Enhancement of Boiling Heat Transfer and Critical Heat Flux using Surfaces with Thermally-Induced Wetting Transition

Abstract

Boiling heat transfer (BHT) and critical heat flux (CHF) are important factors for boiling heat transfer systems. The factors govern the efficiency and safety of the system. In this reason, there have been many studies for the enhancement of BHT and CHF. A surface modification is a promising technique to enhance BHT and CHF without changing the working fluid. Especially, the surface modifications affect wettability of the surface that changes boiling characteristics significantly. However, there is a trade-off between BHT and CHF when the wettability is modulated. A hydrophobic surface induces a nucleation at low wall superheats with increased nucleation site density; it results in an improvement of BHT. At the same time, a premature CHF occurs on the hydrophobic surface due to the vigorous bubble generation. In contrast, a hydrophilic surface enhances CHF with supplying liquid on the heated surface while BHT deteriorates in comparison to the hydrophobic surface. In this respect, if a surface changes its wettability by itself during boiling, then, the enhancements of BHT and CHF are attainable. In the present study, TiO2 and ZnO thin films were used as coating materials on a silicon heating substrate in pool and flow boiling. TiO2 and ZnO thin films were initially hydrophobic, but they became hydrophilic as the temperature increases. For an achievement of the wetting transition at a low temperature which corresponds to the wall temperature during boiling, the radio frequency (RF) sputtering condition was adjusted. For the fabricated surfaces, the wetting transition was confirmed by heat treatment for 10 h in air atmosphere; the contact angles was 83.1 on TiO2 and 101.3 on ZnO after the heat treatment at 100 C. The complete wetting transition was achieved under the heat treatment near 200 C. After the heat treatment at 200 C, they became hydrophilic showing the contact angle of 32.7 and 26.7 on TiO2 and ZnO, respectively. Using the surfaces and an SiO2-deposted surface as a reference, the pool boiling and flow boiling experiments were conducted with deionized (DI) water. The pool boiling experiments were conducted at 1.0, 2.0, 3.3, and 4.1 bars for the regulation of the saturated temperature of the working fluid because the complete wetting transition was postulated to occur near 200 C. BHT was improved on both TiO2 and ZnO regardless of the pressure conditions; CHF on TiO2 was also enhanced at 4.1 bar. Furthermore, additional enhancements of CHFs on TiO2 and ZnO were achieved by the time effect test that maintained heat flux of 8090 % of CHF (at high wall temperature) for 20 min to induce complete wetting transition. As a result, CHFs were enhanced on both surfaces. The time effect of the wetting transition was also examined by heat treatment tests. An empirical correlation for the dependence of the receding contact angle on the heat treatment temperature and time was made and combined with a CHF prediction model. Using the empirical correlation, the CHF enhancement ratio was predicted. In flow boiling experiment, the wetting transition effect was investigated in a macrometer channel (hydraulic diameter was 7.5 mm) using DI water. The mass flux was ranged from 600 to 1200 kg/m2s and the inlet temperature was set 97 C for the prevention of damage in the acrylic channel. BHTs of TiO2 and ZnO were improved in comparison with SiO2 for the examined mass flux conditions. In addition, CHF degradation was compensated on TiO2, and CHF was enhanced on ZnO at the mass flux of 1200 kg/m2s. Also, CHFs on TiO2 and ZnO were more enhanced by the time effect tests. However, the enhancement ratio of CHF was lower than other results in literature. The flow and structure effects are possible reasons. Some of previous studies showed that CHF enhancement ratio decreased as mass flux increased on the modified surface. In the present study, although TiO2 and ZnO became hydrophilic as the wall temperature increased at high mass flux, the increased mass flux may attribute to the reduction in an increase of CHF. Another reason could be that structures in previous studies contributed to the additional enhancement of CHFs on the modified surface. In summary, the thermally-induced wetting transition of TiO2 and ZnO was adopted to pool and flow boiling for the enhancement of both BHT and CHF. There was no defect on the surfaces and contamination of the working fluid. Furthermore, the roughness was nanometer scale which should not provide nucleation sites. The wetting transition was the only plausible factor for the explanation of the boiling characteristics. For both pool and flow boiling, the nucleation on TiO2 and ZnO was more easily activated at the low wall temperature in comparison to SiO2 due to their hydrophobicity. Also, at the high temperature, BHT was enhanced without degradation of CHF. It is explained by that TiO2 and ZnO became hydrophilic at a temperature near CHF, based on the observed variation of the contact angle after the heat treatment. Therefore, TiO2 and ZnO are suggested as potential surfaces which enhance both BHT and CHF without any external control during an operation.