Oscillatory thermocapillary convection in a liquid bridge: Part 1—1 g Experiments

Effect of liquid bridge form on oscillatory thermocapillary convection under 1 g and μg conditions

Marangoni convection when the surface tension increases with the temperature in normal and low gravity conditions

Oscillatory thermocapillary convection in a liquid bridge: Part 2—Drop shaft experiments

The effects of gravity on the combustion characteristics and microstructure of metal-ceramic composites (HfB2/Al and Ni3Ti/TiB2 systems) were studied under both normal and low gravity conditions. Under normal gravity conditions, pellets were ignited in three orientations relative to the gravity vector. Low gravity combustion synthesis (SHS) was carried out on a DC-9 aircraft at the NASA-Lewis Research Center. It was found that under normal gravity conditions, both the combustion temperature and wave velocity were highest when the pellet was ignited from the bottom orientation; i.e., the wave propagation direction was directly opposed to the gravitational force. The SHS of 70 vol pct Al (in the Al-HfB2 system) was changed from unstable, slow, and incomplete when ignited from the top to unstable, faster, and complete combustion when ignited from the bottom. The hydrostatic force (height × density × gravity) in the liquid aluminum was thought to be the cause of formation of aluminum nodules at the surface of the pellet. The aluminum nodules that were observed on the surface of the pellet when reacted under normal gravity were totally absent for reactions conducted under low gravity. Buoyancy of the TiB2 particles and sedimentation of the Ni3Ti phase were observed for the Ni3Ti/TiB2 system. The possibility of liquid convective flow at the combustion front was also discussed. Under low gravity conditions, both the combustion temperature and wave velocity were lower than those under normal gravity. The distribution of the ceramic phase, i.e., TiB2 or HfB2, in the intermetallic (Ni3Ti) or reactive (Al) matrix was more uniform.

The current research is focused on the study of methane-air flame stabilization under the flow geometry variation for normal and upside-down (reverse) flame orientation. The experimental studies of plane-symmetrical rod-stabilized flames under the normal and reverse-oriented gravity conditions were carried out. The results of numerical simulation are presented. The blow-off is shown to be the function of stabilization-body position. We consider both V-shaped and M-shaped plane-symmetrical open flames for different Reynolds numbers, fuel-air ratio and flame orientation relative to the gravity direction. Blow-off limits appear to be independent on the gravity for the lean methane-air mixtures while the quenching processes are different for the normal and reverse-oriented gravity conditions. Blow-off is also accompanied by the chemiluminescence intensity decrease under reverse-oriented gravity conditions, which is exhibited in localized plume extinction and gradual quenching. While under the normal gravity, it appears via stepwise vortex separation from the lateral plume parts. Under such conditions chemiluminescence intensity remains almost constant. The blow-off time scale under the normal gravity conditions is bigger as compared to reversed-oriented ones by several times.

The effect of gravity on the combustion synthesis characteristics and the resultant microstructures of the synthesized metal matrix composites (MMCs) were studied for the HfB2/Al and Ni3Ti/TiB2 reaction systems conducted under both normal (1 g) and low gravity conditions. Under normal gravity conditions, the pellets were ignited at three orientations to the gravity vector. The low gravity combustion synthesis reactions were conducted on a DC-9 aircraft at NASA Lewis Research Center (NASA-LeRC). It was found that under normal gravity conditions, both the combustion temperature and wave velocity were highest when the pellets were ignited from the bottom. Both the combustion temperature and wave velocity were lower when conducting the reactions under low gravity than under normal gravity conditions. It is believed that the convective flow of argon gas was responsible for this phenomenon. Gravity-induced, density-driven fluid flow (sedimentation) of the heavier phases in the MMCs was also observed for both re...

Two-phase flows of gas and liquid are increasingly paid much attention to space application due to excellent properties of heat and mass transfer, so it is very meaningful to develop studies on them in microgravity. In this paper, gas-phase distribution and turbulence characteristics of bubbly flow in normal gravity and microgravity were investigated in detail by using Euler–Lagrange two-way model. The liquid-phase velocity field was solved by using direct numerical simulations (DNS) in Euler frame of reference, and the bubble motion was tracked by using Newtonian motion equations that took into account interphase interaction forces including drag force, shear lift force, wall lift force, virtual mass force and inertia force, etc. in Lagrange frame of reference. The coupling between gas–liquid phases was made with regarding interphase forces as a momentum source term in the momentum equation of the liquid phase. Under the normal gravity condition, a great number of bubbles accumulate near the walls under the influence of the shear lift force, and addition of bubbles reduces turbulence of the liquid phase. Different from the normal gravity condition, in microgravity, an overwhelming majority of bubbles migrate towards the centre of the channel driven by the pressure gradient force, and bubbles have little effect on the turbulence of the liquid phase.

Alkali cyanides in the iron blast-furnace

An experimental and numerical investigation on the hot surface ignition of premixed gases under microgravity conditions

In this article, flame spread phenomena over cylindrical fuel geometry are studied in spacecraft environments and contrasted with those on Earth. An in-house 2D axisymmetric computational fluid dynamics model is used to study the flame spread phenomena. Experimental flame spread results, in normal gravity and microgravity conditions, are used for model validation. The effect of uniform external opposed flow (0–25 cm/s) and surrounding oxygen concentrations (17.5–35%) is investigated in detail for a thin fuel rod of diameter 1 mm. The flow fields around the flame spreading in normal gravity and microgravity are very different. In normal gravity, buoyancy accelerates the flow and entrains surrounding air into the flame. In microgravity, the flow velocities increase only moderately due to thermal expansion. The change in the flow field results in significantly larger flames in microgravity, which spreads faster than the normal gravity flames. A detailed heat transfer analysis is carried out for the solid fuel. In general, the radial heat conduction of heat from the flame to fresh fuel ahead of the flame is the most prominent mode of heat transfer that controls the flame spread rate. However, in normal gravity conditions, near the low oxygen extinction limit, the contribution of axial heat conduction through the solid fuel also becomes significant and may contribute to about 20% of the total heat input to the fresh solid fuel. In normal gravity, the flame spread rate decreases with the increase in external flow speed. On the other hand, in microgravity, an increasing and decreasing trend in flame spread rate is observed with the increase in external flow speed. The flame spread rate increases with the increase in oxygen in normal gravity and microgravity environments.

Themocapillarity is of fundamental importance in material processing. The floating-zone method is a material processing for producing and purifying single crystals of metals and oxides. It is widely known that, using this method, a transition to three-dimensional oscillatory thermocapillary convection takes place in the melt. The oscillatory convection causes detrimental striations in the crystal structure. In this study, flow transition points (critical Marangoni numbers) and flow structures were investigated in a thermocapillary convection in a model of floating-zone method (full-zone liquid bridge) of the high Prandtl number fluid (Pr =28.1) under the normal gravity condition. In the liquid bridge, the convection changes from two-dimensional steady flow to three-dimensional oscillatory one at a flow transition point. The convection was visualized by tracer particles in order to find the flow transition point and the shape of the modal structures in the oscillatory flow. The dominant modal structures near the flow transition point were estimated using the shape of the particle free zone on a horizontal plane of the liquid bridge and superposition of several waves with azimuthal wave numbers by the method of the least squares. In the present study, the critical Marangoni number of the full-zone liquid bridge was almost one-half of that of the half-zone liquid bridge in aspect ratio 0.45 to 1.3 (aspect ratio: half-height of liquid bridge over radius of rods). The dominant modal structures were combination of azimuthal wave numbers with 1, 2, and 3 in a range of the aspect ratio concerned. It was found that the azimuthal wave numbers of the dominant modal structures did not depend on the aspect ratio. The dominant modal structures were a standing or a travelling wave, and changed from one to another irregularly. Trade-off of a power and a phase locking between each modal structure were observed.

BackgroundBacterial phenotypes result from responses to environmental conditions under which these organisms grow; reduced gravity has been demonstrated in many studies as an environmental condition that profoundly influences microorganisms. In this study, we focused on low-shear stress, modeled reduced gravity (MRG) conditions and examined, for Escherichia coli and Staphlyococcus aureus, a suite of bacterial responses (including total protein concentrations, biovolume, membrane potential and membrane integrity) in rich and dilute media and at exponential and stationary phases for growth. The parameters selected have not been studied in E. coli and S. aureus under MRG conditions and provide critical information about bacterial viability and potential for population growth.ResultsWith the exception of S. aureus in dilute Luria Bertani (LB) broth, specific growth rates (based on optical density) of the bacteria were not significantly different between normal gravity (NG) and MRG conditions. However, significantly higher bacterial yields were observed for both bacteria under MRG than NG, irrespective of the medium with the exception of E. coli grown in LB. Also, enumeration of cells after staining with 4',6-diamidino-2-phenylindole showed that significantly higher numbers were achieved under MRG conditions during stationary phase for E. coli and S. aureus grown in M9 and dilute LB, respectively. In addition, with the exception of smaller S. aureus volume under MRG conditions at exponential phase in dilute LB, biovolume and protein concentrations per cell did not significantly differ between MRG and NG treatments. Both E. coli and S. aureus had higher average membrane potential and integrity under MRG than NG conditions; however, these responses varied with growth medium and growth phase.ConclusionsOverall, our data provides novel information about E. coli and S. aureus membrane potential and integrity and suggest that bacteria are physiologically more active and a larger percentage are viable under MRG as compared to NG conditions. In addition, these results demonstrate that bacterial physiological responses to MRG conditions vary with growth medium and growth phase demonstrating that nutrient resources are a modulator of response.

Oscillatory thermocapillary convection features in a liquid bridge under normal gravity and microgravity conditions—Drop shaft experiments

Enhancement of ethanol production efficiency in repeated-batch fermentation from sweet sorghum stem juice: Effect of initial sugar, nitrogen and aeration

The effect of ambient air flow on flow-transition points in thermocapillary convection was investigated using a floatingzone method (full-zone liquid bridge) with a high Prandtl number fluid (Pr = 28.1) under normal gravity conditions. In the liquid bridge, convection changes from two-dimensional steady flow to three-dimensional unsteady flow at a flowtransition point. A pair of partition plates was employed to suppress the ambient air flow. To understand the flow and thermal fields of the ambient air, flow was visualized using smoke and temperature was measured using a thermocouple. Thermocapillary convection was stabilized by suppressing ambient air flow. The primary stabilization factor is heat transfer from the ambient air to the liquid bridge through the free surface. These results suggest that flow-transition point was controllable by modifying ambient air temperature.