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61.
Water electrolysis is carried out in both alkaline (25 and 2 wt.% KOH) and acidic (0.1N H2SO4) solutions for 8 s under a microgravity (μ-G) environment realized in a drop shaft. The effects of gravitational strength on gas bubble evolution behavior are analyzed in consideration of various factors (bubble movement, bubble assembly and single bubble). Under a μ-G environment, a collection of fine gas bubbles forms a froth layer in alkaline solutions, whereas bubbles frequently coalesce in acidic solution. Moreover, H2 gas bubbles in alkaline jump from a cathode surface and O2 bubbles often coalesce on an anode. In acidic solution both H2 and O2 bubbles frequently coalesce on electrode surfaces. Such gas bubble movements are reflected in the coalescence number and bubble residence time. A single bubble is characterized by the bubble size and the dynamic contact angle between a gas bubble and a Pt electrode, however, these factors are not essentially influenced by the gravitational strength. 相似文献
62.
Water electrolysis was conducted in both alkaline (25 wt.% KOH, 2 wt.% KOH) and acid (0.1N H2SO4) solutions for 8 s under microgravity environment realized in a drop shaft. The gas bubble formation of hydrogen and oxygen on platinum electrodes was observed by CCD camera. In alkaline solutions, a bubble froth layer grew on the electrode surface. Hydrogen bubble size was smaller than that of oxygen. The current density at constant potential decreased continually with time. In spite of the growth of a bubble froth layer on the electrode, the electrolysis never stopped, apparently because fresh electrolyte is supplied to the electrode surface by microconvection induced by the gas bubble evolution. In acid solution, hydrogen gas bubbles frequently coalesced on the cathode surface, yielding a larger average bubble than that of oxygen. The current density did not vary at constant potentials from -0.4 to −0.8 V versus reversible hydrogen electrode (RHE), because the effective electrode surface area was significantly reduced by the larger bubble size compared to alkaline electrolyte. The present experiments indicate that, especially in a microgravity environment, the bubble evolution behavior and the resultant current-potential curves are significantly influenced by the wettability of the electrode in contact with the electrolyte. 相似文献
63.
C. Lecoutre Y. Garrabos E. Georgin F. Palencia D. Beysens 《International Journal of Thermophysics》2009,30(3):810-832
Light transmission measurements performed in SF6 close to its liquid–gas critical point are used to obtain turbidity data in the reduced temperature range (T is temperature, T
c is the critical temperature). Automatic experiments (ALICE 2 facility) were made at a near critical density, i.e., , in the one-phase homogeneous region, under the microgravity environment of the Mir Space Station ( is the average density, ρ
c is the critical density). The turbidity data analysis verifies the theoretical crossover formulations for the isothermal
compressibility and the correlation length ξ. These latter formulations are also used to analyze very near T
c thermal diffusivity data obtained under microgravity conditions by Wilkinson et al. (Phys. Rev. E 57 436, 1998). 相似文献
64.
The burning characteristics of a biodiesel droplet mixed with diesel or alkanes such as dodecane and hexadecane were experimentally studied in a reduced-gravity environment so as to create a spherically symmetrical flame without the influence of natural convection due to buoyancy. Small droplets on the order of 500 μm in diameter were initially injected via a piezoelectric technique onto the cross point intersected by two thin carbon fibers; these were prepared inside a combustion chamber that was housed in a drag shield, which was freely dropped onto a foam cushion. It was found that, for single component droplets, the tendency to form a rigid soot shell was relatively small for biodiesel fuel as compared to that exhibited by the other tested fuels. The soot created drifted away readily, showing a puffing phenomenon; this could be related to the distinct molecular structure of biodiesel leading to unique soot layers that were more vulnerable to oxidative reactivity as compared to the soot generated by diesel or alkanes. The addition of biodiesel to these more traditional fuels also presented better performance with respect to annihilating the soot shell, particularly for diesel. The burning rate generally follows that of multi-component fuels, by some means in terms of a lever rule, whereas the mixture of biodiesel and dodecane exhibits a somewhat nonlinear relation with the added fraction of dodecane. This might be related to the formation of a soot shell. 相似文献
65.
Jae Hyuk Choi Junhong Kim SangKyu Choi Byoung-Ho Jeon Osamu Fujita Suk Ho Chung 《Journal of Mechanical Science and Technology》2009,23(3):707-716
A numerical study on soot deposition in ethylene diffusion flames has been conducted to elucidate the effect of thermophoresis
on soot particles under a microgravity environment. Time-dependent reactive-flow Navier-Stokes equations coupled with the
modeling of soot formation have been solved. The model was validated by comparing the simulation results with the previous
experimental data for a laminar diffusion flame of ethylene (C2H4) with enriched oxygen (35% O2 + 65% N2) along a solid wall. In particular, the effect of surrounding air velocity as a major calculation parameter has been investigated.
Especially, the soot deposition length defined as the transverse travel distance to the wall in the streamwise direction is
introduced as a parameter to evaluate the soot deposition tendency on the wall. The calculation result exhibits that there
existed an optimal air velocity for the early deposition of soot on the surface, which was in good agreement with the previous
experimental results. The reason has been attributed to the balance between the effects of the thermophoretic force and convective
motion.
This paper was recommended for publication in revised form by Associate Editor Ohchae Kwon
Jae Hyuk Choi received his B.S. and M.S. degrees in Marine System Engineering from Korea Maritime University in 1996 and 2000, respectively.
He then went on to receive a Ph.D. degrees from Hokkaido university in 2005. Dr. Choi is currently a BK21 Assistant Professor
at the School of Mechanical and Aerospace Engineering at Seoul National University in Seoul, Korea. Dr. Choi’s research interests
are in the area of reduction of pollutant emission (Soot and NOx), high temperature combustion, laser diagnostics, alternative
fuel and hydrogen production with high temperature electrolysis steam (HTES).
Junhong Kim received his B.S., M.S., and Ph. D degrees in Mechanical Engineering from Seoul National University in 1998, 2000, and 2004,
respectively. His research interests include lifted flames, edge flames, and numerical simulation.
Sang Kyu Choi received his B.S. degree in Mechanical Engineering from Seoul National University in 2004. He is a Ph. D student in the School
of Mechanical Engineering, Seoul National University. His research interests include edge flames, oxy-fuel combustion, and
numerical simulation.
Byoung ho Jeon received his B.S degrees in Mechanical Engineering from kangwon University in 1998, and M.S., Ph. D. degrees in Mechanical
Engineering from Hokkaido University in 2002, 2008, respectively. Dr Jeon is working at Korea Aerospace Research Institute
from 2007. June. as Gasturbine engine developer. Jeon’s research interests are in the area of reduction of pollutant emission
(Soot and Nox), High temperature combustion, combustion system (Furnace, Combine Generation system, IGCC, CTL), and Fire safety
in building.
Osamu Fujita received his B.S., M.S., and Ph. D. degrees in Mechanical Engineering from Hokkaido University in 1982, 1984, and 1987, respectively.
Prof. Fujita is currently a Professor at the division of Mechanical and space Engineering at Hokkaido University in sapporo,
Japan. Prof. Fujita’s research interests are in the area of reduction of pollutant emission (Soot and Nox), solid combustion,
catalytic combustion, high temperature combustion, alternative fuel and fire safety in space.
Suk Ho Chung received his B.S. degree in Mechanical Engineering in 1976 from Seoul National University, and his M.S. and Ph. D. degree
in Mechanical Engineering in 1980 and 1983, respectively from Northwestern University. He is a professor since 1984 in the
School of Mechanical and Aerospace Engineering, Seoul National University. His research interests cover combustion fundamentals,
pollutant formation, and laser diagnostics. 相似文献
66.
J.H. Bae 《Combustion and Flame》2006,145(3):607-620
An experimental study of droplet combustion of nonane (C9H20) at elevated pressures burning in air is reported using low gravity and small droplets to promote spherical gas-phase symmetry at pressures up to 30 atm (absolute). The initial droplet diameters range from 0.57 to 0.63 mm and they were ignited by two electrically heated hot wires positioned horizontally on opposite sides of the droplet. The droplet and flame characteristics were recorded by a 16-mm high-speed movie and a high-resolution video camera, respectively. A photodiode is used to measure broadband gray-body emission from the droplet flames and to track its dependence on pressure. Increasing the pressure significantly influences the ability to make quantitative measurements of droplet, soot cloud, and luminous zone diameters. At pressures as low as 2 atm, soot aggregates surrounding the droplet show significant coagulation and agglomeration and at higher pressures the soot cloud completely obscures the droplet, with the result being that the droplet could not be measured. Above 10 atm radiant emissions from hot soot particles are extensive and the resulting flame luminosity further obscures the droplet. Photographs of the luminous zone in subcritical pressures show qualitatively that increasing pressure produces more soot, and the mean photodiode voltage output increases monotonically with pressure. The maximum flame and soot shell diameters shift to later times as pressure increases and the soot shell is located closer to the flame at higher pressure. The soot shell and flame diameter data are correlated by a functional relationship of reduced pressure derived from scaling the drag and thermophoretic forces on aggregates that consolidates all of the data onto a single curve. 相似文献
67.
Guangwen Xu Senji Honma Xiaoxun Ma Daniel L. Dietrich 《International Journal of Heat and Mass Transfer》2003,46(7):1155-1169
Isolated droplet burning were conducted in microgravity ambiences of different temperatures to test the initial diameter influence on droplet burning rate that shows a flame scale effect and represents an overall thermal action of flame in balance with heat loss. The coldest ambience examined was room air, which utilized a heater wire to ignite the droplet. All other ambiences hotter than 633 K were acquired through an electrically heated air chamber in a stainless steel can. An inverse influence of initial droplet diameter on burning rate was demonstrated for the cold and hot ambiences. That is, the burning rate respectively decreased and increased in the former and latter cases with raising the initial droplet diameter. The reversion between the two influences appeared gradual. In the hot ambiences the burning rate increase with increasing the initial droplet diameter was larger at higher temperatures. A “net heat” of flame that denotes the difference between “heat gain” by the droplet and “heat loss” to the flame surrounding was suggested responsible for the results. In low-temperature ambiences there is a negative net heat, and it turns gradually positive as the ambience temperature gets higher and the heat loss becomes less. Relating to luminous flame sizes and soot generation of differently sized droplets clarified that the flame radiation, both non-luminous and luminous, is determinative to the net heat in microgravity conditions. In addition, the work identified two peak values of soot generation during burning, which appeared respectively at the room temperature and at about 1000 K. The increase in ambience temperature made also bigger soot shells. The heat contribution of flame by both radiation and conduction was demonstrated hardly over 40% in the total heat required for droplet vaporization during burning in a hot ambience of 773 K. 相似文献
68.
Gas tungsten arc (GTA) welding was performed both in a microgravity environment and in a terrestrial environment,and the arc shapes in both environments were compared. A microgravity condition was obtained using the free fallsystem at the Japan Microgravi 相似文献
69.
70.
If humans are to explore space beyond low-earth orbit, their health and welfare must be ensured, not only for survival in harsh environments but also so that they can work productively. The requisite technologies, and human physiology itself, are subject to reduced levels of gravity that are indigenous to space travel. Numerous studies have shown that it will require many years of intensive research to develop reliable, efficient, and self-sustaining technologies and to understand the effects of gravity on humans. The research community that was developed to provide crucial specific information has essentially been deactivated because of budget constraints. Thus, the great engineering challenge—to develop advanced and novel technologies that will enable further space exploration—will remain for future generations. 相似文献