Soon Ho Kang

Effect of liquid spreading due to nano/microstructures on the critical heat flux during pool boiling

It is well known that nanoparticles deposited on a heating surface during nanofluid boiling can change the characteristics of the heating surface and increase the critical heat flux (CHF) dramatically. We considered a new approach to investigate the nanoparticle surface effect on CHF enhancement using surfaces modified with artificial micro/nanostructures similar to deposited nanoparticle structures. We examined the effect of the surface wettability and liquid spreading ability on the CHF. The results demonstrated that the CHF enhancement on the modified surfaces was a consequence of both the improved surface wettability and the liquid spreading ability of the artificial micro/nanostructures.

INVESTIGATION ON EFFECTS OF ENLARGED PIPE RUPTURE SIZE AND AIR PENETRATION TIMING IN REAL-SCALE EXPERIMENT OF SIPHON BREAKER

To ensure the safety of research reactors, the water level must be maintained above the required height. When a pipe ruptures, the siphon phenomenon causes continuous loss of coolant until the hydraulic head is removed. To protect the reactor core from this kind of accident, a siphon breaker has been suggested as a passive safety device. This study mainly focused on two variables: the size of the pipe rupture and the timing of air entrainment. In this study, the size of the pipe rupture was increased to the guillotine break case. There was a region in which a larger pipe rupture did not need a larger siphon breaker, and the water flow rate was related to the size of the pipe rupture and affected the residual water quantity. The timing of air entrainment was predicted to influence residual water level. However, the residual water level was not affected by the timing of air entrainment. The experimental cases, which showed the characteristic of partical sweep-out mode in the separation of siphon breaking phenomenon [2], showed almost same trend of physical properties.

Experimental Study of Nucleate Boiling Performance on Several Types of the Mixed-Wettability Pattern Surface by Micro/Milli-Sized Hydrophobic Patterns

To increase the nucleate boiling efficiency, many nucleate boiling experiments have been conducted and could get brilliant and challengeable results. A consensus was that CHF and heat transfer were affected by a modified heating surface which change the micro roughness, thermophysical properties of heating surface, or the wettability. Of the many parameters, the wettability study is regarded as the most powerful factor. For finding the optimized condition at the nucleate boiling (high heat transfer and high CHF), we design the special heaters to examine how two materials, which have different wettabilities, affect the boiling phenomena. The special heaters have several types of hydrophobic patterns which have the precise size because they were made by MEMS techniques on the silicon oxide surface. In the experiments with patterned surface, hydrophobic dots lead to an early bubble inception and induce the better heat transfer. These experiments are compared with classic and recent models for bubble inception. The all experiments are conducted under the saturated pool boiling condition with distilled water at 1 atm pressure. The peculiar Teflon (AF1600) is used as the hydrophobic material. The hydrophilic part is performed by silicon oxide through the furnace procedure. The experiments using the micro-sized patterns and milli-sized patterns are performed, and the results are compared with the reference surface. These mixed-wettability studies are expected to induce the development of the nucleate boiling condition.Copyright

Pool Boiling CHF Enhancement on a Zircaloy-4 Surface With Micro/Nano Scale Modification

The need to operate thermal systems, including those in nuclear power plants, below the critical heat flux (CHF) requires inconvenient compromises between economy and safety. This has led to much research aimed at enhancing the CHF. CHF enhancement was recently noticed in nanofluids where the heating surface was fouled by nanoparticles. We attempted to imitate the micro/nano multi-scale structure of nanoparticle fouling by surface modification. In this study, surface treatment was used to change the surface wettability of a Zircaloy-4 heater. A pool boiling apparatus was developed based on the results of the surface treatment, and an experimental study was performed to investigate the increase in CHF.Copyright