An Experimental Investigation of the Effects of Surface Conditions on Pool-Boiling Heat Transfer for Various Material

In this research, minimum film boiling temperature (Tmin ) is quantitatively investigated under various experimental conditions. Since Tmin is defined as the boundary between film and transition boiling regimes, its value is significant for the design of an emergency core cooling system following a hypothetical loss-of-coolant accident (LOCA) in a nuclear power plant. When a sufficiently heated surface is plunged in a water pool, a vapor blanket is generated around the test section acting as a heat transfer insulator due to the poor thermal conductivity of the vapor. At temperatures lower than Tmin, the heat transfer is dramatically enhanced because of the vapor film collapse and the physical contact between the water and the heated surface. Therefore, it is very important to explore methods and techniques that increase this temperature in order to improve the safety of nuclear reactors. A parametric study was performed by varying the initial rod temperature, liquid subcooling, surface thermophysical properties, and surface conditions.;A Tmin facility was designed and constructed to conduct quenching experiments using vertical rods. Cylindrical test samples were fabricated with three embedded thermocouples inside the cladding material. The thermocouples were connected to a data acquisition system in order to measure the temperature history during the experiments. The temperature and heat flux at the surface were calculated using an inverse heat conduction code (DATARH). The diameter and height of the test sections were 9.5 mm and 25 cm, respectively. The diameter was chosen to approximate the size of fuel rods in commercial nuclear reactors. Seven test samples were used in this study. 316-stainless steel (SS), zirconium-702 (Zr), and Inconel-600 were used as the bare substrate claddings. The other four test samples were made of Inconel-600 that have experienced hundreds of heating and cooling cycles in a 7x7 Rod Bundle Heat Transfer (RBHT) test facility. The latter was constructed at the Pennsylvania State University (PSU) that was sponsored by the U. S. Nuclear Regulatory Commission (NRC). In one of the final test series conducted in the RBHT facility, several of the heater rods became fouled and test data suggested that quench rate was significantly faster where surface deposits had formed. The affected rods were removed from the RBHT bundle, and the surface morphology of the heater rods were determined. The affected rods were also fashioned into the test samples for the quench experiments in the Tmin facility. The quenching results of the test samples showed a linear relation between the liquid subcooling and the T min value. In addition, it was found that as krhoc p of the substrate material decreases, the Tmin value increases. The fouled surfaces were observed to quench faster than the bare ones, indicating higher Tmin values.;An advance image processing technique was applied to quantitatively characterize the liquid-vapor interfacial waves, vapor layer thickness, T min, quenching temperature (TQ), quenching time, and quench front velocity in the film boiling heat transfer regime. Visualization of the boiling behavior was captured by a high-speed camera at a frame rate of 750 frames per second (fps) from which the vapor film thickness and the behavior of the liquid-vapor interface in the film boiling regime were analyzed frame by frame. The vapor-liquid interfacial waves as well as their temporal evolution are visualized for a range of wall superheats and various degrees of liquid subcooling. The thermocouple data and the captured videos were synchronized to couple the behavior of the vapor layer with the thermal behavior of the heated rod. Through the intensive image analyses, it was concluded that the vapor film thickness decreases contributing to a higher Tmin..;A characterization study of the surface condition was performed in order to understand the surface conditions. The behavior of the surface oxidation kinetics of SS and Inconel-600 was examined. The SS was oxidized for 2 hrs at 550 °C and then plunged in a saturated distilled water pool. Whereas, the Inconel-600 test samples were used in RBHT and have experienced a total of 200 heating and cooling cycles under system pressure and cladding temperature that ranged between 0.138-0.414 MPa and 315-1093 °C, respectively. Various characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM) associated with Energy-dispersive X-ray spectroscopy (EDS), and field emission scanning electron were employed to identify the phases, chemical composition, and surface microstructure of the Iconel-600 before and after being used in RBHT. Micro- and nanoparticles composed of NiO, Cr2O3, and Fe2O 3 were observed at the surface of the used heater rods. An absence of a clear trend was revealed between the power profile and the oxide thickness.