I’m reading though the Rolls-Royce jet engine book and it states:
“With Vjet fixed at Mach 1, the new term for pressure thrust allows thrust to be increased by raising exit pressure”
My question is: If velocity can no longer be increased (this is in a choked scenario), then how can an increase in exit pressure increase the thrust? I see how this works in the equation, but it just seems rather counter-intuitive. I thought an increase in pressure meant a decrease in velocity? It also then says that by running the engine hotter, the speed of sound can be increased. So Mach 1 is no longer really 1234.8 km/h?
Another question I have, is that if Vjet (exhaust) is fixed at Mach 1, how is the exit pressure meant to increase? Hasn’t the critical pressure ratio already been reached?
Thanks
In: 2
The way I’ve always managed to make it make sense is like this:
Imagine you have a vast reaction going in a contained space that manages to fully contain the reaction. The exit is small, so the actual velocity is limited. If you increase the size of the exit, as long as the reaction is still under some amount of contained pressure, the velocity should increase. However, if the exit is increased to the point to where the total output of the reaction is allowed to escape the exit, then the velocity should decrease.
But, it’s been a while since I’ve read into that stuff in any detail, so I could be completely fucking wrong.
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