The understanding of critical heat flux (CHF) phenomenon and an accurate prediction of the CHF condition are important for safety and economic design of nuclear reactor and other applications. It is also important to enhance the CHF. Therefore many researches were performed to enhance the CHF by using nanofluid as a working fluid and to understand the effects of nanofluid having influence on CHF phenomenon. However, several kinds of nanofluids were used to improve CHF, most experiments were performed at atmospheric pressure. Therefore, it is required to assess the CHF enhancement using nanofluids and understand the effect of pressure on CHF enhancement with nanofluid at higher pressure in order to broadly apply the nanofluids to nuclear power plant like a coolant of emergency cooling system. This paper described the effect of pressure on CHF enhancement using water-based nanofluids at higher pressure. The pool boiling CHF experiments with a Ni-Cr wire were performed at atmospheric pressure to 1.1 MPa in distilled water to assess the only effect of pressure on CHF and validate the experimental results compared with the existed correlations. The results were well matched with Zuber's correlation. After that, pool boiling experiments were conducted at same ranges of system pressure in water-based nanofluids with alumina (Al_2O_3) and magnetite (Fe_3O_4) nanoparticles. From these experiments, we found that the CHF was generally increased with increasing the system pressure and the CHF using nanofluids can be enhanced with increasing the system pressure. Based on the characteristics of CHF results, there were several attempts to understand the mechanism of CHF occurrence according to the changes of system pressure. First, bubble behaviors (i.e. the bubble frequency, bubble size, and nucleation site density) were analyzed by using the images, captured by high speed camera. Secondly, the contact angle was measured with sessile drop of distilled water on the heated surface. Lastly, the weight of deposited nanoparticles on the heating surface was measured after boiling. We discussed the effect of pressure on bubble behaviors and surface characteristic in detail based on these analyses.
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