Earth Gravity Conditions Research Articles

Cancellation of the heating piston effect by convective enhancement of a cooling piston effect

This work brings new insight to the question of the piston effect, which has been found to be the main cause of temperature equilibration in the vicinity of the liquid–vapor critical point under weightlessness conditions. The thermalization process of a near-critical fluid confined in a cavity and submitted to local heating is modeled with special emphasis on the role of gravity and boundary conditions. The solution of the unsteady Navier–Stokes equations written for a hypercom-pressible low-heat-diffusing van der Waals gas is obtained in a 2-D configuration by means of a finite-volume numerical code. Under Earth gravity conditions, the results show that the thermal plume rising from a heat source strongly decreases and rapidly cancels bulk fluid heating when it strikes the top thermo-stated wall. It is proved that convection does not prevent heat transfer by the piston effect but that it causes a sudden enhancement of the cooling piston effect generated at the thermostated top boundary, which leads to an early equilibrium between the cooling and heating piston effects.

Bubble Behavior in a Horizontal Narrow Divergent Passage. One Dimensional Approximate Analysis.

The authors investigate the behavior of a bubble which bridges a gap in a cross section of a horizontal narrow divergent passage under the Earth's gravity condition (1G). In a narrow passage, inertia forces are known to be small compared with viscous forces. Also, gravity force is not dominant for bubble behavior in a horizontal narrow passage. In this sense, the bubble behavior in the passage is similar to that under a microgravity condition. It is important to understand the bubble behavior in relation to separating gas from a gas-liquid two-phase flow and controlling a gas-liquid interface under a microgravity condition. A one-dimensional momentum equation for the bubble behavior was derived. The equation was converted into an ordinary differential equation with respect to the upstream interface and was solved. Analytical results were compared with experimental data. As a result, effects of gap size, bubble projected area, and divergent angle on the bubble behavior were explained qualitatively.

Experiments on Bubble Behavior in a Horizontal Narrow Divergent Passage.

This study investigates a bubble which is placed such that it bridges a cross section of a horizontal narrow divergent passage under the Earth gravity condition (1-G). In a narrow passage, inertia forces are known to be small compared with viscous forces. Also, gravity force is not dominant for bubble behavior in a horizontal narrow passage under the 1-G condition. In this sense, the bubble behavior in the passage is similar to that under a low-gravity condition. It is important to understand the bubble behavior in relation to separating gas from a gas-liquid two-phase flow and controlling a gas-liquid interface under a low-gravity condition. A single bubble was placed such that it bridged the cross section of a horizontal narrow passage under the 1-G condition. The bubble geometry and its behavior were studied experimentally for gap sizes ranging from 0.5 mm to 2 mm, divergent angles of 1 to 5 degrees, and using ethyl alcohol as a working fluid. The following results were obtained : (1) bubble was found to move to greater cross sectional area, (2) the gas-liquid interface geometry from the top view could be expressed as a "contact circle model" which takes the maximum radius in the passage, and (3) effects of the gap size and the bubble projected area on the bubble behavior were clarified.

Gas Evaporation in Space

Silver (Ag) was evaporated in argon (Ar) and xenon (Xe) gases at various pressures in the low-gravity environment aboard the space shuttle. Four glass bulbs with filament tips coated with 50 mg of silver were filled with Ar gas of 6.7 Pa (A) or 40 Pa (B) or Xe gas of 0.67 Pa (C) or 1.33 Pa (D) and ignited one by one in the low gravitational field of space. The evaporation temperatures were maintained at 1150°C at which smoke plumes were barely detectable in all cases in the ground experiment. A ball of smoke particles appeared to grow around the evaporation source instead of rising as it would under earth gravity conditions. No smoke was observed in (A), but it was observed in (B) and (C) and bursts of smoke extended in various directions from the smoke ball in the case of (D). The experiment suggested that vapor could be confined locally around the source with high pressure and temperature by the surrounding gas in the low gravity. This suggestion cannot be derived from any conventional model of evaporation in the gas.

Analytical comparison of condensing flows inside tubes under Earth-gravity and space environments

The heat transfer behavior of flow condensation inside horizontal tubes under zero-gravity and Earth-gravity conditions is modelled and analyzed. For Earth conditions for wetting fluids the annular flow region changes to a stratified flow pattern owing to gravity drainage of the condensate from the upper portion of the tube. The stratified condensate layer is considered inactive in the heat transfer process; its magnitude is determined along the tube length using the analytical results of Rufer and Kezios. Under zero-gravity conditions no such gravity drainage occurs and so the flow is considered to be annular along the complete length of the tube. The analytical approach of Bae was used to evaluate the heat transfer rates under zero gravity conditions. The results indicate a substantially poorer condensing performance under zero-gravity conditions. These results can be simply explained in terms of the smaller condensate film thickness over the upper portion of the tube periphery at any axial location under earth-gravity conditions because of gravity drainage of the condensate.

Centrifuge modeling of fault propagation through alluvial soils

The behavior of alluvial deposits subjected to fault movements in the underlying bedrock is of major concern for critical structures located within fault zones. An understanding of fault propagation through soils would assist in design of such structures, but could also be utilized in geological interpretation of fault displacement history. On the premise that alluvial fault morphology contains shear patterns characteristic of modes and rates of fault displacements, a study was undertaken involving centrifugal and numerical models of reverse faulting. This paper describes the centrifuge model testing performed to backup simultaneous numerical studies (Geognosis Report, 1980), which will be described elsewhere. A comprehensive model test series under earth gravity conditions (1g) involving reverse and normal faulting under different angles has recently been undertaken by Cole (1979). However, model tests performed under 1g-conditions are limited to rather thin soil layers because of their inability to simulate realistic gravity stress conditions. Furthermore, it is not possible to simulate faulting fast enough to include inertial effects with such models.