Now that all concepts have been defined, a choice has to be made. The concepts are rated on a couple of criteria. The criteria should be noted such that they can be rated on a scale of 1-5 and have been generated from the strengths of the different systems.
• Reliability
• Flexibility
• Simplicity
• Modularity
• Does the system have any moving parts?
For the first four criteria, a rating of 1-5 is given to the system, where 1 means that the system rates poorly, and a 5 means that the system has an excellent rating to the criteria.
For the last criteria, 2 bonus points are given to a system that doesn’t have any moving parts.
Addition of all the ratings gives a total score with a maximum of 22 points. The concept with the highest score is chosen and will be built. The ranking is displayed in Table 3.2, and the motivations for each concepts will be explained in the next chapter.
3.3.1 Motivation rating concepts
As stated before, a machine should be reliable. The reliability of the concept is rated in the first column in Table 3.2. Concept 1a has the lowest ranking in reliability since the principle of metering relies on a concept that has not been proved yet. The reliability of this concept is very low. Concept 1b is proven, but only on particle scale. There is a chance that the apparatus can’t be scaled up. This needs to be investigated further, and therefore this concept does not score good on reliability. Concept 3 has a metering concept that has been proven with aluminium particles, but in another environment and with other aerosol densities, therefore it also scores low on reliability. Concept 4 has been proven on a large scale, it does have some problems with aerosol density, therefore it scores slightly inferior to concept 2, which has been designed for reliability.
Design of a dispersion system for metal fuels 23
CHAPTER 3. GENERATING DISPERSION CONCEPTS
Concept Reliable Flexible Simple Modular Moving parts Total
1a: No moving parts 1 4 3 1 2 11
1b: No moving parts 2 2 2 1 2 9
2: Reliability 4 3 3 5 2 16
3: Flexibility 2 4 4 3 0 13
4: Simplicity 3 3 4 4 0 14
Table 3.2: Ranking of all the concepts to the criteria. A ranking of 1-5 is given in the first four criteria, and an additional 2 bonus points can be given for a system without any moving parts.
The second column rates the flexibility, the ability of a concept to create an aerosol with a wide variety in density. Concept 1b can produce arosols with a really low density, but high density-aerosols are most likely difficult. Therefore, it scores the worst of all concepts.
Concept 2 and 4 both score three points on flexibility, Concept 2 is highly flexible, but the limitations of the venturi will most likely result in limitations for high density aerosols. Con-cept 4 has the opposite problem, since large densities are no problem, but smaller densities may be a problem. Concept 1a and 3 both have a high degree of flexibility, and will therefore get 4 points.
The simplicity of the design is also taken into account. This applies to both the effort it will take to make the machine, and the effort it will take to operate the machine. For this requirement, Concept 1b scores the worst, since it is a quite complicated design, and it will most likely prove very difficult to operate the machine to maintain a constant aerosol density.
concept 1a and 2 are still quite complex, but are rated a bit better since the machines will most likely be simple to operate. Concept 3 rates equally good to concept 4, because both designs will be reasonably simple to make and simple to operate. The reason that these con-cepts do not receive the maximum score is because both the concon-cepts still consist of different parts, which means that it will still take some effort to make the machine.
A modular system is desirable, as modularity gives the ability to swap out components.
This increases the usability of this machine in other projects, or it can increase the flexibil-ity of a certain system. The first two concepts 1a and 1b are not modular, which is why these have been given a score of 1. Concept 3 has some modularity, since the actuator can be swapped out, which is why this concept has a modularity score of 3. Concept four is already a bit more modular, since the hopper and the auger can be seen as seperate from the nozzle and tube. Concept two is also highly modular with a seperate feed system and me-tering system that can be swapped out. These concepts are scored with a 4 and 5 respectively.
In conclusion, adding up these scores results in a score of 16 for concept 2: reliability. This will be the concept which will be investigated further both theoretically and experimentally, in order to determine if this concept can be made into a dispersion system.
Chapter 4
Building a dispersion system
This chapter elaborates on the design of the dispersion system, and the choices that have been made in the design process. Both of the major component of the dispersion system, the vibrating feeder system and the venturi vacuum ejector, are discussed in this chapter, and the setup of the system is shown.
4.1 General design considerations
In order to design an apparatus, some parameters need to be considered. These parameters have come forth from the experience with the previous dispersion systems, along with some calculations in Matlab. A parameter, β, is introduced. This parameter shows the molar fraction of iron in the fuel. Thus, when β is 0, the fuel contains no iron and is pure methane.
When β is 1, the fuel only contains iron. The assumptions that have been made in these calculations are:
• Diameter of the burner is 3 cm.
• The flame speed of the methane / air flame (β = 0) is assumed to be 0.37ms .
• The flame speed of the iron / air flame (β = 1) is assumed to be 0.05ms. This assumption is based on the limited simulation results that are available in literature (eg.[23] ).
• The equivalence ratio φ = 1.
• All the iron powder in is converted to Hematite according to the reaction: 4F e+3O2 = 2F e2O3.
• The flame speed is assumed to decline linearly with an increasing amount of iron powder.
Using these assumptions, the reaction for the iron-air-methane flame can be written as a function of β;
(1− β)CH4+ βF e + (2− 5
4· β)O2 = (1− β)CO2 + 2(1− β)H2O+β
2F e2O3 (4.1)
25
CHAPTER 4. BUILDING A DISPERSION SYSTEM
Using these reaction rates, the stoichiometry of CH4 : F e : O2 can be described as (1− β) : β : (2− 54 · β). These factors are, of course, also dependent on the equivalence ratio. Since the assumption that the equivalence ratio is 1 has been made, the equation simplifies. The other reaction rates describe the molar fractions Nspecies, and using the molar volume of these species and the diameter of the burner, the mass flux of iron powder can be calculated.
Qtot = π
4 · D2 (4.2)
Vmolaravg = (X
Nspecies· Vmolarspecies)/Ntotal (4.3) QF e= VmolarF e· NF e· Qtot
Vmolaravg
(4.4)
˙
mF e= ρF e· QF e (4.5)
Similarly, the mass and volume fluxes for the oxidisers can also be calculated. A dust cloud density can then be calculated by:
ρaerosol= m˙total
Qtotal (4.6)
All these parameters are plotted in Figure 4.1. The iron mass flow shows a spike at β≈ 0.8.
This shows that, as the mixture changes from a 100% iron powder to a mixture with a little bit of methane, the velocity increase of the mixture is larger than the iron powder decrease of the mixture, causing an increase in iron powder flow up to a maximum at β≈ 0.8.
Figure 4.1: Modelling of the first assumptions. These figures give a first estimation of the mass flow and air flow that is required for the dispersion system.
CHAPTER 4. BUILDING A DISPERSION SYSTEM
As can be seen in Figure 4.1, the dispersion system will need to be designed for an iron mass flow of approximately 5− 40mgs , and an oxidizer flow of approximately 2− 15minl . As stated before, the hopper system should be designed such that it can sustain an aerosol for at least 10 minutes, and the system should be designed such that is both modular, and the outflow is as constant as possible.