Simulation Of Dynamic Behaviors Of Emerging Gas Bubbles In A Mini-channel With Liquid Flow

In recent years, as the demand of the efficient and clean energy utilization is ever-increasing, various types of fuel cells have been significantly put forward. In such type of fuel cells, many complicated two-phase flow phenomena usually occur and affect the reactor performance. In particular, as the sizes of the flow channel shrink to micrometer scale, the size effect on the two-phase flow in the microchannel becomes more dramatic and exhibits its distinctive behavior at microscale that is different from that at macroscale. Taking direct methanol fuel cells (DMFCs) as an example, gases generated by the anodic electrochemical reaction easily block the flow channels in the anode and pores of diffusion layer, which will increase the resistance of mass transfer and then influence the performance of a DMFC. Therefore, in order to develop high-efficiency and compact continuous-flow reactor, it is necessary to study the effect of air bubble on the two-phase in microchannels, and the bubble behaviors with different designs and operating conditions as well as its influence on the flow and mass transfer. The outcome will help to optimize the reactor design and to improve the performance of the reactor.Currently, existing researches have been focused on the gas bubble behavior in the statistic liquid phase of large space or vertical pipeline. Research about the bubble in the mini-channel is limited, especially the case of a gas bubble emerging from pores of the permeable wall into a liquid flow mini-channel. To meet the need of the practical application, in this work, we employ the VOF (volume of fluid) method to simulate the dynamic behaviors of a single bubble and bubbles emergence in the mini-channel with the liquid flow, where the PLIC (Piecewise Linear Interface) method and continue surface force model are involved. With this method, The characteristics of gas bubble growth, deformation, detachment and movement, the interaction between the bubbles, bubble coalesce, and variation of the pressure difference in the mini-channel are explored. The effects of gas evolution rate, flow rate of the liquid phase, size and shape of the mini-channel along with pore size and its distribution on the dynamic behaviors of gas bubbles are obtained.Main conclusions of this thesis are summarized as follows:①The wettability of the channel bottom play an important role in the bubble growth and detachment. The highly-hydrophilic surface is beneficial for the bubble detachment, whereas the bottom with the hydrophobicity tends to make bubbles adhere to the bottom. The gas coverage increases with increasing the contact angle, but the variation trend for the dimensionless flow resistance coefficient reverses.②Increasing the gas flow rate facilitates the gas bubble detachment. In the case of the contact angle of the bottom to be 90o, when Reg is small, the departure volume of the gas bubbles increases with the gas flow rate. However, once Reg is over 14, further increases of the gas flow rate show little effect on the departure volume, gas coverage and flow resistance coefficient. Such phenomena are not found when the surface of the bottom is highly hydrophilic.③When the bottom is strongly-hydrophilic surface, the mixed wettability shows ignorable influence on bubble dynamic. For the remaining cases, the mixed wettability will dramatically affect bubble movement, which not only influences the increment speed of gas coverage but also causes the vibration of the pressure drop between the inlet and outlet.④Maintaining the other conditions, it is shown that the detachment time is the shortest and the detachment volume is the smallest when the pore is set at the side wall. For the case of the pore at the top of the channel, the detachment time is the longest and the detachment volume is the largest. The detachment time, detachment volume and removal time will increase and the gas bubble saturation ratio and flow resistance coefficient will decrease as the channel height increases. With respect to the aspect ratio, the detachment time and detachment volume will first increase and then decrease with decreasing the aspect ratio. Besides, the variation in the shapes of the mini-channels will cause significant influence on the detachment time, detachment volume and dimensionless friction coefficient.⑤It is found that the shape and position of the two adjacent pores will influence the gas coverage, gas void fraction and flow resistance coefficient. When the bottom contact angle is 90o and Dx=0.8mm, the orifice pore will facilitate the gas bubble expel from the channel. The pores with tandem arrangement will induce fewer blockages than those with parallel arrangement.⑥Gas bubble emergence from the porous layer largely depends on the resistance resulting from the pores. It is shown that gas bubbles firstly emerge into the mini-channel from the pores with the smallest flow resistance that is dependant of the pore size and position.