Simulating transport in and entrainment form nonwoven fibrous, knitted, and opne-cell foam filters

The movement and re-entrainment of liquid droplets from three different filter media, namely fibrous, knitted, and open-cell foam was investigated numerically using computational fluid dynamics (CFD). A range of face velocities were considered which resulted in a range of oil transform rates and steady state saturation levels. It will be shown that liquid volume fraction in filters depends on velocity and time. The minimum velocity required for detachment of droplets was also identified. The main purpose of this research is to investigate the behaviour of pre saturated oil-mist filters in different geometric configuration and also different air flow conditions. In this study, all the produced filter geometries have a packing density (solidity) of 2 % with fiber/element diameter of 9 μm and overall dimensions of: 2mm (z), 0.5 mm (x), and 0.5 mm (y). In order to capture the gas-liquid interface, the Volume of Fluid (VOF) method will be applied. To perform the simulations, the open source computational fluid dynamics (CFD) toolbox, OpenFOAM is used. To verify the accuracy of computations, the calculation of clean pressure drop is compared against well-established pressure approximation in the literature. This work has examined the movement and re-entrainment of droplets in fibrous, knitted, and open-cell foam media with a range of different face velocities. It was found that by increasing in velocity and time, liquid volume fraction in the filters reduced though re-entrainment once a threshold of 2 m/s in all three cases. Furthermore, it has been shown that knitted media produced largest re-entrainment and the fibrous media the least. It is worth mentioning that other factors such as saturation, initial droplet position, temperature may play an important role in re-entrainment form filter which are not investigated in this study. It is important to note however that these results would need to be validated in real media. The large drops entrained from knitted media would be advantageous in many cases as they would readily settle under gravity

Optimisation of wet pressure drop in nonwoven fibrous, knitted, and open-cell foam filters

This work investigates the relationship between wet (multi-phase) pressure drop, saturation and filter face velocity in mist (or coalescing) filters in order to evaluate optimum operating conditions. Three different structural types of oleophobic filter media (fibrous, knitted and open-cell foam), with equivalent clean filtration efficiency and pressure drop, were investigated numerically over a wide range of inlet velocities (0.05–2 m·s-1). The numerical results indicate the balance between saturation/flow resistance and pressure drop of the filter media with increasing velocity. Experiments were also conducted to validate the trends found in simulations. Both the numerical and experimental results show a local optimum in the pressure drop profile, due to flow regime transition. This effect has not previously been documented due to a lack of parametric studies in this region. This region may be an optimal operating regime for some applications