Thermal Conductivity Determination of Octadecane and Eicosane near Phase Transition for Solid Specimen Prepared Subject to Controlled Freezing and Explaining the “Anomalous” Measurements in Terms of Solid-Solid Transition

Abstract

The effects of the freezing time and vacuum oven time (degasification time) associated with processing of two paraffins specimen, i.e. octadecane (C18H38) and eicosane (C20H42) on the temperature-dependent thermal conductivity in solid phase have been studied. Two distinct freezing routes, i.e. ice-water and liquid nitrogen routes have been utilized to control the freezing time of eicosane and octadecane specimen (melting temperatures of 37 oC and 26.5 oC, respectively) in a novel experimental setup. Four vacuum oven times (0, 5, 10 and 20 hours) have been utilized during the preparation process of the two materials for both freezing routes. In order to relate the rate of progress of the freezing front to the freezing time, a one-dimensional heat transfer model associated with the proposed experimental configuration was developed. The freezing/solidification time of the octadecane and eicosane specimen initially decreases with the vacuum oven time until 10 hours is reached, after which it increases for both processing routes. Theoretical predictions of the dimensionless thickness of the solidifying specimen (ɛ+) as a function of t+ were generally in great agreement with the visually-observed quantities. Thermal conductivity of the solid octadecane specimen from 11.5 oC to 24.8 oC and eicosane specimen from 20.9 oC to 35.5 oC are evaluated for samples prepared following the ice-water and liquid nitrogen routes and all vacuum oven times by means of the transient plane source method. Thermal conductivity of both paraffins associated with the liquid nitrogen route are smaller than the ice-water route in most cases, however deviation of this behavior is recorded for nearly all vacuum oven times. Thermal conductivity of octadecane and eicosane exhibited enhanced values for both ice-water and liquid nitrogen routes and four degasification times as the temperatures of the specimen nears the solid-liquid phase transition points. This behavior recorded for both eicosane and octadecane is explained by inclusion of solid-solid phase transition characterized by possession of a greater thermal conductivity than the solid phase just below the solid-liquid phase transition in a computational model of the simplified transient hot-wire method by utilizing the ANSYS Fluent code. The greater thermal conductivity associated with the solid-solid phase transition causes the thermal conductivity to ascend with respect to temperature before the solid-liquid phase transition point similar to the climb in the experimental results of both octadecane and eicosane proving that rotator phase can be responsible for the recorded trends.