Discussion of flowability results

Flowability of a certain powder is inversely proportional to agglomeration. A powder with excellent flowability properties will most likely have very few or no agglomeration. It was hypothesised in studies [9][10] that the particle size, sphericity, and particle size distribution are parameters which influence the flowability of a powder. It was shown that at an approx-imately constant sphericity, smaller particles are more likely to agglomerate under the same conditions, and thus have a lower flowability. A clear indication that a spread in particle size distribution also increases the agglomeration rate was not found. It should be noted that the current study made use of the iron powders available at the Eindhoven University of Technology. A more thorough investigation of the individual parameters with more powders is neccesary if a better understanding of flowability is to be obtained.

As can be seen in some of the results, especially those which use powder with a smaller diameter such as the Sigma Aldrich powder in Figure 2.4b, the agglomerations are severe and can grow up to 100 micrometer. If these clumps of powder get through the dispersion system, they could give some problems in the heat flux burner. It would therefore be bene-ficial if the dispersion system could break these large agglomerates. Furthermore, this study has only taken the physical properties particle size, sphericity and particle size distribution into account. Studies have shown that the moisture content is also an indication for flowa-bility, and decreasing the moisture content in a powder by employing drying methods might be beneficial for the flowability of the powder [11].

Design of a dispersion system for metal fuels 13

CHAPTER 2. DISPERSION & IRON PARTICLE PARAMETERS

(a) (b)

(c) (d)

Figure 2.4: Images from different types of powder, captured by the scanning electron micro-scope (SEM). Image (a) and (b) show the Sigma Aldrich powder, which has a reported mean diameter of 5-9 micrometer. Image (c) shows the Gongyi City Meiqi Industry powder, which has an average diameter of 10 micrometer. Image (d) shows TLS Technik powder that has a mean diameter of 20-50 micrometer. From these images it is clear that the relatively large TLS Technik powder is very spherical when compared to the other powders. This powder also shows no sign of agglomeration.

Chapter 3

Generating dispersion concepts

This chapter describes and rates concepts for the dispersion of iron powder based on the literature. One system is chosen to be built.

3.1 Morphological chart

Concepts are generated using a morphological chart. This chart enables the generation of new ideas by using the information of current dispersion systems to create a list with the four basic functions of a dispersion system; storing powder, metering powder, dispersing powder and deagglomeration and/or distribution of the powder. For more information on the references to the different authors in this chapter, the reader is referred to Appendix 7.1.

The chart with all the possible options for the functions of the dispersion system can be seen in Table 3.1. These options will be elaborated in following sections.

Storing Metering Dispersing Distributing Hopper Rotating disk Flushing Nozzle

Bed Linear actuator Fluidized bed Dispersion pipe Valve Venturi effect

Auger Mixing in flow Electrostatic

Vibration

Table 3.1: Morphological chart with all the options for a dispersion system that have been introduced in Chapter 2.

Storing powder

Storing of powder indicates the means to store the powder that is about to be dispersed.

This can be done in one of two ways. The first method is using a hopper, a hopper has a large storage capacity, but it has a problem of being guided by gravity. Therefore, a hopper can not be applied in all systems. For systems that cannot contain a hopper, a fluidized bed is usually the only option. The fluidized bed is a small storage where the powder on the bed will be dispersed continuously. A disadvantage of the bed in contrast with the hopper is the relatively small powder storage capacity, an advantage is the minimal space required compared to a hopper.

15

CHAPTER 3. GENERATING DISPERSION CONCEPTS

Metering powder

Metering of powder is needed to ensure a constant volume flux. In an ideal scenario, the metering of the powder is as accurate and as flexible as possible. Metering of powder is one of the most important parts in a dispersion system. There are a lot of possible methods for accurate metering of powder. The current demonstrator that is described in Section 2.2, and the TSI dispersion system as tested by Chen et al. [12], use a rotating disk to meter the powder. This disk system relies on gravity to fill the disk from a hopper, and the rotation can be varied to vary the volume flux of the powder. Another way to vary the flux is with the use of a linear actuator. this concept has been demonstrated by Risha et al. [13], the actuator acts as a packed bed where the powder is continuously flushed into the outlet nozzle.

If a powder is free flowing, a valve can be used to meter the powder. This concept is shown by Prenni et al. [14]. In their work, a solenoid valve is used to control the volume flux of powder. An Auger was also used in the literature [15] [16]. An auger works really well with a hopper, but has the disadvantage that metering accurately is rather difficult. Some examples of electrostatic metering have been shown by Stiphout [17], Chen et al. [18] and Olansen et al. [19]. While most of these methods rely on a packed bed instead of a hopper, the results show that metering of iron powder can be done accurately. However, since this method is relatively new, testing of accurate metering is required. Matsusaka et al. [20]

showed a method of metering using vibration, and showed that this is a reliable and accurate way of metering.

Dispersion of powder

There are several methods of dispersion. Dispersion of a powder is a way to entrain the metered powder in a stream of gas, after which it can be ejected. One way that is often used is flushing the powder. This method is employed by Chen et al. [12], student team SOLID [5] with a rotating disk design and by Risha et al. [13] with a linear actuator design.

This method is relatively simple and ensures that all powder will be displaced. One downside however, is that it is uncertain whether all the powder ends up in the nozzle, since the inserted gas will be flushing throughout the system. A good design is needed to prevent leakage in the system. Another method of dispersion is the fluidized bed which is demonstrated by Prenni et al. [14]. A packed bed of powder is fluidized and the aerosol of nitrogen and particles is gradually fed out through the outlet. While this is a good way to achieve a constant aerosol, the packed bed does not enable a sustained aerosol for an extended period of time.

The venturi effect, or the sucking in of powder into an air flow, has been demonstrated by Sympatec in their powder dispersion system [21]. The suction of particles does have the advantage of gaining more control over the trajectory of the particles, however, the inflow of air has to be accurately controlled to maintain a sufficient vacuum flow. The last option for dispersing powder is the mixing in a flow of air. This principle is demonstrated by Atkins [15], who used gravity to drop the powder from an auger into an air stream. This is the simplest solution, but it can be a problem with powders that are likely to agglomerate, since they might stick to the walls.

CHAPTER 3. GENERATING DISPERSION CONCEPTS

Deagglomeration / distribution of the aerosol

As suggested in the previous sections, it would be preferable to deagglomerate powder clumps so that the powder is distributed in a constant aerosol. Distribution of the aerosol is every-thing that happens from the dispersion to the point where the aerosol exits the system. One of these methods is using a nozzle. A nozzle is a small orifice which turns the aerosol into a constant spray, it is often used in paint spray and coatings. The other method is a simple dispersion pipe. The length of the pipe helps in dispersing the powder since agglomerations are believed to be reduced as clumps of particles collide with other clumps or the wall.