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Non-expert summaryThe use of pressurised air to displace a viscoplastic liquid from a complex duct geometry is simulated using detailed numerical modelling. The transient displacement of Newtonian and viscoplastic liquids by air in cylindrical tubes of finite length with a concentric expansion followed by a contraction in their cross section is considered. The change in diameter is not sudden. Various expansion and contraction ratios are studied. Papanastasiou's formula is employed to regularize the discontinuous Bingham model. Results are presented for a range of fluid and geometrical parameters, and some cases are compared to analytical results.
Non-expert summaryBroad review of the fluid mechanics involved in the cleaning and decontamination of surfaces using liquid flows. Three main phases are reviewed: the contacting phase, which requires the liquid cleanser to reach the soil; the action phase, which studies the various physico-chemical effects of the cleanser on the soil; and the removal phase, where the cleanser transports the soil away from the surface through various phenomena.
Non-expert summaryModelling study of the decontamination of a porous material. An immiscible cleanser react with a contaminant, producing reactants which can be partially soluble in either phases. The diffusive-reactive transport problem is solved taking into account the moving boundary through asymptotic analysis and compared with numerical simulations. It is found that the partition coefficient of the product is more important in determining the removal rate than the reaction rate, as products of the reaction can prevent further reaction if they accumulate at the interface.
Non-expert summaryTheoretical study of the decontamination of a 2D porous material. The decontaminant is immiscible with the contaminant and neutralize it through interfacial reaction. Two scenarios are considered with either the contaminant filling partially the pore space, or fully. This reactive-diffusive problem is solved using a homogenisation technique that separates the local pore-scale dynamics from the macro-scale temporal evolution. The effect of the porous material is accounted for by the model.
Non-expert summaryExperimental study of the permeation of CWA simulants through different layers of porous building materials: brick, cinder block, wall board, wood. The breakthrough time is measured in a specially designed cell. Anisotropy of the permeation is observed for anisotropic material such as wood. Surface decontamination is tested and shown effective.
Non-expert summaryExperimental exploration of the exchange of two immiscible fluids in a wedge. Due to antagonist principal curvature, the more wetting fluid exchange with the less wetting fluid in the wedge, through a capillary instability. The instability develops in the form of fingers pinching into droplets. The less wetting fluid is thus displaced out of the wedge. Measurements are performed for varying geometry and viscosity. Discussion but no conclusion of the physical mechanisms are proposed in light of the more well known viscous fingering instability.
Non-expert summaryImpinging liquid jets are widely used to clean unwanted soil layers from the walls of structures and vessels. This paper investigates what is observed when a coherent, turbulent, water jet impinges normally on a thin layer of an immiscible viscoplastic material. Removal involves the growth of a cleared area (which is circular for a jet impinging normally) bounded by a berm of displaced material. Previously Glover et al. [2016, J. Food Eng.. 178, 95-109] presented a semi-empirical model relating the rate of removal (location of the berm) to the momentum flow rate in the liquid film. The authors present a first-order model for cleaning thin layers of these materials based on the rate of viscous dissipation in a shallow wedge of material at the cleaning front. This yields a result of the form of the Glover et al. model, with expressions linking the kinetic parameters to measurable quantities including the rheology of the soil. The fully coupled problem is not solved: the wedge angle and residual layer thickness need to be specified and in this work they were obtained by fitting to the data. New and existing experimental results are compared with the model for three soft solids immiscible with water: two petroleum jellies and a soft paraffin, which exhibited Bingham plastic behaviour and creep, for jet Reynolds numbers between 10,000-37,000. The ratio of average film depth and layer thickness was in the range 0.1-1.5.
Hydrodynamic systems for assessing surface fouling, soil adherence and cleaning in laboratory installations
Non-expert summaryThis is a short review presenting and comparing five apparatuses that are used in laboratory scale investigations of cleaning: the parallel plate flow cell, the impinging jet, the radial flow cell, the rotating disk and fluid dynamic gauging. These systems are of particular relevance to the study of surface fouling, surface cleaning or adhesion on solid surfaces in laboratory environment. The key features of their hydrodynamics, as well as their practical advantages and drawbacks, are discussed. Examples of applications fields are also listed. A useful introduction to these devices.
Non-expert summaryThis is a relatively early experimental study of mass transfer of a sparingly soluble material from a flat, solid surface when it is exposed to a normally impinging turbulent jet of water. The nozzle Reynolds numbers ranged from 25,000 to 125,000. The surface was coated with trans-cinnamic acid, and thickness profiles were measure over time to determine the local rate of mass transfer. The mass transfer flux is used to calculate the local Sherwood number (dimensionless mass transfer coefficient). In the wall-jet region these were found to be independent of the nozzle to plate distance, and were correlated as Sh = 1.3*Re^0.84*(x/d)^- 1.27. The authors found reasonable agreement with published heat transfer data . The average Sherwood numbers in the impingement region were found to decrease rapidly beyond a transition zone of 6.5 diameters from the nozzle: mass transfer rates are thus weak beyond this zone.
Non-expert summaryThe prediction of flow behaviour in complex geometries representative of industrial and other practical systems often requires the use of computational fluid dynamics (CFD) simulations. Two important pipe geometries are considered in this paper: a sudden and a gradual expansion or contraction. Steady state simulations using the STAR-CD package were used to predict the distribution of the mean shear stress imposed by a turbulent liquid flow from 1 inch to 2 inch cylindrical geometries (Reynolds number in the 1 inch pipe of 50,000), and the fluctuations in the shear stress. These calculated values are compared with estimates based on electrochemical mass transfer measurements. The data sets provide useful benchmarking results for other studies.
Non-expert summarySmall particles may be removed from a surface by the motion of a contact line over the particle or by the passage of a bubble along a channel, which generates a thin film of liquid at the wall of the channel. In both cases the capillary forces induced on the particle can exceed the adhesion force holding the particle on the wall. The origin of this cleaning mechanism is discussed and the removal of bacteria (S. aureus) from the walls of microchannels with square cross section by the passage of bubbles is investigated experimentally. The organisms are not left for long periods to form strong adhesive bonds to the wall.
Non-expert summaryA simple, dynamic model, supported by experiments, is presented for the thinning (removal) of a viscous liquid film from the inside of the smooth interior surface of a long cylindrical tube. The model is based on the motion of the film generated by the shear stress imposed on it by the turbulent flow of air through the tube. The model gives estimates of the mean thickness of the film (an olive oil and a castor oil). The authors extend the investigation to include removal from roughened surfaces, modelling these as regularly spaced triangular cavities.