Phase change of nitrogen as primary flow in gas channel model of directly drained CW DF/HF chemical laser

To study the starting characteristic of directly drained CW DF/HF chemical laser (DDCL), a double-nozzle small-scale model of DDCL gas channel was set up, and a series of experiments without secondary flow was conducted with nitrogen and helium as primary flow respectively. The data of experiment, which is conducted with nitrogen and primary nozzles of large area ratio, are far away from the theoretical result. It is believed that, a part of nitrogen has phase transition while it is expanding in the experiment, and then the one-dimension treatment is out of place. In addition, the states of cavity fuels and secondary diluent helium will not change in a CW DF/HF chemical laser, but the area ratio of secondary diluent nozzle must not be very large if the secondary diluent is nitrogen.