Abstract: In order to understand the operation mechanism of a thermodynamic vent system (TVS) better and optimize its operating parameters, a thermodynamic model is established for a throttling device. The variations of the state parameters during throttling are discussed. The throttling performance of single-phase gas and single-phase liquid is compared. The effects of the pressure and temperature of the pre-throttle fluid (hydrogen and oxygen) on the throttling performance under different throttling back-pressures are discussed and the principle of the Joule-Thomson effect is further revealed. In combination with the practical application of TVS, the utilization of the maximal throttling cooling capacity is illustrated and an optimized TVS operating interval is proposed. The results show that the cooling effect of single-phase gas throttling is more significant than single-phase liquid in the case of no phase change in the throttling process; the temperature of the liquid throttling to the two-phase must be reduced, the latent heat of the two-phase fluid can be utilized, and thus the available cooling capacity is larger; as the heat absorption through gasification leads to the decrease of the fluid temperature such that the pressure drop of 0.5 MPa can produce a temperature drop close to 3 K for liquid hydrogen. For liquid throttling, the pre-throttle temperature and the throttling back-pressure play the leading role in the throttling process, while the pre-throttle pressure can be neglected. Considering the full utilization of throttling cooling capacity and a guarantee of no higher than 90% gas volume fraction for heat exchange, TVS is recommended to have throttling back pressure range of 75- 143 kPa for hydrogen under the in-orbit operation.

Key words: Fluid thermal management, Thermodynamic vent system (TVS), Throttling effect, Thermodynamic analysis, Cooling capacity