The effect of secondary boundary layer combustion of hydrogen on rocket plume heat release characteristics

The secondary combustion between unburnt fuel in rocket plume and ambient air significantly affects the stealth characteristics of rocket motors. In order to reveal the effect of secondary boundary layer combustion of hydrogen on rocket plume heat release characteristics, a two-dimensional axisymmetric model for boundary layer combustion between the rocket plume and air is established. The results identifies significant exothermic effect due to secondary combustion reaction between the plume and air. Compared to the nozzle plume without considering secondary combustion, the average temperature of the plume with secondary combustion increases by 47%. As hydrogen content in the nozzle plume increases from 2.7% to 4.2%, the average temperature of the nozzle plume increases by 17.2% due to enhanced combustion heat release. The secondary combustion process is weakened with the increase of flight altitude. The average temperature of the nozzle plume decreases by 11.8% from 2 km to 8 km of flight altitude. This is because both the pressure and temperature of the ambient air decrease when the flying altitude rises, which leads to lower secondary combustion reaction and heat release rate. Moreover, as the ambient pressure decreases, the hydrogen concentration in the plume decreases due to plume expansion effect. The results indicate that hydrogen content and flight altitude affect secondary combustion by different mechanisms. The hydrogen content directly affects the concentration term of the reaction rate of secondary combustion. However, the flight altitude affects the kinetic rate of secondary combustion by changing both the temperature and partial pressure of oxygen. The present study provides better insight into the interaction mechanism between nozzle plume and ambient air.