Integration of liquid film thickness microsensors, thermocouples and thin film heaters for annular flow investigations in microchannels

Abstract

The liquid film thickness (LFT) is a key parameter to characterize annular flows during flow boiling and condensation. Heat transfer rates, pressure drop and wall dryout are highly influenced by liquid film characteristics; thus, this parameter must be correctly estimated for a solid and reliable design of heat exchangers, heat sinks and thermal management devices based on phase-change processes. Therefore, phenomenological models usually include LFT estimation methods to calculate the heat transfer coefficient (HTC), the interfacial friction factor (fi) and the minimum thickness below which dryout occurs. In microchannels, reliable data are scarce due to intrinsic difficulties related to small scale measurements, in general, correlations are based on results obtained under adiabatic air-water flows and speculations about the behavior of the film, predicting only its mean value. Consequently, these models are only suitable to the conditions for which their parameters were adjusted. In this context, a microsensor based on electrical conductance is under development to be applied to LFT measurements for the annular flow pattern in microchannel under diabatic conditions close to wall dryout. During the proposed internship in the Micro Energy System Laboratory at the University of Tokyo, the LFT sensor will be integrated with heaters and thermocouples in the test section and flow boiling experiments will be carried out. The student will be trained to use the lab's facility and build his own devices using MEMS processes, resulting in low thermal inertia for the thermocouples allowing fast-response temperature measurements. The test section will be brought to Brazil for further testing. It is expected to gather unique experimental data with this device regarding both LFT and temperature oscillations that can be incorporated in heat transfer models to improve their accuracy. (AU)