Experimental And Numerical Simulation Study On Bubble Behavior In Passive Dmfc

The performance of Passive direct methanol fuel cell(DMFC) is greatly influenced by efficiency of the removal of the carbon diocxide from the catalyst layer/diffusion layer and the anode channel. Thus, the study of gas-liquid two-phase flow behavior in the anodic channel is necessary for the optimization of the passive DMFC.The research was carried out by using experimental methode and simulation.In the experiments part, visualization methods are used to study bubble behaviors which can be described by detaching time and detaching diameter. The effects of the orifice submergence, gas flux, gas nozzle size, liquid concentration were investigated. Meanwhile, the process of the contact angle during the bubble growth and the path of bubble rising were recorded by using CCD camer. Bubble behaviors were characterized by bubble centre of gravity, bubble endpoint, bubble equivalent diameter, bubble inclination angle, bubble/wall contact diameter and bubble detachment time. Effects of buoyancy and surface tension on bubble behaviors were investigated. Use the bubble diameter and contact ring diameter calculation the force, analysis bubble detachment mechanism. Also examine the bubble growth diameter, shape shifting and the rate of bubble rise with inclination angle change, to find the influential factors to bubble departure. In the simulation, FLUENT was used to simulate the effect of carbon dioxide bubble behaviors in the anode flow field of DMFC by changing the interior stuucture of the anode channels, bubble behaviors were characterized by bubbles detachment time, detachment diameter and contact ring diameter, the aperture size, inclination diffusion layer orifice, wettability and gas flow rate. Research the ring diameter bubble contact with diffusion layer, bubble shape and the liquid flow field around bubble, compared with experiment content and the former research results, for further verification. The mainly results by using high speed camera through visualization method are as follows:(1) At the beginning, buoyancy have a little effect on formation of bubble, the bubbles grow in approximate spherical shape due to surface tension; During the growth of bubble, the results show that portrait of bubble is elonged under the force of buoyancy and surface tension, at the same time, bubble tail is cut under the shear force of liquid, and then departure. Gas velocity is lesser, bubbles initial diameter is small; the bubble diameter of initial formation increases whth increasing gas velocity, before bubble departure, the gas-liquid interface formation a shape similar to "neck" structure, with gas keeps into, the gas-liquid interface formed a small bubble. The former stage bubble through the interface structure contact with orifice, along with the later bubble growth, happened with once fusion or multiple fusion between bubbles, bubble detachment diameter larger after its formation. Fusion process makes bubble unstable, gas-liquid interface produce fluctuation, appear square, mushroom, elliptic and cap shape, etc. With positions of bubble height, cann’t fusion with the tiny bubbles, volatilita and rise in solution finally.(2) When the orifice is turned upside down, bubbles will slide on the surface of diffusion layer, critical Angle decrease with increase in methanol concentration, sliding velocity increases with Angle. Surface tension and viscosity influence the bubble formation diameter, bubble diameter decrease with increase in methanol concentration. In the same condition, the bubble diameter from double orifice is bigger than the bubble detach from single orifice, detachment time is longer than bubble from single orifice. The process of bubble detach from vertical diffusion layer surface, when gas into1M methanol solution, bubbles on the diffusion layer surface sliding upward, rise velocity is0.075m/s. when concentration of methanol is5M, bubbles detach from the diffusion layer into the solution upward, spiral, hit the wall and compressed, rebound and then detach from the wall. The force down the two sides of bubble become uneven in solution, bubble move on path as "glyph" or "spiral" rise, in this process bubble shape is changing with bubble rise velocity. Bubbles that produced at the same orifice, will be affected by the liquid flow field, whtch generated by the former stage bubble, so bubble diameter changing with detachment order constantly.FLUENT was used to simulate the formation of bubbles under various conditions, the results are shown as follows.(3) The process of bubble formation, internal pressure first decreases rapidly, and then slows down. Along with bubble growth, internal gas maximum velocity vector is less than the gas into velocity, the moment of bubble departure, gas max-velocity vector first increase rapidly and then decrease. The bubble generated from orifice on inclination diffusion layer surface, its formation cycle is shorter than horizontal diffusion layer surface, have little diameter. Under the same gas velocity, bubble diameter increase with orifice diameter, detachment time decrease with increase in orifice diameter. The angle of bubble contact with diffusion layer more close to90°, larger contact area, harder for bubble detach from diffusion layer. Intake velocity can change bubble generation frequency and growth process, when gas velocity lowered, bubbles detach from diffusion layer one by one, separation position lower, detachment diameter smaller; Gas velocity is higher than the former, bubbles are able to merge with gas rate increasing, form a larger bubble. The process of bubble rising the amplitude changes with increasing diameter, little bubble diameter move on a path as "spiral" rise in solution, bubble shape changs with the increasing of bubble diameter. Bubbles that with the same diameter, the oscillation amplitude of gas-liquid interface decrease with increasing in methanol concentration.