The Experts below are selected from a list of 31881 Experts worldwide ranked by ideXlab platform
Johannes Straub - One of the best experts on this subject based on the ideXlab platform.
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boiling heat transfer and bubble dynamics in Microgravity
Advances in heat transfer, 2001Co-Authors: Johannes StraubAbstract:Abstract This article presents results for pool boiling heat transfer under Microgravity conditions that the author and his team have gained in a succession of experiments during the past two decades. The objective of the research work was to provide answers to the following questions: Is boiling an appropriate mechanism of heat transfer for space application? How do heat transfer and bubble dynamics behave without the influence of buoyancy, or more general, without the influence of external forces? Is Microgravity a useful environment for investigating the complex mechanisms of boiling with the aim of gaining a better physical understanding? Various carrier systems that allow simulation of a Microgravity environment could be used, such as drop towers, parabolic trajectories with NASA’s aircraft KC-135, ballistic rockets such as TEXUS, and, more recently, three Space Shuttle missions. As far as the possibilities of the respective missions allowed, a systematic research program was followed that was continuously adjusted to actual new parameters. After a general survey concerning the importance of boiling for technical applications, an introduction is given especially for those individuals not closely familiar with the fields of Microgravity and boiling. Surprising results have been obtained: not only that saturated pool boiling can be maintained in a Microgravity environment, but also that at small heater surfaces and lower values of heat fluxes, even higher heat transfer coefficients have been attained than under terrestrial conditions. The bubble departure can be attributed to surface tension effects, to “bubble ripening” and coalescence processes. Under subcooled conditions only, thermocapillary flow was observed that transports the heat from the bubble interface into the bulk liquid, but does not enhance the heat transfer compared with boiling at saturated conditions. Direct electrical heated plane surfaces lead to a slow extension of dry spots to dry areas below bubbles, the increasing surface temperature suggesting transition to film boiling. The critical heat flux in Microgravity is lower than under earth conditions, but considerably higher than the hitherto accepted correlations predict when extrapolated to Microgravity. The nearly identical heat transfer coefficients received for nucleate boiling under Microgravity as well as terrestrial conditions, and for both saturated and subcooled fluid states, also suggest identical heat transfer mechanisms. These results lead to the conclusion that the primary heat transfer mechanism must be strongly related to the development of the microlayer during bubble growth. Secondary mechanisms are responsible for the transport of enthalpy in form of latent energy of the bubbles and hot liquid carried with them. Under terrestrial conditions, that mechanism is caused by external forces such as buoyancy; under Microgravity conditions, the self dynamics of the bubbles and/or thermocapillary flow under subcooled conditions are responsible. The results demonstrate clearly that boiling can be applied as a heat transfer mechanism in a Microgravity environment and that Microgravity is a useful means to study the physics of boiling.
Markus Braun - One of the best experts on this subject based on the ideXlab platform.
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Analysis of Statoliths Displacement in Chara Rhizoids for Validating the Microgravity-Simulation Quality of Clinorotation Modes
Microgravity Science and Technology, 2018Co-Authors: Lars Krause, Markus Braun, Jens Hauslage, Ruth HemmersbachAbstract:In single-celled rhizoids of the green algae Chara , positively gravitropic growth is governed by statoliths kept in a dynamically stable position 10–25 μ m above the cell tip by a complex interaction of gravity and actomyosin forces. Any deviation of the tube-like cells from the tip-downward orientation causes statoliths to sediment onto the gravisensitive subapical cell flank which initiates a gravitropic curvature response. Microgravity experiments have shown that abolishing the net tip-directed gravity force results in an actomyosin-mediated axial displacement of statoliths away from the cell tip. The present study was performed to critically assess the quality of Microgravity simulation provided by different operational modes of a Random Positioning Machine (RPM) running with one axis (2D mode) or two axes (3D mode) and different rotational speeds (2D), speed ranges and directions (3D). The effects of 2D and 3D rotation were compared with data from experiments in real Microgravity conditions (MAXUS sounding rocket missions). Rotational speeds in the range of 60–85 rpm in 2D and 3D modes resulted in a similar kinetics of statolith displacement as compared to real Microgravity data, while slower clinorotation (2–11 rpm) caused a reduced axial displacement and a more dispersed arrangement of statoliths closer to the cell tip. Increasing the complexity of rotation by adding a second rotation axis in case of 3D clinorotation did not increase the quality of Microgravity simulation, however, increased side effects such as the level of vibrations resulting in a more dispersed arrangement of statoliths. In conclusion, fast 2D clinorotation provides the most appropriate Microgravity simulation for investigating the graviperception mechanism in Chara rhizoids, whereas slower clinorotation speeds and rotating samples around two axes do not improve the quality of Microgravity simulation.
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ground based facilities for simulation of Microgravity organism specific recommendations for their use and recommended terminology
Astrobiology, 2013Co-Authors: Raul Herranz, Markus Braun, Ralf Anken, Johannes Boonstra, P C M Christianen, Maarten De Geest, Jens Hauslage, Reinhard Hilbig, Richard J HillAbstract:Research in Microgravity is indispensable to disclose the impact of gravity on biological processes and organisms. However, research in the near-Earth orbit is severely constrained by the limited number of flight opportunities. Ground-based simulators of Microgravity are valuable tools for preparing spaceflight experiments, but they also facilitate stand-alone studies and thus provide additional and cost-efficient platforms for gravitational research. The various Microgravity simulators that are frequently used by gravitational biologists are based on different physical principles. This comparative study gives an overview of the most frequently used Microgravity simulators and demonstrates their individual capacities and limitations. The range of applicability of the various ground-based Microgravity simulators for biological specimens was carefully evaluated by using organisms that have been studied extensively under the conditions of real Microgravity in space. In addition, current heterogeneous terminology is discussed critically, and recommendations are given for appropriate selection of adequate simulators and consistent use of nomenclature.
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differential gene regulation under altered gravity conditions in follicular thyroid cancer cells relationship between the extracellular matrix and the cytoskeleton
Cellular Physiology and Biochemistry, 2011Co-Authors: Claudia Ulbrich, Jens Hauslage, Jessica Pietsch, Jirka Grosse, Markus Wehland, Herbert Schulz, Kathrin Saar, Norbert Hubner, Ruth Hemmersbach, Markus BraunAbstract:Extracellular matrix proteins, adhesion molecules, and cytoskeletal proteins form a dynamic network interacting with signalling molecules as an adaptive response to altered gravity. An important issue is the exact differentiation between real Microgravity responses of the cells or cellular reactions to hypergravity and/or vibrations. To determine the effects of real Microgravity on human cells, we used four DLR parabolic flight campaigns and focused on the effects of short-term Microgravity (22 s), hypergravity (1.8 g), and vibrations on ML-1 thyroid cancer cells. No signs of apoptosis or necrosis were detectable. Gene array analysis revealed 2430 significantly changed transcripts. After 22 s Microgravity, the F-actin and cytokeratin cytoskeleton was altered, and ACTB and KRT80 mRNAs were significantly upregulated after the first and thirty-first parabolas. The COL4A5 mRNA was downregulated under Microgravity, whereas OPN and FN were significantly upregulated. Hypergravity and vibrations did not change ACTB, KRT-80 or COL4A5 mRNA. MTSS1 and LIMA1 mRNAs were downregulated/slightly upregulated under Microgravity, upregulated in hypergravity and unchanged by vibrations. These data indicate that the graviresponse of ML-1 cells occurred very early, within the first few seconds. Downregulated MTSS1 and upregulated LIMA1 may be an adaptive mechanism of human cells for stabilizing the cytoskeleton under Microgravity conditions.
Osamu Fujita - One of the best experts on this subject based on the ideXlab platform.
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ignition of electrical wire insulation with short term excess electric current in Microgravity
Proceedings of the Combustion Institute, 2011Co-Authors: Osamu Fujita, Hiroyuki Ito, Takeshi Kyono, Yasuhiro Kido, Yuji NakamuraAbstract:Abstract Ignition phenomena of overloaded electric wires have been investigated in Microgravity as basic information for fire safety in space. Microgravity experiments were conducted at MGLAB (Micro Gravity Laboratory of Japan) to provide 4.5 s of Microgravity time. In the experiments the current supply duration was selected as the main test parameter to simulate the status of the circuit breaker shortly after the overload occurs. Other important test parameters were the surrounding oxygen concentration and the supplied electric current amount. The results showed that the Microgravity environment significantly increases the ignition probability, including the occurrence of delayed ignition and extended ignition limits, with large electric currents when compared with the situation under normal earth based gravity. The increase in the ignition probability is explained by decreases in the minimum ignition energy in Microgravity interacting with the ignition mechanism.
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observation of flame spreading over electric wire under reduced gravity condition given by parabolic flight and drop tower experiments
Transactions of The Japan Society for Aeronautical and Space Sciences Space Technology Japan, 2010Co-Authors: Yosuke Onishi, Osamu Fujita, Kei Agata, Hiroyuki Takeuchi, Yuji Nakamura, Hiroyuki Ito, Masao KikuchiAbstract:Ground-based, Microgravity experiments attained by aircraft parabolic flight and drop tower on flame spread phenomenon over electric wire are performed. These are the preliminary tests for expected long-term Microgravity experiments by sub-orbital or on orbit Microgravity experiment. The main objectives of this study are (1) to confirm the apparatus can be work properly in Microgravity and (2) to show the necessity of long-term Microgravity experiments in order to observe the unsteady phenomenon. The flame spread rate and the total soot volume are important items as fundamental characteristics of the spreading flame. From the parabolic flight test, which can provide relatively long Microgravity period, it is confirmed that the apparatus can work properly in Microgravity. On the other hand, the quality of Microgravity provided by aircraft is fair including G-jitters, and dependable data in the slow external flow velocity regime is hardly expected. Flame spread rate and the total soot volume are measured by drop tower, which can provide 10-4G Microgravity environment. Although, in some conditions, the flame spread phenomenon seems to reach steady-state within the available Microgravity time with the drop tower (∼ five seconds), the phenomenon includes periodical change in flame shape in reality. Consequently, to confirm the actual steadiness of the spread phenomenon, at least 10-4G Microgravity environment and long-term Microgravity environment is necessary.
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experimental study on flame spread over wire insulation in Microgravity
Symposium (International) on Combustion, 1998Co-Authors: Masao Kikuchi, Atsuki Sato, Osamu Fujita, Takashi SakurayaAbstract:An experimental study of flame spread phenomena over ETFE (ethylene-tetrafluoroethylene)-insulated wires has been performed in Microgravity to obtain basic data on the fire safety of wire insulation. Three samples with different wire diameters, d w (0.32–0.51 mm) and the same insulation thickness, δ (0.15 mm) were investigated. The effects of the parameters thought dominant for wire combustion in fires: the ambient oxygen concentration, wire initial temperature, T i , wire diameter, d w , pressure, and dilution gas were investigated in the Microgravity experiments. A series of comparative experiments were also conducted at normal gravity. The results show that flame spread rates in Microgravity are higher than vertically downward spread rates at normal gravity when oxygen concentration is greater than 30% O 2 . However, with wire preheating, the spread rate in Microgravity is higher than the downward spread rate at normal gravity even at lower O 2 concentrations. The increase in flame spread rates in Microgravity became larger with decreases in d w . The effect of pressure on the flame spreading appeared very small, and lower pressure caused extinction of the flames in Microgravity. The increase in flame spread rates in Microgravity was especially large with CO 2 dilution, and this must be taken into account when selecting extinguisher gas. The Microgravity experiments with CO 2 dilution gave rise to a new unsteady flame spread phenomenon for flame spreading over the wire: this phenomenon involves discontinuous flames partly occurring ahead of the spreading flame front.
Yuji Nakamura - One of the best experts on this subject based on the ideXlab platform.
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ignition of electrical wire insulation with short term excess electric current in Microgravity
Proceedings of the Combustion Institute, 2011Co-Authors: Osamu Fujita, Hiroyuki Ito, Takeshi Kyono, Yasuhiro Kido, Yuji NakamuraAbstract:Abstract Ignition phenomena of overloaded electric wires have been investigated in Microgravity as basic information for fire safety in space. Microgravity experiments were conducted at MGLAB (Micro Gravity Laboratory of Japan) to provide 4.5 s of Microgravity time. In the experiments the current supply duration was selected as the main test parameter to simulate the status of the circuit breaker shortly after the overload occurs. Other important test parameters were the surrounding oxygen concentration and the supplied electric current amount. The results showed that the Microgravity environment significantly increases the ignition probability, including the occurrence of delayed ignition and extended ignition limits, with large electric currents when compared with the situation under normal earth based gravity. The increase in the ignition probability is explained by decreases in the minimum ignition energy in Microgravity interacting with the ignition mechanism.
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observation of flame spreading over electric wire under reduced gravity condition given by parabolic flight and drop tower experiments
Transactions of The Japan Society for Aeronautical and Space Sciences Space Technology Japan, 2010Co-Authors: Yosuke Onishi, Osamu Fujita, Kei Agata, Hiroyuki Takeuchi, Yuji Nakamura, Hiroyuki Ito, Masao KikuchiAbstract:Ground-based, Microgravity experiments attained by aircraft parabolic flight and drop tower on flame spread phenomenon over electric wire are performed. These are the preliminary tests for expected long-term Microgravity experiments by sub-orbital or on orbit Microgravity experiment. The main objectives of this study are (1) to confirm the apparatus can be work properly in Microgravity and (2) to show the necessity of long-term Microgravity experiments in order to observe the unsteady phenomenon. The flame spread rate and the total soot volume are important items as fundamental characteristics of the spreading flame. From the parabolic flight test, which can provide relatively long Microgravity period, it is confirmed that the apparatus can work properly in Microgravity. On the other hand, the quality of Microgravity provided by aircraft is fair including G-jitters, and dependable data in the slow external flow velocity regime is hardly expected. Flame spread rate and the total soot volume are measured by drop tower, which can provide 10-4G Microgravity environment. Although, in some conditions, the flame spread phenomenon seems to reach steady-state within the available Microgravity time with the drop tower (∼ five seconds), the phenomenon includes periodical change in flame shape in reality. Consequently, to confirm the actual steadiness of the spread phenomenon, at least 10-4G Microgravity environment and long-term Microgravity environment is necessary.
Shuiming Shu - One of the best experts on this subject based on the ideXlab platform.
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prediction of liquid hydrogen flow boiling critical heat flux condition under Microgravity based on the wall heat flux partition model
International Journal of Hydrogen Energy, 2020Co-Authors: Yao Zheng, Huawei Chang, Yinan Qiu, Chen Duan, Jianye Chen, Hong Chen, Shuiming ShuAbstract:Abstract Critical heat flux (CHF) of liquid hydrogen (LH2) flow boiling under Microgravity is vital for designing space cryogenic propellant conveying pipe since the excursion of wall temperature may cause system failure. In this study, a two-dimensional axisymmetric model based on the wall heat flux partition (WHFP) model was proposed to predict the CHF condition under Microgravity including the wall temperature and the CHF location. The proposed numerical model was validated to demonstrate a good agreement between the simulated and experimentally reported results. Then, the wall temperature distribution and the CHF location under different gravity conditions were compared. In addition, the WHFP and vapor-liquid distribution along the wall under Microgravity were predicted and its difference with terrestrial gravity condition was also analysed and reported. Finally, the effects of flow velocity and inlet sub-cooling on the wall temperature distributions were analysed under Microgravity and terrestrial gravity conditions, respectively. The results indicate that the CHF location moves upstream about 5.25 m from 1g to 10−4g since the void fraction near the wall reaches the breakpoint of CHF condition much earlier under the Microgravity condition. Furthermore, the increase of the velocity and decrease of the sub-cooling have smaller effects on the CHF location during LH2 flow boiling under Microgravity.
