Structural integrity investigation of optical window under thermal environment of scramjet intake.

Abstract

The evaluation of thermal integrity of the sapphire optical window was conducted through numerical prediction and experimental confirmation approach under thermal environment of the scramjet intake wall. Step shaped optical window was selected as the geometry of the optical window for this study to be installed in the scramjet intake wall in a flush-mount method so as not to disturb the internal flow-field in the scramjet intake. The test model for the evaluation of the thermal integrity of the optical window was considered as assembly type of the optical window and support considering that the optical window is mounted in an optical sensor such as TDLAS and the optical window with the optical sensor is to be installed in the scramjet intake. The numerical prediction for the evaluation of the thermal integrity of the optical window was performed through the finite element method using a commercial ANSYS transient thermal analysis and transient thermal-structural analysis. Through the finite element method, the temperature distribution, maximum temperature increment, and thermal stress were predicted under the heat flux condition corresponding to the scramjet intake wall. In addition, the principal stresses distribution was obtained using the calculated thermal stress, and the material failure occurrence was evaluated using the calculated principal stresses distribution and the material failure theory. The experimental confirmation for the evaluation of the thermal integrity of the optical window was carried out using the electrical heater. Morphology studies were conducted to verify the thermal integrity of the optical window using a visual inspection, SEM, AFM and EDS analysis. The occurrence of the brittle material failure phenomena such as fracture, macro-crack, and micro-crack was investigated through visual inspection, SEM, AFM and EDS analysis, and EDS analysis was performed to analyze the chemical composition change on the heated surface of the optical window. Through the numerical prediction and experimental confirmation, it is found that the material failure of the optical window did not occur under the heat flux condition corresponding to the scramjet intake wall. Application Studies of the sapphire optical window to TDLAS were additionally carried out using the blowdown tunnel with an electrical heater and the blow down tunnel with vitiating heater to experimentally investigate the occurrence of the optical window damage under the aerodynamic heating condition corresponding to the scramjet intake wall. It was experimentally confirmed that the sapphire optical window did not undergo material failure under the aero-heating condition.