# A Study on Combined Variable Geometries Regulation of Adaptive Cycle Engine during Throttling
## 3 Study on the Method of Variable Geometries Modulation
### Study on the Variable Geometries Modulation during Subsonic Cruise
Although it is beneficial to turn down VSV\({}_{\text{HPC}}\) for constant airflow during throttling, VSV\({}_{\text{HPC}}\) will remain unchanged due to the minimum regulation constraint. At triple bypass mode, turning up VAN\({}_{\text{LPT}}\) will increase thrust due to lower bypass ratio. Directly turning up VAN\({}_{\text{LPT}}\) leads to the rise of HPT expansion ratio. This increase may improve HPT output work. Then, the high pressure rotor work balance is broken and as a result the HPRS tends to rise. Therefore, CDFS suction capacity increases, while the variable fan outlet back pressure decreases. Finally, low pressure compression work reduces. Further, an increased trend of LPRS occurs because of the excess of LPT output work and less low pressure compression work. Hence, for a constant LPRS, less fuel should be injected, which leads to T\({}_{\text{H}}\) declining. Finally, due to the combined effects of lower bypass ratio and higher T\({}_{\text{H}}\), net thrust rises. Because of the changeless pressure ratio of the front fan, airflow is almost the same as shown in Figures 15 and 16. Since airflow is insensitive to VAN\({}_{\text{LPT}}\), it is beneficial to turn down VAN\({}_{\text{LPT}}\) for constant airflow during throttling. When the engine operates at triple bypass mode and LPRS is maintained at a constant level, turning up A\({}_{\text{8}}\) decreases T\({}_{\text{H}}\) and therefore reduces thrust. The effect of turning up A\({}_{\text{8}}\) on airflow and net thrust is similar to the effect at the double bypass mode. According to the histogram in Figures 15 and 16, turning up A\({}_{\text{8}}\) can reduce net thrust and increase airflow effectively. Consequently, under the premise of satisfying the constraints, it is worth turning up A\({}_{\text{8}}\) during throttling. When the engine operates at triple bypass mode and LPRS is constant, as stated earlier, turning up Rear-VABI will increase the thrust due to higher T\({}_{\text{H}}\). Directly, turning up Rear-VABI drives more air flowing into core bypass and increases B\({}_{\text{1}}\), while reducing engine core airflow. Meanwhile, turning up Rear-VABI leads to the decline of LPT expansion ratio. Therefore, a decreased trend of LPRS occurs  
Figure 16: Sensibility analysis of area on airflow during the subsonic cruise. Figure 15: Sensibility analysis of area on thrust during the subsonic cruise. because of the lower LPT output work. Hence, for a constant LPRS, more fuel should be injected. Further, as shown in Figure 15, higher T\({}_{\text{H}}\) leads to an increased net thrust. Because of the changeless pressure ratio of the front fan, airflow is almost the same, which is shown in Figure 16. Since airflow is insensitive to Rear-VABI, it is beneficial to turn down Rear-VABI for constant airflow. Based on the sensitivity analysis mentioned above, the variable geometry adjustment is researched, combining the basic engine working principle analysis during the subsonic cruise in this section. According to the basic engine working principle, the essence of maintaining airflow during throttling is to decrease the specific thrust by modulating variable geometries along with increased total bypass ratio, lower exit total pressure and lower temperature of LPT. Turning down VSV\({}_{\text{Fan}}\) and VSV\({}_{\text{CDFS}}\) can induce a larger percentage of the total airflow through the bypass streams with increased total bypass ratio. As the total bypass ratio increases, flow through the engine core is reduced. Then, LPT output work decreases. This is caused by declined working airflow. The decreased LPT output work will lead to a break in the low pressure rotor work balance. As a result, turning up A\({}_{8}\) can enlarge the LPT expansion ratio to maintain a low pressure rotor work balance. Due to the constraint of the reasonable value of the turbine expansion ratio, turning down VAN\({}_{\text{LPT}}\) appropriately can let LPT work efficiently and obtain suitable T\({}_{\text{H}}\).