Search

In this paper, based on the stability criteria described by the B–E inequality, E-inequality and T–C inequality, the region of stability is evaluated for common compressible isotropic hyper-elastic models by using the deformation modes of simple shear and volumetric change. For models with three model constants, the region of stability will be that of deformation induced by the inequalities due to the normalization conditions. For the models with four constants, for example, the region of stability will be two-folded. For the case in which the free constant is given, the region of deformation will be restricted. For the case in which the limiting deformation is given, the region of the free constant will be restricted. The current work describes how the region of deformation and/or free constant is determined when a compressible hyper-elastic model is utilized in practical engineering.

The diffraction patterns of submicron fluorescent particles were obtained using an air immersion objective (M = 40, NA = 0.6) under non-design optical conditions through a thick cover glass with a mismatched refractive index. The particle diffraction patterns changed with the defocus distance out of the front focal plane in the optical axis direction. A scalar diffraction model was applied to analyze the diffraction patterns for various optical conditions and to determine the defocus distance of the particle according to the measured intensity distribution of the diffraction pattern image. The results showed that the defocus distances have a submicron accuracy (0.2–0.3 µm), which can be used to determine the three-dimensional flow velocity distribution in a micro-channel and to track the three-dimensional Brownian motion of the particles for the specified defocus conditions.

Microscale boiling displays some quite unusual characteristics, such as the occurrence of so-called fictitious boiling. This article discusses some arguments on the thermodynamic and dynamic characteristics of the phase change and bubble nucleation in microchannels or microstructures. The cluster model is proposed to describe the density fluctuation in superheated liquid, and furthermore, the kinematics of a cluster are analyzed using Brownian motion. The analyses explore that the scale of microstructures has importance for the kinematics of a cluster and the boiling nucleation.

A criterion for phase change in microchannels has been proposed and developed. This criterion utilizes homogeneous nucleation and cluster theory to approximate the superheat required for phase change to occur in microchannels. The analysis performed herein indicates that while the fundamental phase-change mechanisms in micro- and macrostructures are the same, the relative importance of some aspects of these mechanisms is significantly different. In particular, because the mechanism of phase change is more strongly dependent upon molecular interaction and kinetics, the rate at which phase change occurs in microchannels ranges from milliseconds to microseconds, while for macrostructures it is of the order of seconds. This leads to a phenomenon referred to as ''flashlike evaporation'' in microchannels, something not previously observed in larger-scale channels. The good agreement between the predicted superheat and the values observed in previous experimental investigations demonstrates that this criterion c...

This investigation provides a theoretical description of the mechanisms that govern the rewetting of the surfaces, and presents an experimentally verified method by which the location of the rewetting front in the evaporator section of high capacity heat pipes can be determined as a function of the applied heat flux. The rewetting velocity was found to be a function of thermal properties of the liquid nd the channel, the input heat flux, the radii of the groove and the channel, and the location of the liquid front

An experimental program was conducted to investigate the rewetting characteristics of liquid flowing through multiple layers of screen mesh covering a heated vertical plate. Data obtained for acetone and R-11 were analyzed and compared with previously developed theoretical expressions for the rewetting velocity, the maximum liquid rise height, and the maximum heat flux for which rewetting could occur. The experimental results indicate that the thickness of the screen mesh layer and the liquid thermophysical properties are the most important factors affecting the rewetting behavior. As predicted by the theoretical model, small increases in the thickness of the screen layer initially caused the maximum permissible rewetting heat flux to increase. Once the layer thickness had reached a critical value, continued increases resulted in a decrease in the maximum permissible rewetting heat flux.

An analytical investigation was conducted to determine the rewetting characteristics of thin, surface tension driven liquid films over heated plates as a function of the fluid properties, the film thickness, and the applied heat flux. Analytical expressions for the maximum sustainable heat flux and the rewetting velocity were developed for both flat and grooved plates and were compared with data from previous investigations. The results indicated good agreement for low film velocities; however, at high velocities the experimental data deviated significantly from the theoretical predictions. It was hypothesized that this deviation was due to the presence of liquid sputtering near the liquid front. To compensate for this liquid sputtering, the expressions for maximum sustainable heat flux and rewetting velocity were modified using an empirical correction factor developed from the data of previous thin film thickness investigations. The resulting modified expressions were found to compare very favorably with available experimental data over a large range of flow conditions and velocities.

Analysis T WO external artery heat pipes are currently under consideration for use as radiator elements on Space Station Freedom.* Both of these utilize a liquid and vapor channel connected by a longitudinal slot to provide axial pumping, Fig. 1. Although previous analytical and experimental investigations have demonstrated that these heat pipes will prime properly iii a zero-g environment, little is known about the effect of accelerations caused by orbital attitude adjustments or docking maneuvers. These accelerations, particularly longitudinal accelerations, could result in redistribution of the working fluid and dry out of the evaporator. This would require a reduction in the evaporator heat flux to allow rewetting. In the present work, an analytical investigation was conducted to determine what reduction in heat flux would be required and the effect of short-term longitudinal accelerations on the liquid/vapor interface. Assuming the heat pipe is properly primed and operating normally in a zero-g environment, the most damaging accelerations will be those occurring along the longitudinal axis because these are most likely to deprime the liquid channel. When the magnitude and/or duration of the acceleration is sufficient, liquid will flow out of the liquid channel through the connecting slot and accumulate in the end of the liquid and vapor channels, opposite the direction of acceleration. Because this flow may occur quite rapidly, some of the liquid may remain in the slot or adhere to the surface of the liquid and vapor channels forming a thin liquid film flowing along the length of the heat pipe. Although the flow of liquid out of the liquid channel and into the vapor channel is the most significant, the flow