When a turbo fan is bolted to the air-frame, it's expected that the engine will operate at flying conditions, and thus the inlet is a divergent nozzle.
https://upload.wikimedia.org/wikipedia/commons/b/b7/Inlet_of_CF6-80C2B2_turbofan_engine.jpg
The nozzle slows down the airflow so that it can be worked on by the engine. On a test bench however, there is no ram effect, there is no need to slow down the airflow entering the engine, rather the airflow itself needs to be sped up. On a test bench the engine is fitted with a bell-mouth duct.
https://www.airlineratings.com/wp-content/uploads/uploads/delta-test-cell.jpg
The bell mouth duct is not only a converging duct, but the thick radius lip takes advantage of the Coanda effect to further accelerate airflow. Instead of having a large stagnation region with sharper radius at the leading edge. The airflow sticks and becomes laminar at the leading edge, which actually reduces losses. This is necessary in order for the engine to ingest enough airflow.
When a turbo fan is bolted to the air-frame, it's expected that the engine will operate at flying conditions, and thus the inlet is a divergent nozzle.
https://upload.wikimedia.org/wikipedia/commons/b/b7/Inlet_of_CF6-80C2B2_turbofan_engine.jpg
The nozzle slows down the airflow so that it can be worked on by the engine. On a test bench however, there is no ram effect, there is no need to slow down the airflow entering the engine, rather the airflow itself needs to be sped up. On a test bench the engine is fitted with a bell-mouth duct.
https://www.airlineratings.com/wp-content/uploads/uploads/delta-test-cell.jpg
The bell mouth duct is not only a converging duct, but the thick radius lip takes advantage of the Coanda effect to further accelerate airflow. Instead of having a large stagnation region with sharper radius at the leading edge. The airflow sticks and becomes laminar at the leading edge, which actually reduces losses. This is necessary in order for the engine to ingest enough airflow.
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