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 17th Dec 2005, 11:53 #1 (permalink) Join Date: May 2005 Location: varies..a lot Posts: 340 Question on compressors and diffusers Hi all, hope it's ok to ask this question here, if not I am sure I shall be moved... When air enters the inlet, it is compressed by the compressor, increasing its mass and temperature, so pressure energy increases and volume decreases. So in this case does the static pressure increase and dynamic decrease? In the case of a centrifugal compressor, the air is accelerated radially outwards, and my AGK notes state that velocity increases and pressure increases. Now if you apply the mass flow is equal equation, in this case acceleration will increase dynamic but as a result, static must decrease, so total pressure essentially remains the same? So I don't get what goes on. The vanes of the centrifugal compressor diverge, which would slow down the air and decrease dynamic and increase static, but all this does essentially nothing to the total pressure of the air? Can someone in VERY BASIC terms tell me what is happening to the pressure? How is it compressed to begin with? Confused, confused, confused!!!! Would appreciate any help on this, Thank you in advance PM
 17th Dec 2005, 13:13 #2 (permalink) Join Date: May 2001 Location: ask petf Posts: 25 Firstly, let me apologise for what happens next as it's a while since I've flexed the muscles of my engineering degree. To tidy up your first statement, compressing air does not increase it's mass. Also, discussing pressure change across jet engines can get confusing rather rapidly so I'll try my best until someone better steps in. Here goes: IIRC, total pressure is the key, and in particular, keeping as much as possible through compression, expansion, etc. 1. Air inlets are designed to present non-turbulent air at a manageable speed to the compressor. Ideally, no work is done and thus total pressure is unaffected. 2. Both axial and radial compressors are doing work on the air and thus the total pressure of the system will increase. If 100% efficiency is assumed, this air is passed across a turbine and imparts all of it's energy as work, thus leaving the system in equilibrium, hence no thrust. Combustion at high pressure is used to fix this. All is neatly covered by a temperature-entropy (T-S) plot, which sounds horrid but is the one thing that underpins most of this and does not require degree maths to interpret. Ultimately, if total pressure is constant, a change in dynamic pressure will drive a change in static pressure. If it is not constant, i.e. work done on/by the air, both may change. So we have: 1. I should read my books more often 2. Mass does not increase, but total pressure changes as it does work, and is worked on. 3. TS plots are useful, try one at this link until a more meaningful answer is posted. http://en.wikipedia.org/wiki/Jet_engines 4. You WILL get a better answer if you put this on the Tech forum. SR
 17th Dec 2005, 14:36 #3 (permalink) Join Date: May 2005 Location: varies..a lot Posts: 340 Hi Slow Roll Thank you so much for that. Yes, you are right , MASS FLOW is increased, not MASS.... I read the text wrong...which almost never helps. I understand most of what you have said, I guess what I don't understand is the two stages of the compression stage. Accelerating the air then diffusing it. How is the total pressure increased? I don't get how work is done to the air and how work done increases its total pressure! 1. When you accelerate the air, you increase its kenetic energy. This would mean a decrease in static energy right? So how is total pressure raised. 2. At the diffuser stage, the air is slowed and is allowed to expand, therefore it looses kenetic , so static increases. Still don't see how the pressure rises, it just seems to change from one state into another. I am sure I am holding on to the wrong end of the stick here. I will try the other forums, but if you have any more to add about this work done to the air I would really appreciate it. Thank you for your previous response. PM
 17th Dec 2005, 15:36 #5 (permalink) Join Date: May 2005 Location: varies..a lot Posts: 340 Thanks! YOu are right, its just the constant not so constant bit I don't get, the rest makes sense. Thank you for a VERY valliant effort! Shouldn't you be doing something more interesting on a Saturday afternoon a week before Xmas than answering tedious questions for people like me? Cheers and Happy Christmas PM
 17th Dec 2005, 16:19 #6 (permalink) Join Date: Nov 2002 Location: uk Posts: 53 powder monkey couple of things to poss make things make sense 1) with pressure increase you get temp increase but velocity decrease 2) conversly then with a velocity increase you get a temp and pressure decrease. Thats how convergent and divergent ducts in compressors work but keep in mind the area that the air is traveling to is decreasing thus maintaining the overall speed of the air through the compressor. I can take you through the compressor in more detail if you wish, ie how the air is behaving at every stage through it or the whole engine.
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