查看更多>>摘要：Understanding bubble behaviors and thermal characteristics under various working conditions is of great significance to the efficient utilization of cryogenic propellants. In the present study, simulation models of pool film boiling at horizontal plate and cylinder surface are proposed with Volume of Fluid (VOF) method combined with Lee's phase change model. Based on the physical and thermal performance, the gravity levels, physical properties, and heater structure dimensions are investigated respectively, which could satisfy the common application conditions of cryogenic propellants. The results show that the movement of bubbles at gas-liquid interface is critical to the pool film boiling heat transfer, and also the heat transfer mechanism affects the bubble motion behaviors. The critical wavelength and the most dangerous wavelength are significant length scales for the bubble behaviors in pool film boiling. When the cylindrical heater size is larger than the most dangerous wavelength, the bubble behaviors as well as the heat flux appear the similar characteristics with that at the plate heater surface. Moreover, the critical wavelength is approximately two times of the bubble departure diameter in the present cryogenic liquid boiling events, and the parameters, including liquid-vapor density difference, surface tension, and gas thermal conductivity, could affect the boiling heat flux significantly. In addition, under 0.03g condition, the bubble departure diameter could reach 27 mm and the boiling heat flux decreases to 800 W/ m2, which are significantly different from that in normal gravity. In general, the present CFD model could execute a film boiling study accurately, and a series of detailed results on the bubble behaviors as well as the heat flux could be revealed.
查看更多>>摘要：Hysteresis effect on thermal contact conductance due to load cycling is well documented for joints at ambient temperature. However very limited information is available at cryogenic temperature. The thermal cycling of joints to cryogenic temperature is also an unexplored area. In addition, the change in heat flow direction on thermal contact conductance across joints formed by dissimilar materials, which is referred as rectification, is an important parameter in maintaining desired thermal contact conductance across the joints. In the present study, these factors are experimentally evaluated for joints formed by stainless steel, aluminium alloy and titanium alloy over a temperature range of 150 K to 300 K at low interface loading. The present study reveals that the thermal contact conductance at cryogenic temperature is less affected by hysteresis by an average of 8% and 16% with thermal and mechanical loading cycles respectively. However, Rectification effect in thermal contact conductance is prominently observed in bi-metallic joints. More than 60% change in thermal contact conductance is observed with change in direction of heat flow. The study points out that rectification can be effectively utilised for controlling the thermal contact conductance at cryogenic temperature in joints subjected to low contact pressure, where the use of other controlling methods is constrained.
查看更多>>摘要：Superconducting high-field magnets working in He II are usually immersed in a pressurized static He II bath. This pressurized He II bath is then connected through a heat exchanger to a saturated He II bath acting as a cold source. Due to the peculiar He II properties (very high heat conductivity), the conventional heat exchanger designs are not suitable here. Additionally, a compact design is often mandatory, as space inside cryostats is always an issue. In a recent paper [1], we presented different configurations of optimized multi tube heat exchangers depending on the available horizontal or vertical space. Such an optimized compact heat exchanger prototype has been built to fulfill the cooling needs and the integration constraints of the HL-LHC superconducting D2 recombination dipole [2]. The chosen design with hundred oxygen-free high-purity copper horizontal tubes penetrating inside the extremity of the D2 cold mass vessel is an efficient solution offering significant operating margins. From the thermal analysis, it appears that the profile and value of the temperature difference across the heat exchanger really depends on the ratio of the transverse conductance to the longitudinal conductance, the first being determined by the conductivity of the copper and the Kapitza conductance. Therefore, optimization and prediction of the thermal performance of such a heat exchanger is only possible when these thermal properties (i. e., the heat conductivity and Kapitza conductance of the copper tubes) or the sum of these three thermal resistances in series are well known. For that purpose, we built a dedicated test bench to perform these transverse resistance measurements on a pipe sample. The experimental results obtained on a single pipe were then used to infer the behavior of the complete heat exchanger. The comparison of the predictive code and the cryogenic measurements shows excellent agreement over a wide range of cryogenic operating conditions (temperatures, pressures, heat loads). This paper presents a guideline for designing compact and efficient He II/He II heat exchanger as well as a method to predict the performance of such heat exchanger accurately.
查看更多>>摘要：Collision of nitrogen droplets is a basic phenomenon in an array nitrogen spray cooling system, while the understanding on which is still lacking. We conduct a numerical simulation to investigate the collision dynamics of two nitrogen droplets in a low-temperature environment under various Weber numbers, Reynolds numbers and impact parameters, and simulation of collision of two water droplets is conducted for comparison. Incompressible Navier-Stokes equations are solved to simulate the colliding process, and Volume of Fluid (VOF) method and adaptive mesh refinement technique are used to capture gas-liquid interface. Three regimes of collision are found for nitrogen droplet collision, including coalescence, reflection separation and stretching separation. Upon collision, a lower surface tension and lower viscous dissipation consume less initial kinetic energy, which is conducive to the separation of droplets. Compared with water droplets, collision of nitrogen droplets easily enables breakage due to the lower surface tension and viscous dissipation. Separation of nitrogen droplets leads to the increase of secondary droplet number and enlarges heat transfer area of droplets and gas, which would enhance droplet vaporization and temperature descendence of the environment.
查看更多>>摘要：A launch vehicle that operates using cryogenic liquid propellants will require a pressurization process for a propellant tank. Usually, helium stored at cryogenic temperatures passes through the engine, is heated, and pressurizes the tank. Low-temperature helium is heated in a heat exchanger using high-temperature combustion gas from an engine. For the accurate performance prediction and weight optimization of the launch vehicle, the performance of the heat exchanger should be evaluated before launching. This study presents an evaluation method for predicting the performance of a heat exchanger in a launch vehicle. The heat exchanger performance was measured at a constant helium flow rate under actual engine operating conditions. A numerical analysis model for predicting the heat exchanger performance was developed and compared to the test results. The tests were conducted at different inlet temperatures. The test results and analysis confirmed that the effectiveness increased with the inlet temperature. Further analyses revealed that a change in the heat transfer coefficient, as well as the inlet temperature, changed the effectiveness by 6%. To accurately predict the performance of the launch vehicle, it was necessary to accurately predict the change in the heat transfer coefficient with temperature. A design method for the heat exchanger of a launch vehicle was also discussed. The heat-transfer area and weight should be optimized under the required conditions.
查看更多>>摘要：As a newly-developed microchannel heat exchanger, printed circuit heat exchanger (PCHE) is employed as the primary LNG vaporizer in floating storage and re-gasification unit for high efficiency and compactness. In this paper, thermal-hydraulic characteristics of trans-critical NG through the improved staggered S-shaped fin channels were numerically investigated. Variation tendencies of fin performance under different operating pressures and a wide range of Reynolds numbers were analyzed. Sensitivity analyses on the effect of various thermophysical properties on heat transfer around pseudo-critical temperature revealed that heat transfer capacity were deeply affected by specific heat, and effects of thermal conductivity and viscosity could be canceled each other. Additionally, to further estimate forced convective heat transfer mechanism, common features of HTC and f factor distributions were analyzed and some reasonable criteria were also discussed. The results showed that except for a few data points across critical temperature affected by entrance effect, the peak HTC always appeared in the vicinity of the pseudo-critical points. Furthermore, the Bo of 10-5 and q+ of 5 x 10 - 4 could be adopted to analyze buoyancy and flow acceleration influences qualitatively. The simulated results agreed with the predictions from Ngo Correlation within 16.32% and 12.36% errors respectively for HTC and f factors, which implied that though the drastic local changes of performance tendency could not be predicted well, suitable correlations were sufficiently accurate for engineering applications.
查看更多>>摘要：The European Dipole (EDIPO) test facility is undergoing a major upgrade of its magnet assembly, which will provide ground for testing both fusion and high-energy physics superconducting samples at variable temperature and with a background field of 15 T. In view of a cooling of the superconducting magnet based on a saturated helium bath, a helium vessel has been designed and procured. In this work, we present a preliminary design of the pressure relief system of the helium vessel. After having introduced the main requirements of the cryogenic circuit, we describe a staged pressure protection concept. A bursting disc, chosen as safety pressure relief device, is then sized following the appropriate European norms and according to credible accidental scenarios. Finally, we introduce the need of pressure relief and helium recovery following a quench of the superconducting magnet, after having determined the actual heat load transferred to the helium bath. Numerical simulations are also employed to study the dynamics of the pressure and temperature evolutions, and ideas for future studies are briefly discussed.
查看更多>>摘要：A series of lanthanide-rich oxyborates LiLn(6)O(5)(BO3)(3) (LLnOB, Ln = Gd, Tb, Dy, and Ho) have been synthesized using high temperature solid state method. Their magnetic and magnetocaloric properties were investigated upon magnetic susceptibility (chi), magnetization (M), and isothermal magnetic entropy change (delta S-m) measurements. The maximum delta S-m, max of LiGd6O5(BO3)(3 )is 44.7 J kg(-1) K-1 at 4 K and delta mu H-0 = 9 T, which is higher than the one of commercial Gd3Ga5O12 (GGG, delta S-m,( max )= 41.8 J kg(-1) K-1 at 2 K for delta mu H-0 = 9 T). For LiHo6O5(BO3)(3), delta S-m, (max) is 14.12 J kg(-1) K(-1 )at 3 K and delta mu H-0 & nbsp; = 2 T, which is also larger than that of Ho3Ga5O12 (HoGG, -delta S-m, max = 4.38 J kg(-1) K-1 at 2 K for delta mu H-0 & nbsp;= 2 T) and Dy3Ga5O12 (DGG, delta S-m, max = 11.0 J kg(-1) K-1 at 2 K for delta mu H-0 & nbsp;= 2 T). These results indicate that LiGd6O5(BO3)(3) and LiHo6O5(BO3)(3) may be competitive candidates for applications as magnetic refrigerants. Moreover, thermal stability, infrared spectrum (IR), and ultraviolet-visible-near-infrared diffuse reflectance spectrum (UV-Vis-NIR) were carried out to characterize the title compounds.
查看更多>>摘要：Recently, combination models, in which two or more advanced methods are employed, have been studied to explore various approaches to simulating large-scale ReBCO coils efficiently. However, modeling the combina-tions could be more complex if the methods are not integrated well. In this paper, an ingenious combination of thin-strip, homogeneous and multi-scale methods is presented. In particular, we build the thin strips as both the analyzed HTS tapes and the boundaries of the homogeneous bulks where the non-analyzed tapes are merged. Thus, the coil geometry is re-constructed with several bulks, but the bulks' boundaries and domains are tackled with different electromagnetic properties, and solved by T and A formulations, respectively. Firstly, we introduce the modeling process and highlight the differences and advantages over the previous models. Then, the accuracy of the proposed model is validated by comparing the results with those from the reference model based on a 2000-turn coil. The distributions of normalized current density, magnetic flux density and hysteresis losses from the two models are highly consistent, and the error of the total loss is less than 1%. Besides, the combination model is the most time-saving among all the advanced models. Furthermore, the proposed method can be applied in 3D simulations, and the high accuracy and efficiency are validated by simulating a 50-turn racetrack coil. The study shows that the combination method is a feasible approach to simulating large-scale HTS coils, and can be a powerful tool to design and optimize HTS systems.
查看更多>>摘要：In this paper, a novel flowmeter with a double-stage perforated plate structure is proposed, and its pressure drop and flow coefficient are numerically compared with the commonly used single-stage perforated plate structure for the cryogenic fluid measurement. The numerical calculations are based on the Realizable k-epsilon turbulence model and consider the influence of cavitation with thermal effects, which is validated by modeling the cavitating flow of liquid nitrogen over the hydrofoil in the publications. The influence characteristics of the thickness of the single-stage perforated plate on the performance are firstly clarified. Then the emphases are focused on the analysis of the effects of the gap of the double-stage perforated plates on the discharge coefficient, the pressure loss coefficient and the deviation coefficient. The results reveal that compared with the single-stage perforated plate, the double-stage one with the optimized gap has a smaller pressure drop and more horizontal flow coefficient profile with the Re number. The reason for the performance improvement is primarily ascribed to the "thickness effect ", which is explained in detail. While, the increase in the number of stages makes the cavitation more likely to occur, which leads to a smaller Re range of the horizontal flow coefficient profile. The results can be used as a useful reference for the design of the cryogenic differential pressure flowmeter.
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