitude and high speed, the calculation of turbulent convection heat transfer over its dome is the key to designing this kind of aircraft. RANS equations-based turbulence models are of high computation efficiency and their calculation accuracy can satisfy the engineering requirement. But for the calculation of the flow in the shock layer of strong entropy and pressure disturbances existence, especially of aerodynamic heat, some parameters in the RANS energy equation are necessary to be modified. In this paper, we applied turbulence models on the calculation of the heat flux over the dome of sphere-cone body at zero attack. Based on Billig’s results, the shape and position of detached shock were extracted in flow field using multi-block structured grid. The thermal conductivity of the inflow was set to kinetic theory model with respect to temperature. When compared with Klein’s engineering formula at the stagnation point, we found that the results of turbulent models were larger. By analysis, we found that the main reason of larger values was the interference from entropy layer to boundary layer. Then thermal conductivity of inflow was assigned a fixed value as equivalent thermal conductivity in order to compensate the overestimate of the turbulent kinetic energy. Based on the SST model, numerical experiments showed that the value of equivalent thermal conductivity was only related with the Mach number. The proposed modification approach of equivalent thermal conductivity for inflow in this paper could also be applied to other turbulence models.
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