Turbocharger compressor maps
Turbocharger compressor maps
(OP)
Hi,
I am having a difficult time wrapping my head around a question regarding turbocharger compressor maps. If anyone could provide an explanation it would be very much appreciated!
Turbocharger compressor maps essentially plot 4 variables on a 2D plot - mass flow, pressure ratio, rotor speed, and efficiency.
Many compressor maps show what I would expect to see. For a given rotor speed the mass flow increases as pressure ratio decreases (inversely proportional). If you follow a line of constant rotor speed, you can pick a specific mass flow OR pressure ratio and only ONE unique point exists. This is what I would expect. See map1.jpg (attached) for an example.
On other compressor maps the mass flow and pressure ratio both increase as we follow a constant rotor speed line starting at the surge line and moving to the right. This means we have two different mass flow rates for the same pressure ratio while holding a constant rotor speed! How can this be?
See map2.jpg (attached). If we look at the green dots both points have a pressure ratio of ~2.45 and a rotor speed of 111981 rpm yet the left point has a mass flow rate of 19 lb/min and the right point is flowing 34 lb/min. What is causing the difference in mass flow rate between the points? Efficiency?
Thanks in advance for any clarification.
http://files.engineering.com/getfile.aspx?folder=8...
http://files.engineering.com/getfile.aspx?folder=f...
I am having a difficult time wrapping my head around a question regarding turbocharger compressor maps. If anyone could provide an explanation it would be very much appreciated!
Turbocharger compressor maps essentially plot 4 variables on a 2D plot - mass flow, pressure ratio, rotor speed, and efficiency.
Many compressor maps show what I would expect to see. For a given rotor speed the mass flow increases as pressure ratio decreases (inversely proportional). If you follow a line of constant rotor speed, you can pick a specific mass flow OR pressure ratio and only ONE unique point exists. This is what I would expect. See map1.jpg (attached) for an example.
On other compressor maps the mass flow and pressure ratio both increase as we follow a constant rotor speed line starting at the surge line and moving to the right. This means we have two different mass flow rates for the same pressure ratio while holding a constant rotor speed! How can this be?
See map2.jpg (attached). If we look at the green dots both points have a pressure ratio of ~2.45 and a rotor speed of 111981 rpm yet the left point has a mass flow rate of 19 lb/min and the right point is flowing 34 lb/min. What is causing the difference in mass flow rate between the points? Efficiency?
Thanks in advance for any clarification.
http://files.engineering.com/getfile.aspx?folder=8...
http://files.engineering.com/getfile.aspx?folder=f...





RE: Turbocharger compressor maps
- Steve
RE: Turbocharger compressor maps
Regardless of the boundary conditions, like any contour map, in general, there is no fundamental requirement for the contours to be monotonic with respect to any of the axes.
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Turbocharger compressor maps
- Steve
RE: Turbocharger compressor maps
Yes. Alternatively, you could state that the compressor is operating in a partially "stalled" condition when operating away from the best efficiency line.
RE: Turbocharger compressor maps
RE: Turbocharger compressor maps
I think if you track down a good turbomachinery textbook and read the chapter on "Energy transfer between a fluid and a rotor", or something similar that explains the Euler Equation as applied to turbomachines, the answers to your questions will make more sense. If you are not familiar with dimensional analysis (Buckingham Pi Theory), you may want to read up on that, also.
Many books out there explain things well, but my favorite is a book by Shepherd called "Principles of Turbomachinery". Very practical explanation of how these things actually work, yet detail enough to facilitate quantitative performance analysis.
Based on your questions, I think you will enjoy the reading.
Dick
RE: Turbocharger compressor maps
The point is, the compressor map is what it is, there is nothing mysterious or freaky about there being coordinates of identical pressure ratio and speed at two different flow rates.
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Turbocharger compressor maps
For the speed line to dip to the left is unusual because this is an unstable operating condition (surge). That is what the surge line is for - all the speed lines fall if continued to lower flows. Left of the surge line is not plotted because this is a no-go zone - the instability will wreck the compressor if operated continuously in surge. In some cases (like Map 2) the manufacturer may allow operation in an area where the surge is "mild".
To visualise surge, think about gradually restricting the discharge at constant speed, starting at the right of the map, moving the operating point to the left and increasing the pressure as you go - this is stable. If you continue to the point where the pressure starts to fall, the reduction in pressure produces a further reduction in flow through the restriction - leading to a further reduction in flow - leading to . . . . . and so on. The flow goes to zero or thereabouts, as does the pressure and suddenly we have a situation where flow can restart, to fill the low pressure volume between the discharge and your restrictor - and the then the whole cycle starts again. This repetitious cycle is "compressor surge"
Engineering is the art of creating things you need, from things you can get.