Two-phase choking conditions of real gases flow at their critical stagnation temperatures and closely above

Homogenous flow choking conditions as pressure, temperature, speed of sound, and the consequent mass flux are derived for stagnation temperatures at the vicinity of the critical temperature and for a broad range of stagnation pressure (from 0.1 and up to 30 times the critical pressure). Results are presented and studied as relative as well as absolute deviations from the ideal gas predictions. It was found that there is a broad domain of isothermal stagnation states for which the corresponding choking states are of single-phase at saturated conditions. However, there is a narrow domain of stagnation states for which choking occurs inside the two-phase dome. For this domain, the homogenous flow choking is compared to that of the slip flow. Mass fluxes are discussed in view of the principle of corresponding states. On the basic level, a quite unified presentation is obtained by the proper normalization of the choked mass flux. The principle is refined by the observation that the remaining deviation may be organized according to the magnitude of the acentricity factor of each gas. Some practical examples are listed for demonstrating the relevance of results for Joule-Thomson cryocooling.     

Absolute dynamic viscosity measurements of subcooled liquid oxygen from 0.15 MPa to 1.0 MPa

New absolute dynamic viscosity measurements of subcooled liquid oxygen are presented which were acquired in the pressure and temperature domains from 0.15 MPa to 1.0 MPa and from 55.20 K to 90.19 K, respectively. The measurements were acquired with an uncertainty of 1% at a 95% confidence level using a pressurized gravitational capillary (PGC) viscometer specifically designed for subcooled liquefied gases. The measurements are summarized by Arrhenius–Eyring plot parameters (μ = Ae^E/RT), and interpreted with respect to the chemical reaction rate theory of viscosity by Eyring. The Arrhenius–Eyring plot parameters reproduce the dynamic viscosity measurements with only a 2% RMS error, which is remarkable considering just two parameters are involved, A, the factor which includes the weak pressure dependence of the dynamic viscosity, and E/R, the barrier energy of the flow, where R is the universal gas constant. Although the Arrhenius–Eyring plot parameters do not have a discernible pressure dependence in the present work, the pressure coefficient versus temperature for the dynamic viscosity was determined from line plots of the dynamic viscosity versus pressure. The pressure coefficients suggest that the pressure dependence is very weak, yet positive, and increases with decreasing temperature. Measurements at pressures an order-of-magnitude higher are required to confirm this suggestion.     

Study on unsymmetrical phenomena of vapor cooled current leads

Unsymmetrical phenomena of gas flow and temperature distributions between a pair of vapor cooled current leads (VCCL) often occur in superconducting systems, which makes the VCCL depart from the optimum operating status, consequently results in an increased heat leak to the cryostat and even a destroyed safety operating condition of the VCCL. To analyze this problem, a numerical model for the VCCL was built, which is based on the conservation equation of energy for the solid and the conservation equations of mass, momentum, and energy for the fluid. With this model, unsymmetrical phenomena between a pair of VCCL were analyzed. Unbalanced gas flow distribution in a single multi-channel VCCL was also studied. Some conclusions were made for the design of VCCL, and a new type of VCCL with combined positive and negative poles and a helical cross section structure was developed. Test results showed that the unsymmetrical phenomena can be well restrained by using the new type of VCCL.     

Capacitance based mass metering for cryogenic fluids

This paper describes a method for measuring the mass of cryogenic fluids in on-board rocket propellant tanks or ground storage tanks. Linear approximations to the Clausius-Mossotti relationship serve as the foundation for a capacitance based mass sensor, regardless of fluid density stratification or tank shape. Sensor design considerations are presented along with the experimental results for a capacitance based mass gage tested in liquid nitrogen. This test data is shown to be consistent with theory resulting in a demonstrated mass measurement accuracy of ±0.75% and a deviation from linearity of less than ±0.30% of full scale mass. Theoretical accuracies are also shown to be ±0.73% for hydrogen and ±1.39% for oxygen for a select range of pressures and temperatures.     

A coupled numerical analysis of shield temperatures, heat losses and residual gas pressures in an evacuated super-insulation using thermal and fluid networks: Part II: Unsteady-state conditions (cool-down period)

This paper analyses the cool-down period of a 300 L super-insulated cryogenic storage tank for liquid nitrogen. Storage tank and evacuated shields are the same as described in part I of this paper where stationary states were investigated. The aim of the present paper is to introduce thermal resistance networks as a tool to quantitatively understand and control also unsteady-states like cool-down of super-insulations. Numerical simulations using thermal resistance networks have been performed to determine time dependence of local shield temperatures and heat loss components. Coupling between radiation and solid conduction is investigated under these conditions. Using the numerical results, we have checked an experimental method suggested in the literature to separate heat losses through the insulation from losses through thermal bridges by measurement of unsteady-state evaporation rates. The results of the simulations confirm that it takes the outer shields much longer to reach stationary temperature; cool-down does not proceed uniformly in the super-insulation. Coupling between different heat transfer modes again is obvious. Thermal emissivity is important also during the early phase of cool-down. Using the obtained numerical results, the experimental method to separate heat loss components could only roughly been confirmed for thick metallic foils.     

A coupled numerical analysis of shield temperatures, heat losses and residual gas pressures in an evacuated super-insulation using thermal and fluid networks: Part III: Unsteady-state conditions (evacuation period)

This paper analyses the evacuation period of a 300 L super-insulated cryogenic storage tank for liquid nitrogen. Storage tank and radiation shields are the same as in part I of this paper. The present analysis extends application of stationary fluid networks to unsteady-states to determine local, residual gas pressures between shields and the evacuation time of a multilayer super-insulation. Parameter tests comprise magnitude of desorption from radiation shields, spacers and container walls and their influence on length of the evacuation period. Calculation of the integrals over time-dependent desorption rates roughly confirms weight losses of radiation shields obtained after heating and out-gassing the materials, as reported in the literature. After flooding the insulation space with dry N₂-gas, the evacuation time can enormously be reduced, from 72 to 4 h, to obtain a residual gas pressure of 0.01 Pa in-between shields of this storage tank. Permeation of nitrogen through container walls is of no importance for residual gas pressures. The simulations finally compare freezing H₂O-layers adsorbed on shields, spacers and container walls with flooding of the materials.