High-resolution internal temperature measurements and X-ray imaging of sodium heat pipes

Abstract

A heat pipe incorporating internal fiber optic temperature sensors was fabricated and charged with high purity sodium. The heat pipe was tested in start-up and steady-state operation with heat loads up to 1500 W. Internal temperature data was collected to characterize the axial temperature profile of the heat pipe at different experimental conditions. The novel application of time-resolved X-ray imaging at 30 Hz in combination with fiber optic temperature measurements allowed for tracking of the liquid sodium and revealed internal flow phenomena and temperature fluctuations. X-ray imaging of the heat pipe showed that excess liquid accumulated in the condenser during start-up and remained there during steady-state operation. It was found that continuum flow in the vapor regime began at a temperature of 450℃. For steady-state operation at heat loads of less than 1500 W, and inactive region of the condenser was identified, characterized by a sharp decrease in temperature, indicating negligible condensation in this region. The inactive region was found to decrease in length with increasing heat load and to increase in length with increasing heat removal from the condenser. Surface waves were observed on the pool of liquid sodium in the condenser, with magnitude depending on the applied heating load. Pairing time-resolved X-ray images with high-resolution temperature data indicated a relationship between the passing of the surface waves and fluctuations in the local internal temperature, with both seeming to occur at a frequency of 0.4 Hz.