Capillary rise in porous fibrous media during liquid immersion

The modified Washburn equation has been further developed in this work, to examine capillary rise in porous media which are being immersed in liquid at a continuous rate. The capillary rise rate and/or properties of the media can be determined by a series of differential equations which govern the processes. The model was applied to oleophillic and oleophobic porous, fibrous media with generally good results, however it was found that the model needed to be fitted separately to the initial rise phase and to the longer-term (near-constant slope) phase. It was also found that a good agreement between the theory and experiment could be found if the porous media was contained inside a glass tube so that inflow could only occur from below and not from the sides of the filter. In order to model the insertion of media without this tube, a combination of a horizontal and a vertical capillary model was needed. The model could not be easily fitted to very thin, oleophillic media, and reasons for this are discussed; one likely reason being the inflow of liquid from the sides of the media. However, good agreement between the model and experiment could be found for equivalent media with an oleophobic coating and the same oil

Influence of flow-interruption on filter performance during the filtration of liquid aerosols by fibrous filters

This study investigated the influence of flow interruptions on the filtration performance of two different multi-layered fibrous filters during liquid aerosol filtration. It was found that both types of filters experienced a significant secondary loading stage, though they had reached a steady state for continuous flow. The filters showed distinctive increases in pressure drop and filter saturation until a second equilibrium state was reached. This second equilibrium state was attributed to a rearrangement of liquid in the filter during the breaks, clogging previously free passages. The ratio of shear and capillary forces was found to determine whether these passages were able to be “cleaned” once airflow was recommenced. Based on these findings, the airflow required to clean fully saturated filters was investigated and a phenomenological model developed to describe this behaviour. Furthermore, it was found that filters could be cleaned and reused, whereby they would return to one of the previous steady states (continuous or discontinuous flow). The experiments conducted in this work represent a more realistic test for oil-mist (or coalescing) filters than typical laboratory testing. Furthermore, it is hoped that they will help to bridge the gap between laboratory and field test results

Modelling the Influence of Filter Structure on Efficiency nd Pressure Drop in Knitted Filters

Fibrous filters are used extensively in a range of applications, including process engineering, automotive filtration and for worker (respiratory) protection. These filters are usually a felted, nonwoven structure of randomly arranged fibres. However, a special class of such filters exists - knitted filters. These filters are advantageous for many applications, as their knitted structure imparts significant mechanical strength. The structure of the fibres in such filters can be described by the classical strophoid equation. There has been relatively little study on the pressure drop and efficiency of such filters. This work has developed a geometric model of a knitted metal filter, by applying the strophoid equation. The geometric model thus allows a range of geometries to be generated, based on the strophoid variables, and also fibre/wire diameter, then the knits layered at a given bulk porosity (packing density), to create a geometry of desired properties. The geometric model outputs can then be coupled with a novel computational fluid dynamics (CFD) model for fibrous filtration (developed by the authors). This then allows, the relationship between the aforementioned structural properties and critical filter properties such as particle capture efficiency and pressure drop to be investigated. This work examined the pressure drop and efficiency of a knitted filter geometry at 3 different packing densities. The CFD results were compared to classical single fibre efficiency theory for conventional fibrous filters. The CFD results showed increased capture efficiency and pressure drop compared to fibrous filter theory

Thermogravimetric analysis of carbon black and engine soot - Towards a more robust oil analysis method

This work examined the thermal behaviour of diesel engine produced soot and commercial carbon black using thermogravimetric analysis (TGA). It was found that during TGA analysis of the carbon matrices (at a temperature range commonly used for soot-in-oil content determination), a gradual mass loss occurred. This was attributed to pyrolysis effects and combustion processes occurring due to poor hydrodynamic design of some commercial thermobalances. This process resulted in a significant mass loss of the carbon during TGA. This finding may strongly effect soot-in-oil analysis conducted using current methods. Experiments were conducted using a range of soot-in-oil mixtures according to the widely used thermogravimetric standard method ASTM 5967-08 which showed a significant underestimation of the soot content in the oil as a result of carbon mass loss due to combustion and/or pyrolysis effects. An improved oil analysis method is proposed which provides a significantly increased accuracy of soot determination in lubricant oils

Comment on "Performance evaluation of 3 optical particle counters with an efficient multimodal calibration method" (Heim et al.,2008) - Performance of improved counter

This comment adds performance data for a modified version of one of the optical particle counters investigated in Heim et al. (2008), namely the WELAS 2100. The new version was found to have a counting efficiency much closer to unity for larger particle sizes as well as some improvement in the lower 50% detection limit

Development and Validation of a Computational Fluid Dynamics (CFD) Solver for Droplet-Fibre Systems

Droplet-fibre interactions are found in many natural and anthropogenic systems. A common industrial example is fibrous filtration - used to capture liquid (e.g. oil) mists. The filters used consist mostly of highly porous arrays of randomly layered fibres. Given the random (complex) nature of these filters, the existing models describing their behaviour are mainly empirical in nature and thus only applicable over a narrow range of parameters and operating conditions. Therefore simulation of these filters using computational fluid dynamics offers a viable alternative to the existing models. In this work we will detail the development of a solver that couples the Lagrangian tracking of particles with a volume-of-fluid (VOF) solver. This solver is built on the existing open-source OpenFOAM CFD libraries, which have been modified to allow the physically accurate modelling of small particles. The solver also models the collection of these particles, where there is a transition from the discrete treatment (as in the Lagrangian tracking of the particles) to the volume-of-fluid treatment. The solver allows the simulation of the motion of small liquid droplets, the capture of these droplets by filter fibres, the coalescence of these captured droplets, into films and the subsequent break up of these films into droplet arrays by Plateau-Rayleigh instability. Also simulated is the movement of these coalesced droplets within the filter, leading to the drainage of oil from the filter.A validation of the fundamental physical mechanisms in the filter was performed, by comparing the simulated conformation of liquid droplets and films on the fibre to Plateau-Rayleigh instability theory. The model showed general agreement with both theory and observations. The simulated capture efficiency was also compared to capture efficiencies predicted by the single fibre efficiency (SFE) theory. A good agreement between the two was found

Comparison of breathing models for determining flow and particle deposition in the lungs

Collection and deposition of particles in the upper airway and lungs is of considerable importance – for example, when studying chronic diseases, or when determining the efficacy of aerosol drug delivery. Modelling of particle deposition usually assumes either constant flow (typically at maximum inspiration), or oscillating flow – ignoring any effects of the lung’s motion. This paper presents a preliminary examination of the effects of ignoring mesh motion when modelling the lungs. Initially, an idealised lung model was created, corresponding to generations 0 to 3 of Weibel’s morphology[14]. Simulations were then made using this geometry for steady flow, oscillating flow, and flow developed by expanding the lung. The expansion of the lung was modelled using a mesh motion library developed by the authors. This model allowed the expansion of the lung to be prescribed. Results from the simulations show significant differences between the three modelling options – relating to both the predicted flow field, and particle deposition sites. Robustness of the moving mesh modelling technique is demonstrated on a high-resolution geometry created from CT scans of a Sprague-Dawley rat model lung

Simulating Plateau-Rayleigh instability and liquid reentrainment in a flow field using a VOF method

Plateau-Rayleigh Instability (PRI) is the well known phenomena of the breakup of a liquid column or cylinder. Such a process is integral to the operation of a range of natural and anthropogenic systems, such as gas-liquid and liquid-liquid separators, fuel cells, the accumulation of dewdrops on spider webs, and many more. Volume Of Fluid (VOF) methods, such as available in OpenFOAM, should be able to accurately resolve PRI in such systems. One such system, in which PRI is integral, is the filtration of oil or water aerosol mists using fibrous filters. In many cases, entrainment (or carryover) of liquid from fibers occurs. The mechanisms behind such entrainment are poorly understood. This work will validate the OpenFOAM VOF against classical PRI theory, both with and without a secondary fluid phase flowing through the system (e.g. air). Furthermore, the work will utilise the validated two-phase VOF solver to examine the phenomena of liquid reentrainment from mist filters

Bloated bodies and broken bricks: power, ecology, and inequality in the political economy of natural disaster recovery

Disaster recovery efforts form an essential component of coping with unforeseen events such as earthquakes, hurricanes, floods, and typhoons, some of which will only become more frequent or severe in the face of accelerated climate change. Most of the time, disaster recovery efforts produce net benefits to society. However, depending on their design and governance, some projects can germinate adverse social, political, and economic outcomes. Drawing from concepts in political economy, political ecology, justice theory, and critical development studies, this study first presents a conceptual typology revolving around four key processes: enclosure, exclusion, encroachment, and entrenchment. Enclosure refers to when disaster recovery transfers public assets into private hands or expand the roles of private actors into the public sphere. Exclusion refers to when disaster recovery limits access to resources or marginalize particular stakeholders in decision-making activities. Encroachment refers to when efforts intrude on biodiversity areas or contribute to other forms of environmental degradation. Entrenchment refers to when disaster recovery aggravates the disempowerment of women and minorities, or worsen concentrations of wealth and income inequality within a community. The study then documents the presence of these four inequitable attributes across four empirical case studies: Hurricane Katrina reconstruction efforts in the United States, recovery efforts for the 2004 tsunami in Thailand, Typhoon Yolanda in the Philippines, and the Canterbury earthquakes in New Zealand. It then offers three policy recommendations for analysts, program managers, and climate researchers at large: spreading risks via insurance, adhering to principles of free prior informed consent, and preventing damage through punitive environmental bonds. The political economy of disaster must be taken into account so that projects can maximize their efficacy and avoid marginalizing those most vulnerable to those very disasters