I am studying the thrust distribution of jet engines. But I'm now a bit confused.
In Rolls-Royce's "The Jet Engine" book, http://aeromodelbasic.blogspot.com/2012/05/thrust-distribution-distribution-of.html
At the start of the cycle, air is induced into the engine and is compressed. The rearward accelera- tions through the compressor stages and the resultant pressure rise produces a large reactive force in a forward direction. On the next stage of its journey the air passes through the diffuser where it exerts a small reactive force, also in a forward direction
I understand the first part of the paragraph that the compressor is providing forward thrust, as it is pushing (so compressing) air rearward. But why is the diffuser also providing forward thrust? And also why the nozzle is providing rearward thrust?
The similar conculsion is also shown here: http://www.pulse-jets.com/phpbb3/viewtopic.php?t=2183, that the diffuser is providing positive thrust by calculating the pressure force.
From my understanding of basic fluid mechanics, shouldn't a nozzle be providing forward thrust, like the sprinkler in the garden or a fire hose? And shouldn't a diffuser be providing rearward thrust, as the outlet speed is slower than the inlet speed, and so m dot X (v - u) is negative?
What's wrong with my understanding?
It seems like I don't know when I need to add a pressure force term P_out A_out - P_in A_in, in addition to the change of momentum term (m dot x (v-u)), while calculating the force of a control volume.