In this publication, we report on a robust and scalable process flow for fabrication of valves, compatible with other microfluidic components using silicon microfabrication techniques. The valves require an abrupt change in both the depth and width of a rectangular microchannel for proper functioning. We report on the process that allow for a very sharp interface between the two areas of the channel. A non-sharp interface will results in leaky valves. We show the variation in silicon etch depth as function of feature size. For small features (3–10)μm there is roughly a 50 percent increase in etch depth compared to 40μm features, while features above 40 μm show minimal differences in depth. For the formation of valves, this provides important design rules allowing one to make structures with precise dimensions. Initial characterization, demonstrating the functionality of the fabricated valves is also be presented.

To analyze the interaction between the turbulent flow structure with the cavitation shedding dynamics, a three-dimensional unsteady cavitating turbulent flow around the three-dimension NACA009 hydrofoil is investigated in this study. The cavitating flow in has been modeled with a homogeneous mixture of liquid and vapor using LES. The interaction between the cavitation and the fluid vortex is analyzed and discussed. The results demonstrate that the vortex stretching is mainly in the center of the cloud cavity and changes quasiperiodically as the cloud cavity evolves. As a result, the mechanism of the inception of cavitation, re-entrant jet and cavitation cloud shedding are accurately captured and predicted by LES in accordance with the experiment data.

Capillary flow traversing a backward facing step (BFS) in a microchannel at low Capillary and Weber numbers is investigated in detail using analytical, numerical and experimental techniques. The BFS's under study included both open surface, where a free surface is formed at the top to the channel, and closed surface, where a lid with a different contact angle than the base material is used. An analytical model valid for both geometries was derived to determine the capillary pressure as a function of the liquidgas interface position as it traverses the BFS. The model was validated against two different numerical simulation techniques: (1) surface energy minimization of the meniscus shape and (2) CFD simulation using the volume of fluid method. Comparison between the simulations and analytically derived model for a range of aspect ratios (0.5-3) and contact angles of the base material (60 °-80 °) revealed that the analytical model works best at high contact angles (> 70 °) and high aspect ratios (> 2). Furthermore, an analytically derived geometric condition required for spontaneous capillary flow over a BFS was developed. To validate the flow condition, experimental measurements were performed on microchannels with BFSs fabricated in silicon using deep reactive ion etching with aspect ratios ranging from 1.5 to 3.8. The contact angle of surfactant/water solutions ranged from 50 °to 85 °on the silanetreated silicon surfaces and from 93 °to 100 °on the PDMS top surface for the closed structure experiments. The experimental results were in good agreement with the analytically derived condition. The developed model is an enabling tool for designers of capillary-driven microfluidic systems.

The paper handles the subject of the modelling and simulation of the flow inside a centrifugal pump through non-cavitating and cavitating conditions. Operating under cavitation state is so perilous to a pump and can considerably reduce its lifetime service. Hence, to provide highly reliable pumps, it is essential to comprehend the inner flow of pumps. The investigated centrifugal pump comprises five backward curved-bladed impeller running at 900 rpm. The modelling process started with an unsteady numerical analysis under non-cavitating conditions to validate the numerical model and the solver comparing with the available testing data. Due to high Reynolds numbers, turbulence effects have been taken into account by unsteady RANS methods using an SST-SAS turbulence model. The obtained pump performances were numerically compared with the experimental ones, and the outcome shows an acceptable agreement between both. The temporal distribution of the internal flow parameters such as pressure and velocity was then studied. Furthermore, basic investigations of cavitating flow around 3D NACA66-MOD profile using a recently developed and validate cavitation model was established. The verification of the numerical simulation validity was based on comparing calculated and experimental results and presented good agreement. Finally, a 3D simulation of the inception of the cavitating pocket inside the centrifugal pump is performed to analyze the impact of the cavitation in the decrease of the head and efficiency.