Fluid transport and entropy production in electrochemical and microchannel droplet flows

Abstract

The growth of energy demand in the world requires addressing the increasing power requirements of industrial and residential consumers. Optimizing the design of new and existing large power producing systems can efficiently increase energy supply to meet the growing demand. Hydrogen as an energy carrier is a promising sustainable way to meet the growing energy demand, while protecting the environment. This thesis investigates the efficient production of hydrogen from the electrolysis of copper chloride, by predicting entropy production as a result of diffusive mass transfer. Also, this thesis investigates the possibility of producing electrical energy from waste heat produced by industrial or other sources. The thermocapillary motion of fluid droplet in a closed rectangular microchannel is used to generate electrical energy from waste heat in a piezoelectric membrane by inducing mechanical deformation as a result of the droplet motion. Modeling, fabrication, and experimental measurement of a micro heat engine (MHE) are investigated in this study. Analytical and experimental results are reported for both circular and rectangular microchannels. A novel fabrication technique using lead zirconate titanate (PZT) as substrate in microfluidic application is presented in this study. This thesis develops a predictive model of the entropy production due to thermal and fluid irreversibilities in the microchannel. Thermocapillary pressure and friction forces are modelled within the droplet, as well as surface tension hysteresis during start-up of the droplet motion. A new analytical model is presented to predict the effect of transient velocity on the voltage production in the MHE. In order to predict the effect of the applied stress on voltage, the different layers of deposition are considered for thin film laminates. The highest efficiency of the system from simulated taking into iv account the electromechanical coupling factor is about 1.6% with a maximum voltage of 1.25mV for the range of displacement considered in this study. In addition, new experimental and analytical results are presented for evaporation and de-pinning of deionised water and toluene droplets in rectangular microchannels fabricated from Su-8 2025 and 2075.