## 4 Regulation Method and Characteristics Analysis  
### Pump Bypass Model and Control Characteristics Subsection  
For the parallel-bus-type single-phase fluid loop thermal control system, due to the large number of downstream terminals and the internal components, heat dissipation and resistance characteristics of various loads are not consistent. In addition, the power consumption and operating mode of each load are different, and the flow and temperature demand of each load branch will be different. The key to the optimal design and reliable operation of the thermal management system is to establish a stable and reliable control strategy and method to ensure high robustness and strong stability of the system under complex and variable conditions. The TCB of single-phase fluid circuit is a multi-parallel branch thermal-fluid network driven by centrifugal pump, and the drive unit is the main power equipment of the system. It takes the role of providing the drive force for the fluid network and the basic control parameters of the bus, whose working characteristics determine the basic key parameters of the whole system, such as the operating pressure, the pressure difference and the flow rate. According to the design principles of the drive unit in Section 2, the abstract simplified model of the basic pressure flow characteristics is shown in Figure 11a below:  
According to the design principle of the drive unit, the basic fluid network can be composed of the drive pump and the resistance element in the unit; equalize the components with fixed flow resistance coefficient such as flow sensor, filter, bubble capture and separation device, one-way valve, etc. The equivalent resistance characteristics model of the drive unit is  
\[\Delta P_{o}=\Delta P_{p}-(\xi_{1}+\xi_{2})Q_{mp}^{2} \tag{1}\]  
where \(\Delta P_{o}\) is pressure difference in TCB (kPa); \(\Delta P_{p}\) is pressure difference in pump (kPa); \(\xi_{1}\) is equivalent flow resistance coefficient of pump outlet; \(\xi_{2}\) is equivalent flow resistance coefficient of pump inlet; \(Q_{mp}\) is the flow rate of the pump. Figure 10: Design principle of secondary circuit thermal control terminal. From Formula (1), it can be concluded that the output pressure characteristics of the drive unit are related to the characteristics of the working point of the pump and the equivalent flow resistance coefficient of the main flow path. In order to ensure the relative independence and stability of the thermal control parameters between each rack when the thermal control system switches the operating conditions, a pump bypass structure is constructed to control and regulate the pressure of TCB. If the working speed(\(n\)) of the drive pump is constant, the relationship between the output flow rate and the pressure difference between the inlet and outlet of the pump is  
\[\Delta P_{p}=f_{p,n}\left(Q_{mp}\right) \tag{2}\]  
When the flow rate of the thermal bus \(Q_{m0}\) is adjusted in the range of \([Q_{m0,min},Q_{m0,max}]\), assuming that the thermal bus working pressure difference \(\Delta P_{o}\) is set in \(\Delta P^{*}\):  
\[\Delta P_{0}=\Delta P^{*} \tag{3}\]  
The flow pressure output characteristics of the drive pump at the rotational speed(\(n\)) are as follows:  
\[\Delta P_{p}=f_{p,n}\left(Q_{mp}\right) \tag{4}\]  
The relationship between the working flow rate of the drive pump and the pressure difference in the TCB is obtained as follows:  
\[\Delta P_{o}=\Delta P^{*}=f_{o,n}\left(Q_{mp}^{*}\right)=f_{p,n}\left(Q_{mp}^ {*}\right)-(\xi_{1}+\xi_{2})Q_{mp}^{*2} \tag{5}\]  
The operating parameters (\(Q_{mp}^{*}\), \(\Delta P_{p}^{*}\)) are determined. The speed of the drive pump needs to be increased to ensure \(Q_{mp}^{*}>Q_{m0,max}\).