Математическое моделирование процессов кипения в невесомости

The main outcomes of the worldwide experimental activity dealing with pool boiling in reduced gravity are summarized. The currently available experimental facilities and experimental opportunities are examined, the main results obtained by the various experimental teams are reviewed, and highlights of current and future applications of boiling in space systems are given. The work initiated by several groups around the world seems to indicate that pool boiling (especially the subcooled one) may be safely sustained in micro-g conditions with appropriate measures and that improvements in performances (e.g. by application of other force fields) are possible. However, due to the high cost and low availability of flight opportunities, and to their limitations in space and time, a final assessment has yet to be completed, and some results are still controversial. In the second part of the paper, a review of the main pool boiling features and of the related models is carried out. The main effects of gravity and other force fields are stressed and compared with the above mentioned experimental results on earth and under reduced gravity conditions. The most commonly accepted viewpoints are reported for each aspect. The empirical correlations developed for boiling heat transfer in terrestrial conditions do not trivially extend their validity outside their range of application. Thorough experimentation in microgravity is thus needed to assess the performance of boiling heat transfer in such conditions. It is believed that, after further experimental activity, it will be possible to design efficient boiling systems for future spacecrafts. The research in microgravity, by eliminating the dominant effect of the buoyancy forces, may also help clarify the role played by the various mechanisms in the boiling phenomenon.

The present paper mainly describes the latest progress in the thermal management devices with using “self-rewetting fluids” as a working fluid. The self-rewetting fluids are those dilute aqueous solutions of high carbon alcohols, which show a particular surface tension behavior—an increase in the surface tension with increasing temperature. In the case of phase change of these particular solutions, the Marangoni effects caused by both temperature gradient and concentration gradient along vapor/liquid interface are expected to induce a strong liquid inflow to local hot or dry spot at heater wall. Such a behavior is more pronounced in reduced gravity conditions and micro-scale heat transfer. The paper contains a series of reduced gravity and terrestrial experimental results on thermal management devices with self-rewetting fluids, wickless heat pipes and wicked heat pipes in reduced gravity and terrestrial conditions, respectively. In addition, preliminary experimental data for pool boiling characteristics of self-rewetting fluids are given.

Containerless rapid solidification of hypereutectic Al–8wt%Fe is investigated experimentally using the Impulse Atomization technique (IA), as well as ElectroMagnetic Levitation (EML) under terrestrial and reduced gravity conditions. The samples were analyzed using scanning and transmission electron microscopy, X-ray and neutron diffraction, as well as electron backscattered diffraction. In both EML and IA, the samples experience some undercooling for the solidification of the primary intermetallic phase, which is likely metastable AlmFe (m = 4.0–4.4). After recalescence, the solidification path then continues with the nucleation and growth of stable Al13Fe4. While Al13Fe4 dominates in EML samples, it becomes minor in favor of AlmFe in IA droplets. The morphology differences of the primary intermetallics growing under terrestrial and microgravity conditions in EML are clear with acicular morphology for the former and a star-like morphology for the latter. The α–Al has a strong texture in microgravity EML and in IA samples while a weak one is observed on terrestrial EML. This difference is attributed to the weaker fluid flow occurring under reduced gravity conditions and in IA droplets.

Previous data on amorphous NdFe2 have shown that it has some sort of phase transition at approximately 330 K. Previous magnetization studies on amorphous DyCu have shown that this alloy undergoes some sort of phase transition at approximately 17 K. Unusual field cooling and time relaxation effects were also observed in these measurements on Dycu below this temperature. these phase transitions and Both the exact the resultant nature of magnetic structures of these alloys are not yet fully understood. This study was undertaken to obtain additional information about them. Small-angle neutron scattering data- on amorphous NdFe2 were taken in zero field and in applied fields up to 18 kOe. In zero field, the lineshape of the scattering was Lorentzian above the transition temperature and became increasingly deviated from this form below this temperature. The spin correlation length showed a broad maximum near to, but lower than, the transition temperature and then decreased below this temperature. SANS data obtained with the applied field showed a sharp decrease in intensity with increasing field. The spin correlation length also decreased with increasing field. The response of the SANS from amorphous DyCu to applied fields up to 15 kOe was examined. Application of the field caused a decrease in SANS intensity with increasing field and also caused a decrease in the correlation length which was obtained from the lineshapes which were Lorentzian to 6 A from the zero field value of c 13.5 a Time relaxation effects were observed in the field-cooled data. The SANS measurements showed that amorphous Dycu and NdFe2 do not have a magnetic phase transition to long-range ferromagnetism. .They indicated that these alloys undergo a transition to a state that has some features of a spin glass.

Gas—liquid annular flow under microgravity conditions: a temporal linear stability study

Flow film boiling collapse and surface rewetting in normal and reduced gravity conditions

Numerical analysis of the competitive influence of Marangoni flow and evaporation on heat surface temperature and molten pool shape in laser surface remelting

Thermodynamics of Transport Phenomena in Field-Flow Fractionation: Organized Structures Formation

According to the diffusion equation and the hypothetical relationship between vapour pressure, moisture density and temperature, a differential file method is used to calculate the inner vapour pressure of concrete at high temperatures. When concrete is heated, a distribution field of vapour pressure will be formed. If the temperature changes, the value and the location of the peak pressure in concrete will change. However, the temperature at which the peak pressure is obtained is nearly the same, ranging from 220°C to 240°C. Calculation results show that the vapour pressure reaches peak values at a distance of less than 10 mm from the heating surface when the heating time is not more than 1 h according to the ISO 834 heating regime. The peak pressure values are from 1 MPa to 2 MPa depending on heating time. Some explanations about concrete spalling are given, which indicate that vapour pressure is the main reason for the spalling of concrete.

In the frame of ESA-MAP (Microgravity Application Promotion) project entitled XRMON (In situ X-Ray MONitoring of advanced metallurgical processes under microgravity and terrestrial conditions), a microgravity (μg) experiment in the XRMON-GF (Gradient Furnace) setup was successfully launched in 2012 on board MASER 12 sounding rocket. During this experiment, in situ and real time observations of the formation of the solidification microstructures in diffusive conditions were carried out for the first time by using X-ray radiography. In addition, two reference experiments with the same control parameters but in ground-based conditions were performed to enable us a direct comparison with the μg experiment and therefore to enlighten the effects of gravity upon microstructure formation. This communication reports on fragmentation phenomenon observed during those experiments. For 1g upward solidification, fragmentations mainly take place in the upper part of the mushy zone. After their detachments, dendrite fragments are carried away by buoyancy force in the bulk liquid where they are gradually remelted. For μg experiment and horizontal solidification, this type of fragmentation is not observed. However, a great number of fragmentations are surprisingly revealed by in situ observation in the deep part of the mushy zone, when the liquid fraction is very small. Moreover, as soon as they are detached, the dendrite fragments move toward the cold part of the mushy zone, even in the case of μg experiment. The observations suggest that sample shrinkage may be at the origin of this fragment motion.

Gas‐liquid flows through packed bed reactors (PBRs) are challenging to predict due to the tortuous flow paths that fluid interfaces must traverse. Experiments at the International Space Station showed that bubble and pulse flows are predominately observed under microgravity conditions, while the trickle and spray flows observed under terrestrial conditions are not present in microgravity. To understand the physics behind the former experiments, we simulate bubble flow through a PBR for different packing‐particle‐diameter‐based Weber numbers and under different gravity conditions. We demonstrate different pore‐scale mechanisms, such as capillary entrapment, buoyancy entrapment, and inertia‐induced bubble displacement. Then, we perform a quantitative analysis by introducing new dynamic scales, dependent upon the evolving gas‐liquid interfacial area, to understand the dynamic trade‐offs between the inertia, capillary, and buoyancy forces on a bubble passing through a PBR. This analysis leads us to define new dimensionless Weber‐like numbers that delineate bubble entrapment from bubble displacement.

The present research program is centered on the experimental and numerical study of two instabilities that develop at the interface between two different fluids when the interface experiences an impulsive or a constant acceleration. The instabilities, called the Richtmyer-Meshkov and Rayleigh-Taylor instability, respectively, adversely affect target implosion in experiments aimed at the achievement of nuclear fusion by inertial confinement by causing the nuclear fuel contained in a target and the shell material to mix, leading to contamination of the fuel, yield reduction or no ignition at all. The laboratory experiments summarized in this report include shock tube experiments to study a shock-accelerated bubble and a shock-accelerated 2-D sinusoidal interface; and experiments based on the use of magnetorheological fluids for the study of the Rayleigh-Taylor instability. Computational experiments based on the shock tube experimental conditions are also reported.

The present research program is centered on the experimental and numerical study of two instabilities that develop at the interface between two different fluids when the interface experiences an impulsive or a constant acceleration. The instabilities, called the Richtmyer-Meshkov and Rayleigh-Taylor instability, respectively, adversely affect target implosion in experiments aimed at the achievement of nuclear fusion by inertial confinement by causing the nuclear fuel contained in a target and the shell material to mix, leading to contamination of the fuel, yield reduction or no ignition at all. The laboratory experiments summarized in this report include shock tube experiments to study a shock-accelerated bubble and a shock-accelerated 2-D sinusoidal interface; and experiments based on the use of magnetorheological fluids for the study of the Rayleigh-Taylor instability. Computational experiments based on the shock tube experimental conditions are also reported.

We experimentally and theoretically investigated the rapid solidification and microstructural evolution of freely falling droplets of the ternary Fe45Cu40Co15 peritectic alloy, with the critical undercooling temperature of metastable liquid phase-separation initiation being measured to be 38 K. We found that liquid phase separation occurs when the droplet diameter ranges from 80 to 980 μm, resulting in the formation of either microscopically or macroscopically segregated microstructures. The peritectic solidification microstructure was produced when the droplet diameter either exceeded 980 μm or was below 80 μm. The dispersed morphology of the large alloy droplets possessed nonuniformly dispersive characteristics, i.e., proximity to the droplet surface is negatively associated with the size of Cu-rich globules. With the further reduction of the droplet size, the phase-separated morphology first transformed into a core–shell structure and finally displayed a homogeneously dispersed structure. Our theoretical calculations showed that the residual Stokes motion, Marangoni convection, and surface segregation are the dominant dynamic mechanisms for the phase separation and microstructural evolution under reduced-gravity conditions inside the drop tube.

The problem of radiation-conductive heating and subsequent melting of ice in a climatic chamber in a single-phase approximation of the Stefan problem with allowance for the emerging thin water film on the irradiated surface was solved by mathematical modeling methods. Fields of temperature and flux density of the resulting radiation are obtained, as well as the rate of melting and heating of the non-irradiated ice surface. Comparison with experiment showed satisfactory agreement.