Question about the 40K speed line.
What is happening when it drops off to the right?
It starts around 60 CFM 1.2 pressure ratio and then drops to 1.0 pressure ratio at around 250 CFM.
Are they adjusting anything on the test stand to accomplish this?
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$\begingroup$ Yes the air delivery is changing. $\endgroup$– Solar MikeCommented Sep 12, 2020 at 3:08
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$\begingroup$ Thank you.What do you mean 'the air delivery is changing'? Are they turning a control while doing the test at 40K? Reducing air supply to the compressor?I have read that the RPM is being kept constant at RPM test points for speed lines.Are they changing something else at the 40K test point? $\endgroup$– The CalifornianCommented Sep 12, 2020 at 4:21
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$\begingroup$ Follow the x-axis. $\endgroup$– Solar MikeCommented Sep 12, 2020 at 4:22
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$\begingroup$ Okay.Are they 'drawing' more CFM from the output side at different test points? While keeping the RPM constant.The pressure ratio is dropping as the CFM increases.Is this arrived at mathematically or are they adjusting some controls on the gas stand? $\endgroup$– The CalifornianCommented Sep 12, 2020 at 4:41
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$\begingroup$ If it is on an engine you would think the engine draws more CFM as the RPM's increase.Thus the RPM of the turbocharger rises.This test seem to be without an engine.This book does have compressor maps with an engine though.This one seems to be turbocharger only.So.How are they varying the CFM during this test.Regarding the 40K speed line? $\endgroup$– The CalifornianCommented Sep 12, 2020 at 17:56
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1 Answer
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If you plot pressure ratio vs air velocity at a constant speed of a compressor, you find that there is some peak pressure ratio, with a decrease if the the air velocity is increased or decreased. This peak pressure is the surge limit, the curve on the left of the chart. Pressure ratio decreases on either side of the maximum because turbine blades are designed for a specific air velocity. This is an inherent property of turbines, not due to any action taken during testing.
This property is important for turbocharger operation, and leads to the importance of operating above the surge limit at a given flowrate. Consider two points on the curve, point (A) to the right of the surge limit (higher flow) and point (B) to the left (lower flow). Note that (B) is not represented on the compressor map (we'll show why in a bit).
- Say that the air flowrate drops at point (A) due to a change in engine load. Then the exit pressure rises, leading to an increase in power drawn by the compressor. This causes the blade speed to drop, maintaining a stable equilibrium
- Say that the air flowrate drops at point (B). Then the exit pressure drops, leading to a decrease in power demand and an increase in blade speed, creating an unstable equilibrium that can damage the compressor or other systems.
Due to its instability, surge is an immensely complicated phenomenon, but this should be enough to start to explain the mechanics behind the compressor map.
