Numerical Modelling Of Flow And Transport Characteristics In Microfluidic Fuel Cells With Fuel Micro-jet

The rapid development of portable mobile electronic devices requires high-performance micro power sources.Membraneless microfluidic fuel cell（MMFC）exploits co-laminar flow at low Reynolds number to separate the fuel and oxidant without using the solid ion-exchange membrane.In addition,the MMFC has shown unique advantages about simple structure,low cost,high energy density,and variable configuration.Thus,the MMFC has been regarded as a research focus in the energy field.Previous researches have made considerable progresses in electrode materials,catalyst preparation,cell configuration,and optimal operating conditions.However,there are still some challenges such as low cell performance and low fuel utilization.Therefore,from the perspective of engineering thermophysics,the construction of a new type high-performance microfluidic fuel cell with strengthening of mass transport as the guiding ideology is of great significance to the promotion of microfluidic fuel cell.To overcome these challenges,the present work considered novel microfluidic fuel cells with fuel micro-jet,which used the fuel micro-jet to convectively replenish the fuel concentration boundary layer over the anode.Multiphysics modelling were performed to 1)characterize the fuel flow and transport and electrochemical reaction in a 3D single-phase model for the cells with alkaline electrolyte;2)investigate the gas-liquid two-phase flow coupled with electrochemical reactions in a 3D two-phase model for the cells with acidic electrolyte.In addition,to better understand the dynamic behavior of CO₂ bubbles in microfluidic fuel cells,especially in operation conditions with micro-jet,a 2D two-phase numerical model was developed using the phase field theory to capture the gas-liquid interface during transients.The effects of micro-jet on the growth,deformation,and detachment of CO₂ bubbles were studied.The influence of wall wettability on the bubble growth dynamics was discussed.The main conclusions of this study are as follows:（1）A three-dimensional single-phase numerical model was developed for the microfluidic fuel cell with fuel micro-jet in alkaline conditions.The modelling results showed that as compared with the microfluidic fuel cell with flow-over anode,the directional vertical injection of the micro-jet can replenish the concentration boundary layer on the anode to enhance local fuel transport convectively.As a result,the cell performance can be improved by about 30%.In addition,the cell performance can be improved by increasing either the fuel flow rate or fuel concentration.At constant total fuel flux,there was an optimal ratio（60:140）for the fuel flow rate fed into the micro-jet and microchannel to achieve best cell performance.Relatively high cell performance and fuel utilization can be simultaneously achieved by increasing the fuel concentration while reducing flow rate.Moreover,the geometrical parameters of micro-jet were numerically studied.Replenishment to the fuel concentration boundary layer can be enhanced when the micro-jet opening was located in the middle and downstream of the cell.Also,the cell performance can be improved when the number of micro-jet openings was increased to 2 since the affected electrode area was extended.（2）A three-dimensional two-phase computational model was developed to research the gas-liquid two-phase flow coupled with electrochemical reaction for the microfluidic fuel cell with micro-jet in acidic electrolyte.It was found that the effective fuel diffusivity and effective electrolyte conductivity on the anode surface were reduced because of the existence of gas phase occupied some volume fraction.The modelling results suggested that conventional single-phase assumptions（and models）could overestimate the cell performance as the two-phase effects were neglected.The fuel micro-jet structure can not only enhance fuel transfer but also drive the carbon dioxide to the channel corners,thereby reducing fuel transport resistance.In addition,increased flow rate can accelerate bubble discharge,effectively reducing the gas volume fraction in the cell.Although the cell performance can be improved by increasing the fuel concentration,the gas volume fraction increased accordingly.（3）A two-dimensional two-phase computational model was developed using the phase field theory to investigate the dynamic behavior of CO₂ bubbles in the microchannel.The results showed that the buoyancy force was much smaller than the surface tension for CO₂ bubbles in the microchannel of microfluidic fuel cell with flow over anode,letting the bubbles to adhere to the bottom wall,which could decrease electrode reaction area in cell operations.By contrast,for the bubbles in the microchannel with micro-jet,the disturbance of high-speed micro-jet flow could enhance bubbles detachment,reducing the coverage on the electrode surface.In addition,it was found that the super-hydrophilic surface can reduce the detachment time and the diameter of detached bubbles.