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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. |
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This design can help when there is a mismatch in the fuel and oxidizer stream momentums, which may result in an unfavorable resultant angle (e.g. that could spray flames towards the chamber wall instead of the nozzle exit). Additionally, increasing the amount/momentum of propellant impinging generally also improves the mixing of the propellants.
Pintle
The pintle injector design typically consists of just one element and works by impinging a central radial flow with an outer axial flow as shown below.
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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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