Abstract
Enhancing the critical heat flux (CHF) of external reactor vessel cooling (ERVC) to achieve the strategy of in-vessel retention (IVR) has been deemed as an essential requirement for the security of modern commercial PWR nuclear power plants. A new type of multi-scale conical pin fin structures had been designed to meet this need. These conical structures of four different axial heights (1.0, 1.5, 2.0 & 2.5mm) and similar radial geometric parameters were fabricated on four heating surfaces of copper substrates by supersonic cold gas spray technique (SCGS), and the conical pin fin structures were combined with a thin layer of micro porous structures. Transient-state (quenching) and steady-state (heating) pool boiling experiments had been respectively conducted on 6 different downward-facing surfaces under different inclination angles including 5°, 30°, 45°, 60°, 75° & 90° (in vertical), which included above-mentioned four pin fin surfaces, a plain surface and a coated surface covered with a 1mm-thick layer of uniform porous coating fabricated by SCGS with copper particles. The experimental results showed that the CHF increased with the inclination angles. The CHF of all the SCGS-modified surfaces were significantly enhanced compared with those of the plain surface. The maximum CHF value and the maximum CHF enhancement belonged to the 2.0mm-height pin fin surface whose CHF values were more than 1.7 times of those of the plain surface, and up to 2.1 times in 90°. The 1.0mm pin fin surface had the greatest performance in boiling heat transfer coefficient (HTC) and its CHF-enhancing performance also greatly exceeded that of 1.0mm uniform porous coating surface. Remarkable performances of multi-scale conical pin fin structures had been experimentally confirmed, hence the safety margin of IVR-ERVC will be greatly enhanced after adopting them. In addition, the CHF enhancement mechanism of the multi-scale conical pin fin structures was analyzed.
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