NOVEL COOLING MEANS FOR A CRYOGENICWIND TUNNEL
用于低温风洞的新颖制冷方法

Existing cryogenic wind tunnels with cooling accomplished by spraying liquid nitrogendirectly into the tunnel circuit must equally discharge cryogenic gaseous nitrogen for keeping thesteadiness of the test flow. The tremendous "cool energy" carried away not only leads to heavydemands on liquid nitrogen and hence refrigerating power but also causes pollution of the localatmosphere.An alternative means of cooling air to cryogenic temperature by relying alone on the compres-sion energy of the air is presented. A bi-directional heat exchanger is installed at the exhaust exitof a wind tunnel. After the cryogenic exhaust and the compressed air pass in opposite directionsthrough which in the meantime, the former is heated to near ambient temperature and the latteris cooled deeply. By means of a thermal separator, the precooled compressed air is cooled furtherto the required cryogenic temperature and then to supply the wind tunnel.For cooling air from 320K to 100K, the thermal separator inlet pressure needs to be 2.5, 3.5or 8.2 times of the exit pressure, if the heat exchanger efficiency is assumed to be 90% and thethermal separator efficiency assumed to be 80%, 60% or 40% respectively.An experimental device for concept studies was set up in the lab. The experimental data areas follows: The pressure and temperature of compressed air at the inlet of the heat exchanger arel.162 MPa and 293.5 K respectively. The temperature and pressure at the inlet and the exit of thethermal separator are 140K, 118.5K and 1.119MPa, 0.13MPa respectively. In test section, theMach number and the temperature of the test flow are approximately one and 100K. The exhausttemperature at the exit of the heat exchanger is 283.9 K. The pressure loss of airflow through thetunnel circuit except the thermal separator is lower than 0.1 MPa. On the basis of the measureddata, it can be calculated that the efficiency of the heat exchanger and the thermal separator areequal to 90% and 33.4% respectively. The abnormal low efficiency of the thermal separator inuse is attributed to bad manufacturing. Otherwise the temperature of test gas and the requiredpressure of compressed air would be lower.The particular virtues of the novel cooling means for a wind tunnel are to save refrigeratingenergy and to protect environment in comparison with existing one by liquid nitrogen.