## 6 Conclusions  
A bus-type two-stage thermal control system based on fluid loop is proposed for the thermal management of large-scale and diversified scientific experimental loads in the payload lab module of China Space Station. The working principle of the thermal management system and the internal design of the key units are introduced in this paper. A kind of control method for the two-stage thermal control system based on \"pump bypass\" and the \"valve resistance ratio\" is put forward. The performance of the thermal control system in CSS is tested and verified. The key parameters on orbit are compared to those on ground. Through the on-orbit test, a large number of data of typical working conditions are obtained.  
After the design of the thermal control system and the theoretical analysis of the control method, it is concluded that:  
* The two-stage thermal management system based on single-phase fluid circuit and parallel bus can effectively solve the system heat dissipation and diversified thermal control requirements of the cluster space payloads.
* Based on the pump bypass structure of the system, the proper increase in working pressure difference in TCB is conducive to improve the flow rate control capacity of the downstream rack, and also to reduce the work flow rate of the circulating pump under the same flow rate output capacity of TCB, which will reduce system power consumption.
* A reasonable valve resistance ratio is the key to control ability in the rack. Too large a valve resistance ratio will lead to an increase in resource costs under the same flow rate demand, and too small a valve resistance ratio will lead to insufficient control and compensation ability.  
Taking the above-mentioned thermal control system applied in module 2 of CSS as an example, after test verification on orbit, the following conclusions can be drawn regarding the design frame of the module and the working parameters of the pump valve:  
The effectively constructed thermal control system supports the thermal management of the payloads in module 2. The pressure control accuracy on orbit is up to 2%. The active heat exchange efficiency of the thermal control system in the rack is more than 80% and may reach 90%. The heat exchange efficiency of the TCB is more than 83%. Compared with the ground data, the parameters on orbit have good consistency. So far, the thermal management systems of the three modules in CSS are working in good condition. The research in this paper also plays an active role in promoting the development of space thermal control technology and thermal management in China.  
Figure 24: Result of heat exchange data analysis of payload rack 1.  
**Author Contributions:** Conceptualization, D.G., Q.S. and J.X.; methodology, D.G. and J.C.; validation, D.G. and Q.S.; formal analysis, D.G.; resources, D.G. and W.W.; data curation, D.G. and J.C.; writing--original draft preparation, D.G. and Z.W.; writing--review and editing, M.X., X.X. and W.W.; visualization, D.G. and Z.W.; supervision, M.X., X.X. and K.W.; project administration, K.W.; funding acquisition, K.W. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This study is supported by a research project undertaken by Technology and Engineering Center for Space Utilization, Chinese Academy of Science, project name: Space Station Applied Fluid Loop System, project number: Y3140621 PN.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.  
**Acknowledgments:** This work was supported by Technology and Engineering Center for Space Utilization, Chinese Academy of Science. All flight products were developed by the scientific research institute.  
**Conflicts of Interest:** The authors declare no conflicts of interest.