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C = 2000 ppm and x = 0.1 wt.%, the SER for SDS is averagely 31.8% larger than that for CTAB, and is averagely 33.4% larger than that for Span-80. The order of the SER values for SDS, CTAB, and Span- 80 is opposite to the order of the density values for SDS, CTAB and Span-80, meaning that the surfactant with smaller density gives more enhancement of the nucleate pool boiling heat transfer. The possible reason for this phenomenon is as follows. The diffu- sion velocities of surfactant molecules increase with the decrease of surfactant density, causing a larger number of surfactant mole- cules approach and accumulate on the growing bubble surface and reduces the surface tension faster [10], which leads to the larger decrease of bubble departure diameter and the larger increase of bubble departure frequency. Therefore, the SER increases with the decrease of surfactant density. 5. Nucleate pool boiling heat transfer correlation for refrigerant-based nanofluid with surfactant As there is no published literature on nucleate pool boiling heat transfer correlation for refrigerant-based nanofluid with surfactant, the development of a new correlation is needed. The surfactant concentration, the nanoparticle concentration and the surfactant type are three important factors influencing the nucle- ate pool boiling heat transfer, and should be reflected in the new correlation. The nucleate pool boiling heat transfer coefficient of refriger- ant-based nanofluid with surfactant (hr,n,s) can be obtained by using surfactant enhancement ratio (SER) and nanoparticle enhancement ratio (NER) to correct the nucleate pool boiling heat transfer coefficient of pure refrigerant (hr). In order to reflect the influences of surfactant concentration, nanoparticle concentration and surfactant type on the nucleate pool boiling heat transfer, the SER should be expressed as the function of the surfactant con- centration, the nanoparticle concentration, the molecular weight of surfactant and the heat flux; while the NER should be expressed as the function of the nanoparticle concentration and thermophysical properties. The nucleate pool boiling heat transfer coefficient of pure refrigerant (hr) can be calculated by Stephan and Abdelsalam correlation [30] which is widely used in the prediction of nucleate boiling heat transfer coefficient of refrigerant. In the new correlation, the nucleate pool boiling heat transfer coefficient of refrigerant-based nanofluid with surfactant, hr,n,s, is expressed as Eq. (7). hr;n;s 1⁄4SERNERhr ð7Þ where SER, NER and hr can be calculated by Eqs. (8)–(10), The seven coefficients of m1, m2, m3, m4, m5, n1 and n2 in Eqs. (8) and (9) are fitted based on 360 experimental data of Cu-R113 nanofluid with three different types of surfactants (i.e., SDS, CTAB and Span-80) in this study, covering the surfactant concentrations from 200 to 5000ppm, and the nanoparticle concentrations from 0.1 to 1.0 wt.%. By nonlinear programming solution method, the seven coefficients of m1, m2, m3, m4, m5, n1 and n2 can be determined as -2691, 27.1, 3517, 0.5, 1290, 0.69 and 0.25, respec- tively. Therefore, the nucleate pool boiling heat transfer correlation for refrigerant-based nanofluid with surfactant is expressed Eq. (11). !0:581 0:533 mr;f ar;f 3517 # respectively. 2 m3 SER1⁄4exp ðm1C þm2CÞðqMxÞn1 ð8Þ "# H. Peng et al. / Experimental Thermal and Fluid Science 35 (2011) 960–970 969 Fig. 10. Comparison of the predicted values of the new correlation with the experimental data of Cu-R113 nanofluid with surfactant. 0:745 ð2691C2 þ 27:1CÞ qr;g qr;f qDb Db kr;fTsat h 1⁄4207kr;f r " exp ( "k ðqCÞ#) 1þx0:25 0:5 n 1290 p n ð11Þ Fig. 10 shows the comparison of the predicted values of the new correlation with the experimental data of Cu-R113 nanofluid with surfactant. From Fig. 10, it can be seen that the predicted values agree with 92% of the experimental data within a deviation of ± 25%. The thermophysical properties of refrigerant and nano- particles, and the molecular weight of surfactant are reflected in the new correlation, so the new correlation can be used to other kinds of refrigerant-based nanofluids with or without surfactants. But this nucleate pool boiling heat transfer correlation was verified only by Cu-R113 nanofluids with three different types of surfac- tants in the present study, and more verifications are needed in the future in order to ensure the accuracy of this correlation for other kinds of refrigerant-based nanofluids with or without surfactants. 6. Conclusions Effect of surfactant additives on nucleate pool boiling heat transfer of refrigerant-based nanofluid is investigated experimen- tally, and some conclusions are obtained. kr;f ðqCpÞr;f ðqMxÞ 0:69 NER1⁄41þxn2 m kn þm ðqCpÞn ð9Þ 4 k 5 ðqC Þ r;f p r;f k qD 0:745 q !0:581m 0:533 hr1⁄4207r;f b r;g r;f ð10Þ Db kr;f Tsat qr;f ar;f In Eqs. (8)-(10), Cpr,f, kr,f mr,f ar,f are the isobaric specific heat, the thermal conductivity, the kinematic viscosity, and the thermal dif- fusivity of pure refrigerant, respectively; qr,g and qr,f are the vapor and liquid densities of pure refrigerant, respectively; Cpn, kn, qn are the isobaric specific heat, the thermal conductivity and the den- sity of nanoparticle, respectively; M is the molecular weight of sur- factant; Db is the bubble departure diameter, an is defined as Db = 0.0146b[2r/g(qr,f qr,g)]0.5 with a contact angle, b, of 35°; q is the heat flux; Tsat is the saturated liquid temperature.PDF Image | Experimental Thermal and Fluid Science 35
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