Impact of network structure and fluid properties on two-phase flows in complex networks
Immiscible two-phase flows in porous media is of great importance to many fields, such as water management in fuel cells, micro-reactors, electronic chips, microbubbles-aided drug delivery, oil recovery, CO2 sequestration, and underground water remediation. The transport of two-phase flows in the porous media directly affects the system performance, heat and mass transfer efficiency. Resistance to a two-phase flow is normally regarded as the criterion to evaluate the transport performance of two-phase flows, and it is measured by the pressure drop for fluids flowing through the system. This PhD thesis experimentally and theoretically investigates the effect of two-phase interfaces on pressure resistance in both constricted microchannels and complex capillary networks, the effect of fluid properties, network structure and channel geometry on fluid transport and the pressure drop, evaluates the pressure drop required for dislodging a bubble from the complex capillary network and estimates the pressure distribution in a complex capillary network. Two-phase interfaces play a significant role for two-phase flow in porous media, especially when the porous media possess pore with a diameter less than “the effective pore throat”, irregular pore shape, complex flow path structure, and interconnected channels, etc. Investigations of the effect of interface on the pressure drop in constricted microchannels (i.e. microchannel with a gradually decreasing pore size) indicate that the capillary force applied to a two-phase flow is mainly due to the interface. If a flow channel with a diameter is larger than the “effective pore throat”, the capillary resistance to the interface, or to the two-phase flow, is almost zero. If the flow channel has a pore size less than the “effective pore throat”, capillary force to two-phase interfaces takes significant effect, and the resistance to two-phase flows increases suddenly when the two-phase interface reaches the effective pore throat. The ‘effective pore throat’ is between 150 to 650 microns depending on capillary tip size and it is very different from the geometrical throat of a channel. To predict the pressure drop for a two-phase flow in constricted capillaries, a new equation has been derived based on Darcy-Weisbach equation to calculate the frictional pressure drop in constricted capillaries. The effect of the capillary tip diameter, capillary gradient, surface tension, viscosity, gas type, and contact angle on the effective pore throat of constricted capillaries has been studied in detail. Experimental measurement indicates that effective pore throat depends on fluid surface tension and the capillary geometry, but not on liquid viscosity. The higher the fluid surface tension, the larger the diameter of the effective pore throat. A channel with a large tip diameter or gradient will give a large effective pore throat diameter. Fluid viscosity only affects the magnitude of the resistant pressure drops of fluid flows in constricted capillaries, but does not affect the effective pore throat diameter. The effective pore throat and the pressure profile measured in this study can be explained by the pore contact angle, but cannot be explained by the contact angle measured on a flat surface of the same materials. Resistance to two-phase interface in complex capillary network has been investigated further by measuring the pressure drop required to dislodge a bubble (i.e. dislodging pressure) from microfluidic networks with multiple bifurcations. More than 500 individual experiments have been conducted to quantitatively characterize the factors affecting the dislodging pressure from a lab-on-a-chip network. The experimental results indicate that the dislodging pressure is determined by bubble length, channel dimensions, bifurcation, bifurcating angle, surface tension and fluid viscosity. Based on the experimental results, the network structure is a dominant factor. The dislodging pressure increases with the increase in network complexity. The effect of the network structure, bubble position, proximal channels on the bubble dislodgement in microfluidics networks has been further studied by employing networks with one bifurcation. The results indicate that bifurcations, multiple channels, and curvature of channels, all affect bubble dislodging pressure and the pressure distribution in capillary networks. A parameter cj is used to characterize how the overall pressure applied to the system distributes to an individual channel. The cj value is smaller for the channel with complicated surrounding network, such as multiple bifurcations and multiple microchannels with varied diameters. The cj value increases with bifurcating angle of microchannel j, and a high bifurcating angle results in a decrease of cj value in the proximal microchannel. Theoretical investigation on resistance to two-phase interfaces has been conducted in both constricted microchannels and complex capillary network. Theoretical equations have been derived based on Darcy-Weisbach equation to predict the pressure drop in the constricted structure of microchannel. Combined with homogenous flow model and separated flow model, our newly-derived equation can be used to predict the pressure drop for two-phase flows in constricted microchannels with the deviation of below ±20%. For complex capillary network, a theoretical equation has been derived, and it indicates that the bubble dislodging pressure is the function of bubble length, channel dimension, and network structure. The equation theoretically agrees well with the experimental results. The effect of network structure on the pressure drop was characterized by introducing the parameter, cj. The analysis of model parameters NBj and MAj shows that parameter cj, rather than the channel size, dominates the dislodging pressure for bubbles with a length greater than 2 mm, and the increase rate of the dislodging pressure is significantly affected by both channel size and parameter cj.