| comp wheel to "hang on" to the air as it is being driven to higher pressures, it requires that the gas velocity leaving the comp wheel be at sufficient velocity/inertia to prevent the pressure from blowing back through the compressor wheel. I've seen people talking about surge a bit more than usual lately and there is a misconception that I believe they have. When you hear the flutter at the end of a blowoff event, this is not compressor surge. What you are hearing is the buffeting of the blowoff's valve. At the end of a blowoff event the valve is closing to its seat. Part of what opens the BOV is the pressure within the pipe. As the valve seats at the end of a BO event, there is still rotational inertia within the turbo's rotating group and it will cause the pressure within the charge pipes to slightly increase - enough that the valve will slightly open again to release a little more air. It will do this in quick succession at the tail end of a BO event. This is not compressor surge - this is valve flutter. Compressor surge, the thing that destroys turbobearing assemblies, will occur at really low airflow and higher pressure ratios. An improperly sized compressor/turbine/housing arrangement can result in spoolup rates that drive the compressor to high pressure ratios at really low flow volume and can result in compressor surge. When this occurs, it happens at a frequency much much higher than what you hear in a blowoff valve flutter. In my small gas turbine projects, I built a turbojet engine using a stock 5-speed turbocharger. The problem with using these smaller gas-engine turbos is that the turbine is undersized for the compressor wheel. This results in a condition where the compressor wheel is operating in surge, and I went through a few rotating groups/bearings before I discovered this was occuring. It would very quickly destroy the bearings - in less than a minute running in this condition. I could also feel a very high-rate "blow-back" from the compressor inlet with my hand placed about 8" directly in front of the compressor inlet - this was surge at its best - literally where the compressor wheel can no longer hold back the pressure and the air will violently shuffle back and forth as it moves through the compression stage. Compressor surge and blowoff valve flutter are two entirely different phenomena and should not be confused.... =)
Enthusiasts soon understand each other. --W. Irving.
Are you an enthusiast? If you are out to describe the truth, leave elegance to the
tailor.
Albert Einstein
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