Distributed Sapphire Fiber Bragg Grating-Based Thermal Profiling of Submerged Entry Nozzles

This article focuses on the application of sapphire fiber Bragg gratings (FBGs) for instrumentation in submerged entry nozzles (SENs) within the steelmaking industry. The SEN is pivotal for transferring molten steel from a tundish to a mold, while preventing the infiltration of oxygen and nitrogen from the surrounding environment. Maintaining optimal flow conditions in the mold is crucial for ensuring casting process stability and maintaining high-quality steel. Sapphire FBG sensors have been instrumented in SENs to enable distributed thermal mapping for monitoring the health of the SEN. The optical sensor comprises three cascaded sapphire FBGs inscribed using femtosecond (FS) laser technology into a 1-m-long sapphire crystalline fiber. The sensor underwent characterization in a laboratory setting up to $1600~^{\circ }$ C and was tested for long-term stability over 40 h under extreme environmental conditions. The coupling between silica and sapphire fibers was investigated and implemented during sensor packaging. The sensor successfully captured the preheat sequence of the SEN in real-world steelmaking operations. Compared to conventional thermocouples, sapphire FBG sensors demonstrated exceptional efficiency and precision. They offer potential benefits such as increased productivity, reduced energy consumption, and minimized carbon footprint in the steel industry.