Experimental and computational investigation of droplet-particle interactions

Abstract only

An experimental study of droplet-particle collisions

When spray drying a liquid slurry such as milk, collisions between droplets, partially dried particles and completely dry particles are important because coalescence, agglomeration and breakup events influence the size and morphology of the produced powder. When modelling such a spray drying process, it is therefore important to be able to predict the outcomes of individual binary collisions. Both binary dry particle collisions and binary droplet collisions have individually been thoroughly researched over the years due to their widespread occurrence. The importance of understanding binary particle-droplet collisions has been emphasized more recently. However, the number of available studies is limited and simulation studies usually focus on relatively high capillary number. A theory explaining the transition between different regimes is still lacking. The goal of this study is to provide an experimental data set at low capillary number. These results can be used to validate future theories and simulations. To produce and record particle-droplet collisions, an experimental setup that enables synchronized release of both a particle and a droplet was used. One single hanging droplet was released from above onto a particle that initially was held in place by vacuum suction. A high speed camera was synchronized with the setup, and recorded the collisions. Image files were then analysed in Matlab to find velocities and sizes of the particle and droplet before and after impact. The contrast of particle and droplet against the illuminated background was a key factor in succeeding with this. Different collision outcomes were identified as either agglomeration (merging), where the whole droplet would stick to the surface of the particle, or a stretching separation (breaking), where the droplet collides with the particle in an oblique position and stretches out until a part of the droplet detaches from the liquid sticking to the particle. The formation of satellite droplets, i.e. droplets with a radius significantly smaller than the leaving droplet, was also detected. The relation of these collision outcomes to impact conditions such as Weber number and impact parameter was reviewed and put into regime maps

Experimental study of the heat transfer in a falling film evaporator: influence of the co-flowing vapor

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.A large number of industrial processes are based on the concentration of liquid products by means of falling-film evaporation. In the dehydration of dairy products, concentrating a fluid by evaporating represents one of the most important steps of the whole drying process. Among the advantages of this technology is that it is possible to operate within small temperature differences which results in low heat consumption. In this sense, it is necessary to increase the amount of removed water during the falling-film evaporation to reduce the energy cost of the overall process. However, reducing the fraction of the solvent leads to an increase in viscosity of the product which can show non-Newtonian features. This aspect significantly affects the heat transfer, that is to say, the higher the solid content, the lower the heat transfer coefficient. One of the possible solutions to this drawback consists in drawing maximum benefit from the interaction between the fluid film and the co-flow of the gaseous phase resulting from the evaporation process. Unfortunately, accurate studies of the effect of co-flow on evaporative falling films are very rare and difficult to perform because of the high costs of the implementation of a suitable experimental apparatus. In this work, the experimental study of the influence of the co-flow on the heat transfer coefficient is presented as a function of both the solid content and the mass flow rate of the feed. The experimental set-up, consisting in a unique industrial pilot scale evaporator, provides the possibility to obtain results useful for realistic industrial conditions. Tests were conducted with varying dry solid content from 10 to 50%. The results show that the co-flow has the effect to decrease the potential for fouling/poor wetting. Above this, the influence on the heat transfer is not as large as expected because of the dominant influence of the viscosity.cf201

Visualization of the drop deformation and break-up process in a high pressure homogenizer

For the creation of sub-micron emulsions in fluids of low viscosity the high pressure homogenizer (HPH) is usually chosen. One way of obtaining deeper knowledge of exactly what happens in the active region is to visualize it. In this work, a drop deformation and break-up visualization system based on a modified Particle Image Velocimetry (PIV) system is described. The system reproduces the gap in a HPH and has been used with pressures up to 18 MPa and drops as small as 5 mu m. The optics of the system are analyzed taking into account limiting factors such as the lens resolving power, the focal depth, and the duration of the laser pulses. It is shown that it is possible to resolve drops down to a few mu m moving in excess of 100 m/s, and that the main limitations are the resolving power and in the focal depth of the objectives. Examples are shown from capillary drop creation and from the deformation and break-up of drops in a HPH. It can be concluded that in a HPH, the drops are only deformed to a limited extent in the inlet of the gap, and that all drop break-up occurs far downstream of the gap

Qualitative investigation of the flow behaviour during falling film evaporation of a dairy product

Falling film evaporation is an important technology in the dairy industry for producing powders. In this paper, flow details of liquid falling films have qualitatively been investigated using a pilot evaporator, and in particular using a high-speed camera. Variations with different dry solids contents, flow rates and driving temperature differences were investigated. The flow characteristics were seen to be considerably affected by all three variables. Two of the main observations were the formation of bubbles under evaporative conditions and that the flow, bubble formation and evaporative heat transfer coefficient was observed to be heat flux dependent

Heat transfer for falling film evaporation of industrially relevant fluids up to very high Prandtl numbers

In many industrial applications, falling film evaporation is an attractive technique for solvent removal due to high heat transfer and low residence times. Examples are the powder production in the dairy industry and in kraft pulp production process to remove water from so called black liquor. Common for both applications is that the fluids exhibit high viscosities in industrial practice. In this paper, results from experimental studies on both black liquor and a dairy product are reported for Prandtl numbers up to 800. The results are compared with several existing correlation in literature, and the need for a modified correlation is recognized especially to cover higher Prandtl-numbers. The following correlation for the turbulent flow region with

Large Eddy Simulation of the flow and heat transfer in a half-corrugated channel with various wave amplitudes

Large Eddy Simulation (LES) of turbulent flow and convective heattransfer over a half-corrugated channel is presented in this paper. Simulationsare performed for various ranges of the normalized wave amplitudes,AM = 0 − 0.15 (the ratio of wave height to wave length). The Reynoldsnumber based on the bulk velocity is chosen as Reb = 10 000 and thePrandtl number is Pr=0.71. A comparison between the DNS and LESresults of a plane channel (AM = 0) at Re = 395 is also performed.The obtained results indicate that the region of recirculating flow dependsstrongly on the wave amplitude. This study shows that the Nusseltnumber (Nu) increases by increasing the wave amplitude until a specificvalue then it remains approximately constant. The thermal performanceparameter (JF) is used as a measure for the heat transfer enhancementrelative to the pressure drop and it is found that the maximum values ofNu and JF appear at AM = 0.1, which hence correspond to the optimumvalue of the wave amplitude

Prediction of regions of coalescence and agglomeration along a spray dryer-Application to skim milk powder

Spray drying is a well-established method used in the food industry for production of powders, such as dry milk, coffee, tea, and soup. The operating conditions of spray dryers depend on the product and chamber design. Thus scaling up the process is complex, and simulation tools are needed to reduce time and cost, and to enhance quality of the final product. In this study a validated distributed-parameter model for predicting drying of single particles was combined with a CFD simulation model of an eight-meter pilot dryer to investigate drying kinetics of skim milk powders. A qualitative assessment of the effect of water diffusivity values on preconditions for agglomeration and a multi-scale analysis of coalescence and agglomeration regions were performed. Results showed that accurate water diffusivity values and sticky conditions have to be implemented when modelling spray drying to investigate preconditions for coalescence and agglomeration. The results also showed the need to simulate surface conditions of particles during spray drying to predict probable regions of coalescence and agglomeration. In conclusion, the applied methodology allows understanding of the stickiness ability of a product and size of the chamber. The methodology can be used to support preliminary design of spray dryers

Heat transfer for falling film evaporation of industrially relevant fluids up to very high Prandtl numbers

In many industrial applications, falling film evaporation is an attractive technique for solvent removal due to high heat transfer and low residence times. Examples are the powder production in the dairy industry and in kraft pulp production process to remove water from so called black liquor. Common for both applications is that the fluids exhibit high viscosities in industrial practice. In this paper, results from experimental studies on both black liquor and a dairy product are reported for Prandtl numbers up to 800. The results are compared with several existing correlation in literature, and the need for a modified correlation is recognized especially to cover higher Prandtl-numbers. The following correlation for the turbulent flow region with

Estimation of the effective diffusion coefficient of water in skim milk during single-drop drying

This paper presents a new approach combining experimental methodology and modelling, developed to evaluate the effective diffusivity of water in skim milk during drying over a full range of water contents and temperatures. This parameter is important to support modelling of spray-drying processes and designing of equipment. The effective diffusion coefficient is evaluated using a combination of nuclear magnetic resonance (NMR) and parameter estimation. NMR is used to determine the temperature dependence and parameter estimation is used to estimate the water concentration dependence of the effective diffusivity of water in skim milk (0.90 on total weight basis) during drying by comparing the experimental data obtained using a suspended-drop method, which allows the recording of weight and temperature changes during drying, with the results of a distributed heat and mass transport model. The results indicate that the free-volume theory best predicts the dependence of the effective diffusion coefficient of water in skim milk. A mathematical correlation of effective diffusivity over a full range of water contents and temperatures (from 50 to 90 degrees C) was obtained and experimentally successfully validated for concentrated skim milk (0.70 on total weight basis)