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H. Peng et al. / Experimental Thermal and Fluid Science 35 (2011) 960–970 967 Fig. 8. Nucleate pool boiling heat transfer of Cu-R113 nanofluid with nonionic surfactant (Span-80). (a) Boiling curves (heat flux vs. excess temperature) (b) Heat transfer coefficients vs. heat flux From Figs. 6–8, it can be concluded that the surfactant concen- tration and the surfactant have effects on the nucleate pool boiling heat transfer coefficient of refrigerant-based nanofluid. These ef- fects will be quantitatively analyzed in the following section. 4.4. Effects of surfactant on the nucleate pool boiling heat transfer of pure refrigerant and refrigerant-based nanofluid In order to analyze the effect of surfactant on the nucleate pool boiling heat transfer coefficient quantitatively, surfactant enhance- ment ratio, SER, is defined in this paper, as shown in Eq. (6): SER 1⁄4 hf;s=hf ð6Þ where hf,s and hf are the nucleate pool boiling heat transfer coeffi- cient of fluid with surfactant and that of fluid without surfactant, respectively. The fluids in the present study include pure R113 and Cu-R113 nanofluid. Fig. 9a–d show the surfactant enhancement ratio (SER) for three types of surfactants (i.e., SDS, CTAB, Span-80) at nanoparticle con- centration (x) of 0 wt.%, 0.1 wt.%, 0.5 wt.% and 1.0 wt.%, respec- tively. It can be seen from Fig. 9a that SER at x of 0 wt.% (i.e., the fluid is pure R113) are in the ranges of 1.36–1.81, 1.20–1.51, and 1.18–1.39 for SDS, CTAB and Span-80, respectively. Fig. 9b shows SER at x of 0.1 wt.% are in the ranges of 1.25–1.67, 1.11–1.39, and 1.09–1.29 for SDS, CTAB and Span-80, respectively. Fig. 9c shows SER at x of 0.5 wt.% are in the ranges of 1.17–1.57, 1.07– 1.31, and 1.04–1.26 for SDS, CTAB and Span-80, respectively. Fig. 9d shows SER at x of 1.0 wt.% are in the ranges of 1.12–1.46, 0.94–1.24, and 0.85–1.17 for SDS, CTAB and Span-80, respectively. In order to isolate the effects of surfactant and the nanoparticles on the nucleate pool boiling heat transfer, the effect of surfactant on the nucleate pool boiling heat transfer of pure refrigerant are firstly analyzed as follows. From Fig. 9a, it can be seen that for pure R113, the value of SER increases with the increase of surfactant concentration and then decreases, presenting the maximum values at 2000ppm, 500 ppm and 1000 ppm for SDS, CTAB and Span-80, respectively. There are following three enhancement factors of surfactant on the nucleate pool boiling heat transfer of pure refrigerant. (1) The surfactants can be absorbed by the liquid–vapor interface and form an orientation-arrange molecular layer, decreasing the surface ten- sion especially at low surfactant concentration [6]. The decrease of surface tension causes the decrease of the superheat degree of the bubble nucleation, the decrease of bubble departure diameter, and the increase of bubble departure frequency. (2) The surfactants accumulate at the heating surface and form the excess layer, reducing the surface-energy between the liquid and the heating surface [13], thus the active nucleation sites increase. (3) The sur- factants may aggregate in the fluid and form the large particles [28]. The interaction between these hot particles and bubbles may induce the secondary nucleation on the bubbles [29]. How- ever, the surfactants increase the viscosity especially at high sur- factant concentration, causing reduced micro-convection near the heating surface [28], which may lead to the deterioration of thePDF Image | Experimental Thermal and Fluid Science 35
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