Search

We report the thermal transition and concomitant morphology change of polyimide films, investigated through dynamic mechanical analyses, thermo-mechanical analyses, and X-ray diffraction spectroscopy. For this study, highly transparent polyimide films composed of 3,4,3′,4′-biphenyltetracarboxylic dianhydride and fluorinated diamines, 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl, are prepared through a solvent-casting method. The films are annealed at different temperatures, and two kinds of reaction solvents (n-methyl-2-pyrrolidone and N,N-dimethylacetamide) for the polyimide precursors are compared. The mechanical analyses during thermal transition along with spectroscopic measurements reveal unusual transition behaviors and morphology differences between the polyimide films at various annealing temperatures. The results can be applied to the quantitative analysis of the imidization ratio in highly imidized polyimides when the films are imidized thermally at temperatures above 300 °C. [ABSTRACT FROM AUTHOR]

The submerged helium injection process results in the heat and mass transfer between the helium bubble and the cryogenic liquid. The objective of this paper is to analyze the dynamics of the heat and mass transfer process. It is observed that during the helium injection process the dynamics of mass transfer is dominant and the transient heat transfer is negligible. The helium bubble shape and rising patterns are observed with a visualization device that helps to discern the dominant process between heat transfer and mass transfer. The clustering patterns such as coalescence of helium bubbles are observed with the visualization device. The visualization results indicate that, it is very difficult to determine the representative size of bubbles due to the irregular shape of the helium bubbles. The shape and size of the helium bubbles are important parameters for evaluating the overall mass transfer coefficient ( k G A ) which is the essential parameter for calculating the evaporation rate of the bulk liquid into the helium bubbles. In this paper, the simplified lumped model is considered to fairly approximate the evaporation rate of the cryogenic liquid into the bubbles and the cooling rate of helium injection. The empirical correlation for the average concentration ( C ‾ A ) of evaporated cryogenic liquid into the helium bubbles is presented and the overall mass transfer coefficients ( k G A ) are calculated as the result of the lumped model. The proposed model and empirical correlations are compared with the experimental results, and the comparison result shows good agreement with differences that are less than ±0.4 K. [ABSTRACT FROM AUTHOR]

Highlights: [•] Thermal expansion for rigid and planar polyimides depends on film thickness. [•] Charge transfer complex is formed more favorably at thinner films. [•] Variable thermal expansion originates from complex morphological effects. [•] Variable thermal expansion provides optimum film thickness for display substrates. [Copyright &y& Elsevier]

In this paper, we propose the use of deep learning-based image super-resolution to accelerate structural topology optimization. Topology optimization suffers from iterative computation and a time cost that increases with the number of elements. Recently, there have been attempts to accelerate topology optimization using deep learning-based models, but they often do not address a wide range of physical conditions or require extensive training data. This highlights the need for an approach that can effectively solve different problem conditions with less training data. Our approach first starts topology optimization at a low resolution to quickly obtain an optimized structure. It is converted to the structure at a higher resolution by using an image super-resolution model. Then, the final structure is obtained by performing topology optimization at this high resolution by using the converted structure as the starting configuration. The super-resolution model learns how to transform low-resolution structural features into high-resolution ones regardless of optimization conditions. As a result, it can be applied to any problems different from the problem used to generate the data for training the model. The proposed approach's excellent acceleration and generalization performance is demonstrated for three representative problems in structural topology optimization with a small number of training datasets. Furthermore, it has been shown to be effective in the deck arch bridge problem and the natural frequency maximization problem. Finally, the proposed approach is also found to be effective on the three-dimensional cantilever beam problem. [ABSTRACT FROM AUTHOR]

• Cryogenic natural circulation loop analyzed experimentally and mathematically. • The circulation flow-rate calculated by the two-phase flow model: HEM and TEDFM. • The TEDFM estimated the circulation flow rate with 5% error. • The two-phase flow pattern of cryogenic liquid verified with flow visualization. This paper presents the application of cryogenic natural circulation loop for engine cooling of launch vehicles, and the experimental measurements of the natural circulation system. The experimental results indicate that the circulation flow rate increases with an increase in heat input and a decrease in system pressure, on varying them. The study includes the temperature and pressure profiles of the natural circulation loop. The steady-state flow rate calculation was developed with two-phase models: the homogeneous equilibrium model (HEM) and thermal equilibrium drift-flux model (TEDFM). The TEDFM revealed a 5% error in the experimental circulation flow rate, and it is agreeable with the experimental results. The small-scale experimental setup verified the two-phase flow pattern in the cryogenic natural circulation loop. A fully liquid flow was observed in the downward section. The horizontal section had a bubbly flow, where the speed of the gas and liquid were similar. The slug and churn flows were present in the vertically upward section, where gas velocities were faster than liquid. Both HEM and TEDFM can be applied in the horizontal heated section. However, it is more appropriate to apply TEDFM in the vertically upward section during the pressure drop calculation for the cryogenic natural circulation loop. [ABSTRACT FROM AUTHOR]

• Cryogenic propellant is pressurized with hot helium in typical launch vehicle. • Depressurization of ullage was observed with cold helium input to the LN 2 tank. • Cryogenic liquid droplet propagation observed during the pressurization. • The ullage temperature and pressure decreased due to the ullage cooling effect. • The ullage depressurization is possible during the helium gas with liquid droplets. In liquid propellant propulsion systems, helium pressurization is required to transfer liquid propellant from the tank to the engine. When a cryogenic liquid propellant is used, the tank becomes pressurized by relatively high temperature helium. Typically, the ullage pressure increases with the helium inflow. However, in one particular case, the ullage pressure reduced while cold helium was supplied through direct gas injection. High-velocity gaseous helium strikes the cryogenic liquid surface that generates the liquid splash in the ullage. The cryogenic liquid splash in the ullage then transfers heat between the gas and the liquid. Therefore, the ullage pressure decreases because of the decrease in the ullage temperature. The pressure behavior of the cryogenic tank during the gas inflow is experimentally examined, and the depressurization during the pressurization process is analyzed. The results indicate that the gas–liquid interface disturbance decreases the ullage pressure. The observed gas–liquid interface disturbance effect can be employed in pressure decrease applications in cryogenic liquid storage tanks. [ABSTRACT FROM AUTHOR]

Toughness and high elastic elongation are important requirements for polyimide films used as substrates in flexible displays. However, these properties can be altered by imidization during the thermal treatment. In the present study, we investigated a probable mechanism to enhance the toughness by shrinking fluorinated polyimide films containing benzidine moieties. A higher degree of fluorination leads to weaker intermolecular charge transfer and optically clearer films, but it also degrades the mechanical properties. Moreover, β -transition originating from the molecular rotation around benzidines makes the film dependent on the elongation conditions during solvent drying. According to frequency-sweeping measurements and simulations based on density functional theory and molecular dynamics, the β -transition temperature increases and the phase lag in elastic energy loss decreases with increasing shrinkage. Both experimental and theoretical findings suggest that the change in frequency dependence can be attributed to the variation in elasticity retardation, which possibly originates from the chain-entanglement density. [ABSTRACT FROM AUTHOR]