Since the rocket engine for high altitude operates in low pressure enviornment, a nozzle with high expansion ratio is used. if we progress a performance test on the ground, there happens flow separation inside of the nozzle, which makes it difficult to test exactly. Also, there is a possibility of destroying the nozzle because of the flow separation. Therefore, the performance test is progressed when low pressure environment is made around the nozzle. One of the low pressure simulation methods is the Supersonic diffuser. When doing the actual test, the burnt gas is used, which requires the cooling to make sure the safety of the wall of the diffuser. What is analyzed in this paper is the changing performance of the diffuser and flow characteristic inside of it according to the level of the cooling.


1) It was started in the initial pressure P0 = 40 bar, which happened before cooling. However, after cooling, there was heat transfer between the hot burnt gas and the cooling water on the wall of the diffuser, which caused the reduction of the thermal energy of flow itself. Therefore, it did not start.

2) After cooling, the pressures of the vacuum chamber that were started in 50 bar and 60 bar at first were 1380 Pa, 1550 Pa, Respectively, which makes it possible to test at higher altitudes in that the lower pressure inside of the vacuum chamber can be simulated than before it is cooled.

3) There were changes of the figuration of the shock wave on the center of the back side of the diffuser depending on the coolant mass flow rate. As the coolant mass flow rate increased, the magnitude and intensity of shock train from the flow separation that occurred near the second throat to the divergence that showed pressure recovery steadily decreased.
4) The decrease in the power of the shock train has positive influences on the vibration and noise, which can happen inside the diffuser. As the flow rate of coolant increases, the diffuser has an advantage in obtaining the integrity.

5) Through the numerical analysis, the value of the heat flux on the wall of the diffuser depending on the Coolant mass flow rate was drawn and by using this the minimum mass flow rate not to be evaporated according to the operating pressure can be decided. Furthermore, the flux that has the optimal cooling efficiency can be predicted.