Coupling Mechanism Stufy On Two-phase Flow Characteristics And Vibration Effect Of Air-breathing Microfluidic Fuel Cell

With the rapid development of mobile communication technology,portable electronic devices are closely related to the people daily life,and high requirements of the standby time and low-cost in micro portable power supply are proposed due to the development of various high performance electronic devices.Microfluidic fuel cell（MFC）is considered to be an important direction of sustainable development in portable power in the future because of the cleaning product and high efficiency,and it is also an ideal approach for the solution of environmental pollution problems.Moreover,it is capable of fulfilling the requirements of extended operating time and minimisation of power supply,and are comparable to a macroscopic battery system in terms of current density and has promising potential applications.At present,the research of MFC is still in the stage of theoretical analysis.To accelerate the manufacturing and commercialisation of MFC,it is necessary to consider factors that affect real-world cell performance in practical application scene.The portable electronic devices are bumped with the movement of human body activities,the MFC will operate under various disturbing forces,resulting in the fluid crossover.Besides,when organic fuel is used as reductant to participate in electrochemical reaction,the released carbon dioxide（CO₂）will disturb the laminar flow,leading to the convective mixing and fuel penetration.It is,therefore,important to research the impacts of gas-liquid two phase flow and vibration effect on cell characteristics and performances of acidic MFCs for the commercialisation purpose.Nevertheless,literature correlated with vibration and two-phase research in regard to MFC analysis remains sparse,which cannot provide insightful guidance for performance improvement and structural optimisation.To deepen the realisation of the cleaner energy conversion technology and accelerate its practical application,this study aims to further investigate the coupling effects of two-phase flow and vibration on the fluid flow and mass transfer characteristics of MFC.The numerical calculation models of single-phase and two-phase MFCs,which the former ignore the gas existence and the latter assumes the gas presence,were developed in the study.The main research contents include:（1）The three-dimensional single-phase computational models of two types of MFCs（flow-over and flow-through）are developed.A comprehensive parametric analysis which includes the design parameters and gravity effect is conducted,and a sensitivity analysis is performed to evaluate the influence of the main indicators on the cell performance.（2）A novel two-phase air-breathing MFC model was developed,the processes of CO₂bubble growth,separation,and migration were simulated under the different operation parameters via the phase field method,and the cell performance responses were predicted at different time.（3）The novel two-phase computational model was developed for the MFC employing the flow-over electrode based on Euler-Euler model,while considering the vibration effect.Effect of vibration on the fluid flow characteristic was simulated,and the influences of vibration effects on the cell performances and two-phase flow were analysed in detail under the different vibration and operation parameters.（4）A vibration and two-phase flow coupling computational model is developed for the MFC which employs a flow-through porous anode to further evaluate the coupling effects of vibration and two-phase flow on cell characteristics under different work conditions.The normalised sensitivity analysis is conducted to determine the contribution of influence factors to the main outputs.The major findings of this study are summarised as follows:（1）Gravity had a significant effect on the flow field and fuel concentration distribution in the flow-over MFC,increasing the feed liquid flow rate can reduce the interference of gravity effect on the velocity domain of the inlet section.The current and power outputs of the flow-over MFC first increased with the angle increasing from 0°to 30°,and then decreased as the angle continues to increase beyond 30°.The flow-through MFC can resist the gravity effects compared with the flow-over MFC,the favorable performance stability was remained at different angles as well.（2）The CO₂ bubbles generated by the oxidation reaction reduced the anode activation reaction area,hindered the fuel transfer,increased the ohmic internal resistance,and decreased electrochemical reaction rate,resulting in the cell performance degeneration.Increasing the contact angle contributed to a reduction in the gaseous volume fraction,accelerated the gas diffusion,and provided active reaction sites for the electrochemical reaction in time.（3）The cell performance was greatly affected by the vibration effect.The aggravated vibration intensity and frequency lead to a negative effect comprising a critical fuel crossover and delayed gaseous discharge,moreover,the proportion of parasitic current density in the current output was increased,leading to the cell performance deterioration.（4）Increased feed liquid flow rates played an important role in the resisting vibration effect,alleviating fuel crossover,and accelerating gas phase discharge;however,these were achieved at the sacrifices of fuel utilisation and exergy efficiency.The optimal supply of the fuel flow rate s and fuel concentration for the MFC was a trade-off between enhanced cell performance and sacrificial fuel utilisation.（5）The fuel utilisation was closely related to the current density,while the exergy efficiency was proportionate to the power output.Increasing the electrochemical reaction rate is an effective approach to improve the fuel conversion efficiency and promote the sustainable development of MFC.（6）The MFC employing a flow-through porous anode,comparing with the MFC using a flow-over anode,can resist the effects of vibration to a certain extent,and operate effectively in an emergency braking situation with a vibration acceleration of 6.0 m s^-2,while the cell failure generated in the flow-over MFC when the vibration acceleration exceeded 0.05 m s^-2 in the flow-over MFC.