I think there may also be some mixing here of the term "pressure". Static pressure and flow pressure (or total pressure) are somewhat different things.
"evaporating along the way, perhaps explosively. Locally, liquid water is being transformed into gas, along with the air being compressed."
When thinking of this problem remember that the turbocharger's compressor wheel only accellerates the gas. The compression takes place in the diffuser scroll in the housing. Most of the evaporation/boiling should take place towards the tips of the impeller as that is where the static pressure should be lowest (though velocity pressure quite high). Thank the Bernoulli principle for that one.
While I'm not sure if the water's evaporation would increase or decrease the density of the gas, I think it's fairly safe to say that the evaporation of water in the compressor would significantly change the flow through the compressor.
The reason I say that is that on the forward facing edges of the blades there is much higher static pressure than on the backwards facing edges. Since more water will evaporate in lower pressure, the low pressure regions may either get MUCH lower, or get much LESS lower, depending on what effect the water has on static pressure when it evaporates. In either case, it would dramatically change the flow field through the impeller.
Adrian~
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