Instrumentation and operation of NDA cryogenic wind tunnel

The NDA´s (National Defense Academy of Japan) 0.06-m×0.3-m cryogenic wind tunnel was constructed in 1985 for testing transonic airfoils and for other basic research of fluid mechanics. Stainless steel SUS 304 was chosen as the material of the pressure shell, and a centrifugal compressor was chosen as the compressor. External insulation was adopted for the tunnel. Although no information was available on problems of using SUS 304 as the externally insulated tunnel pressure shell at cryogenic temperature and the thermal conductivity of SUS 304 is worse than that of aluminium alloys, only eight thermocouples were installed to monitor the thermal condition of the shell. The original temperature control was achieved by manually controlling the mass flow of liquid nitrogen injected into the tunnel circuit, but that system was found to be inadequate because the settling time of the total temperature took about 15 min in the change of rotational speed of the compressor. The total pressure control systems were modified to simple automatic PID (proportional, integral, derivative) controls. As a result, the pressure control can be achieved almost perfectly, and the temperature control is also greatly improved as the settling time of temperature is greatly reduced