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The way we plan on controlling our engine's mass flow rate is by placing ball valves upstream of the combustion chamber that will regulate the mass flow into the combustion chamber. In the case of the nitrous, a reduction in valve opening area will decrease the mass flow across the valve, which will cause the chamber pressure to decrease. The decreased chamber pressure will then induce a greater dP across the valve, which will cause more nitrous crossing the valve to flash boil. This means that the nitrous entering the injector manifold will be at a lower density, since more of it will be a gas. And since mdot = Cd*A_inj * sqrt(2 * rho * dP), a decreased rho will cause a greater dP per unit of massflow. The result is Less massflow but more dP per unit of massflow means that the nitrous injector pressure drop will remain somewhat constant over a wide range of throttle levels. In the case of the IPA, a reduction in valve opening area will also decrease the mass flow across the valve, which will then decrease the chamber pressure. However, although this reduced chamber pressure will cause a higher dP across the valve, the IPA will not boil, which means its density will not change. So, the dP across the IPA injector will decrease. This means that the limiting propellant (in regards to stiffness) is the IPA.
For regenerative cooling, a another limiting factor is cooling efficiency, which is what we aim to get valuable data on for our research. When an engine throttles down, there is less fuel (and ox) massflow, which means less fuel in the regenerative channels cooling the engine walls. We aim to characterize the effect of less fuel flow on the cooling efficiency of our engine.
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