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I hope you are doing fine. I'm currently designing an injector for a hydrogen peroxide - ethanol engine, and the following formula is confusing me:

Q = Cd * A * sqrt( 2 * dP / rho); where Cd is the discharge coefficient, A the total area, dP the pressure drop, and rho the fluid's density.

Supposing Cd, A, and rho are fixed once we built the injector, the only way to throttle the engine would be changing delta p (dP). What does this pressure differential represent? The difference between the entry and exit pressure of the fluid through the injection plate as a consequence of sheer, gravitational, viscous, etc forces, OR the difference between the injection pressure (taking into account the pressure drop through the plate) and the design chamber pressure? How would you change dP?

Best,

Roy S. Ramirez