The effect of corrugation on heat transfer and pressure drop in channel flow with different Prandtl numbers

Large Eddy Simulation and Direct Numerical Simulation are applied to study the turbulent flow field in a wavy channel at two Prandtl numbers, Pr = 0.71 and Pr = 3.5, and Reynolds number Re b = 10,000. The characteristics of the separated shear layer and the near wall recirculating zone are discussed in relation to the turbulent heat transfer. Special attention is paid to the behavior of the flow and thermal boundary layers and various turbulent characteristics and their effects on the distribution of the Nusselt number and friction coefficient in the separation and reattachment regions. The results indicate that the thickness of the thermal boundary layer rather than the turbulent fluctuations has a significant effect on the local variation of the averaged Nusselt number. The results are compared with Direct Numerical Simulation results of a plane channel at the same Reynolds number. \ua9 2013 Elsevier Ltd. All rights reserved

Drop Break-up in High-Pressure Homogenisers

The overall aim of this project was to investigate the drop break-up process in milk homogenisers. This was done by measurements and calculations of the flow fields in the gap region and by visualisation of drops being broken up. To make visualisation and measurements possible, two scale models of a homogeniser gap were developed. The full-scale model was a direct copy of the gap in a production-scale homogeniser, but with optical access. Normal operational homogenisation pressures could be tested, and drops down to 5µm in diameter could be visualised. The second model was scaled-up about 100 times ensuring that the relevant dimensionless groups were kept constant, so that the same factors governed the drop break-up process. The scaled-up model was made of transparent plastic and was used for both velocity field measurements and drop visualisation. From these measurements it was concluded that the drops did not break up in the entrance of the gap. Larger drops were elongated to some extent and smaller ones remained spherical. Not much happens in the gap itself. The velocity profile is very flat throughout the gap in a production-scale homogeniser. In a pilot-scale homogeniser the boundary layers have time to grow and the velocity profile is almost developed at the gap exit. The growing shear layers seem to have a limited effect on the drops. During passage through the gap small drops will have time to relax back to their spherical shape, while large ones will leave the gap with almost the same aspect ratio as when they entered it. This study shows that drop break-up takes place in the turbulent jet at the gap outlet. The flow velocity measurements show a very unsteady jet breaking down faster than a jet in a free liquid. Depending on the geometry of the chamber at the gap outlet, the jet can attach to either of the 45-degree walls and become a wall jet. The turbulence in the jet is very high, with turbulence intensities of 50-100%. Indications were found that flow structures of the size of, or slightly smaller than, the gap height, have very high intensities. Drop deformation experiments and theoretical analyses show that the eddies breaking up the drops range in size from much larger than, to just smaller than, the drop. The larger eddies deform the drop viscously by the velocity gradient created by the eddy. The smaller eddies deform the drop by fluid inertia. The critical phase of the drop break-up process is the initial deformation. If the drop is deformed to an aspect ratio of 3-5, the drop is then very quickly elongated into one or more filaments which may be bent, coiled and further deformed before they break up into many small droplets

Steam condensation dynamics in annular gap and multi-hole steam injectors

AbstractIn direct UHT (Ultra high temperature) treatment, milk is pumped through a closed system where it is preheated, high temperature heated, cooled, homogenised and packed aseptically [1].Continuous direct steam injection is used to quickly raise the temperature of a product, either for pure heating or for a sterilization process. The injector can be of either annular gap type (also called ring nozzle steam injector), or multi hole type. In this study we have analyzed the details of the condensation process by visualizations and by mapping the temperature fields. It was found that steam condensation in steam injectors is an intense process with heat transfer rates in the order of 1 MW/(m2 K). The steam is always condensed at the equilibrium temperature and the turbulence created in the condensation zone mixes the hot condensate and heated product with the cold product. The efficiency of this turbulent transport determines the sufficient heat transfer area and thus the size of the steam/product interface. If the condensation rate is faster than the steam addition rate the condensation process is unstable which results in detachments, fluctuations, noise and vibrations

Analysis of the flow field in a high-pressure homogenizer

Numerous theories have been developed describing the drop break-up in a high-pressure homogenizer (HPH), but they are generally based on quite rudimentary descriptions of the flow fields. As the flow in a real HPH is very extreme, with gaps of 10–100 μm and velocities of hundreds of m/s, it is practically impossible to measure the velocity fields. In this study, a scale model of an HPH made of acrylic plastic has been developed making measurements possible. Great care was taken to keep the relevant dimensionless numbers constant during the scale-up. The flow field at the gap entrance shows a steady acceleration and total turbulence suppression, in the gap the flow field is flat with thin boundary layers, and at the exit a turbulent jet is formed. The jet was found to be very unsteady, and in very similar flow situations could either be attached to the walls or continue straight ahead after the gap

The dissipation rate of turbulent kinetic energy and its relation to pumping power in inline rotor-stator mixers

The theoretical understanding of inline rotor-stator mixer (RSM) efficiency, described in terms of the dissipation rate of turbulent kinetic energy as a function of mixer design and operation, is still poor. As opposed to the correlations for shaft power draw, where a substantial amount of experimental support for the suggested correlations exists, the previously suggested correlations for the dissipation rate of turbulent kinetic energy have not been experimentally validated based on primary hydrodynamic measurements. This study uses energy conservation to reformulate the previously suggested dissipation rate correlations in terms of pumping power which allows for empirical testing. The dimensionless pumping power of three investigated geometrically dissimilar inline RSMs were found to be qualitatively similar to that of centrifugal pumps and decrease linearly with the inline RSM flow number. The previously suggested models for turbulent dissipation in inline RSMs are inconsistent with this observation. Using this reformulation approach, the previously suggested correlation for power-draw is extended to a correlation for dissipation. A new model is suggested based on conservation of energy and angular momentum, and the empiric pumping power relationship. The new model compares well to CFD simulations of total dissiaption and show reasonable agreement to emulsification drop size scaling

Effects of large particles in pipe flow at low and moderate Reynolds numbers

The presence of solid particles in a Newtonian liquid flow will affect the properties of the flow. For small particles these effects are fairly well understood. However, the behaviour of liquids laden with large particles are less well understood and even more so if the carrier liquid is a non-Newtonian fluid. In the present study we consider large particles of spherical shape. By large is here meant particles that are of the same size as the large scale length scales of the flow and larger. We are considering how particles volume fraction affects parameters such as pressure drop and velocity distribution in the pipe flow. The simulations are performed using a finite difference based in-house software and the particles are represented using an virtual boundary method. The size of the spherical particles is about 1/6 of the pipe diameter and the volume fraction is varied between 5and 20%. The fluid is either Newtonian or shear thinning modelled using a power law expression

Dynamic modelling of the deformation of a drop in a four-roll mill

The deformation of a drop flowing along the centre streamline of a four-roll mill (4RM) has been investigated. The velocities and elongation rates along the centre streamline in the 4RM were measured using particle tracking velocimetry. The deformation and position of the deforming drops were photographed with a video camera. A dynamic, one-dimensional, analytical simulation model describing the drop deformation has been developed. The model is based on Taylor's [1964. International Congress on Applied Mechanics, vol. 11, 790-796] static conical drop shape model, but has been extended to include elliptic drops undergoing rapid deformation. The model was incorporated into a numerical scheme using Matlab and the drop deformation in the 4RM was simulated. The simulations were compared with the results of the experiments with the help of a dynamic Weber number incorporating the exact effect of the continuous phase stress on the deformation of the drop. With a dynamic Weber number of 0.42 the agreement between the experiments and the simulations along the whole deformation process was excellent for all three drop diameters studied. With this model the deformation of drops of all sizes in different elongation fields can be calculated, for example sub-micron-sized drops in a high-pressure homogeniser. (c) 2005 Elsevier Ltd. All rights reserved

Theoretical and experimental analyses of drop deformation and break-up in a scale model of a high-pressure homogenizer

High-pressure homogenizers (HPHs) are used to create sub-micron emulsions in low-viscosity fluids. As the flow in a real HPH is very extreme, with gaps ranging from 10 to 100 pm and high velocities, it is almost impossible to visualize the drop break-up process. In this study a plastic scale model of a HPH was made, allowing the visualization of drop deformation and break-up. Great care was taken to keep the relevant dimensionless numbers constant during the scaling-up. The experimental data were interpreted in terms of theoretical drop break-up theory. It was found that both viscous and inviscid mechanisms can deform the drop. When a drop is exposed to a high-energy eddy, the deformation process proceeds rapidly. The deformed drop offers very little resistance to the eddies surrounding it, and it is drawn out, coiled and finally broken up into smaller droplets of various sizes. (C) 2010 Elsevier Ltd. All rights reserved

Identification and Mapping of Three Distinct Breakup Morphologies in the Turbulent Inertial Regime of Emulsification—Effect of Weber Number and Viscosity Ratio

Turbulent emulsification is an important unit operation in chemical engineering. Due to its high energy cost, there is substantial interest in increasing the fundamental understanding of drop breakup in these devices, e.g., for optimization. In this study, numerical breakup experiments are used to study turbulent fragmentation of viscous drops, under conditions similar to emulsification devices such as high-pressure homogenizers and rotor-stator mixers. The drop diameter was kept larger than the Kolmogorov length scale (i.e., turbulent inertial breakup). When varying the Weber number (We) and the disperse-to-continuous phase viscosity ratio in a range applicable to emulsification, three distinct breakup morphologies are identified: sheet breakup (large We and/or low viscosity ratio), thread breakup (intermediary We and viscosity ratio > 5), and bulb breakup (low We). The number and size of resulting fragments differ between these three morphologies. Moreover, results also confirm previous findings showing drops with different We differing in how they attenuate the surrounding turbulent flow. This can create ‘exclaves’ in the phase space, i.e., narrow We-intervals, where drops with lower We break and drops with higher We do not (due to the latter attenuating the surrounding turbulence stresses more)

Identification and Mapping of Three Distinct Breakup Morphologies in the Turbulent Inertial Regime of Emulsification—Effect of Weber Number and Viscosity Ratio

Turbulent emulsification is an important unit operation in chemical engineering. Due to its high energy cost, there is substantial interest in increasing the fundamental understanding of drop breakup in these devices, e.g., for optimization. In this study, numerical breakup experiments are used to study turbulent fragmentation of viscous drops, under conditions similar to emulsification devices such as high-pressure homogenizers and rotor-stator mixers. The drop diameter was kept larger than the Kolmogorov length scale (i.e., turbulent inertial breakup). When varying the Weber number (We) and the disperse-to-continuous phase viscosity ratio in a range applicable to emulsification, three distinct breakup morphologies are identified: sheet breakup (large We and/or low viscosity ratio), thread breakup (intermediary We and viscosity ratio > 5), and bulb breakup (low We). The number and size of resulting fragments differ between these three morphologies. Moreover, results also confirm previous findings showing drops with different We differing in how they attenuate the surrounding turbulent flow. This can create ‘exclaves’ in the phase space, i.e., narrow We-intervals, where drops with lower We break and drops with higher We do not (due to the latter attenuating the surrounding turbulence stresses more)