High Temperature Gas Velocity Profile Measurement Using Fiber Optic Hot-Wire Velocimetry

Abstract Most advanced reactors operate at high temperatures; however, there is a lack of local velocity measurement techniques that survive in the high-temperature, corrosive, and irradiation environments present in these reactors. In this experimental study, a Fiber Optic Hot-wire Anemometer (FO-HWA) using Rayleigh Backscattering is developed and applied to high-temperature airflow in a circular tube operating at temperatures exceeding 600 °C as the first step to developing a velocity profile measurement technique for high-temperature gas-cooled reactors. The circular tube is made of transparent borosilicate glass to allow for flow visualization measurement techniques to be applied at the desired temperature ranges. Additionally, the flow channel will be equipped with thermocouples to monitor the temperature of the flow. Also, inlet conditions were well controlled by applying a nine-to-one contraction and a flow straightener, which generates a uniform inlet velocity profile. A Time-Resolved Particle Image Velocimetry (TR-PIV) system consisting of a high-speed camera capable of recording at up to 9,000 frames per second and a high-power continuous green (532 nm) laser is applied simultaneously to the flow to generate validation data. The flow rate was controlled to cover laminar, transition, and turbulent flow regimes (Re: 1,800, 5,500, and 7,300). High-temperature air velocity profiles are obtained at those different flow rates by the FO-HWA and compared with the PIV data and existing experimental data in the literature. Considering the relatively thick sensor diameter compared with the traditional hot-wire sensors, the velocity data is post-processed to generate a time-averaged velocity profile, and the accuracy of the time-averaged velocity profile was discussed by comparing the results with the PIV data. The time-resolved capability of the FO-HWA is assessed. Also, the second moment of the temperature fluctuation measured by the FO-HWA and the velocity fluctuation measured by PIV were compared. The FO-HWA technique demonstrated in this study will be applied to higher temperatures, targeting over 800 °C, found in helium-cooled reactors and potentially the sodium vapor core of heat-piped cooled microreactors. In the present setup, the oxidation issue is not thoroughly resolved, although acceptable stability of the measurement is achieved. This may cause an issue for the long-term operation of the current setup. However, in the realistic target conditions mentioned above in advanced reactors, with the same stainless steel capillary tubing protection of the current setup, the oxidation issue can be mitigated compared with the present experimental condition with airflow. Eventually, with additional coating/protection techniques, the FO-HWA concept can be applied to other high-temperature energy-transporting fluids such as molten salt that can be used in nuclear fusion & fission, and solar energy applications.