Instrument measurement

0 Introduction Low-temperature liquids have a wide range of applications in aerospace technology; everything from rocket fuel to the oxygen used by astronauts for breathing is stored in the form of low-temperature fluids. Supercritical storage involves storing a cryogenic fluid in a container at a pressure higher than the critical pressure. The container outputs fluid under constant pressure to the supply system. When the pressure in the container becomes too high, the safety valve opens to relieve pressure from the container. Compared to high-pressure gas storage, supercritical storage has a lower system mass. During subcritical storage, the fluid in the container is initially two-phase; upon pressure increase it becomes liquid-phase, and after the fluid is drained, the container is again two-phase. Compared to subcritical storage, in supercritical storage the fluid remains in a single phase at all times (liquid below the critical temperature and gas above it), making it easier to accurately measure the amount of fluid in the system. 1 Experimental study http://co.163.com/netea «Prohibited content; please delete» vp/resource/paper/doc/20061241138094786164/image001.gif Figure 1 Schematic diagram of the experimental setup. The experimental setup is shown in Figure 1. The installation includes: 1 low-temperature storage tank with a volume of 2 cubic meters and multi-layer insulation for high vacuum, a vacuum system, a data acquisition system, and auxiliary facilities. The vacuum system includes: a mechanical pump, an oil diffusion pump, and a vacuum gauge. The data acquisition system includes: a computer, Keithley multi-channel boards and data display instruments, thermocouples, pressure transmitters, flow transmitters, and level gauges. The analog signals collected by the instruments are displayed on the computer monitor via Keithley and the computer’s universal interface bus, with the results stored on the computer’s hard drive. The test medium is liquid nitrogen. A Φ0.1mm copper-constantan thermocouple was used to measure the temperature field of liquid nitrogen in the low-temperature storage tank. A pressure transmitter is used to measure the pressure in the space at the top of the container. At the start of the experiment, the tank was purged with nitrogen and then filled with liquid nitrogen to 90% of its capacity. The static daily evaporation rate is measured, from which the heat loss rate and heat flux density of the storage tank are calculated. The daily evaporation rate was measured; the storage tank was sealed, and the distribution of the temperature field began to be measured. After the system reaches thermal equilibrium, data collection for the experiment begins. The test steps are as follows: (1) Fill the storage tank with liquid nitrogen from a tank truck until 90% of its effective volume is filled. (2) During the filling process, liquid nitrogen absorbs a large amount of heat and boils. Once the liquid level drops and stabilizes, add more liquid nitrogen until the level reaches 90%. Measure the standard static daily evaporation rate. (3) Close the fill and vent valves. The pressure rises until it exceeds the critical pressure, and the pressure continues to increase until the safety valve activates. Throughout the process, the pressure in the container is measured. The working process of supercritical storage of cryogenic liquids is described as follows: after the liquid is filled and the container is sealed. Heat from the environment enters the system through the insulation layer, causing the pressure of the fluid inside the container to rise; the liquid expands and the gas condenses, until the liquid fills the entire pressure vessel. As heat continues to flow into the system, the pressure inside the container increases continuously, exceeding the critical pressure until it finally reaches the desired operating pressure. Once the system reaches the operating pressure, it can begin to function properly. By opening the system’s supply valve, the low-temperature fluid flows out of the container through the temperature control heat exchanger and pressure regulator to meet the required needs. After the fluid is discharged, the pressure in the container decreases. To maintain a constant pressure, heat must be supplied to the container in order to ensure that the system operates at a constant pressure. When the pressure in the container exceeds the discharge pressure of the safety valve, the container is released through the safety valve. The entire process is shown in Figure 2: the process of increasing pressure at constant volume for low-temperature liquids http://co.163.com/netea《Prohibited content; please delete》vp/resource/paper/doc/20061241138094786164/image002.gif, the process of discharging low-temperature liquids in a supercritical state at constant pressure http://co.163.com/netea《Prohibited content; please delete》vp/resource/paper/doc/20061241138094786164/image003.gif, and the process of releasing pressure from low-temperature liquids in a supercritical state http://co.163.com/netea《Prohibited content; please delete》vp/resource/paper/doc/20061241138094786164/image004.gif. http://co.163.com/netea「Content prohibited from being published; please delete it」vp/resource/paper/doc/20061241138094786164/image005.gif Figure 2: The three processes involved in storing cryogenic liquids in a supercritical state. 2. Test results and analysis http://co.163.com/netea「Content prohibited from being published; please delete it」vp/resource/paper/doc/20061241138094786164/image006.gif Figure 3: The variation of pressure over time. As shown in Figure 3, there is a turning point B in the increase of pressure over time. The boosting process is divided into two stages, I and II, with B as the boundary. The pressure rise rate of the container in phase II is higher than that in phase I. In Phase II, the container filling rate is 100%. The pressure rise rate decreases as the initial filling rate increases. http://co.163.com/netea「Content prohibited from being published; please delete it」vp/resource/paper/doc/20061241138094786164/image007.gif Figure 4: Variation of pressure with heat flux. As the heat input into the container increases, the liquid expands and gradually fills the tank, causing the pressure to rise until it exceeds the critical value. The relationship between the pressure inside the container and the heat absorbed by the cryogenic medium is shown in Figure 4. The more heat absorbed, the higher the pressure inside the container ； The higher the initial filling rate, the less heat is required to reach the critical pressure. Figure 5 shows the discharge processes of three liquid nitrogen tests: I, II, and III. Among them, Ⅰ and Ⅱ represent supercritical pressure discharge, while Ⅲ represents subcritical discharge. FG refers to the discharge process after the relief valve is opened. The average pressure drop rates for the three processes are: 1.178 MPa/hr, 0.379 MPa/hr, and 0.310 MPa/hr. That is, the pressure release rate is related to the initial pressure. When the discharge piping is the same, the higher the initial pressure, the higher the average pressure drop rate. http://co.163.com/netea「Content prohibited from being published; please delete it」vp/resource/paper/doc/20061241138094786164/image008.gif Figure 5: Relationship between pressure and time during the supercritical pressure release process. During supercritical pressure release, the amount of liquid remaining in the container continues to decrease. By measuring the liquid level, it is possible to indirectly determine how the amount of liquid in the container and the total mass of the fluid change over time. The decrease in the amount of material inside the container is the amount of liquid discharged. The curve of the discharge volume rate of change over time is shown in Figure 6. The container pressure gradually decreases over time. The average pressure drop rate is approximately 1.2 MPa/hr. The test conditions are: initial pressure of 3.6 MPa, ambient pressure of 87 kPa, and a safety valve diameter of 19 mm. At this time: the maximum discharge flow rate is 220 kg/hr. The discharge flow gradually decreases as time increases. http://co.163.com/netea「Prohibited content; please delete」vp/resource/paper/doc/20061241138094786164/image009.gif Figure 6: Curve showing the variation of fluid discharge rate over time. 3 Conclusions This study investigated the storage properties of cryogenic liquids at supercritical pressures, and the following conclusions were drawn: During the process of increasing pressure in a constant-volume container for cryogenic liquids, once the critical pressure is exceeded, the pressure inside the container increases linearly with the heat absorbed ； The higher the initial filling rate, the less heat is required to reach the critical pressure, and the faster the container pressure rises. Pressure relief process: The pressure inside the container gradually decreases as time passes. The discharge flow gradually decreases as time increases. The higher the initial pressure, the greater the average pressure drop rate. References: 1 Mueller, P.J.; Batty, J.C.; Zubrin, R.M. High-pressure cryogenic hydrogen storage system for a Mars sample return mission. Cryogenics, 1996, 36(10): 815-822. 2 Salerno, Louis J.; Kittel, Peter. Cryogenics and the human exploration of Mars. Cryogenics, 1999, 39(4): 381-388. 3 Heydenreich, R. Cryotanks in future vehicles. Cryogenics, 1998, 38(1): 125-130. 4 W. Frost. Cryogenic heat transfer. Beijing: Science Press, 1982. 5 D.S. Lubbe, Gerrit Kornelius. Modeling the boiling delay upon venting vapor from a saturated liquid-vapor system. The Canadian Journal of Chemical Engineering, 1998, 76(10): 952-958. 6 Pierorazio A J; Birk A M. Dynamic behavior of transportation pressure relief valves under simulated fire impingement conditions. Journal of Pressure Vessel Technology, 2000, 122(1): 60-65. 7 Marino di Marzo. A simplified model of the BWR depressurization transient. Nuclear Engineering and Design, 2001, 205(1-2): 107–114. 8 Birk A M, Pierorazio A J. Effects of pressure relief valve behavior on 2-phase energy storage in a pressure vessel exposed to fire. Journal of Pressure Vessel Technology, 2002, 124(2): 247-2

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