Boiling heat transfer is essential in energy systems as it impacts safety and efficiency. In pressurized water reactors, the coolant is water under high pressure, and the heat transfer is in single-phase convection without boiling. When the heat flux from the fuel rod exceeds the critical heat flux (CHF) of boiling heat transfer, the fuel rod's temperature will increase sharply due to boiling regimes changing from nucleate boiling to film boiling. Fuel cladding failure will be likely to occur. In a design-based accident scenario of reactivity insertion accident, the fuel is pulsed heated to a temperature.&nbsp; The rapid heating of the fuel rod may cause the departure from nucleate boiling (DNB). However, the relationship between transient heating rate and CHF at different sub-cooling levels remains a topic for research.Auracher and Marquardt have performed transient boiling experiments on a flat surface at heating rates lower than 50K/s. Their results indicated that the heat transfer coefficient increase with the heating rate. Visentini conducted a series of experiments to explore the influence of a power excursion on rapid boiling on semi annulus. The results show high power at different shapes will influence the CHF greatly. Besides, subcooling experiments with triangular power pulses show CHF (0.22-0.25MW/m2) in experiments are in good agreement with Katto and Ohno's correlation (0.19MW/m2).&nbsp; In the square power pulse experiments, the CHF increases by 25%, and it can be predicted by Sakurai model. Fau's pulsed heating experiment found that CHF's mechanism at large powers relates to an explosive vapor wave. At lower powers, the CHF is similar to steady boiling.The paper aims to explore the pool boiling CHF changes at different transient heating rates. The experiment is conducted within a small boiling tank with glass walls for high-speed video recording. A 127um diameter Pt wire, with controlled electric heating, is submerged in deionized water at atmospheric pressure. The wire resistance is recorded to reflect the average wire temperature. To accurately measure the resistance, 4- wire resistance measurement is adopted. Before every experiment, the wire resistance is measured six times, three for normal connection and three for reversed connection. Then these results are averaged to be the initial resistance. A PID-controlled heater controls the water temperature, which is monitored by a thermocouple. The result shows that the heating rate will influence the CHF in the subcooling situation. Many parameters, such as wire diameter, subcooling level, and wire heat rate, strongly influence the CHF. The next step is to conduct the experiment in a high-pressure environment.

The Influence of Rapid Transient Heating in Critical Heat Flux of Boiling Heat Transfer

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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