Problems to note when using the nozzle to nozzle test method

摘要

The strange behaviors of the discharge coefficient of the critical nozzle were found when gradually changing the back pressure ratio (Pd/Pu) from the almost zero to the critical back pressure ratio in the nozzle to nozzle test method. When the test gas is Helium, the change amount of the discharge coefficients at these strange behaviors is quite large and seems significantly to influence to flow measurements. The causes of the strange behaviors are different in the ranges of Pd/Pu<0.1 and of Pd/Pu>0.1, and in either case are considered to be due to the measurement principle of the nozzle to nozzle test method that the mass flow rate through the system is constant. Under the condition that the mass flow rate is constant, the discharge coefficient of the critical nozzle on the downstream side in the nozzle to nozzle test method must definitely change in inverse proportion to its upstream pressure change. The flow models are suggested to explain the reason why the upstream pressure changes when changing the back pressure ratio. The more important point of the present results is that the equivalence of the discharge coefficient determined by the general calibration methods and by the nozzle to nozzle test method may be not always hold.