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Currently, we are thinking of two ways to do this. The first is to make "mock injectors" that have smaller/less orifices to account for the increase in dP. So, we would run fluid through the system at nominal pressures, but with a smaller orifice area. The orifice area , or simply less orifices. This can be sized designed to perfectly offset the greater dP, such that a smaller total injector area perfectly offsets the larger dP. The other method we are thinking of is to cold-flow test . For nitrous, this approach is feasible, as we can just decrease the number of slots on the pintle. However, for the IPA, making the annulus smaller than it already is (it is nominally 10 thou) will be a machining headache. Therefore, for the IPA, we will use another method. This other method consists of water flowing at off-nominal conditions, i.e. with manifold pressures equal to injector dP during hotfire + atmospheric pressure so that the dP across the injector for this cold flow is the same as the hotfire. We prefer the second option much more than the first, as we wouldn't need to make mock injectors with even smaller orifices (since our engine is small, it might not even be possible to make our fuel annulus smaller). But we haven't seen people do it this way, so we need to look deeper into whether there's anything wrong with this method before we decide to do it. I don't see anything wrong with the strategy at the moment though – as long flowrates in a cold flow are equal to the flowrates during a hotfire, I would expect valve calibration for that coldflow to also work similarly for a hotfire.We could also combine these two methods. The main reason why we don't like the "mock injector" method is because it would require us to make our annular gap even smaller than it currently is (6 thou) which is borderline impossible. We have talked with some engineers and they said the main concern with method #2 is cavitation. We still don't understand how cavitation would be different for method #2, as our current understanding is that it only depends on injector pressure drop. If we find that different amounts of cavitation are a concern, we can use method #2 to test our annular flow (since that's incompressible fuel) and method #1 to test our nitrous mass flow. For the nitrous, we will just make an aluminum cylinder with less radial holes to offset the greater pressure drop. This would be really easy to test – another testament to why pintle injectors are so awesome.
All of this, however, is just to obtain the Cd of our flow at nominal flow rates. When throttling down, our flow rates will change. Since our fuel is incompressible, its Cd will remain constant when we throttle down, so we can use the same Cd for all throttle levels. However, for the nitrous, this is not the case. This is because we are modeling the nitrous as an incompressible fluid (which it is not) and wrapping all of its flash-boiling into a very low Cd. However, the amount of flash-boiling is governed by the dP across the injector. If the dP is high, more flash-boiling will occur; if the dP is low, less flash-boiling will occur. This means that as we throttle down, our Cd will increase because the injector dP decreases, which makes the nitrous behave more like an incompressible fluid. Currently, we are thinking of obtaining Cd at a bunch of different injector dP's, and then fitting that data to a curve. If we know how Cd changes with mass flow, we can characterize our throttle valves.