I am studying the thrust distribution of jet engines. But I'm now a bit confused.

In Rolls-Royce's "The Jet Engine" book, enter image description here 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?

  • $\begingroup$ Is the text you are quoting the text from RR's book, or text written by the person who posted the picture on the blog site you link to? Your understanding seems perfectly fine to me, I would like to verify the context of the diagram. $\endgroup$
    – Penguin
    Jun 12 '18 at 10:52
  • $\begingroup$ It's from RR, See P.218 Paragraph 3 in docs.google.com/file/d/0Bx0MqOfev7dnS01DMmtqSmhoVmc/edit $\endgroup$
    – Jono
    Jun 12 '18 at 13:04
  • $\begingroup$ because the inner surface of the nozzle faces front so the gas pressure pushes it back, very simple. $\endgroup$ Jun 12 '18 at 22:33

But why is the diffuser also providing forward thrust?

The diffuser slows down the flow to ease fuel-air mixing and combustion a bit later. If you only focus on entry and exit speeds, there would be no thrust.

However, if you look at the pressures on the diffuser walls, a different result emerges. Slower flow means higher static pressure, and the total pressure right at the compressor exit is already the highest within the whole engine. The pressure on those widening diffusor walls does indeed push the engine forward because of the forward slant of the pressure vector (which acts perpendicularly to the diffusor walls). Your linked pulse jet page explains this quite well.

Of course, no thrust would result if the flow were not heated and thus accelerated further downstream. So the diffuser all by itself will not create thrust; this happens only when it is placed inside a working jet engine.

And also why the nozzle is providing rearward thrust?

This is not always the case, but here the nozzle has a converging shape which helps to accelerate the subsonic flow and converts the remaining pressure to speed. The walls now have a backward-facing slant, so the pressure vector on them will contribute a backward-facing component. In addition, the high flow speed along the large nozzle walls causes some friction, which needs to be considered, too.

For a comparison, look at the cone behind the turbine wheels. Its thrust contribution only stems from the forward-facing pressure acting on it.

  • $\begingroup$ But can you explain why don't i need to consider the pressure difference when i am calculating the thrust of a fire hose or garden sprinkler, but i need to consider the pressure difference when i am calculating the thrust contribution of a nozzle or diffuser? What makes them so different? $\endgroup$
    – Jono
    Jun 13 '18 at 11:18
  • 1
    $\begingroup$ @Jono: What makes you think that you should not consider the pressures in garden hoses? What makes them different??? $\endgroup$ Jun 13 '18 at 16:48
  • $\begingroup$ It seems like the engine nozzle and garden hose nozzle have a lot of similarities to me. Garden hose: 1. High pressure created by water pump/tap at inlet; 2. Low pressure at outlet (atmospheric?) 3.Water accelerates from inlet to outlet.. and Engine: 1. High pressure created by compressor/ombustor/turbine at inlet, 2. Low pressure at outlet (atmospheric?) 3. Air accelerates from inlet to outlet... But why is the nozzle in engine creating thrust in the same direction as the fluid flow (to the right as the picture above), while the garden hose is creating thrust in the opposite direction? $\endgroup$
    – Jono
    Jun 13 '18 at 21:52

And shouldn't a diffuser be providing rearward thrust, as the outlet speed is slower than the inlet speed, and so $\dot{m}\times(v - u)$ is negative?

Conservation laws in physics are an excellent tool. They let you calculate a lot without looking at the minutiae details of the actual process. And this is great example: you can trivially calculate thrust of the whole engine from the change of momentum of the working fluid. But that won't tell you how the force is actually applied, only the sum of the forces on the entire engine.

The thrust break-down is the minutiae details of the process. And at that level, the only way to create a force is by pressure of the fluid, and since pressure always acts perpendicular to the surface, only the aft-facing surfaces can have forward thrust act on them, while any forward-facing surfaces have negative thrust act on them.

And it's no different in the sprinkler. The pressure inside is acting on all the walls, but there is a bit missing in the nozzle where the water flows out, so the force on the opposite wall prevails.

  • $\begingroup$ ,I also think that diffuser produce rearward thrust,because walls from diffuser are facing backwards and pressure allways act prependicular to the surface.. $\endgroup$ Jul 11 '20 at 15:16

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