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But how would we find it, anyway? In some cases, we can estimate the droplet size using published empirical formulas (based on the chosen injector design/flow characteristics). We can also use computational fluid dynamics methods (abbr. CFD) to computationally model the flow and estimate it that way. CFD is a little difficult to learn and use properly, though, but it is a very popular tool for verifying flow characteristics.
Impingement distance
The impingement distance can be defined as the axial distance the propellant travels before impinging with the other propellant. This is an important metric because it tells you at about what point in the combustion chamber you can expect most mixing to begin, and subsequently, a general idea of after where combustion will happen, which is good to know for thermal analysis and making sure your injector doesn’t melt. |
There’s no real desire to have it very close to the injector (which can unnecessarily heat the faceplate) or to have it very far from the injector (which means your combustion chamber will have to be even longer). There are some rules of thumb found in the literature for what impingement distance you should aim for based on your injector type, and they are usually measured relative to the diameter of the orifices in the injector.
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Like impinging jets (also known as self-impinging) allow the separate propellants to atomize very well and break up into very small droplets, and result in fan streams that then mix the two propellants for combustion.
Doublets/Triplets/etc.
It may also be beneficial to increase the number of streams impinging at a single point from 2 to 3 or more.
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The injector gets its name from the pintle-shaped bit (that green knob thing on the far right of the figure below) that allows the central flow (the red below) to turn from going towards the nozzle exit to spraying radially, more towards the chamber walls. It then impinges directly onto the outer flow (the blue below) which is going axially down the chamber. The result of their combined flow is a cone of liquid film.
→ From feed system | → Nozzle exit |
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On the other hand, this design is not good for very viscous propellants because of the behavior of the swirlers, which reduces atomization of these kinds of propellants. In addition, modeling the flow behavior inside the swirlers can become quite complex, especially in the inner swirler because of the presence of a central gas core that forms as the propellants swirl around.
Inner/External Mixing A lot of design choices can be made that affect the overall behavior of this type of injector. One of the major choices is choosing whether to design for inner or external mixing. External mixing is when the inner and outer cones are able to exit directly into the combustion chamber and mix that way. Internal mixing is when the outlet for the inner swirler is recessed far back enough such that the inner cone collides with the wall of the outer swirler, pictured to the right. |
This choice primarily affects the flow behavior (internal mixing can be more difficult to model), and the resultant angle, but there isn’t very much research to back up giving a more specific description than this.
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