Rheological Analysis of Binary Eutectic Mixture of Sodium and Potassium Nitrate and the Effect of Low Concentration CuO Nanoparticle Addition to Its Viscosity

Abstract

This paper is focused on the characterisation and demonstration of Newtonian behaviour of salt at both high and low shear rate for sodium and potassium nitrate eutectic mixture (60/40) ranging from 250 °C to 500 °C. Analysis of published and experimental data was carried out to correlate all the numbers into one meaningful 4th order polynomial equation. Addition of a low amount of copper oxide nanoparticles to the mixture increased viscosity of 5.0%–18.0% compared to the latter equation.

Rheological Characteristics of Molten Salt Seeded with Al2O3 Nanopowder and Graphene for Concentrated Solar Power

“…The decrease of viscosities was mainly due to the uncertainty of the measurement. Similarly, the addition of 0.1 wt.% copper oxide into solar salt led to an increase of 4.7~18.3% in viscosity with the temperature range of 250~450 • C, as shown in the literature [27]. While in the research of Munoz-Sanchez et al [28], the refined grade solar salt (RSS) nanofluid containing 1.5 wt.% alumina nanoparticles had a variation of −14.71~0.63% in viscosity with the temperature range of 300~400 • C. On the contrary, the RSS nanofluid containing 1.5 wt.% Silica nanoparticles had an increase of 3.49~21.75% in viscosity with the temperature range of 300~400 • C.…”

“…On one hand, the shear rates might affect the interactions among nanoparticles, inducing the agglomeration. On the other hand, the uncertainty of the device may cause the deviation of the results, as the concentration of Al 2 O 3 nanopowder larger than those in the literature [27] affects the flow of liquid salt significantly. Furthermore, the viscosities of the nanocomposites with the addition of 2 wt.% Al 2 O 3 nanopowder were generally larger than those with 1 wt.% Al 2 O 3 nanopowder no matter what the shear rate was, especially at high temperature.…”

“…Figure 4c,d show the viscosities and shear stresses against the shear rates below 25 s −1 . Large fluctuation can be found both for the viscosities and shear stresses, and the Newtonian behaviour is not obvious in the period due to low torque resolution, where the same phenomenon can be seen in the literature [27,28]. Thus, the values below 25 s −1 were discarded, and only the shear rates between 25 s −1 and 250 s −1 were taken into account in the following discussion.…”

“…The present results of the viscosities were compared with the correlations in Table 2, as shown in Figure 6a,b. The curves in Figure 6 based on the correlations from the literature [25,27,[40][41][42][43][44][45][46][47][48] were obtained from seven temperature points, including the minimum and maximum temperatures of the temperature range in Table 2, five temperature points among the range. It can be seen that the viscosities in the present study are larger than those in the literature to some extent, especially under the low temperature.…”

“…Jin [25] measured the viscosities of ternary nitrates using the viscometer based on the rotating cylinder method, and it was found that the addition of lithium nitrate to eutectic salt affected the viscosity slightly, while calcium nitrate would increase the viscosity of eutectic salt to some extent, e.g., the viscosity of solar salt adding calcium nitrate increased by about 53.2% at 550 K. Chen et al [26] experimentally and theoretically studied the rheological behaviours of TiO 2 seeded ethylene glycol nanofluids, and pointed out that the concentration and structure of nanoparticles would greatly influence the viscosities of the nanofluids. Lasfargues et al [27] measured the viscosities of solar salt and salt doped with 0.1% CuO, and it was found that the viscosities of the nanofluids increased by about 5.0~18.0% in the temperature range of 250~500 • C. Munoz-Sanchez et al [28] measured the viscosities of solar salt seeded with alumina and silica nanoparticles at various conditions, and found out the influences of salt purity, concentration of nanoparticles, and measuring configuration of rheometer on the viscosities of solar salt. The results indicated the Newtonian behaviour of the nanofluids in spite of different rheometer configurations, and the Arrhenius model was suitable to explain the relationship between the viscosity and temperature.…”