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 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.
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 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.
Non-expert summaryAn important part of hygienic design is knowledge of the forces imposed on a surface by the flow of a process liquid, and whether those forces are large enough to remove any contaminants. This is a chiefly experimental study investigating the numbers of adhering Bacillus cereus spores before and after cleaning in place of part of a dairy processing line. Non-intrusive flow characterization, of local wall shear stress and velocity profiles, were measured using electrochemical and ultrasonic Doppler velocimetry techniques. The geometries studied inlcuded gradual asymmetric pipe contractions and pipe expansions and bends. The recirculation zone resulting from the flow detachment after the expansion resulted in a large number of adherent spores downstream of this geometrical change. Welding and gasket regions also retained spores, proving difficult to clean despite high-shear stress forces involved. The flow measurements are compared with some theory so the level of modelling is not high but the data sets present important cases for new investigations, e.g. using CFD methods.
Non-expert summaryThis is an experimental and modelling study of the removal of a passive tracer contained in small, thin, viscous drops attached to a flat inclined substrate using the flow of a thin gravity-driven film. The drop cannot be detached either partially or completely from the surface by the mechanical forces exerted by the cleaning fluid on the drop. Convective mass transfer is established across the interface between the drop and the flowing liquid film and the (dilute) tracer diffuses into the film flow, which takes it away. The Peclet number, comparing the rate of mass transfer in the drop to the rate in the liquid film, is small (< 1) . Two models are presented: a simple empirical model based on film mass transfer coefficients; and a fuller theoretical model solving the quasi-steady two-dimensional advection–diffusion equation in the film, coupled with a time-dependent one-dimensional diffusion equation in the drop. A range of values of the Peclet number (0.01 to 1) is considered in the fuller model. Good agreement is observed between the experimental data and the models.
Non-expert summaryThe paper presents a model to describe the results of an experimental investigation of removal of thin (several microns thick) Xanthan gum layers from flat horizontal plates by a coherent water jet impinging normally on the plate. In the experiments, the impingement point of the jet moved across the plate, replicating the action of a moving or rotating nozzle which causes the point of impact to move across a wall or other surface to be cleaned. Removal was monitored by a fluorescence technique: the Xanthan gum layers contained fluorescent ZnS crystals which allowed the location of the cleaning front to be monitored in situ and in real time. The curved shape, and final width, of the steady state cleaning front was predicted by a first order peeling model based on the momentum flow in the film. The governing equation for the steady state shape is formulated in terms of a general cleaning rate. For these experiments, the parameters of the cleaning model which were obtained from experiments involving static nozzles gave good predictions of the shape of the cleaning front observed with moving nozzles.
Non-expert summaryThe Lattice-Boltzman numerical method is used to solve the Navier-Stokes equations describing the displacement of a viscoplastic fluid from a cylindrical pipe by the injection of a Newtonian liquid. The equations are written in dimensionless form so no particular time or length scale is specified. Gravity and surface tension effects are included. The average velocity of the Newtonian liquid is such that its flow is laminar. The numerical code is able to resolve several features of the process: the growth of a 'finger' of Newtonian liquid as it pushes the viscoplastic fluid out and the development of waves at the liquid-fluid interface. The impact of key dimensionless groups, particularly the Bingham number (ratio of critical or yield stress to stress induced by the flow) and Reynolds number.
Non-expert summaryThis paper describes an experimental study of the removal of thin (less than or equal to 1 mm) layers of (i) dried polyvinyl alcohol, PVA, and (ii) petroleum jelly (a viscoplastic fluid) from vertical glass or polymethyl methacrylate (PMMA) surfaces by the normal impingement of a turbulent coherent water jet (passing through air). The PVA experiments feature a static nozzle and surface so a circular region of cleared material grows steadily over time: the petroleum jelly also features experiments where the surface moves relative to the the nozzle, replicating a moving jet. In the latter case, the jet generates a rounded cleaning front upstream of the impingement point. The viscoplasticity of the petroleum jelly limits the extent of the cleaned area. A quantitative model is presented where the cleaning rate is related to the flow of momentum per unit length in the liquid film, modified for yield stress behaviour. This model is able to describe the results obtained for static and moving configurations. An expression is obtained for the momentum flow rate associated with viscoplastic behaviour.