BOILING CHARACTERISTICS DURING QUENCHING OF METAL RODS UNDER POOL AND FLOW BOILING CONDITIONS

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Date
2023-08
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Saudi Digital Library
Abstract
Knowledge of the post-critical heat flux (post-CHF) heat transfer and accurate prediction of the boiling characteristics are important during nuclear safety assessments for the hypothetical Loss of Coolant Accident (LOCA). The minimum film boiling temperature (Tmin) separates the film boiling regime and the transition boiling regime, and its value is utilized to determine the boiling heat transfer in various thermal-hydraulics codes. Quenching experiments under pool boiling conditions as well as constant reflood experiments were performed, respectively, at the PSU Pool Quenching Test Facility and the NRC-PSU Rod Bundle Heat Transfer (RBHT) Test Facility to investigate the behavior of Tmin under pool and flow boiling conditions. Pool boiling experiments are conducted by quenching a heated rod in a liquid pool under atmospheric pressure. The surface heat flux and Tmin are quantitatively investigated by varying the initial surface temperature, liquid subcooling, and surface conditions. All test samples are 9.5 mm in diameter, simulating a typical nuclear fuel rod. Each test sample has embedded thermocouples, and an inverse heat conduction code (DATARH) is used to calculate the surface temperature and the surface heat flux from the temperature transients recorded by the embedded thermocouples. The results indicate that Tmin increases with increasing liquid subcooling. On the other hand, the effect of the initial surface temperature during film boiling regimes was found to be insignificant. Visualization of the transient quenching process was captured by a high-speed camera, and the vapor film thickness and behavior of the liquid-vapor interface in the film boiling regime were analyzed. Heat transfer enhancement of Chromium (Cr) coating on Zirconium cladding is investigated as one of the candidates for accident tolerant fuel (ATF). During high subcooling experiments, it was found that the Cr-Coating increased the heat transfer rate, specifically during film boiling, due to the surface microstructure of the Cr coating. SEM analysis was used to investigate the heat transfer enhancement of chromium coating. When the pool temperature is closer to saturation, a thick vapor film is observed during film boiling, which plays a crucial role in suppressing the effect of Chromium coating. Reflood experiments are performed in the Rod Bundle Heat Transfer (RBHT) at the NRC-PSU facility. The 7×7 bundle is electrically heated, and Inconel was used as the wall material. Quenching and boiling curves, curves as well as the minimum film boiling temperature (Tmin), or precisely, referred to as the quenching temperature (Tq ) in the context of reflood experiments, are determined at various elevations throughout the bundle under different reflood conditions. The impact of pressure, subcooling, flow rate, and flooding rate on Tq has been carefully investigated. Under the same initial rod temperature, the bundle tends to quench faster for higher flooding rates, higher pressure, and higher degrees of liquid subcooling. A model was developed to predict quenching temperature (Tq), considering factors such as flooding rate, liquid subcooling, wall heat flux, and system pressure. The model shows a good agreement with RBHT experiments with relative errors within ± 15%.
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Keywords
Flow Boiling, Pool Boiling, Quenching, Minimum Film Boiling Temperature, Rod Bundle, Two-Phase Heat Transfer
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