FIBER BRAGG GRATINGS FOR HEAT FLUX MEASUREMENTS IN THERMAL PROTECTION SYSTEMS UNDER A STEADY CONDUCTIVE THERMAL LOAD

摘要

This paper examines the performance of fiber Bragg gratings (FBGs) as embedded heat flux sensors for ablative thermal protection systems (TPS). Ablative TPS materials are currently used for reentry spacecraft applications because ablative TPS materials are able to withstand the higher temperatures present in ballistic reentry than non-ablative materials. It is important to measure the through-the-thickness temperature profile in-situ to verify the heat shielding performance. FBG sensors were chosen to monitor the temperature in the TPS primarily because of a good match in the thermal properties of the silica fibers and the TPS material. A TPS specimen was subjected to a conductive and steady thermal load. Two FBG sensor arrays were embedded in the TPS specimen. One array was embedded horizontally in the specimen near the surface where the heat was applied, and the second array was embedded diagonally through the thickness of the specimen. The temperature load was produced using the lower platen of a hot press, and the maximum temperature was below the ablation temperature of the material. After the specimen was heated, it was removed from the hot press and allowed to passively cool on a metal table. The peak wavelength output from each sensor was monitored and recorded during the loading cycle, and the wavelength measurements were converted to temperature data over time. The test was completed three times, and excellent repeatability was present across the three tests and the temperature response of the FBGs was reasonable, even though the test setup was torn down and reassembled between two of the tests. Thermal images of the specimen were also collected during the test with an IR camera. The thermal images were used to provide a temperature map of the specimen which showed good agreement with the FBG data.