Pool Boiling Heat Transfer Characteristics Of Nanofluids And Nanocoatings
Kwark, Sang Muk
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This research is a qualitative and quantitative investigation to understand the behaviors of nanofluids and nanocoated surfaces during pool boiling heat transfer. The pool boiling behavior of low concentration nanofluids, a mixture created by dispersing nanoparticles in pure water, was experimentally studied over a flat heater. A majority of this work was conducted using Al₂O₃ nanoparticles dispersed in water and some minor work was performed with others (CuO and diamond nanoparticles). Results from this study are consistent with those previously reported in demonstrating that boiling of nanofluids produces a nanocoating on the heater surface, and which in turn increases the critical heat flux (CHF). This study also investigates the possible causes responsible for the deposition of nanoparticles on the heater surface. Through experimental, it was shown that microlayer evaporation, during nanofluid boiling, was responsible for the nanoparticle coating formed on the heater surfaces. Subjecting the heater surfaces to extended periods of nanofluid boiling has shown an eventual degradation in BHT that has been attributed to modifications in surface conditions that are continuously being altered through additional nanoparticle deposition. The wetting and wicking characteristics of the nanocoating are investigated by measuring the apparent contact angle and by conducting vertical dip test. It is found that the CHF enhancement mechanism is dominated by the wetting characteristics of the nanocoating and a relationship between the quasi-static contact angle and the CHF value is provided. The fundamental pool boiling test of nanofluid exhibited same unique characteristics like an enhanced CHF, transient boiling behaviors, and nanoparticle deposition on the heater surface. After this fundamental study, further investigation was conducted to understand the effects of the nanocoating in pool boiling heat transfer. The thickness and the uniformity of the nanocoating dictated the BHT and the CHF conditions based on this. A methodology for an optimal nanocoating development is provided. The optimal nanocoating provided unique pool boiling characteristics and was generated by controlling the thickness and uniformity of the nanoparticle precipitation on the heater's surface. Parametric tests on pool boiling using this nanocoated surface are investigated. The parametric test involved variations in nanoparticle size, system pressure, heater orientation, and heater size. For this, different Al₂O₃ nanoparticles sizes (75 ± 50, 139 ± 100, and 210 nm ± 200 nm), system pressures (20 ~ 200 kPa), heater orientations (0 ~ 180º), and heater sizes (0.75 × 0.75 ~ 2 cm × 2 cm) were used. Results indicate that the pool boiling performance is dependent on the parameters tested, except the particle size, for both uncoated and nanocoated surfaces. The nanoparticle coated heater consistently showed a dramatic CHF enhancement relative to the uncoated surface at all tested conditions.