From Stealth316:
Quote:
The wheel rotational speed, in revolutions per minute (rpm), at various values is shown on the flow map as a function of air flow rate and pressure ratio. When air flow is held constant (a vertical line on the flow map), faster rotation means a higher pressure ratio. When the PR is constant (a horizontal line on the flow map), faster rotation generally means more air flow. However, air flow does not appreciably increase after the outer tips of the compressor wheel are moving faster than the speed of sound. When the air flow reaches sonic speeds, the diffuser becomes choked and only very small increases in flow rate are possible even with large increases in wheel speed. Larger compressor wheels have maximum rotational speeds less than smaller wheels because of this limitation.
On the flow map above, the air flow regime to the right of the dotted line marking maximum wheel speed is called the choke area. The choke area is almost never noted on a flow map. To determine the choke area, you can drop a vertical line from about where the fastest wheel speed curve ends on the right side of the map. This vertical line is the approximate maximum air flow the compressor is capable of, regardless of efficiency or pressure ratio.
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The key phrase here is just as you said, SaabTuner, it's not the wheel speed it's "when
air flow reaches sonic speed." Big difference.
I also like the sentence about how the choke area is almost never noted on a compressor map (the map usually stops at 60% efficiency). That's great for us, we have some room to spare.
I wrote to a "turbomachinary" professor in England. His take is as we have noted, the gain in work with "wet compression" over and above cooling the air after compression (intercooling) is that the more air is compressed with the same amount of work energy applied--it's the shovels of air analogy. To recover, the same mass of compressed air without water injection, your need to turn the compressor more to make up for the loss in efficiency. He also said there is no combustion penalty up to 5% water:air mass. Unfortunately, he did not give much of a molecular take on things in fluid dynamics terms.