# Dynamic Modeling and Full-Process Simulation of the Core Engine Test Rig Main Test System
## 5 Results and Discussion
### Low-Temperature Operating Condition  
As shown in Figure 10a, similar to the normal-temperature operating condition, the variation trend of the pressure simulation curve at GP1 middle grid is consistent with that of the normal-temperature air supply pressure test curve in the 1240 s rig-regulating process of the low-temperature operating condition, and the maximum error is less than 3%, except that the test curve with only 0.01 MPa measuring accuracy moves down in staircase form and by an amplitude a little bigger than the simulation curve. In addition, the test appears to have air replenishing operation around 1050 s.  
As shown in Figure 10b, the variation trend of the flow-rate simulation curve at GP22 outlet grid is consistent with that of the flow test curve at the engine inlet, and the mean error between simulation and test is less than 10%. In addition, as in the normal-temperature operating condition, the low-temperature operating condition also has the problem of inaccurate test flow rate. In the low-temperature condition, the low-temperature air flow rate is controlled by the valve in front of FS1 and not affected by F11, which remains fully open during 0-1184 s; after the main control valve F4 of the normal-temperature air supply line opens at 125 s, its opening varies around 0.2 and the variation range is less than 0.1, so the changes in the F4 opening and the low-temperature air flow rate only control the total flow rate after mixing of the low-temperature and normal-temperature air, while the variation trend of the flow rate at the engine inlet is similar to that in the normal-temperature operating condition and is mainly affected by the change in the openings of F9 and F10. In low-temperature condition, the modeling accuracy of F9 and F10 valves is higher than that in normal-temperature condition. The Vanessa 30,000 valve throttling model may be associated with fluid temperature. As shown in Figure 10c, the overall variation trend of the total temperature simulation curve at GP22 outlet grid is consistent with that of the total temperature test curve at engine inlet in the 0-1240 s whole process of the low-temperature operating condition, but the flow-field total temperature simulation curve descends at a speed much higher than the total temperature test curve during 30-400 s after the engine main line valve F9 is open. To be specific, at about 30 s, the valve F9 opens (Figure 7b), the low-temperature air below 250 K (as shown in Figure 10f) begins to flow into the pipeline behind F9, and the engine inlet total temperature \(T_{\text{tm}}\) begins to drop. At 55 s, when the relative opening of F9 reaches 0.53, the simulation curve has decreased to 256.48 K, close to the incoming flow total temperature 247.85 K (Figure 10f), but the test curve slowly drops to only 284.65 K and then to 258.55 K till about 400 s. By analysis on order of magnitude, the total temperature simulation curve drops from the normal temperature to near the inflow temperature on the order of tens of seconds, while the test curve costs the order of several minutes. The total volume of all components from F9 to GP22 outlet approximates to 0.78 m\({}^{3}\), and it needs to take about 3 s to complete the displacement according to about 0.5 kg/s average flow rate of engine main line during 30-400 s, as shown in Figure 10b. Even considering the heating effect of heat transfer from the pipeline wall to the internal flow field and the mixing effect of the cold incoming flow with the local retained gas, it should not reach the order of several minutes measured in experiment for \(T_{\text{tm}}\) to drop from normal temperature to a level close to the incoming flow temperature. The main reason why the test curve moves down slowly is that the location of the temperature sensor in the measurement rake is near the pipe wall; the measured temperature is the total temperature of near-wall flow field, while the 1D flow-field simulation result is the average temperature of each grid.