Knowledge Base

Scale1e-3 to 1 m / 1e-3 to 1 s, 1 to 60 s

Non-expert summaryIce-pigging is a cleaning method in which a dense slurry (to be removed) is pushed along a channel (often a pipe, but more complex geometries can also be cleaned by this method) by an upstream solid-liquid thick ice slurry. The authors use the commercial CFD code FLUENT to evaluate a numerical model of this multiphase (solid-liquid) material undergoing pipe flow. In this coupled Eulerian-Eulerian description the two phases are treated separately as continuous phases coupled by pressure and interphase forces. They validate their model against experimental data from literature and report the predicted ice volume fraction, wall shear stress and melting rate. The results show strong inhomogeneity in the solid content (ice volume fraction).

Article

Aging is important: Closing the fouling–cleaning loop

Ishiyama, E.M., Paterson, W.R., Wilson, D.I.

2014Heat Transfer Engineering35(3)311-326

Keywordscleaning, heat transfer

DisciplinesChemical Engineering

CategoryTheoretical modelling

Scale1e-3 m, 1e-3 to 1 m / 1 to 24 hours, 1 to 30 days, 1 to 12 months, greater than 1 year

Non-expert summaryThis paper considers the management of heat exchanger units subject to regular cleaning and thus regular cleaning. A quantitative model is needed to support the decision of when to clean an exchanger, giving rise to repeated cycles of fouling and cleaning. The initial stages of fouling are strongly influenced by the effectiveness of the most recent cleaning step and, similarly, the effectiveness and rate of cleaning are determined by the extent and nature of the deposit layer present on the surface. Deposit aging is an important factor in this, as an aged deposit is usually more difficult to clean. Ageing therefore introduces an element of choice into fouling–cleaning operating cycles, between in situ “chemical” methods and ex situ “mechanical” methods, with associated differences in effectiveness, time, and cost. The cleaning scheduling problem is presented in terms of the choice of cleaning method, as well as the timing of cleaning. A process scale model is used, with the performance of units described by lumped parameter (i.e. not detailed) models. Dimensionless groups are obtained which capture the scaling involved. Case studies are used to illustrate the concepts.

Article

Mechanisms of photoresist dissolution

Hunek, B., Cussler, E.L.

2002AIChEJ48(4)661-672

Keywordscleaning, decontamination, mass transfer

DisciplinesChemical Engineering, Chemistry

CategoryLaboratory experiments

Scale1e-6 to 1e-3 m / 1e-3 to 1 s, 1 to 60 s, 1 to 60 min

Non-expert summaryPhotoresists are an example of long chain molecules which dissolve in the presence of alkali. Dissolution involves the transport of hydoxyl ions (OH-) into the layer: they react with the photoresist and, depending on the temperature (above or below the upper critical solution temperature, UCST), it will either be transported by diffusion through the fluid boundary layer (above UCST) or form a gel phase (below the UCST), introducing an additional diffusion step in the reaction and mass transfer processes. A physico-chemical model of the process is presented here, applied to novolak resins, which can be applied to other polymeric systems including printing inks and dairy protein foulants. Experiments are performed with a spinning disk apparatus, which gives well defined mass transfer conditions.

Article

A three-dimensional computer model analysis of three hypothetical biofilm detachment mechanisms

Chambless, J.D., Stewart, P.S.

2007Biotechnology and Bioengineering97(6)1573-1584

Keywordsbacteria, biofilm, cleaning, detachment, flow displacement, shear flow

DisciplinesChemical Engineering, Microbiology

Scale1e-6 m, 1e-6 to 1e-3 m, 1e-3 m / 1e-3 to 1 s, 1 to 60 s, 1 to 60 min

Non-expert summaryThe removal of biofilms by sloughing is a natural phenomenon in biofilm dynamics. Three hypothetical mechanisms of detachment were incorporated into a 3D model of biofilm development. The model integrated processes of substrate utilization, substrate diffusion, growth, cell advection, and detachment in a cellular automata framework. The three detachment mechanisms analyzed represented various physical and biological influences hypothesized to affect biofilm detachment; fluid shear removing protruding material; removal linked to local nutrient availability; and erosion. The detachment mechanisms demonstrated diverse behaviors with respect to the four analysis criteria. The results show that detachment is a critical determinant of biofilm structure and of the dynamics of biofilm accumulation and loss.

Article

A general description of detachment for multidimensional modelling of biofilms

Xavier, J. de B., Picioreanu, C., van Loosdrecht, M.C.M.

2005Biotechnology and Bioengineering91(6)651-669

Keywordsbacteria, biofilm, cleaning, detachment, erosion, shear flow

DisciplinesChemical Engineering, Microbiology

Scale1e-6 m, 1e-6 to 1e-3 m, 1e-3 m / 1e-3 to 1 s, 1 to 60 s, 1 to 60 min

Non-expert summaryThe structure of many biofilms results in a surface layer which does not detach readily and a growing layer that detaches (sloughs) off more readily. The authors present a general method for describing biomass detachment in a multidimensional biofilm modelling framework. Biomass losses from processes acting on the entire surface of the biofilm, such as erosion, are modelled, and discrete detachment events, i.e. sloughing, are implicitly derived from the simulations. This methodology for biomass detachment was integrated with multidimensional (2D and 3D) particle-based multispecies biofilm models by using the level set method. Application of the method is demonstrated by looking at the trends in biofilm structure and activity over time in two case studies: I - a simple model considering uniform biomass; II - a model discriminating biomass composition in heterotrophic active mass, extracellular polymeric substances (EPS) and inert mass.

Article

Three-dimensional advective–diffusive boundary layers in open channels with parallel and inclined walls

Etzold, M.A., Landel, J.R., Dalziel, S.B.

2020International Journal of Heat and Mass Transfer153119504

Keywordscleaning, decontamination, drop, heat transfer, mass transfer, narrow channel

CategoriesNumerical simulations, Theoretical modelling

Scale1e-6 to 1e-3 m, 1e-3 m, 1e-3 to 1 m / 1e-3 to 1 s, 1 to 60 s, 1 to 60 min, 1 to 24 hours

Non-expert summaryThe authors consider the steady laminar advective transport of a diffusive component released at the base of a narrow three-dimensional longitudinal open channel with non-absorbing side walls and rectangular or truncated-wedge-shaped cross-sections: the findings are relevant to heat and mass transfer applications in confined U-shaped or V-shaped channels (or trenches) such as might arise in the decontamination and cleaning of narrow gaps, crevices and boundary features on walls or other surfaces, and well as transport processes in chemical or biological microfluidic devices. The fluid flows along the channel in the laminar regime and there is no flux or slip on the side walls. Numerical simulations are conducted for various duct shapes and the rate of mass transfer from the base is calculated: this is used to evaluate the dimensionless mass transfer coefficient, the Sherwood number. The results for 3-D (constant cross section, long in the direction of flow) are compared favourably with results for a simplified, 2-D, calculation.

Article

Pressure-driven miscible two-fluid channel flow with density gradients

Sahu, K.C., Ding, H., Valluri, P., Matar, O.K.

2009Physics of Fluids21043603

Keywordscleaning, flow displacement, hydrodynamics, miscible liquids, multiphase flow

Non-expert summaryThe displacement of one liquid by another in a channel of constant height occurs in flushing and cleaning operations. The authors study the effect of buoyancy (arising from different fluid densities) on a pressure-driven flow of two miscible fluids in inclined channels using direct numerical simulations DNS). The flow dynamics are governed by the continuity and Navier–Stokes equations, without the Boussinesq approximation for buoyancy, coupled to a convective-diffusion equation for mass transfer between the two liquids. The effect of concentration on viscosity and density is modelled. The effect of varying the density ratio, Froude number, and channel inclination on the flow dynamics is examined, for moderate Reynolds numbers. These detailed simulations give insights into mixing and cleaning behaviour.

Article

Linear stability analysis and numerical simulation of miscible two-layer channel flow

Sahu, K.C., Ding, H., Valluri, P., Matar, O.K.

2009Physics of Fluids21042104

Keywordscleaning, flow displacement, interfacial dynamics, linear stability analysis, miscible liquids, multiphase flow

CategoriesNumerical simulations, Theoretical modelling

Non-expert summaryWhen one liquid is pumped into a channel containing a second liquid, the behaviour depends on the properties of the two fluids and pressure driving force. This paper considers the stability of a flow of two miscible fluids in a horizontal channel. The flow dynamics are governed by the continuity and Navier–Stokes equations, with mass transfer between the two. An analysis of the flow in the linear regime delineates the presence of convective and absolute instabilities, and shows that vertical gradients of viscosity perturbations (caused by mixing) are the main destabilizing influence of the interface (in agreement with previous work). Previous work in the area is reviewed as well. Transient numerical simulations demonstrate the development of complex dynamics in the nonlinear regime, characterized by roll-up phenomena and intense convective mixing.

Article

Sliding, pinch-off and detachment of a droplet on a wall in shear flow

Ding, H., Gilani, M.N.H., Spelt, P.D.M.

2010Journal of Fluid Mechanics644217–244

Keywordsdecontamination, drop, hydrodynamics, interfacial dynamics, shear flow

CategoriesNumerical simulations, Theoretical modelling

Non-expert summaryThis modelling study considers what happens after a droplet is set into motion by the action of an impose shear flow. Inertial effects and contact-angle hysteresis are both considered. A number of flow regimes are investigated, including steadily moving drops, partial and entire droplet entrainment. The critical conditions (capillary number) for the onset of entrainment are determined for pinned as well as for moving drops. The approach to breakup is then investigated in detail, including the growth of a ligament on a drop, and the reduction of the radius of a pinching neck. A model based on an energy argument is proposed for the rate of elongation of ligaments. The paper concludes with an investigation of detachment of a hydrophobic droplet from a solid wall.

Article

Convective mass transport from rectangular cavities in viscous flow

Occhialini, J.M., Higdon, J.J.L.

1992J. Electrochem. Soc.139(10)2845-2855

Keywordscavity flow, cleaning, decontamination, mass transfer, shear flow

Non-expert summaryThe authors conduct a comprehensive numerical study of convective mass transport from 2-D rectangular cavities in low‐Reynolds‐number flows, i.e. the flow set up in a long rectangular trench by the motion of a steady shear flow across the top. They calculate the velocity field in the trench and the associated mass transport (enhancement of diffusion). The flow field is calculated by a high‐order implementation of the boundary‐integral method, while the convective diffusion equation is solved using the spectral‐element method. Results are presented in the form of concentraton contours and local mass fluxes, for cavity aspect ratios from 1:1 to 4:1 and for Péclet numbers from 0 to 100,000. They investigate the effects of inlet flow profile and system boundaries on the system.

Article

Transient displacement of a viscoplastic material by air in straight and suddenly constricted tubes

Dimakopoulos, Y., Tsamopoulos, J.

2003J. Non-Newtonian Fluid Mech.11243-75

Keywordscleaning, flow displacement, moving boundary problem, viscoplastic fluid

Non-expert summaryThe flushing of a viscous fluid from a pipe or cavity by a less viscous one is widely used in cleaning operations. This paper reports the numerical modelling of the transient displacement of a viscoplastic material from straight or suddenly constricted (square entry) concentric cylindrical tubes of finite length, simulating a gas at higher pressure displacing the geometry initially filled with liquid. A mixed finite element method is coupled with a quasi-elliptic mesh generation scheme in order to follow the very large deformations involved. The gas bubble grows in length and leaves a thin liquid film on the duct wall (so it does not remove all the liquid). The shape of the bubble is computed for various Reynolds and Bingham numbers (ratio of yield stress to viscous stress). The 'tip splitting' instability that can arise in flow of a gas along a tube filled with a viscous Newtonian fluids is suppressed with viscoplastic fluids at higher Bingham numbers. The shape of the bubble as it passes through the constriction is also studied.

Review article

The fluid mechanics of cleaning and decontamination of surfaces

Landel, J. R., Wilson, D.I.

2021Annual Review of Fluid Mechanics53147-171

Keywordscleaning, decontamination

CategoryReview

Scale1e-9 to 1e-6 m, 1e-6 m, 1e-6 to 1e-3 m, 1e-3 m, 1e-3 to 1 m, 1 to 10 m, 10 to 1000 m, greater than 1 km / less than 1e-3 s, 1e-3 to 1 s, 1 to 60 s, 1 to 60 min, 1 to 24 hours, 1 to 30 days, 1 to 12 months, greater than 1 year

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.