## 4 Feature Prediction and Full-Condition Simulation  
### Characteristic Prediction Simulation of Multiple Operating Conditions Switching  
By verifying the reliability of the BPNN-MDSG digital twin modeling method, feature prediction, performance research, and full-condition simulation research are carried out. The BMPN-MDSG digital twin system can set the control conditions by the user and realize the feature prediction and simulation of experimental and non-experimental conditions in the virtual space. Figure 14 shows the dynamic change in valve opening during multi-operating condition switching. Red dashed lines indicate valve actuation completion and subsequent pressure/flow responses. Green dashed lines denote surge entry/exit transitions triggered by valve actions. The inlet valve 1# and the exhaust valve 2# are fully opened, and the system is in the empty condition. Then close the exhaust valve 2#, open the air supply valve 3#, and the system switches from the exhaust condition to the air supply condition. High-pressure gas is fed to downstream equipment. At the same  
Figure 13: The predicted flow value of each valve under dynamic conditions: (**a**) The mass flow of each valve in condition 1. (**b**) The mass flow of each valve in condition 2.  
time, reduce the inlet valve 1# to 80% opening, and regulate the air supply through the inlet valve 1#. Then, at 100 s, gradually turn down the air supply valve 3# to increase the air supply pressure. Valve 3# is closed too small so that the compressor flow is too low, causing the system to surge. Then, at t = 150 s, open the anti-surge valve 4#, so that the system is out of the surge condition and can achieve high-pressure gas supply. Figure 15 shows the dynamic prediction results of the above multi-condition transformation in virtual space. Figure 15a,b show the flow prediction and inlet and outlet pressure prediction for each part of the ATF compressor system, respectively. Compressor flow and working conditions are mainly manifested in two aspects of the system: the upstream flow into the compressor and the downstream flow out of the compressor. Compressor inflow primarily comprises intake flow through valve 1# and recirculation flow through the anti-surge valve 4#, while outflow consists predominantly of discharge through the waste gas valve 2# and supply flow through valve 3#. In a steady state, the flow rate entering the compressor, the current flow rate of the compressor, and the flow rate leaving the compressor are equal. The system is in equilibrium. When the valve is operated, the valve resistance changes, and the balance between the pressure of the equipment before and after the valve and the valve flow is broken. Pressure and flow vary concurrently, propagating through the entire system via the working medium, ultimately driving the system toward a new equilibrium state. According to the simulation results, at 50 s, the discharge condition is switched to the gas supply condition, both flow rates belong to the flow leaving the compressor, and the switching has little impact on the overall balance of the system. According to the predicted results, the pressure before and after the compressor did not change significantly at this time. When the total amount of a certain type of traffic is kept constant, the dynamic change in the system is small. On the contrary, when the inlet valve 1# and the supply valve 3# are changed separately, the overall working condition of the system undergoes a great change. When the performance is about 100 s, the compressor outlet pressure rises sharply, until the supply valve 3# is opened too small, causing the system to surge. When a surge occurs, the work capacity of the compressor decreases, and the  
Figure 14: Valve opening for multiple operating conditions switching. gas pressure downstream of the compressor is higher than the pressure rise in the working medium brought by the compressor. Pressure and temperature vibrate periodically.