Specific heat capacity of molten saltbased alumina nanofluid
 MingChang Lu^{1}Email author and
 ChienHsun Huang^{1}
DOI: 10.1186/1556276X8292
© Lu and Huang; licensee Springer. 2013
Received: 18 April 2013
Accepted: 15 May 2013
Published: 21 June 2013
Abstract
There is no consensus on the effect of nanoparticle (NP) addition on the specific heat capacity (SHC) of fluids. In addition, the predictions from the existing model have a large discrepancy from the measured SHCs in nanofluids. We show that the SHC of the molten saltbased alumina nanofluid decreases with reducing particle size and increasing particle concentration. The NP sizedependent SHC is resulted from an augmentation of the nanolayer effect as particle size reduces. A model considering the nanolayer effect which supports the experimental results was proposed.
Keywords
Nanofluid Nanolayer Specific heat capacity (SHC) MoltensaltBackground
where the subscripts nf, np, and f denote nanofluid, NP, and solvent, respectively, and c_{p}, ϕ, and ρ are SHC, volume fraction, and density, respectively. In this work, we investigate SHCs of molten saltdoped with alumina NPs. The material selected is because of the fluid utilized as a heat storage medium in the solarthermal power plants, and the SHC of it determines energy storage capacity in the power plants. Here, the effect of NP addition on the SHC of the molten salt and the underlying mechanisms were examined. Furthermore, a theoretical model supporting the experimental results was proposed.
Methods
Results and discussion
Figure 4b shows the SHCs of the 13nm and 90nm alumina NPs and bulk alumina at various temperatures. The SHCs of NPs were measured using model 7020 of EXSTAR while the values of the SHCs of the bulk alumina were taken from Ginnings and Furukawa [16]. The SHCs of NPs and bulk alumina increases as temperature increases. Meanwhile, the SHC increases as particle size reduces and the SHC of 90 nm is approaching that of bulk alumina at high temperature. The sizedependent SHC of NPs has also been observed from previous studies [17, 18]. This increment of SHC with reducing particle size could be explained by the Debye model of heat capacity of solids, wherein the heat capacity increases as the Debye temperature reduces [18]. The Debye temperature decreases with reducing particle size [17], resulting in an increased SHC.
Figure 4c shows the SHCs of solid salt and solid salt doped with 13nm and 90nm alumina NPs at 0.9, 2.7, and 4.6 vol.%, respectively (measured using model 7020 of EXSTAR). The effect of NP concentration on the SHC of the solid salt doped with NPs is not significant whereas the SHC decreases with increasing NP size. The NPsizedependent SHC might be due to the fact that the larger NPs have a smaller SHC (see Figure 4b). Nevertheless, the effect of NP addition on the SHC of the nanofluid is pronounced (see Figure 4a).
The theoretical prediction using Equation 1 is also shown in Figure 5, where the values of c_{p,np} are obtained from the temperatureaveraged (290°C to 335°C) SHCs of the 13 and 90nm alumina NPs shown in the Figure 4b (i.e., 1.30 and 1.10 kJ/kgK, respectively). The red dash line and blue dashdot line in Figure 5 are the theoretical predictions of Equation 1 for the nanofluids having 13 and 90nm alumina NPs, respectively (where c_{p,13nm}, c_{p,90nm}, and c_{p,f} are 1.30, 1.10, and 1.59 kJ/kgK, respectively whereas ρ_{np} and ρ_{f} are 3,970 and 1794 kg/m^{3}, respectively). It is noted that the alumina NP density was taken from the value of the bulk alumina as an approximation. The existing model (Equation 1) predicts a slight decrease trend of the SHC of the nanofluid with increasing particle concentration since the SHCs of NPs are smaller than that of molten salt. This slight decrease tread is similar to that observed for the solid salt doped with NPs (see Figure 4c). Furthermore, the model (Equation 1) shows that the SHCs of nanofluids decrease with increasing particle size because smaller particles have larger SHC, which is in contrast to the experimental results for the nanofluid. In addition, the experimental results have a large difference from the model prediction of Equation 1, which has also been observed in previous studies [6, 9–12]. This indicates that there might be other mechanisms responsible for the large discrepancy.
The proposed mechanisms for the thermal conductivity enhancement are the following: (1) Brownian motion [19, 20]. It is argued that Brownian motion of NPs in the solvent could result in a microconvection effect that enhances heat transfer of the fluid; (2) Colloidal effect [21–23]. It says that heat transfer in nanofluids can be enhanced by the aggregation of NPs into clusters; (3) Nanolayer effect [24–26]. The solidlike nanolayer formed on the surface of the nanoparticle could enhance the thermal conductivity of the fluid [14]. In light of these studies, we believe that some of these mechanisms might affect the SHC of nanofluid as well.
Particle aggregation was observed when both the solid salt and the molten salt were doped with NPs as shown in Figures 2 and 3. The sizes of the clusters formed from the aggregated NPs are both on the order of 1 μm in the solid salt and molten salt (see Figures 2 and 3). However, the SHC of the solid salt doped with NPs is close to that of solid salt alone whereas the SHC of the molten salt doped with NPs is apparently different from that of molten salt. Furthermore, the NP size effect shows reverse trends in these two cases: the SHC of solid salt increases as NP size reduces (see Figure 4c) whereas the SHC of molten salt doped with NPs decreases as NP size reduces (see Figure 4a). This indicates that the observed large discrepancy between the SHCs of nanofluid and molten salt does not result from the particle aggregation effect. In addition, Ishida and Rimdusit [27] have also shown that the SHC is a structureinsensitive property, provided that formation of different degrees of network do not affect the SHC of the composite. Furthermore, since SHC is not a transport property, the microconvection effect caused by Brownian motion of NPs should not play a significant role in the SHC of the nanofluid. Thus, we conjectured that the nanolayer effect might be the only important factor among these three mechanisms affecting the SHC of the nanofluid. Accordingly, a theoretical model considering the nanolayer effect on the SHC was proposed. Since the solidlike nanolayer formed on the surface of NP is at a thermodynamic state between solid salt and molten salt [26], the value of the SHC of the nanolayer should lay between those of the solid salt (1.04 kJ/kgK) and the molten salt (1.59 kJ/kgK). In other words, the nanolayer has a lower SHC than that of the molten salt. Further, the thermal properties of the nanolayer (e.g., thermal conductivity and SHC) could vary with different combinations of NPs and base fluids, since the structure of the nanolayer is dependent on the interaction of molecules [28]. In addition, Lin et al. [25] also found that the nanolayer structure is sizedependent, resulting in a sizedependent thermal conductivity.
where ρ_{f} and ρ_{np} are solvent density and NP density, respectively.
Using Equation 5, one can obtain the SHC of the nanofluid (c_{p,nf}) at any mass fraction (α’) from the measured SHC of the nanofluid (c_{p,m}) at a certain mass fraction (α) for a given NP size. The predictions using Equation 5 for the SHCs of the nanofluids at various concentrations having 13nm alumina NPs (red solid line) and 90nm alumina NPs (blue dash line) based on the measured SHCs at 4.6 vol.%, along with the experimental results, are also shown in Figure 5. As Figure 5 shows, the predictions from the proposed model agree well with the experimental results.
The large difference between the predictions of Equations 5 and 1 is from the result of the nanolayer effect on the SHC. This could be better understood by looking at the third term in the numerator of Equation 4. Since the weight of nanolayers (W_{layer}’) increases as particle concentration increases, it results in a further reduced SHC, provided that the nanolayer has a lower SHC than that of molten salt. Furthermore, the increase of SHC with increasing particle size is also a result of the nanolayer effect. For a given NP concentration, the nanolayer effect increases as particle size reduces since the number of particle increases with reducing particle size. Thus, one observes a decreased SHC as particle size reduces, and particle concentration increases because of the augmentation of the nanolayer effect.
Conclusions
In conclusion, we have explored the SHC of the molten saltbased alumina nanofluid. The NP sizedependent SHC in the nanofluids had never been reported before and cannot be explained by the current existing model. We found that the reduction of the SHC of nanofluid when NP size reduces is due to the nanolayer effect, since the nanolayer contribution increases as particle size reduces for a given volume fraction. A theoretical model taking into account the nanolayer effect on the SHC of nanofluid was proposed. The model supports the experimental results in contrast to the existing model. The findings from this study are advantageous for the evaluation of the application of nanofluids in thermal storage for solarthermal power plants.
Abbreviations
 NP:

Nanoparticle
 SHC:

Specific heat capacity
 DI:

Deionized
 SEM:

Scanning electron microscope
 EDS:

Energy dispersive spectrometer
 OM:

Optical microscope
 DSC:

Differential scanning calorimetry.
Declarations
Acknowledgements
The authors would like to thank Dr. CW Tu and Dr. SK Wu of the Industrial Technology Research Institute and Prof. Chuanhua Duan of Boston University for the helpful discussion about the heat capacity of the nanofluid. The authors would also like to acknowledge the Green Energy and Environmental Laboratory of the Industrial Technology Research Institute for the use of their equipment for the heat capacity measurement. The funding support for this study is from the National Science Council of Taiwan (Grant no. NSC 1012623E009 001ET).
Authors’ Affiliations
References
 Choi SUS: Enhancing Thermal Conductivity of Fluids with Nanoparticles. In Book Enhancing Thermal Conductivity of Fluids with Nanoparticles, FEDvol. 231/MDvol. 66. New York; Cairo: Hindawi Publishing Corporation; 1995:99–105.Google Scholar
 Xuan Y, Li Q: Heat transfer enhancement of nanofluids. Int Commun Heat Mass 2000, 21: 58–64.Google Scholar
 Patel HE, Das SK, Sundararajan T, Sreekumaran Nair A, George B, Pradeep T: Thermal conductivities of naked and monolayer protected metal nanoparticle based nanofluids: Manifestation of anomalous enhancement and chemical effects. Appl Phys Lett 2003, 83: 2931–2933. 10.1063/1.1602578View ArticleGoogle Scholar
 Liu MS, ChingCheng Lin M, Huang IT, Wang CC: Enhancement of thermal conductivity with carbon nanotube for nanofluids. Int Commun Heat Mass 2005, 32: 1202–1210. 10.1016/j.icheatmasstransfer.2005.05.005View ArticleGoogle Scholar
 Liu MS, Lin MCC, Tsai CY, Wang CC: Enhancement of thermal conductivity with Cu for nanofluids using chemical reduction method. Int J Heat Mass Transf 2006, 49: 3028–3033. 10.1016/j.ijheatmasstransfer.2006.02.012View ArticleGoogle Scholar
 Namburu PK, Kulkarni DP, Dandekar A, Das DK: Experimental investigation of viscosity and specific heat of silicon dioxide nanofluids. Micro Nano Lett 2007, 2: 67–71. 10.1049/mnl:20070037View ArticleGoogle Scholar
 Kulkarni DP, Vajjha RS, Das DK, Oliva D: Application of aluminum oxide nanofluids in diesel electric generator as jacket water coolant. Appl Therm Eng 2008, 28: 1774–1781. 10.1016/j.applthermaleng.2007.11.017View ArticleGoogle Scholar
 Vajjha RS, Das DK: Specific heat measurement of three nanofluids and development of new correlations. J Heat Transf 2009, 131: 071601. 10.1115/1.3090813View ArticleGoogle Scholar
 Zhou SQ, Ni R: Measurement of the specific heat capacity of waterbased Al[sub 2]O[sub 3] nanofluid. Appl Phys Lett 2008, 92: 093123. 10.1063/1.2890431View ArticleGoogle Scholar
 Zhou LP, Wang BX, Peng XF, Du XZ, Yang YP: On the specific heat capacity of CuO nanofluid. Advances in Mechanical Engineering 2010, 2010: 1–4.View ArticleGoogle Scholar
 Shin D, Banerjee D: Enhancement of specific heat capacity of hightemperature silicananofluids synthesized in alkali chloride salt eutectics for solar thermalenergy storage applications. Int J Heat Mass Transf 2011, 54: 1064–1070. 10.1016/j.ijheatmasstransfer.2010.11.017View ArticleGoogle Scholar
 Shin D, Banerjee D: Enhanced specific heat of silica nanofluid. J Heat Transf 2011, 133: 024501. 10.1115/1.4002600View ArticleGoogle Scholar
 Buongiorno J: Convective transport in nanofluids. J Heat Transf 2006, 128: 240–250. 10.1115/1.2150834View ArticleGoogle Scholar
 Hitec Solar Salt, Costal Chemical. http://www.coastalchem.com/
 Carling RW: Heat capacities of NaNO3 and KNO3 from 350 to 800 K. Thermochim Acta 1983, 60: 265–275. 10.1016/00406031(83)802482View ArticleGoogle Scholar
 Ginnings DC, Furukawa GT: Heat capacity standards for the range 14 to 1200 K. J Am Chem Soc 1953, 75: 6359.View ArticleGoogle Scholar
 Avramov I, Michailov M: Specific heat of nanocrystals. J Phys Condens Matter 2008, 20: 295224. 10.1088/09538984/20/29/295224View ArticleGoogle Scholar
 Michailov M, Avramov I: Surface Debye temperatures and specific heat of nanocrystals. Sol St Phen 2010, 159: 171–174.View ArticleGoogle Scholar
 Jang SP, Choi SUS: Role of Brownian motion in the enhanced thermal conductivity of nanofluids. Appl Phys Lett 2004, 84: 4316–4318. 10.1063/1.1756684View ArticleGoogle Scholar
 Prasher R, Bhattacharya P, Phelan P: Thermal conductivity of nanoscale colloidal solutions (nanofluids). Phys Rev Lett 2005, 94: 025901.View ArticleGoogle Scholar
 Prasher R: Effect of aggregation kinetics on the thermal conductivity of nanoscale colloidal solutions (nanofluid). Nano Lett 2006, 6: 1529–1534. 10.1021/nl060992sView ArticleGoogle Scholar
 Gao JW, Zheng RT, Ohtani H, Zhu DS, Chen G: Experimental investigation of heat conduction mechanics in nanofluids. Clue on clustering. Nano Lett 2009, 9: 4128–4132. 10.1021/nl902358mView ArticleGoogle Scholar
 Zhu H, Zhang C, Liu S, Tang Y, Yin Y: Effects of nanoparticle clustering and alignment on thermal conductivities of Fe[sub 3]O[sub 4] aqueous nanofluids. Appl Phys Lett 2006, 89: 023123. 10.1063/1.2221905View ArticleGoogle Scholar
 Xie H, Fujii M, Zhang X: Effect of interfacial nanolayer on the effective thermal conductivity of nanoparticlefluid mixture. Int J Heat Mass Transf 2005, 48: 2926–2932. 10.1016/j.ijheatmasstransfer.2004.10.040View ArticleGoogle Scholar
 Lin YS, Hsiao PY, Chieng CC: Roles of nanolayer and particle size on thermophysical characteristics of ethylene glycolbased copper nanofluids. Appl Phys Lett 2011, 98: 153105. 10.1063/1.3579522View ArticleGoogle Scholar
 Yu W, Choi SUS: The role of interfacial layers in the enhanced thermal conductivity of nanofluids: a renovated Maxwell model. J Nanopart Res 2003, 5: 167–171.View ArticleGoogle Scholar
 Ishida H, Rimdusit S: Heat capacity measurment of boron nitridefilled polybenzoxazine: the composite structureinsensitive property. J Therm Anal Calorim 1999, 58: 497–507. 10.1023/A:1010127805836View ArticleGoogle Scholar
 Xue L, Keblinski P, Phillpot SR, Choi SUS, Eastman JA: Two regimes of thermal resistance at a liquid–solid interface. J Chem Phys 2003, 118: 337–339. 10.1063/1.1525806View ArticleGoogle Scholar
Copyright
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.