High-resolution measurements of thermal diffusivity of water-alcohol mixtures using a photopyroelectric thermal-wave cavity.

摘要

A sensitive ultrahigh-resolution thermal-wave cavity technique for thermal diffusivity measurements of a series of water-alcohol mixtures in the entire range of concentrations including the very low end is introduced. A new design of the cavity measurement cell with fixed dimensions, operating in a frequency scan mode eliminates instrumental errors and substantially improves the precision and accuracy of the measurements, allowing measurements of a series of water mixtures. Thermal diffusivities of water-methanol and water-ethanol mixtures were measured with two techniques: conventional single-pulse photopyroelectric frequency scans and the common-mode-rejection demodulation dual-pulse scheme. A theoretical model describing conduction and radiation heat transfer in the cavity was developed. The model predictions and the frequency-scan experimental data were compared, showing excellent agreement. The fitted thermal diffusivity and effective emissivity versus concentration results of the mixtures were compared to literature theoretical and experimental data. The frequency-scan measurements showed the capability to distinguish between water-alcohol mixtures having a 0.5% v/v difference in concentration. The common-mode-rejection demodulation method improved this result up to the level of 0.2% by volume, which is the highest thermophysical resolution of water-methanol and water-ethanol mixtures reported to date, to our best knowledge. The ultra-high sensitivity of the method can be especially useful in environmental applications, specifically in real-time water pollution monitoring.