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Effect of Boiling Heat Transfer on Surface Wettability

Experimental Setup and Procedure

A schematic view of the experimental apparatus for pool boiling experiments with small heat transfer area is shown in Fig. 10.5. The apparatus consists of a copper block, a furnace, a rectangular container, and a heat exchanger. The copper block has a cylindrical part (10 mm in diameter, 15 mm in height). Several cartridge

Fig. 10.4 Change of contact angle from ultraviolet, γ-ray, and proton beam irradiation to TiO2 sample. (a) Ultraviolet irradiation. (b) γ-Ray irradiation. (c) Proton beam irradiation

heaters are installed in the copper block (3 kW in total). Two different copper blocks were fabricated to investigate the wettability effect on the heat transfer: one is a test section for ultraviolet irradiation and the other is for γ-ray irradiation. Both test sections are illustrated in Fig. 10.5: three thermocouples are inserted into the cylindrical part of each copper block to estimate heat flux and surface temperature.

Fig. 10.5 Schematic of experimental apparatus

Table 10.1 Irradiation conditions

The heat transfer surface for ultraviolet experiments is coated by a TiO2 thin film by a sputtering process in Kyushu University. The surface for γ-ray experiments is polished with #400 emery paper and then heated in air at 200 oC for 1 h. After the thermal oxidation of the copper surface, it is irradiated in the 60Co facility where the integrated dose is 220 kGy. The irradiation conditions for heat-transfer experiments are summarized in Table 10.1.

Results and Discussion

Measured boiling curves with and without ultraviolet irradiation are shown in Fig. 10.6. Calculated lines denote existing correlations for nucleate boiling [12–14] and critical heat flux [15, 16]. As shown in this figure, the boiling curve after irradiation moves to the higher wall superheated side in the nucleate boiling region, which may be caused by inactivation of nucleation sites on the heat transfer surface resulting from hydrophilicity. In this experiment the maximum heat flux after the

Fig. 10.6 Boiling curve before and after ultraviolet irradiation

nucleate boiling region is defined as a critical heat flux (CHF). In both boiling curves, the wall temperature rises rapidly just after the CHF region and the heat flux increases with decreasing wall temperature, resulting in heat flux being larger than CHF. These phenomena can be considered as a microbubble emission boiling (MEB) because many small bubbles are observed in this region. In the MEB region the boiling curve after irradiation moves to the lower wall superheated side, in contrast to the nucleate boiling region.

Measured boiling curves with and without γ-ray irradiation are shown in Figs. 10.7, 10.8, and 10.9. Table 10.2 shows the contact angles before and after irradiation to the oxidized copper surface. As shown in these figures, MEB phenomena are observed with or without irradiation for ΔTsub ¼ 20 , 40 , and 60 K. The boiling curves in the nucleate boiling region (low superheat region) are shifted to the higher superheat side similar to the ultraviolet irradiation experiments. However, in contrast, the boiling curves in the MEB region (higher superheat region) are shifted to the lower superheat side after irradiation, which may be caused by enhancement of thin liquid film at the re-wetting phenomena.

CHF and maximum heat fluxes in the MEB region are plotted against liquid subcooling in Fig. 10.10. Solid and dashed lines denote the predicted values of existing CHF correlations by Kutatekadze [15] and Haramura and Katto [17], respectively. The measured CHF agree well with the predicted values by the aforementioned existing correlations. Measured CHF after the irradiations are slightly larger than those before the irradiations; however, the effect of the irradiation on the CHF is not obvious in the present experimental conditions. Maximum heat flux in the MEB region is almost 50 % larger than the CHF value, and the effect of the irradiation on the maximum heat flux is also not distinct, similar to the CHF within present experimental conditions.

Fig. 10.7 Boiling curve before and after γ-ray for ΔTsub ¼ 20 K

Fig. 10.8 Boiling curve before and after γ-ray for ΔTsub ¼ 40 K

Fig. 10.9 Boiling curve before and after γ-ray for ΔTsub ¼ 60 K

Table 10.2 Contact angles before and after γ-ray irradiations

Subcooling (K)

Before irradiation (o)

After irradiation (o)

20

62.6

31.0

40

61.5

35.7

60

72.3

12.6

Fig. 10.10 Critical heat flux (CHF) and maximum heat flux before and after γ-ray irradiation


Conclusions

Effect of surface wettability on boiling heat transfer was studied by applying photocatalysis effect and radiation-induced surface activation. The main conclusions are as follows.

• Wettability enhancement was observed by proton-beam irradiation as well as the ultraviolet and γ-ray irradiation.

• In comparing the irradiation between that in air and and that in water, no influence of the radiation environment is observed for ultraviolet irradiation. However, the wettability was well enhanced by γ-ray and proton-beam irradiation in water rather than in air.

• The boiling curve with ultraviolet irradiation moves to the higher wall superheated side in the nucleate boiling region. In contrast, the boiling curve with irradiation moves to the lower wall superheated side in the MEB region.

• A similar tendency of the boiling curve was observed with γ-ray irradiation in comparison to the ultraviolet irradiation.

• The effect of irradiation on the CHF was not obvious at present experimental conditions.

Acknowledgments The authors express their sincere gratitude to Prof. Y. Takata and Dr. S. Hidaka of Kyushu University for their valuable comments and technical assistance. A part of this study is the result of “Research and Development for an Accelerator-Driven Sub-Critical System using a FFAG Accelerator” carried out under the Strategic Promotion Program for Basic Nuclear Research by the Ministry of Education, Culture, Sports, Science and Technology of Japan.

 
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