Film thickness effects on calibrations of a narrowband thermochromic liquid crystal

Abstract

Thermochromic liquid crystals (TLCs) have been widely employed by researchers in heat transfer and fluid flow communities as a reliable and non-intrusive temperature measurement tool due to their unique optical properties such as birefringence, optical activity, circular dichroism and selective reflection of colours in the visible spectrum as function of temperature. The use of narrowband TLCs are attractive for temperature and heat transfer measurements due to their higher precision in temperature measurements and due to the fact that narrowband TLCs are less affected by variations in illumination-viewing angles and illumination disturbances. Narrowband TLCs have been used with full intensity-matching methods to provide robust image processing for measurements of thermal parameters in transient heat transfer tests. Calibration of narrowband TLCs is necessary in order to obtain the intensity-temperature relationship of the TLCs. Film thickness is one of the factors which affects calibrations of TLCs. In this research, film thicknesses of 10, 20, 30, 40 and 50 μm were investigated on green intensity-based calibrations of R35C1W TLC during heating and cooling. Results showed an increase in magnitude of peak green intensity with increasing film thickness, with a percentage increase of nearly 18% when film thickness increased from 10 to 50 μm. Results also showed an inconsistent shift in temperature at which peak green intensity occurs, with a maximum shift of 0.40 °C, suggesting that film thickness effects may be insignificant for narrowband TLCs compared with wideband TLCs. A theoretical method for estimating the volume of TLC coating required to achieve a desired film thickness has also been described in this paper, based on the surface area and dry solids content of the TLC. The method is easily implemented and applicable for sprayable TLC coatings.

Liquid Crystal Thermography Measurement Uncertainty Analysis and Its Application to Turbulent Heat Transfer Measurements

“…Rao and Zang [2], Camci et al [5], Farina et al [6], Behle et al [7], Sabatino et al [8], and Chan et al [9] studied experimentally the effects of the lighting angles and viewing angles on the hue-temperature calibration curves of the TLCs. Roth and Anderson [10], Wiberg and Lior [11], Abdullah et al [12], and Rao et al [13] studied experimentally the effects of the coating thickness on the TLC hue-temperature calibration curves. These previous researches only indicated that the lighting angle and the coating thickness can significantly affect the TLC hue-temperature calibration curves.…”

Heat transfer performance of multiple holes impingement cooling technique

“…The temperature was measured based on the observation of the thermochromic liquid crystal (TLC) color play on the target plate and fast response Ttype thermocouple data which gained from the data acquisition system. The TLC was calibrated first before been used in the experiment using the method stated by Abdullah et al [10]. The result acquired from the color changes of the TLC were captured by digital video camera.…”

Use of vapochromic crystals to measure the concentration of a gaseous throughflow at the solid-fluid interface

“… Film thickness -Abdullah et al [7] found that the green intensity calibration curve for their narrowband TLC shifted by 0.4°C when increasing film thickness from 10μm to 50μm; they also observed an 18% increase in peak green intensity.  Ageing -Kakade et al [8] repeatedly heated narrowband TLC through their colour change limit.…”