Development and Applications of Micro Thermal Sensor for Measuring Liquid Level, Concentration, and Thermal Conductivity

Abstract

This dissertation focuses on development and applications of a micro thermal sensor for measuring the level, concentration, and thermal conductivity of the liquid sample. This work is divided into three parts, including the design and development of the sensor and two applications of the sensor.Firstly, a novel micro thermal sensor under harsh conditions, such as inside a vapor compressor, was designed and developed. The sensor employs an ac (alternating current) thermal technique with a single heating/sensing element. It has a simple structure consisting of a metallic thin-film heater on a glass substrate and thus it is compact enough to be installed in a small space. As the sensing scheme is based on the so-called three omega method, the sensing signal is noise-resistant and hardly affected by flow in the liquid being measured. Experiments with DI water, ethanol and ethylene glycol confirm that the sensor performance is satisfactory under atmospheric pressure. The effect of the rotating flow by a magnetic stirrer and the internal flow in a circular tube on the sensor signals was examined theoretically and experimentally.Secondly, the developed sensor was tested in a pressure vessel containing R410A and PVE oil to measure the oil level and concentration under high temperatures and pressures. The transient characteristics and sensitivity of the oil level sensing were analyzed and demonstrated to be appropriate for practical usage. The sensor acquires the thermal response of the mixture to ac heating and then the concentration can be determined using one of two different methods. In the first method, the thermal conductivity of the mixture is obtained using the three-omega method and the concentration is determined using the correlation between the mixture concentration and thermal conductivity. In the alternative fast-detection method, the concentration is determined by directly calibrating the sensor output signal to the mixture concentration. The uncertainty of the oil concentration measurement was estimated to be 5.8 ~ 7.3 wt.%, depending on the sensing scheme.Thirdly, the developed sensor was applied to measure the thermal conductivity of the small volume of the nanofluids. The effect of the KrF excimer laser irradiation (248 nm) on the aqueous suspensions of multiwalled carbon nanotubes (MWCNTs) was experimentally examined for the first time. Transmission electron microscopy (TEM) images of the nanotubes after the laser irradiation indicated obvious fracture and disentanglement of them. The threshold laser fluence to affect the thermal conductivity and viscosity of the suspensions was about 50 mJ•cm-2. As the irradiation time progressed at a laser fluence of 144 mJ•cm-2, both the thermal conductivity and viscosity decreased and ended up being saturated. The thermal conductivity enhancement decreased from 16 % to 5 % and the low shear viscosity decreased dramatically to 200 times smaller value than that of the unirradiated. The Raman spectra and TEM images showed that the defects of the nanotubes increased by the laser irradiation. In conclusion, the excimer laser irradiation on the suspension of the carbon nanotubes was found to be an effective way of adjusting the heat transfer and rheological characteristics.