ETW has a closed aerodynamic circuit contained inside an internally insulated stainless steel pressure shell. The compressor with a drive power of up to 50 MW circulates the nitrogen gas around the circuit.
50 MW Compressor
To achieve the desired low temperature of the gas flow and to compensate for the heat input caused by viscous friction in the flow, liquid nitrogen with a temperature of minus 196 degrees centigrade is continuously injected into the tunnel flow through four rakes with some 230 spray nozzles in the short leg of the circuit upstream of the compressor. This liquid nitrogen vaporizes immediately and thus forms the cold gas flow.
Liquid Nitrogen Injection Rakes
Nitrogen Injection Nozzles
The corresponding gaseous nitrogen exhaust is located in the other short leg of the circuit upstream of the stilling chamber and is controlled by valves to maintain constant pressure inside the tunnel.
The gas in the circuit then enters the wide angle diffuser where it passes through two filling screens into the stilling chamber and on through a honeycomb flow straightener and two anti-turbulence screens prior to entering the 12 to 1 contraction and the nozzle which is adjustable for supersonic operation.
The flow then passes through the test section, which houses the aircraft model, through the second throat area, the high speed diffuser, and through a corner back to the liquid nitrogen injection station.
Downstream view into Test Section
The overall layout of the circuit and especially the design of the stilling chamber area, the nozzle and the test section are consistent with the exceptional flow quality required for high Reynolds number testing.