Determining the Thermal Conductivity of Liquids Using the Transient Hot Disk Method.

 

Part I: Establishing Transient Thermal-fluid Constraints

Methods to determine the thermal conductivity of liquids have come under increased scrutiny recently, particularly in the field of nanofluids, where there is growing disagreement between research groups on whether thermally conductive nanoparticles provide anomalous enhancements in bulk fluid thermal conductivity at volume fractions as low as 0.1%. Recent studies suggest that the presence of natural convection during testing can lead to gross over predictions of thermal conductivity in both steady-state and transient thermal characterization systems. However, few (if any) studies account for this. This is particularly true in newly developed measurement systems, such as the transient hot disk system, which is designed to rapidly measure the thermal conductivity and diffusivity of materials simultaneously and with a high degree of accuracy.

Thus, in the first part of this study, the effects of natural convection are analyzed numerically for the transient hot disk system in order to determine when convection begins to affect the solution used to determine a surrounding fluid's thermal conductivity. A comprehensive analysis of the effect of the fluid's pertinent thermophysical properties, the Rayleigh number and the Prandtl number on the transient solution is completed and a correlation for the onset of natural convection during testing under the aforementioned conditions is developed for use in a new iteration scheme used to determine the thermal properties of fluids over a wide range of Prandtl numbers. The solution is verified using experimentation and a fluid (deionized water) with known volumetric heat capacity. The second part of this study uses the correlation to establish an accurate and repeatable procedure to determine the thermal conductivity/diffusivity of fluids with unknown volumetric heat capacity.

 

Hot Disk Sensor
Fig (a) - Apparatus used for comparing numerical and experimental transient thermal profiles across the transient hot disk sensor Fig (b) - Hot Disk Sensor