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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 a review of heat transfer and flow phenomena during unsubmerged liquid jet impingement on solid surfaces, such as when a water jet passes through air and impinges on a wall. Both axisymmetric and planar jets are considered. The focus is on convective transport without phase change. Results for the stagnation zone are given first, followed by those for the regions downstream. Correlations are presented for flow and heat transfer phenomena. The heat fluxes that can be generated in these systems can be large, so there is a considerable body of work on topic. Splattering - the formation of breakaway droplets from the liquid film - that accompanies turbulent jet impingement is described. Other aspects of liquid jet impingement cooling are discussed briefly.
Non-expert summaryThis is the first paper to present a coherent treatment of the formation of a hydraulic jump by a smooth, coherent liquid film impinging normally on to a smooth horizontal plate. The liquid spreads out radially in a thin layer bounded by a circular hydraulic jump, outside which the depth is much greater. The motion in the layer is analyzed in terms of boundary-layer theory, both for laminar and for turbulent flow, and relations are obtained for the radius of the hydraulic jump. These relations are compared with experimental results obtained for water jets in air.. The analogous problems of two-dimensional flow are also considered.
Non-expert summaryThis paper extends the analysis of Watson (1964) to consider the influence of surface tension on the laminar circular hydraulic jump generated by a smooth liquid jet of finite viscosity passing through air and impinging on a horizontal surface. A hydraulic jump is obtained where the liquid film thickness changes from a thin film to a deeper, slow moving film. They obtain an expression for the curvature force and use it to obtain an expression for the radius of the jump. The analysis is accompanied by experiments. They show that the surface tension correction is generally small for laboratory chases, but is appreciable for jumps of small radius and height.
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.
Numerical simulation of pressure-driven displacement of a viscoplastic material by a Newtonian fluid using the lattice Boltzmann method
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.
Non-expert summaryThis is a review of social science studies on the social problems of environmental contamination and the social factors that affect cleanup processes and outcomes. It does not focus on technical aspects. An interdisciplinary literature review identified 97 articles and identified three themes: effects on cleanup worker health; public engagement and decision making; and societal benefits of cleaned-up sites. The need to link up various societal factors in making environmental cleanup policy is highlighted.