| Summary: | 博士 === 國立清華大學 === 工程與系統科學系 === 106 === Both solar thermal and nuclear power systems are important energy options with low carbon emissions. Molten salt has been proposed as the working fluid in a concentrating solar power system, and it is also suggested to be employed in a molten salt reactor, which is one of six types of Generation-IV power systems. Molten salts present feature of high heat capacity and high thermal stability. Nevertheless, molten salts usually exhibit relatively poor heat transfer performance and non-uniform temperature distribution. Besides, the specific heat capacity of molten salts is not as high as water and there is a need for the enhancement.
This study demonstrates the variation of specific heat capacity and heat transfer performance in the molten salt doped with nanoparticles. Innovative apparatus and procedures have been developed to prepare the specimen of molten salt with high quality. Moreover, a series of thermal-physical characteristics, including melting point, decomposition temperature, thermal diffusivity, thermal conductivity and specific heat capacity for a pure fluoride salt (FLINAK) and a nitrate salt (HITEC) are measured and compared with that reported in the literature to validate the specimen prepared.
At first, this work studied the effect of alumina nanoparticles (mean diameter about 40 nm) concentration on the specific heat capacity of molten HITEC salt, which is called nano-HITEC. The nano-HITEC salt in this study is free of surfactant and an innovative preparation process and sampling apparatus for molten nano-HITEC have been established in the present study to avoid the possible precipitation of nanoparticles. The specific heat of molten HITEC and nano-HITEC are measured using a power compensated differential scanning calorimeter (DSC) at the Industrial Technology Research Institute (ITRI) of Taiwan. A power compensated DSC monitors the endothermic or exothermic reaction of specimen under a constant heating rate to determine the specific heat capacity of specimen. In this work, an optimal concentration of 0.063 wt.% is identified, demonstrating the highest enhancement on the specific heat capacity of 19.9%. Lower or higher concentration of nanoparticles tends to reduce the enhancement. For the concentration of 2 wt.%, the negative effect of doping the nanoparticles on the specific heat capacity appears for all temperatures in the present study. The SEM images after solidification of samples reveal that near uniform dispersion of nanoparticles with negligible agglomeration for the concentration smaller than 0.016 wt.%. The agglomeration becomes significant and the particle clusters seem to be inter-connected for higher concentrations. Moreover, this study conducts a statistical analysis on the number of isolated particles and number and size of clusters using SEM images to develop a simplified model, which describes the area of particle-liquid interface of nano-HITEC fluid with different concentration. The results indicate that the agglomeration of nanoparticles occurs as the concentration is over 0.023 wt.%; clusters with different sizes are formed and the total area of isolated particles is decreased and the clusters begin to contribute to the total interfacial areas with the overall area decreasing at the beginning. The minimum particle-liquid interface take place at concentration of 0.048 wt.%. Subsequently, the total interfacial area increases with increasing concentration. It is found that the optimal concentration is corresponding to approximately the concentration that the contribution of isolated nanoparticles and particle clusters, size ranging from 0.2 to 0.6 μm, to the interfacial area are approximately the same. The interfacial phenomena at the particle (isolated or cluster) fluid interface is proposed in the literature to be the primary reason for the enhancement of specific heat capacity in nano-salts.
Subsequently, the effect of nanoparticle concentration on the laminar convective heat transfer performance for molten nano-HITEC fluid in a mini-circular tube is investigated. An innovative piston driven molten salt apparatus and preparation process of molten HITEC nanofluild are developed to prevent the precipitation of nanoparticles during the measuring process. The piston molten salt pump provides steady flow through the test section which the channel diameter and length are 2.1 mm and 120 mm, respectively. The mean heat transfer of molten salt flow can be evaluated by Newton’s cooling law based on the inlet/outlet temperature and three wall temperatures measured at three axial locations. The results demonstrate that the measurement of mean Nusselt number of the pure HITEC fluid in this study be in good agreement within ±10% with a well-known correlation. A concentration of nanoparticles of 0.25 wt.% in the nano-HITEC, which can be maintained uniform dispersion for about 30 minutes, results in the maximum enhancement of mean Nusselt number of 11.6%. On the other hand, a concentration of 0.063 wt.% provides 9.2% enhancement on the mean Nusselt number of HITEC nanofluid, and precipitation phenomenon is not observed within an hour. In addition, a new correlation with consideration of particle concentration for the laminar convective heat transfer performance of the nano-HITEC fluid in the present minichannel is developed, by which more than 93.9% of the experimental data can be predicted within ±10% of deviation.
A nano-HITEC fluid with concentration of 0.063 wt.% of alumina particle of about 40 nm demonstrates best enhancements of its specific heat capacity of 19.9% as well as near best heat transfer performance in this study. This suggests that nano-HITEC of 0.063 wt.% of nano alumina particles of about 40 nm may serve as a working fluid for applications in a thermal storage system or a concentrating solar power system.
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