Viscosity and density measurements on liquid n-tetradecane at moderately high pressures
Introduction
Thermal energy storage systems are very important to achieve significant energy savings in order to get sustainability in its environmental, economic and social aspects. The storage of energy in adequate forms is a present day challenge to engineers and scientists. In particular, storing thermal energy is presently requiring further research in order to enable its large scale application. Among the materials in use for storing thermal energy, phase change materials (PCMs) are particularly interesting. Paraffins and their mixtures are a group of substances with potential for use as phase change materials (PCM) [1], [2] near room temperature. Having in view large-scale applications of potential PCMs, namely involving the use of solar energy [3], the knowledge of their physical properties assumes an obvious relevance. Normal tetradecane (n-C14) has a melting point near 6 °C [4] which makes it suitable for some “low temperature” energy storage applications, like v.g., cold water tanks. The development of models for the description of energy charge and discharge of PCMs, eventually contained in solid structures (v.g., capsules), require thermophysical properties of both the solid and liquid phases of the PCMs. Among other applications, the development of two-phase, moving boundary models, may require accurate data on the thermophysical properties of the PCM's, namely, their density, viscosity, and thermal conductivity, possibly at pressures above atmospheric pressure, v.g., in the case of containment in capsules.
The thermal conductivity of n-alkanes, including n-C14, have been subject of rigorous measurements since, at least, the 1980's (vd., for instance Calado et al. [5]). However, this might eventually not be the case of viscosity and density measurements at moderately high pressures. In particular, the available viscosity results covering the range from (0.1–10) MPa, which is of great importance for many applications, are restricted to just one set of measurements [6]. In fact, as far as the authors are aware, only one set of viscosity values could be found in that pressure interval [6]. The present article has therefore a definite goal to provide information on the viscosity and density of n-C14 at moderately high pressures, where measurement results are scarce. Ultimately it is aimed to develop a reference viscosity correlation with density for n-C14, which may prove useful for the development of estimation techniques for the properties of paraffin mixtures in use for thermal energy storage. It is believed that this goal is consistent with work that is being developed in the International Association for Transport Properties (IATP).
The present article follows the lines discussed in several recently published works regarding the strategy, the type of property correlation methods [7], [8], [9], [10], [11], [12] and the experimental measurement technique mainly used by the present authors, namely, the vibrating wire method [13], [14], [15], [16].
Section snippets
Materials
The vibrating wire sensor was calibrated with toluene, supplied by Sigma Aldrich, having a nominal purity of 99.8%, without further treatment, except for drying to a water content of ca. 14 mg∙kg−1 and degassing by helium spraying.
The n-C14 sample was obtained from Merck KGaA, batch S7150867 (normal tetradecane GR for synthesis), with a minimum purity of 99%, used without further treatment except for drying to a water content less than 13 mg∙kg−1 and degassing by helium spraying. Drying of both
Density measurements
The results of the measurements of the density, ρ, of n-C14 in our laboratory obtained with an Anton Paar DMA HP densimeter, performed at pressures up to 70 MPa, and at nine temperatures from (283 to 373) K are shown in Table 2. The measurements have an estimated uncertainty of ±0.2% at a 95% confidence level. It should be noted that the (283, 288 and 293) K isotherms contain a restricted range of pressures, due to the low freezing pressures of n-C14 at these temperatures.
Density correlation with temperature and pressure
Since the proposed
Experimental results
In the present work, experimental measurements of the viscosity, η, for n-C14 along eight isotherms between (283 and 358) K and pressures up to 70 MPa, have been performed. The measurements at high pressures were carried out using the vibrating wire technique. The density data needed to compute the vibrating wire viscosity results from the raw data were obtained from Eqs. (1)–(3). All high-pressure viscosity data are presented in the Supporting Information. The viscosity results were correlated
Conclusions
The density and viscosity of n-C14 have been measured in the ranges (283–373) K and (283–358) K, respectively, and at pressures up to 70 MPa. The importance of n-alkanes in general and normal tetradecane in particular call for the establishment of reference values for both properties. This is especially clear for viscosity, which has a significant lack of experimental data, in particular for temperatures lower than 293 K and at pressures greater than 0.1 MPa and lower than 100 MPa. Moreover, a
Acknowledgements and funding
This work was supported by the Strategic Project PEst-OE/QUI/UI0100/2013 funded by Fundação para a Ciência e a Tecnologia (FCT, Portugal). The authors are grateful to FCT (Portugal) for its support.
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