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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. But wait, you may say – isn't the mass flow rate set by the injector? Well, yes, but the mass flow rate that the injector outputs is dependent on the pressure drop across the injector. An upstream valve will create a pressure drop across itself, which will decrease the injector manifold pressure. This will decrease the dP, which will decrease the mass flow of the injector. When the mass flow decreases, the chamber pressure will decrease proportionally. Thus, the main challenge with throttling is relating ball valve pulse width (if using a servo) to mass flow reduction. 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 that the nitrous injector pressure drop will remain somewhat constant over a wide range of throttle levels.
For regenerative cooling, a 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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