The concepts will be chosen with help of the morphological chart. There will be four different concepts, each one of these concepts will be based on a different principle. These four principles are:
• No moving parts.
• Reliability.
• Flexibility.
• Simplicity.
In the sections below, each of these principles will be explained, and a concept will be described based on the choices within the morphological chart. The advantages and dis-advantages of each concept will then be summarized. These dis-advantages and disdis-advantages are mostly qualitative, because accurate quantitative differences cannot be measured at this stage. The advantages and disadvantages are based on observation in current methods and comments in the reference papers as described in Chapter 2 and Appendix 7.1.
Design of a dispersion system for metal fuels 17
CHAPTER 3. GENERATING DISPERSION CONCEPTS
3.2.1 No moving parts
Moving parts are generally not preferred when working with fine powders because the powder can jam the moving parts, or leakage can occur on the entrance of a moving part. Therefore, it is important that a dispersion system has as few moving parts as possible.
Both a hopper and a bed can be used in this concept, since both these parts cannot move. For the metering of the powder, electrostatic metering has a preference over the other options, since there are no moving parts involved in the examples that are shown in Appendix 7.1.
Mixing in the flow has a clear advantage since the powder does not come into contact with any part other than the walls. For the distribution, a dispersion pipe can be used for the deagglomeration of the powder. This option is used because mixing in the flow can cause some agglomerations. All these choices lead to the following concept:
Hopper/Bed−→ Electrostatic metering −→ Mixing in flow −→ Dispersion pipe The concept can be interpreted in multiple ways, and since there are several ways to
imple-(a) (b)
Figure 3.1: (a) Concept 1a: No moving parts with hopper & flow retardation. (b) Concept 1b: No moving parts with a bed & electrodes.
ment electrostatic metering, the concept is divided into two concepts. Both of these concepts can be seen in Figure 3.1.
Concept 1a uses electrostatic retardation of the flow, which is a concept that still needs to be proven. The concept is fairly simple, since it only requires a variable current for the
CHAPTER 3. GENERATING DISPERSION CONCEPTS
Positive
• Simple to operate.
• Hopper allows for aerosol that can be sustained for a long time.
• Large range of aerosol densities by varying current and mass flux.
• No moving parts, so no leakage of powder.
Negative
• Electrostatic retardation of iron powder is an unproven concept
• Dumping powder in air flow can cause particle build up and unsteady aerosol.
Concept 1b is based on the apparatus from Stiphout [17] and the apparatus from Shoshin &
Dreizin [22]. The concept, which can be seen in Figure 3.1b, uses a bed which can be used to fluidize the particles and allow them to bounce up and down between the electrodes. The air comes in from underneath the electrodes and the stream of air takes iron particles with them as it is forced through the outlet. Although this is quite an effective concept, the downsides are that the density of the aerosol cannot be easily controlled, and that this density varies over time. In addition, Shoshin reported that, since the particles that exit the outlet are generally positively charged, the particles will repel each other. This problem can possibly be fixed by making a dispersion tube that is grounded, as is sketched in Figure 3.1b. This, as with the previous concept, will need to be tested in order to validate the effectiveness. The positive and negative factors of this concept are listed here:
Positive
• Proven concept of dispersion.
• No agglomeration since the charged particles will not stick together.
• No moving parts, so no leakage of powder.
Negative
• Very difficult to control aerosol density.
• Particles repelling each other at the outlet.
• Aerosol is not consistent.
Design of a dispersion system for metal fuels 19
CHAPTER 3. GENERATING DISPERSION CONCEPTS
3.2.2 Reliability
Reliability considers the consistency of an aerosol, and aims to decrease the possibility of a concept failing. A reliable concept has parts that are proven to work, and is able to create a constant aerosol that can be maintained for a long time.
A hopper is a great way to get a consistent aerosol that can be maintained for several minutes. A hopper can also be as large as is required. A stirring device can be added in the hopper to prevent blockage through agglomeration [5]. Venturi vacuum ejectors are already implemented in the current generation of powder dispersion systems. Sympatec uses venturi devices in the latest version of their powder dispersion system used in dynamic image analysis [16]. To create a constant reliable aerosol, a dispersion pipe is the last factor to ensure that the iron particles exit deagglomerated and constantly entrained in the flow. These choices have led to the following concept:
Hopper−→ Vibration −→ Venturi effect −→ Dispersion pipe
(a) (b)
Figure 3.2: (a) Concept 2: Reliability. The main components of this concept are the vibration feeder and the venturi vacuum ejector. (b) Concept 3: Flexibility. The linear actuator enables the air stream to draw a constant mass from the pile.
A sketch of the concept can be seen in Figure 3.2a. The concept has the advantage that all of
CHAPTER 3. GENERATING DISPERSION CONCEPTS
and disadvantages for such a system are:
Advantages
• Good control of aerosol density.
• Hopper allows for aerosol that can be sustained for a long time.
• Both feeder and outlet minimize the chance of agglomeration.
• Modular system.
Disadvantages
• More complex with more input parameters.
• Open system.
3.2.3 Flexibility
A dispersion system that can produce a wide range of aerosol densities can be used in many different set ups, which is a clear advantage. For this reason, the most flexible concept is assembled here.
A linear actuator is one of the most flexible devices for metering the powder. Due to the accuracy of the feed speed and the flexibility in bed size of linear actuators that are widely available, a actuator can be chosen based on the needs of the project. The downside to this is that the actuator acts as a bed, and therefore a hopper cannot be an option in this concept.
For the dispersing, flushing the powder is very flexible since the density of the aerosol can be varied with the mass flux of the inflow. A nozzle can be implemented in the design to reduce agglomeration. All these choices lead to the following concept:
Bed−→ Linear actuator −→ Flushing −→ Nozzle
This concept is sketched in Figure 3.2. with only two input parameters, the concept is simple, and the density of the aerosol has a wide range. The accuracy of this density however, is directly dependent of the accuracy of the linear actuator. Another downside is that due to the use of the linear actuator, small particles may penetrate the sealing of the actuator and cause leakage. This can reduce the lifetime of the system. The advantages and disadvantages of this system are summarized below:
• A lot of flexibility in mass flow and aerosol density.
• Simple to operate .
• Compact design.
Disadvantages
• Linear actuator may cause leakage.
• Accuracy is dependent on actuator manufacturer.
Design of a dispersion system for metal fuels 21
CHAPTER 3. GENERATING DISPERSION CONCEPTS
3.2.4 Simplicity
A concept that is simple has the benefit that the construction is relatively cheap and the apparatus is easy to operate. In this concept, the simplest concept will be chosen.
The use of a hopper is simpler than the use of a bed, since a hopper is widely available and can be filled much easier than a bed in most cases. For the metering, an auger is avail-able for purchase and easy to operate. When it is combined with mixing in flow, there are only 2 parameters that determine the aerosol density, namely the Auger and the air flux. A nozzle is the last part of this concept since it is a simple way to control the flow speed and width of the aerosol. These choices lead to the following concept:
Hopper−→ Auger −→ Flushing −→ Nozzle
A sketch of this concept is shown in Figure 3.3. It is flexible and because the powder is kept away from the motor of the auger, it is not likely to cause leakage. The concept is simple to operate and the auger is reliable. It does, however, have a few drawbacks. The auger is generally not suited for very low powder feed rates, so lower densities would be hard to achieve with this concept. On top of that, flushing powder off the auger is likely to provide an aerosol that does not have a constant density. The advantages and disadvantages of this concept are:
Figure 3.3: Concept 4: Simplicity. A simple auger/hopper combination is the simplest form of a dispersion system.
CHAPTER 3. GENERATING DISPERSION CONCEPTS
• Simple to operate.
• Flexible range of aerosol densities.
• Robust, low chance of leakage.
• Auger is reliable.
Disadvantages
• Not suited for low aerosol densities.
• Not suited for low feed rates.
• Inconsistent aerosol density.