## 4 Feature Prediction and Full-Condition Simulation
### Anti-Surge Prediction and Full-Condition Simulation of ATF System
According to the prediction results of starting conditions without anti-surge measures in BPNN-MDSG digital twin virtual space, it is found that surge is difficult to avoid at low power and low flow. Improper start-up operations can cause great damage to the system. Anti-surge valve 4# was used for the simulation and performance prediction of surge prevention and control in start-up conditions. Figure 18 presents the opening of valves, confirming that valve 4# maintains identical opening characteristics to those shown in Figure 16. Open the anti-surge valve in the first 80 s of start-up. Figure 19 shows the starting condition prediction after opening the anti-surge valve. By comparing with Figure 17, Figure 19 reveals stable variations in both pressure and flow rate, confirming the system's avoidance of high-frequency surge instabilities induced by inadequate inlet flow. During the period of surge in Figure 17, the flow of anti-surge valve 4# is used as a supplement to the flow into the compressor to achieve surge avoidance. Figure 20 shows the migration trajectory of operating points at start-up with and without surge protection. In Figure 20, the system did not enter surge, and the operating points migrated from the bottom left to the top right along the performance curve while moving up and down with different altitude test demands. In Figure 20, the operating point moves counterclockwise periodically to form a surge ring at a small inlet flow. At the same time, the compressor characteristic curve obtained by the BPNN-MDSG method is given in Figure 20, and each working condition point on the surge ring is basically distributed near the performance curve. In the modeling theory used, the operating point of the compressor is on the performance curve at steady-state equilibrium. When the boundary conditions change, the system enters an unsteady state. At this time, the operating point breaks away from the performance curve at a time step, and there is an unsteady deviation between the flow and pressure calculated by the conservation equation and the flow and pressure calculated by the performance curve. In the next few steps, the deviation is eliminated through the performance curve until the system reaches a new equilibrium state. When the operating point crosses the surge line on the performance curve, the conservation equations  
Figure 18: Valve opening in start-up condition with surge protection. dominate system dynamics. This results in downstream pressure exceeding upstream pressure, causing progressive flow reduction until flow reversal occurs. Until the flow rate decreases to the negative side of the performance curve, the relationship between pressure and temperature returns to the performance curve. This is also the basic principle of the BPNN-MDSG model to simulate and solve the characteristics of surge conditions. This study comprehensively examines surge prevention strategies and control methodologies for the anti-surge valve 4#. When the anti-surge valve 4# is opened, the downstream flow of the compressor is supplemented to the upstream, and the excessive downstream pressure is released. This causes the pressure and flow obtained iteratively by the conservation equation to run unsustainably to the left of the surge line of the performance curve, always on the right performance curve, as shown in Figure 20.  
Figure 19: Mass flow (**a**) and pressure (**b**) of start-up condition with surge protection: (**a**) Flow forecast result. (**b**) Pressure prediction result. BPNN-MDSG simulation can effectively simulate the surge and anti-surge characteristics of the ATF compressor system. This advantage is fully utilized in virtual space, and the full-condition atlas of the actual physical system is obtained through full-condition simulation. In the whole working condition simulation, the inlet valve opening, outlet valve opening, and anti-surge valve are combined to carry out the whole working condition simulation of the system. The simulation covers almost all conditions of the actual physical system.