The Experts below are selected from a list of 27216 Experts worldwide ranked by ideXlab platform
Osamu Fujita - One of the best experts on this subject based on the ideXlab platform.
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Opposed-Flow Flame Spread and Extinction in Electric Wires: The Effects of Gravity, External Radiant Heat Flux, and Wire Characteristics on Wire Flammability
Fire Technology, 2019Co-Authors: Yusuke Konno, Yoshinari Kobayashi, Carlos Fernandez-pello, Nozomu Hashimoto, Shinji Nakaya, Mitsuhiro Tsue, Osamu FujitaAbstract:Combustion of electric wires is the most probable cause of fire in human space activities. Therefore, the fire performance of electric wires in microGravity Conditions must be thoroughly analyzed. This study investigates the opposed-flow flame spread and its limits in electric wires preheated by external radiation, under both normal Gravity and microGravity, to understand their fire performance when exposed to external heat sources in such Gravity Conditions. The experiments were performed on low-density polyethylene (LDPE)-insulated copper (Cu) wires having an outer diameter of 4 mm and differing in core diameter (2.5 and 0.7 mm, corresponding to insulation thicknesses of 0.75 and 1.65 mm, respectively). Both standard and black LDPE insulations were used to study the effect of radiation absorption on the wire preheating and subsequent flame spread. The comparison of the flame spread limits revealed that the wire with the thicker Cu core was less flammable under both normal Gravity and microGravity; in particular, its flammability further decreased in the case of microGravity, in contrast with thinner electric wires (~ 1 mm outer diameter), which exhibited higher flammability in the same Gravity Condition. These results suggest that different mechanisms, for thicker and thinner wires, determining the critical Conditions to sustain flame spread under microGravity. This study provides valuable information about the fire performance of electric wires in space Gravity.
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effect of ac electric fields on flame spread over electrical wire
Proceedings of the Combustion Institute, 2011Co-Authors: Suk Ho Chung, Osamu FujitaAbstract:Abstract The effect of electric fields on the characteristics of flame spread over insulated electrical wire has been investigated experimentally by varying AC voltage and frequency applied to the wire in the normal Gravity Condition. The polyethylene (PE) insulated electrical wire was placed horizontally on electrically non-conducting posts and one end of the wire was connected to the high voltage terminal. Thus, the electrical system is the single electrode configuration. The wire was ignited at one end and the flame spread rate along the wire has been measured from the images using a video camera. Two distinct regimes existed depending on the applied AC frequency. In the low frequency regime, the flame spread rate decreased with the frequency and voltage. While in the high frequency regime, it decreased initially with voltage and then increased. At high frequency, the spread rate was even over that without applying electric fields. This result implies that fire safety codes developed without considering the effect of electric fields may require modifications.
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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.
Suk Ho Chung - One of the best experts on this subject based on the ideXlab platform.
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Blowout of non-premixed turbulent jet flames with coflow under microGravity Condition
ELSEVIER SCIENCE INC, 2019Co-Authors: Wang Qiang, Suk Ho Chung, Hu Longhua, Wang Shaoming, Wang Shuangfeng, Fujita OsamuAbstract:The blowout behavior of non-premixed turbulent coflow jet flames under microGravity environment was studied experimentally by utilizing a 3.6 s drop tower. Variations of flames leading to liftoff as well as blowout were examined by varying the coflow velocity and compared with those obtained under the normal Gravity Condition. A modeling work was conducted to incorporate the effects of the Gravity (buoyancy) and coflow velocity on blowout behavior. Major findings include: (1) the flame length in microGravity was longer than that in normal Gravity and decreased with increasing coflow velocity. The flame in microGravity showed more intense yellow luminosity with larger sooting zone; (2) the flame liftoff height increased with increasing coflow velocity in both Gravity levels. The flame base was closer to the burner in microGravity as compared with that in normal Gravity; (3) the blowout velocity in microGravity was appreciably larger than that obtained in normal Gravity; and (4) a physical model based on Damkohler number was developed by using similarity solutions to characterize the differences in the blowout limits considering both the coflow and Gravity (buoyancy) effects. The proposed model can successfully predict the experimental data. This work provided new data and basic scaling analysis for blowout limit of non-premixed turbulent jet flames considering both the coflow and Gravity (buoyancy) effects. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved
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effect of ac electric fields on flame spread over electrical wire
Proceedings of the Combustion Institute, 2011Co-Authors: Suk Ho Chung, Osamu FujitaAbstract:Abstract The effect of electric fields on the characteristics of flame spread over insulated electrical wire has been investigated experimentally by varying AC voltage and frequency applied to the wire in the normal Gravity Condition. The polyethylene (PE) insulated electrical wire was placed horizontally on electrically non-conducting posts and one end of the wire was connected to the high voltage terminal. Thus, the electrical system is the single electrode configuration. The wire was ignited at one end and the flame spread rate along the wire has been measured from the images using a video camera. Two distinct regimes existed depending on the applied AC frequency. In the low frequency regime, the flame spread rate decreased with the frequency and voltage. While in the high frequency regime, it decreased initially with voltage and then increased. At high frequency, the spread rate was even over that without applying electric fields. This result implies that fire safety codes developed without considering the effect of electric fields may require modifications.
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stabilization mechanism of lifted flame edge in the near field of coflow jets for diluted methane
30th International Symposium on Combustion, 2005Co-Authors: Sang Hee Won, Jongsoo Kim, K J Hong, Min Suk Cha, Suk Ho ChungAbstract:Abstract The stabilization mechanism of lifted flames in the near field of coflow jets has been investigated experimentally and numerically for methane fuel diluted with nitrogen. The lifted flames were observed only in the near field of coflow jets until blowout occurred in the normal Gravity Condition. To elucidate the stabilization mechanism for the stationary lifted flames of methane having the Schmidt number smaller than unity, the behavior of the flame in the buoyancy-free Condition, and unsteady propagation characteristics after ignition were investigated numerically at various Conditions of jet velocity. It has been found that buoyancy plays an important role for flame stabilization of lifted flames under normal Gravity, such that the flame becomes attached to the nozzle in microGravity. The stabilization mechanism is found to be due to the variation of the propagation speed of the lifted flame edge with axial distance from the nozzle in the near field of the coflow as compared to the local flow velocity variation at the edge.
A A Ranjbar - One of the best experts on this subject based on the ideXlab platform.
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numerical simulation of subcooled flow boiling under conjugate heat transfer and microGravity Condition in a vertical mini channel
Applied Thermal Engineering, 2017Co-Authors: Mohammad Bahreini, A. Ramiar, A A RanjbarAbstract:Abstract In this study, subcooled flow boiling in a vertical mini channel is simulated using Color Function Volume Of Fluid (CF-VOF) in micro Gravity Condition. The continuous surface force model and Lee model are used for analyzing surface tension forces and mass transfer via interface, respectively. The Newtonian flow equations are solved using the finite volume method, which is based on the Pressure Implicit with Splitting of Operators (PISO) algorithm. The existing OpenFOAM two phase solver i.e. interFoam was developed by adding energy equation and the source terms corresponding phase change. Moreover, in order to increasing the accuracy of the simulation, the effect of axial conduction is taken into account by applying the conjugate heat transfer model. The validation of developed solver and mathematical model indicates a proper compliance with experimental data of technical literature. Results show variation in flow patterns and heat transfer coefficients in the micro Gravity (Gravity level of ± 0.05 g ) Condition comparing with the normal Gravity Condition. Moreover, it is found that some imperative parameters such as heat flux and mass flux affect aforementioned patterns and coefficients. In addition, simulation results show that the size of bubbles formed in the micro Gravity Condition is larger than the bubbles in normal Gravity Condition and the variation in the size of bubbles increases the heat transfer coefficient in the micro Gravity (%22) compared to the normal Gravity Condition.
T Morizono - One of the best experts on this subject based on the ideXlab platform.
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robust pd control using adaptive compensation for completely restrained parallel wire driven robots translational systems using the minimum number of wires under zero Gravity Condition
IEEE Transactions on Robotics, 2007Co-Authors: Hitoshi Kino, T Yahiro, Fumiaki Takemura, T MorizonoAbstract:A parallel-wire driven mechanism uses flexible wires instead of heavy rigid links. In this paper, we propose a robust point-to-point (PTP) position control method in the task-oriented coordinates for completely restrained parallel wire-driven robots, which are translational systems using the minimum number of wires under zero-Gravity Conditions. In the cases where parallel-wire driven robots are disassembled/assembled and used outdoors (also applied in space), actuator positions would be uncertain or contain some errors. The error of internal force among wires that results from such uncertainty of actuator positions deteriorates positioning performance. To overcome such a difficulty, adaptive compensation is employed for robust PD control against the error of internal force, in this paper. It is necessary for the adaptive compensation to separate the internal force term linearly into a regressor matrix and a parameter vector concerned with the errors of actuator positions. The internal force term, however, possesses the nonlinear characteristic concerned with the errors of actuator position. Noting the structure of the internal force term, this paper shows that measuring both the position of an end-effector and wire length in real time enables the linear separation. Not only does this robust PD control method ensure precise positioning using external sensors; it enhances the robustness for uncertainty of the Jacobian matrix, which results from the error of actuator installation. First, we explain the linearization of the internal force term. Next, the robust PD control for the parallel-wire driven system using the uncertain Jacobian matrix is proposed; then, we prove the motion convergence to desired points and discuss its robustness based on Lyapunov stability analysis. Finally, the usefulness of the proposed control method is demonstrated through experiments and simulations.
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.
