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
Thrust comes from mass flow.
The flow is choked so the *velocity* is fixed, but if you increase the exit pressure you increase the density so the mass flux at equal velocity goes up, hence thrust goes up.
Increase pressure = decrease in velocity is Bernoulli’s equation…that only describes a constant energy situation. If you’re raising exit pressure you’re increasing energy input, straight Bernoulli doesn’t apply.
And yes, if you crank the temperature Mach 1 is no longer 1234.8 km/h or whatever it was, it gets higher. Mach 1 scales with the square root of temperature. If you hike the temperature Mach 1 is a higher speed. 1235 kph is Mach 1 at standard temperature (68F). Mach 1 in a jet exhaust is *far* faster.
Critical pressure ratio is the minimum to achieve Mach 1 at the exit; there’s nothing preventing you from increasing the pressure upstream of the choke. You’re above (usually well above) the critical pressure ratio.
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