The Influence of Aerodynamic/Geometric Parameters on the Performance of the TBCC Ejector Nozzle

For the hypersonic aircraft exhaust system, the ejector nozzle has its place due to its complexity and reliability. The automatically adjustable "pneumatic wall" formed by the ejector secondary flow can improve the over-expansion of the primary flow in low-altitude conditions.

In this paper, the complex flow characteristics of the TBCC ejector nozzle in different working conditions are analyzed. The influence of key aerodynamic/geometric parameters such as primary flow pressure ratio(NPR), secondary flow pressure ratio(SPR), secondary throat diameter ratio(D_s^*), pitch ratio(L_s^*) and sleeve expansion angle(β) on flow field details and ejector characteristics were explored in severe over-expansion condition and over-expansion condition.

As for the static pressure distribution of the sleeve surface, the flow area of the subsonic secondary flow is reduced rapidly due to the compression of the high-pressure primary flow at the main nozzle outlet. In the initial position of the sleeve expansion section, the static pressure increases slightly. This is due to the common limitation of the primary flow and the secondary throat, which results in the minimum flow area of secondary flow The supersonic secondary flow undergoes a weak compression before it continues to expand.

As the Mach number of the aircraft increases, the total pressure of the secondary flow from the atmosphere will be higher than the primary flow, and the high-pressure secondary flow will form a layer of “pneumatic wall”. This pneumatic wall “lengthens” the main nozzle wall along the secondary throat convergence angle, and the primary flow pneumatic throat is separated from the geometric throat, so the area of the primary flow pneumatic throat is reduced, and the mass flow of the main nozzle is mismatched.

As the NPR increases, when the low-pressure primary flow is attached to the sleeve surface for the first time, the ejector nozzle changes from the under-expansion state of the converging nozzle to the severe over-expansion state of the C-D nozzle, so the thrust coefficient decreases rapidly. As the NPR increases further, the airflow separation zone near the outlet surface of the ejector nozzle disappears gradually, and the airflow changes from over-expansion to under-expansion. The thrust coefficient increases first and then decreases.

The introduction of the secondary flow does not change the flow field structure of the ejector nozzle. However, it can effectively improve the initial expansion state of the primary flow, significantly weaken the shock intensity. But the high-pressure secondary flow will cause the primary flow pneumatic throat to detach, which results in a mismatch in the mass flow of the main nozzle.

The increasing D_s^* can increase the secondary mass flow. The secondary flow can more effectively wrap the primary flow, thereby weakening the shock intensity, and the airflow separation zone near the nozzle outlet gradually disappears, so the performance of the nozzle is improved.

The increase in L_s^* causes that the primary pneumatic throat detaches gradually from its geometric throat. The flow field structure of the airflow in the sleeve expansion section is almost constant, and the over-expansion state of primary flow is less affected, so the performance of the nozzle is less affected by L_s^*.

A smaller β means a longer nozzle length, which can make the airflow expand more fully. The airflow in the expansion section is more uniform, the shock intensity in the nozzle is weakened, and the performance of the nozzle is improved. However, the frictional loss caused by the smaller β ejector nozzle is large, which prevents further increase in performance.

The research shows that the geometric parameters that have a significant influence on the performance of the ejector nozzle are D_s^*, L_s^* and β. Other geometric parameters are negligible compared to the above key parameters. The influence of the key parameters in the over-expansion condition and the influence mechanism of key parameters will be explored detailedly in the final paper.