Masterarbeit, 2019
43 Seiten, Note: 77.00
1 Introduction
2 Problem Statement
3 Aim and objectives
4 Literature Review
4.1 Introduction
4.2 Fluid-solid mixture
4.3 Calculation methods of spherical particles settling velocity
5 Test apparatus
5.1 Chamber
5.2 Air delivering
5.3 Laser diffraction
6 Experimental methodology
6.1 Preparation
6.1.1 Safety
6.1.2 Setup
6.2 Martials
6.2.1 Salt
6.2.2 Glass beads
6.2.3 Air properties
6.3 Experiment
6.3.1 Assumption
6.3.2 Testing
7 Result &Discussion
7.1 Experimental results
7.1.1 Salt results
7.1.2 Glass beads results
7.2 Hand calculation results
7.3 CFD modelling
7.3.1 Methodology
7.3.2 CFD modelling results
8 Conclusion
9 Future work& Recommendations
References
The investigation of fluids containing particles or filaments includes a category of complex fluids and is vital in both theory and application. The forecast of particle behaviours plays a significant role in the existing technology as well as future technology.
The present work focuses on the prediction of the particle behaviour through the investigation of the particle disentrainment from a pipe on a horizontal air stream. This allows for examining the influence of the particle physical properties on its behaviour when falling on horizontal air stream. This investigation was conducted on a device located at the University of Greenwich's Medway Campus. Two materials were selected to carry out this study: Salt and Glass Beads Nano particles. The shape of the Slat particles is cubic where the shape of the Glass Beads is almost spherical. The outcome from the experimental work were presented in terms of distance travelled by the particles according to their diameters as After that, the particles sizes were measured using Laser diffraction device and used to determine the drag coefficient and the settling velocity.
For a verification and more deep insight, the experimental setup was modelled using Computational Fluid Dynamics (CFD) technique and the results were compared with the experimental results in terms of distance travelled. A good agreement was observed between the CFD and experimental results.
The experimental and numerical results showed that the size of the particle has a huge impact on the drag coefficient and the settling velocity. Larger diameters lead to less drag and hence higher settling velocity. Also, the lighter particles tent to travel horizontally further than the heavier ones.
Thank you God for all your blessings to me and my family. For the strength you give me each day and for all the people around me who make life more meaningful. A teacher takes a hand, opens a mind and touches a heart. That is the definition of Dr. Zigan. All that I am or ever hope to be, I owe to my angel mother. My brother Ahmed, Thank you for always giving me the extra push I need.
Figure 1. Test apparatus
Figure 2. Flow over a settling sphere particle and forces acting on it 7
Figure 3. Plate drawing showing main dimensions
Figure 4. CSD 102 air delivery unit
Figure 5. Laser diffraction device
Figure 6. Chamber connected to the air control panel
Figure 7. Preparation for the feeding
Figure 8. Air velocity measurement
Figure 9. Locations of the of air velocity measurements
Figure 10. Finding the size of the chamber
Figure 11. Cleaning the plats of the chamber
Figure 12. Filling the pipe with Nano particle material for injection
Figure 13. Particle collection using fine brush
Figure 14. Using the scale for getting the measurement
Figure 15. Salt test results by trays for experiment 1
Figure 16. Glass beads results by trays
Figure 17. Drag coefficient of the particles against the travelled distance
Figure 18. Particle diameter against distance travelled
Figure 19. 2-D sketch of the geometry used in CFD model
Figure 20. Computational grid and boundary conditions
Figure 21. Air velocity contour
Figure 22. Particle tracking coloured by the particle diameters
Figure 23. Comparison between experimental results and CFD results
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The investigation of fluids containing particles or filaments includes a class of complex fluids and is vital in both theory and application. Several studies have focused on the particle behaviour in a fluid flow. The forecast of particle behaviours plays a significant role in several engineering processes and fields, for example, chemical and metallurgical processes as well as mechanical and environmental engineering. Modelling of the particle in water process requires deep knowledge of predicting the particle velocity. In the gas turbine, blades and nozzles are exposed to erosion which is strongly rely on the particles velocities. A further application is the transmission of bulk material via pneumatic conveying, the assumption made for the particle motion is based on the terminal velocity of the particles. This emphasise the significance of predicting such velocity for selecting the right sizing and the appropriate design of the plant.
Regardless of such significance, the existing investigation do not offer a sufficient insight on such subject and enhancements of the prediction of the particles behaviours which in turns can lead to more accurate mathematical models. Also, the greater part of the existing studies is centred around the investigation of particles having regular shapes such as sphere (the least difficult and most researched shape), cubical or cylindrical shapes. Furthermore, the used material in these studies are fairly limited due to the irregular shape of their particles.
The present work focuses on the prediction of the particle behaviour through the investigation of the particle disentrainment from a horizontal air stream. The objective of the current study is to examine the effect of the particle properties on disentrainment on an air stream. The study was conducted using mechanical assembly which is located at the University of Greenwich's Medway Campus which is shown in Figure 1. Two different materials were considered in the current study, Salt, and Glass beads, were selected for their almost perfect spherical shapes. This experiments were repeat few times in order to verify the results and performances. Later on, the results were compared with the hand calculations and were further used to produce a two-dimensional Computational Fluid Dynamics (CFD) model.
Today, the idea of particle granulomere has been liable to different sorts of research and concentrates instead of what it resembled 20 odd years prior. Researchers have acknowledged that issue comes in 3 unique states; strong, fluid and gas, frequently hidden the states in the middle. This order was handled by powders, which very still are solids, when circulated air through may carry on as fluids and when suspended in gas in may go up against a portion of the properties of gases. The expansion in innovation has made it conceivable to examine molecule conduct anyway there are insufficient gadgets to enable scientific models to foresee such attributes. For molecule measure investigation, granular materials are regularly utilized, which is known to be made of individual strong particles, paying little heed to its molecule estimate. Therefore, granular material can go from coarse colliery rubble to fine measured components.
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Figure 1. Test apparatus.
The main goal of the current study is to investigate the particle interaction with air and the influence of particle properties such as particle size on the settling in a horizontal fluid flow. Another main goal of this study is to determine the drag coefficient of the particle.
To meet the aims, the objectives were defined as follows:
- Conduct a particle segregation experiment.
- Study various properties of particles and the effect of the fluid stream on the particle settling.
- Generate a CFD model to analyse the flow and to gain insight on the particles behaviour.
Particle technology has already been adopted in a many formats and applications all over the globe throughout the history. Many kinds of powders have been produced to improve human life, such as food, detergents, pigments, cements, fertilizers and industrial chemicals.
Particle technology account to a significant contribution in many products, for instant, it contributes by almost one-half to the products within the chemical industry and over 75% of the granular raw materials 1.
In general, particle size can vary from sizes as small as order of nanometres up to 1 mm 2. There is no general agreement on particle classification according its size is not, however, particles with less than 30 μm mean diameter are typically stated as fine particles which their uses in industries are huge. Particle having a diameter below 10 μm are referred to as superfine particles that can split down to: nanoparticles (1-100 nm) and molecular cluster (<1 nm), sub-micron particles (0.1-1 μm), micron particles (1-10 μm) 3.
In the recent years, study of the mixture of fluid and fibres or particles gained an attention due to its importance to wide range of modern applications. The behaviour of particle in flow of fluid has been the focus of a large number of researches. One of the important behaviours of such particle is its velocities and trajectories as it plays a vital role in several engineering fields such as environmental and mechanical engineering besides chemical and metallurgical processes. The crucial problem is correlating the mixture properties at global level as well as local level. The existence of the solid particles in the fluid causes very complicated hydrodynamic phenomena [4-6].
There are two main forces acting on the settling particles as shown in Figure 2. First is the primary force such as gravity or forces due to centrifugal motion. Second force is the generated drag as a result of the particles motion in the fluid. The drag force is usually affected by the velocity of the particle where the applied force is not affected.
When the particle is not in motion, it will not experience any drag force and the particle will start to accelerate by means of the gravity force. After that, the drag force starts to act on the particle in the direction opposite to the motion. As the velocity of the particle build up, the drag force increases until it equates the applied force and the velocity of the particles will remain with no further change. This velocity referred to as settling velocity or terminal velocity of the particle.
There are many parameters that can alter the particle’s terminal velocity. Any parameter affects the drag will directly affect the terminal velocity of the particle. Hence, the grain shape, size and density have the most effect on the terminal velocity in addition to the fluid properties.
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Figure 2. Flow over a settling sphere particle and forces acting on it 7.
In order to understand the natural phenomena of the mixture and various industrial processes, it is essential to determine the flow behaviour at local level as well as the potential interaction between the solid and liquid phases [8-12]. However, reviewing the available in the literature shows that studies been conducted to determine the global behaviour such as average concentration and velocity are relatively larger than the studies conducted to determine the local behaviour of the mixture. This shows that most of the studies are related to a specific application 13.
Using mass diffusion equations approach for modelling fluid flow provide approximation of the flow behaviour and in most cases, ignores some characteristics of the particles such as mechanical and physicochemical in addition to the rheological behaviour of the mixture [14, 15].
In such situations, the terminal velocity of solid particles settling in fluid plays a significant role in the governing equations of such complex fluid flow [16-20]. Varity of relationships can be found to estimate the terminal velocity which makes it difficult to be applied [15, 21-23] . In the experimental investigations, a special interest is given to the effect of particle characteristics, such as density and shape, on sedimentation process [17, 24-29]. Also, the attention is paid to examine the effect of specific parameters of the fluid such as rheological characteristics and density [30-35].
The solid particles’ terminal velocity is usually related to spherical shape. The proposed correlations for settling velocity are more likely to be implemented as a foundation for more complex shapes, which can be applied either directly or indirectly to evaluate the particle’s setting velocity [36-39]. There are two methods to calculate the settling velocity of spherical particles. The direct approach treats the velocity as a function of dimensionless numbers roughly equivalent to an Archimedes number [40-45]. On the other hand, the indirect approach employs iterative process by using Reynold number and the drag coefficient [46-48] . By performing Bibliographic study, it was shown that the relationships are limited by small range of Reynold number [49, 50]. The validity of both theoretical and semi-theoretical treatments are limited to Reynold number less than unity.
When a particle settles in still fluid will accelerate for short period by means of gravity force until it reaches its terminal velocity, which occurs when the drag and gravitational forces are balanced. Therefore, the following relationship is obtained 51.
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Where, V is the volume, A is the surface area, ρp is the particle density, ρf is the fluid density Ws is the settling velocity and Cd is the drag coefficient, which depend on Reynold number, Rep, of the particle:
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Where dp is the diameter of the particle.
Practically, experimental data for terminal velocity along with Ws in Eqs. (1) and (2) t are usually used to generate the curve of Cd=f (Res). While the conventional approaches use experimental data only to generate the curve Cd=f (Res) and employ an iteration process to evaluate Ws. The curve Cd=f(Res) have been described by several empirical correlations [12, 49, 52-56].
Also, there is possibility to use by direct calculation to calculate the value of settling velocity. In such instance, the parameters of Archimedes number, Grs, should be taken into account. Hence, by altering Eq. (1) the following relationship can be obtained.
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The value Ws for spherical particle can be obtained via Res=h (Grs) along with Eq. (2) 40.
Most of the used relationships to determine the coefficient of drag are derived from data for settling velocity of particles in quiescent fluid. Other relationships were obtained for turbulent flow by placing a sphere in wind tunnel and measuring the drag coefficient 49. It is worth noting that the value of the drag coefficient of sphere falling freely in static fluid is larger than when the surrounding fluid is in motion by 15-30%. The reason of this difference is that the when the particle falls in a quiescent fluid, its trajectory is more likely to be altered.
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