Thermodynamic Diffuser: Explaining Dynamic Pressure & Thrust

[jaredokie]

Can someone explain dynamic pressure and the pressure in the outlet of a diffuser nozzle? Its isentropic and follows the Bernoulli energy balance equation. But the pressure in the outlet is close to atmospheric pressure and velocity is significantly less. How can you get thrust from low velocity and atmospheric pressure?

 

[Periapse]

Hello jaredokie,

First, you get thrust anytime there's a transfer of momentum out of the nozzle, which is not necessarily synonymous with a pressure gradient across the diffuser-nozzle system. Especially in compressible flows, the thermodynamics of the flow will also play an extreme part, since the compressible gas equations (from which the compressible Bernoulli equation is derived), are highly coupled with the thermal properties of the flow. If you write down these equations for a typical diffuser-nozzle system you will see that by assuming that the outlet pressure is close to the atmospheric pressure (taken at the end of the nozzle), the exit velocity, which is proportional to the square root of the transferred momentum, is dependent on the temperature ratios between internal components, usually standardized at the location of the fuel injector against the leading cusp of the nozzle. This is essentially how ramjets and scramjets work, so you can google one of those terms if you need more info.