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968 H. Peng et al. / Experimental Thermal and Fluid Science 35 (2011) 960–970 Fig. 9. Surfactant enhancement ratio (SER) for three types of surfactants (i.e., SDS, CTAB, Span-80) at different nanoparticle concentrations: (a) 0 wt.%; (b) 0.1 wt.%; (c) 0.5 wt.%; (d) 1.0 wt.%. (a) ω = 0 wt% (pure R113) (b) ω = 0.1 wt% (c) ω = 0.5 wt% (d) ω = 1.0 wt% nucleate pool boiling heat transfer. The conjunct role of three enhancement factors and one deterioration factor determines the influence of surfactant on the nucleate pool boiling heat transfer of pure refrigerant. For SDS, at surfactant concentration lower than 2000 ppm, the enhancement factors dominate; while at surfactant concentration higher than 2000ppm, the deterioration factor dominates. For CTAB, at surfactant concentration lower than 500 ppm, the enhancement factors dominate; while at surfactant concentration higher than 500 ppm, the deterioration factor dom- inates. For Span-80, at surfactant concentration lower than 1000 ppm, the enhancement factors dominate; while at surfactant concentration higher than 1000ppm, the deterioration factor dominates. Therefore, for each type of surfactant, there exists an optimal surfactant concentration to obtain the maximal SER as well as the highest enhancement effect of surfactant on the nucleate pool boiling. Under the experimental conditions, 2000ppm, 500 ppm and 1000 ppm are the optimal concentrations for SDS, CTAB and Span-80, respectively. From Fig. 9b–d, it can be seen that for Cu-R113 nanofluid, the value of SER also increases with the increase of surfactant concen- tration and then decreases, presenting the maximum values at 2000 ppm, 500 ppm and 1000 ppm for SDS, CTAB and Span-80, respectively. The presence of nanoparticles does not change the tendency of SER changed with the surfactant concentration. From Fig. 9, it can be seen that the value of SER increases with the de- crease of nanoparticle concentration (x) for fixed surfactant type and surfactant concentration (C), and reaches the maximum value at x of 0 wt.% (i.e., the fluid is pure R113). The possible reasons for this phenomenon are as follows. (1) The decrease of nanoparticle concentration weakens the interaction between surfactant mole- cules and nanoparticles, thus the diffusion velocities of surfactant molecules increase and a larger number of surfactant molecules accumulate on the growing bubble surface, which reduces the sur- face tension faster. (2) The faster reduction of the surface tension causes the decrease of bubble departure diameter and the increase of bubble departure frequency to be more obvious, which leads to SER increasing with the decrease of nanoparticle concentration. From Fig. 9, it can be seen that the values of SER are in the order of SDS > CTAB > Span-80 at fixed surfactant concentration (C) and nanoparticle concentration (x). For example, on the condition ofPDF Image | Experimental Thermal and Fluid Science 35
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