## 4 Feature Prediction and Full-Condition Simulation
### Anti-Surge Prediction and Full-Condition Simulation of ATF System
It should be explained that the outlet valve includes the exhaust valve 2# and the air supply valve 3#, and usually only one valve is opened in actual operation. 2# and 3# valves are same. In order to facilitate the discussion of the impact of leaving the compressor flow on the system, the opening of one of them refers to the system outlet valve. Figure 21 shows the simulation results of full conditions according to the surge limit. The curves in the figure represent the minimum anti-surge valve opening from 10% to 100% of the inlet valve opening, and the different outlet valve opening degrees are opened in turn to avoid system surge. When the inlet valve is less than 20% of the small opening, the system is very prone to enter the surge. To explore its characteristics further, add the minimum anti-surge valve opening of the inlet valve at different outlet opening degrees of 15%. The X, Y, and Z in the figure are, respectively, associated with the opening of outlet valve, inlet valve, and anti-surge valve. Any point on the curve in the figure means that, at that point in the valve opening coordinate, the system will just not enter the surge. Figure analysis indicates that, when the inlet valve opening exceeds 20%, the anti-surge valve achieves full closure (0% opening) when the outlet valve is operating at elevated opening positions. This means that the system will not surge at this time, and there is no need to open the anti-surge valve. When the outlet valve is gradually closed, the required anti-surge valve opening gradually increases from 0 to a certain value. This means that the anti-surge valve needs to be opened at least to the above opening for the system to avoid the surge caused by too small a flow away from the compressor. The projection of each curve is given on the XZ plane, in which no special marks are added when the inlet valve is greater than 20%. Observing these projections, it is found that the change trend is the same; basically, when the outlet valve is less than 11% or so, it is necessary to open the anti-surge valve, and, when the outlet valve is almost completely closed, it is necessary to open the anti-surge valve to at least 45% or so to avoid surge. Figure 20: Comparison of performance curve in starting condition. For the three inlet valve curves at 10%, 15%, and 20% opening, with the inlet valve turned off, the cause of system surge shifts from the low flow rate leaving the compressor to the low flow rate entering the compressor. These three curves in turn exhibit properties different from the others. As the inlet valve opening becomes smaller, the outlet valve opening is very large, which will also cause the system to surge. If the opening degree of the inlet valve is 15%, the outlet valve needs to be opened for surge protection when the opening degree is 31%. When the opening degree of the inlet valve is small, no matter how much the opening degree of the outlet valve is, the minimum anti-surge valve opening degree of the system to avoid surge is not 0. If the opening degree of the inlet valve is less than 20%, it is necessary to open the anti-surge valve. Observe the projection of these three curves on the XZ plane, marked by triangles, circles, and squares. It can be more intuitively observed that, with the decrease in the inlet valve opening, the system is more likely to enter a surge caused by too small a flow into the compressor. So, when the opening degree of the inlet valve is 10%, the anti-surge valve needs to be opened at least 50% of the time to avoid surge. As observed in the XZ plane in Figure 21 (specifically comparing the 15% opening: blue triangles, dashed line and 10% opening: cyan squares, dashed line curves), the relationship between the required anti-surge valve 4# opening and outlet valve opening undergoes a drastic shift when reducing the inlet valve opening from 15% to 10%. This pronounced change indicates that the 10% operating point lies critically close to the surge margin.