# Dynamic Modeling and Full-Process Simulation of the Core Engine Test Rig Main Test System
## 4 Numerical Model of the System  
### System Model  
The core engine test rig main test system is complex in constitution, there are physical phenomena such as flow, heat transfer, and mixing, and the combined regulation by multiple regulator valves is needed to meet the requirements of air total temperature, total pressure, and flow rate under different operating conditions. Depending on the system operating conditions, there are two modeling options available:  
(1) Establish a numerical model suitable for a specific operating condition only in consideration of the actually circulating pipelines and ignoring the non-circulating pipelines under such operating condition. (2) Establish a complete simulation model based on the actual configuration of the system, in which different regulation time sequence, initial, and boundary conditions are set for different operating conditions. The second option has the most components. Although the near-zero flow-rate non-circulating pipeline section seems to bring unnecessary calculation workload, the second option is most consistent with the actual debugging of the test rig and is more versatile and general. Therefore, this option is chosen for the modeling. Figure 6 shows the numerical simulation model of the core engine test rig main test system established strictly according to the structural size parameters given by the drawing. In the modularization modeling, the system is divided into three fluid sources (FS1-3), twenty-four gas pipes (GP1-24), six gas volumes (GV01-6), one gas mixing apparatus (GMA1), two gas steady apparatus (GSA1-2), ten gas valves (GV1-10), and one heater exchange cold gas (HECG1) so as to form a 47-components numerical system. Figure 6: Numerical simulation model of the core engine test rig main test system. Numerical simulations are carried out for two test-rig regulating processes in a 0\(\sim\)1400 s normal-temperature operating condition and a 0\(\sim\)1240 s low-temperature operating condition, respectively. In Figure 6, the length (unit: m), the outer diameter, and the wall thickness (unit: mm) of each pipe are given; the flow-field grid of the pipe-type component is divided into 100 mm/grid; the length of the lumped parameter component (such as gas valve, gas volume, etc.) in the direction of pipeline is two standard grids, i.e., 200 mm; and the total volume of the air source (composed of many high-pressure cylinders) is set according to specific operating condition, i.e., 10,000 and 500 m\({}^{3}\) for normal-temperature condition and low-temperature condition, respectively. The simulation gives initial flow field and pipe-wall temperature field distribution which are close to the actual situation, and the test data are used for FS1-3 three fluid source boundary conditions. The pipe-wall radial 1D heat transfer model is used for the GP, GV, and GSA module components; the wall 0D heat transfer model is used for the GVol module components; and the wall adiabatic treatment is given for the GMA module component. The total number of wall radial grid serial number of the components using the 1D heat transfer model is four. The heat exchange between the exterior walls of various components and the outer environment is considered as simple superposition of convection heat transfer and radiation heat transfer, where the convective heat transfer coefficient is set at 6 W/(m\({}^{2}\)K) for the sections of normal-temperature air, at 10 W/(m\({}^{2}\)K) for the section behind GP21, and at 0.1 W/(m\({}^{2}\)K) for the section with coating layer. The outer environment temperature is 291.65 K in normal-temperature operating condition and 289.25 K in low-temperature operating condition.