# A Study on Combined Variable Geometries Regulation of Adaptive Cycle Engine during Throttling
## 2 Structural Analysis and Modeling of ACE  
### Operating Modes  
Airflow from the front fan passes through the second variable fan and the outer bypass duct which is called the third bypass. A heat-exchanger is located in the third bypass which can cool the turbine cooling bleed and increase the HPT inlet temperature maximum limit effectively. Then, airflow from the variable fan passes through the CDFS and the outer bypass duct, which is called the front bypass. Clearly, airflow from the CDFS passes through the HPC and the outer bypass duct named the core bypass. The third bypass airflow is exhausted through the fixed third bypass nozzle, while other airflow will exhaust via the variable main nozzle.  
As shown in Figure 2, VSV\({}_{\text{Fan}}\) is used to modulate the airflow passed through the third bypass and the fan system pressure ratio. VSV\({}_{\text{CDFS}}\) is designed to affect the airflow passed through front bypass and the fan system pressure ratio. VSV\({}_{\text{HPC}}\) is connected with the airflow passed through the core and rotor speed difference. VAN\({}_{\text{LPT}}\) is used to adjust LPT expansion ratio and HPT expansion ratio. Pro-VABI is designed to control the static pressure of the core bypass exit via modulation of the Pro-VABI area in order to avoid flow recirculation to the front bypass. Rear-VABI is used to modulate the airflow passed through bypass duct and mixing loss. As is designed to adjust the LPT expansion ratio to control thrust in a wide range of the flight envelope. Through the combined modulation of various variable geometries, ACE has two different working mode alternatives to guarantee superior performance along a wide flight regime [18; 19; 20].  
The third bypass and core bypass are open all the time. When the front bypass is open, it operates at triple bypass mode. In contrast, if the front bypass is closed, it operates in double bypass mode.  
For subsonic cruise and part power operation, ACE operates at triple bypass mode reasonably well to provide higher bypass ratio. The entire inlet airflow is inducted by the front fan. The airflow is then divided between the variable fan and the third bypass. The third bypass flow is exhausted  
Figure 2: The variable geometries of an adaptive cycle engine (ACE).  
through the fixed third bypass nozzle. Meanwhile, the airflow from the variable fan is divided between the front bypass and CDFS inlet as the front bypass is open. Finally, the CDFS flow is divided between the core bypass and HPC inlet because of the open core bypass. The front bypass airflow is mixed with the core bypass airflow at Pro-VABI, requiring a static pressure balance. Then, the other static pressure balance is required at Rear-VABI between the mixed flows and the core flow which are then exhausted through the main nozzle. The gas paths of ACE at triple bypass mode are shown in Figure 3.  
During max power and supersonic cruise operation, ACE operates in double bypass mode reasonably well to obtain a lower bypass ratio with almost all of the front fan flow being passed through the engine core. All the inlet airflow is inducted by the front fan. The airflow is then divided between the variable fan and the third bypass as the third bypass is open. The decreased third bypass flow compared with triple bypass mode is exhausted through the fixed third bypass nozzle because of the larger VSVran. Meanwhile, all the air flow through the variable fan is inducted by the CDFS with the closed front bypass. Then, the airflow is split between the core bypass and HPC inlet because of the open core bypass. Finally, a static pressure balance is required at Rear-VABI between the decreased core bypass flow because of the larger VSVHPC and the core flow which then are exhausted through the main nozzle. The gas paths of ACE in double bypass mode are shown in Figure 4.