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Sökning: WFRF:(Deng Xiang) > Kungliga Tekniska Högskolan

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1.
  • Deng, Yucheng, et al. (författare)
  • A numerical study on the levitation system for droplet preparation in a fuel-coolant interaction experiment
  • 2023
  • Ingår i: Progress in nuclear energy (New series). - : Elsevier BV. - 0149-1970 .- 1878-4224. ; 159
  • Tidskriftsartikel (refereegranskat)abstract
    • The MISTEE facility at KTH was designed to investigate the process and phenomena of a molten droplet falling into a water pool that may be encountered in fuel-coolant interactions (FCI) during a severe accident of light water reactors. An aerodynamic levitation mechanism is proposed to hold the molten droplet during its preparation (melting and heating up to a prescribed temperature) in an induction furnace. The crucible is flushed with argon through an injection nozzle at the bottom to prevent the droplet from falling out of the crucible. A numerical simulation of the aerodynamic levitation system is performed in the present study with the objective of determining and optimizing the design. The problem was simplified as an isothermal two-phase flow in an axisymmetric geometry. The simulation is realized through ANSYS Fluent v17 platform, which employs the VOF method to track interfaces between two phases and the SST k-omega model to describe turbulence flow of argon gas. The numerical model is validated against tests performed in the MISTEE facility after mesh sensitivity study. It is then applied to investigate the impacts of various parameters on the facility levitation capability and the droplet stability. According to the simulation results, stable molten droplets can be obtained in the designed experimental setup. The simulation also provides the appropriate values of argon inlet velocity and sample mass at which a stable droplet can be obtained inside the crucible before its discharge. Either higher or lower inlet velocity will destabilize the formation of the droplet. Considering the temperature-dependent melt properties, both surface tension and viscosity affect the movement and deformation of the molten droplet. The wettability of melt on the crucible wall is critical to droplet formation, and it is found that a poor wettability can ensure the levitation of droplet.
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2.
  • Deng, Yucheng, et al. (författare)
  • An experimental study on droplet quench and steam explosion in boric acid solutions
  • 2023
  • Ingår i: Progress in nuclear energy (New series). - : Elsevier BV. - 0149-1970 .- 1878-4224. ; 166
  • Tidskriftsartikel (refereegranskat)abstract
    • Boric acid (H3BO3) is widely adopted as an additive in the coolant of light water reactors for reactivity control, but its effect on fuel coolant interactions (FCI) during severe accidents (especially on steam explosion) was rarely investigated. To examine the effect of the boric acid additive in coolant on steam explosion, a series of molten droplet-coolant interaction tests using H3BO3 solutions (with concentration ranging from 0-3.2% by weight) is carried out in the present study. The characteristics of melt-coolant interactions are the occurrence probability of typical phenomena (no fragmentation, minor fragmentation, or spontaneous steam explosion), lateral deformation ratio, quench depth, pressure impulse and debris particle size distribution. The statistical data of such characteristics are obtained through repeating 20 runs of the same test category. The experimental results show that the H3BO3 addition in coolant has various impacts on the above-mentioned characteristics of melt-coolant interactions, depending on the H3BO3 concentration. In particular, the probability of steam explosion sightly decreases as the H3BO3 concentration increases from zero to 1.2 wt.%, but significantly increases as the H3BO3 concentration further increases to 3.2 wt.% trough 2.2 wt.%. Namely, the inhibiting effect of boric acid on steam explosion is diminishing with increasing H3BO3 concentration beyond 1.2 wt.%. It is also found that both melt and coolant temperatures are crucial parameters impacting the likelihood and energetics of steam explosion.
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3.
  • Deng, Yucheng, et al. (författare)
  • An experimental study on the effect of chemical additives in coolant on steam explosion
  • 2024
  • Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier Ltd. - 0017-9310 .- 1879-2189. ; 218
  • Tidskriftsartikel (refereegranskat)abstract
    • In assessment of severe accident risk in light water reactors (LWRs), steam explosion is a nonnegligible phenomenon following a relocation of core melt (corium) into coolant, and thus various research efforts have been paid to steam explosion. There had been numerous studies showing that the occurrence of steam explosions is influenced by several factors such as melt and coolant temperatures, melt materials, non-condensable gasses, etc. However, most of the existing experiments used deionized (DI) water or tap water as coolant, with little consideration of the effect of chemicals (e.g. boric acid, sodium hydroxide, sodium phosphate) commonly applied in reactor coolant. To examine the effect of the chemical additives in coolant on steam explosion, the present study performs a series of molten Tin droplet-coolant interaction tests using DI water and different chemical solutions, including H3BO3 solutions, NaOH + H3BO3 neutral solutions, and Na3PO4 + H3BO3 neutral solutions. The experimental results show that adding NaOH and Na3PO4 in boric acid solution significantly affects the occurrence probability of spontaneous steam explosion, because of the presence of PO43− and H+ ions. When different solutions have equivalent concentrations of H3BO3, the peak pressure values of the spontaneous steam explosion of Sn droplets are similar among various solutions. Compared with those in DI water, steam explosion in the chemical solutions occurs predominantly within a narrow range of depth from 28 mm to 40 mm and produces a much higher peak pressure. This implies that more energetic steam explosions may occur in the chemical solutions.
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4.
  • Fang, Di, et al. (författare)
  • A numerical study of liquid film dynamics in multi-nozzle spray cooling of downward-facing surface
  • 2023
  • Ingår i: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322 .- 1879-3533. ; 161
  • Tidskriftsartikel (refereegranskat)abstract
    • In a consideration of spray cooling as the potential cooling mechanism for the in-vessel melt retention (IVR) strategy of nuclear reactors because of its superior heat removal efficiency, the SPAYCOR experiment has been conducted at KTH to investigate the spray cooling capacity of multiple nozzles applied to a downward-facing heated surface. In the present study, the dynamics of liquid film on the downward-facing surface resulting from the multi-nozzle spray are numerically simulated by using a coupled Eulerian-Lagrangian method implemented in the OpenFOAM platform. Prior to simulation of the SPAYCOR experiment, the numerical approach is used to calculate two theoretical setups which have known analytical solutions, with the objective to validate the models in predicting liquid film dynamics either in spray or on an inclined surface. In the simulation of the SPAYCOR experiment, the predicted film morphology shows a good agreement with the experimental observation. What's more, the influential factors, including the inclination of the downward-facing heater surface, the nozzle-to-surface distance as well as the nozzle-array layout, are also investigated numerically in the present study. The simulation results show that a decreasing nozzle-to-surface distance does not only lead to a thicker liquid film and a lower velocity in the vicinity of each spray coverage, but also increases non-uniformity of the liquid film. The nozzles-array layout has little influence on the average liquid film thickness and velocity, but significantly affects the film morphology.
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5.
  • Jiang, Tao, et al. (författare)
  • Investigation of DC-Biased Optical OFDM With Precoding Matrix for Visible Light Communications : Theory, Simulations, and Experiments
  • 2018
  • Ingår i: IEEE Photonics Journal. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 1943-0655. ; 10:5
  • Tidskriftsartikel (refereegranskat)abstract
    • Orthogonal frequency-division-multiplexing (OFDM) technology is widely used in visible light communication (VLC) to achieve high data rate transmission. However, the traditional direct-current (DC)-biased optical OFDM (DCO-OFDM) VLC systems suffer from the high peak-to-average power ratio (PAPR) which causes signal clipping distortion, and, thus, performance degradation. Furthermore, severe high-frequency fading due to the limited system bandwidth results in poor bit error rate (BER) performance. Precoding matrix (PM) techniques have been proposed to enhance the performance of VLC OFDM transmission, but a little or no work has been carried out in investigating the theory of PM used in OFDM VLC systems. In this paper, we aim to reveal the theory of PM-DCO-OFDM for a VLC system. To figure out the intrinsic laws of a PM method, we investigate the principles of PAPR reduction, clipping distortion optimization, and signal-to-noise ratio (SNR) distribution equalization. Based on the analysis of PAPR, we theoretically proved the simplicity of PM as a method to reduce the possibility of high PAPR by improving the autocorrelation performance of input symbols. The clipping distortion could be improved due to the reduction of high PAPR. Moreover, the relatively uniform SNR distribution can be achieved by PM through equalizing the clipping and channel noise, which is beneficial to improve the BER performance in high-frequency constrained systems. However, the PM method used in a DCO-OFDM VLC system should consider the transmitting power, modulation format, and transmission distance as a whole to achieve the transmission performance improvement. The simulation results demonstrate the complementary cumulative distribution function of PAPR can be reduced similar to 3 dB, while the performance of clipping distortion power and clipping error probability are significantly improved. Furthermore, experiment is carried out with results showing that the PM method can improve the BER performance in the case that VLC OFDM transmission has enough transmitting power, but with the low transmitting power, the PM also can damage the BER performance. The simulation and experiment results are consistent with our theoretical analysis.
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6.
  • Liu, Lingling, et al. (författare)
  • Uniform and perfectly linear current-voltage characteristics of nitrogen-doped armchair graphene nanoribbons for nanowires
  • 2017
  • Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 19:1, s. 44-48
  • Tidskriftsartikel (refereegranskat)abstract
    • Metallic nanowires with desired properties for molecular integrated circuits (MICs) are especially significant in molectronics, but preparing such wires at a molecular level still remains challenging. Here, we propose, from first principles calculations, experimentally realizable edge-nitrogen-doped graphene nanoribbons (N-GNRs) as promising candidates for nanowires. Our results show that edge N-doping has distinct effects on the electronic structures and transport properties of the armchair GNRs and zigzag GNRs (AGNRs, ZGNRs), due to the formation of pyridazine and pyrazole rings at the edges. The pyridazine rings raise the Fermi level and introduce delocalized energy bands near the Fermi level, resulting in a highly enhanced conductance in N-AGNRs at the stable nonmagnetic ground state. Especially for the family of AGNRs with widths of n = 3p + 2, their semiconducting characteristics are transformed to metallic characteristics via N-doping, and they exhibit perfectly linear current-voltage (I-V) behaviors. Such uniform and excellent features indicate bright application prospects of the N-AGNRs as nanowires and electrodes in molectronics.
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7.
  • Wang, Xiang-Gao, et al. (författare)
  • HOW BAD OR GOOD ARE THE EXTERNAL FORWARD SHOCK AFTERGLOW MODELS OF GAMMA-RAY BURSTS?
  • 2015
  • Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 219:1
  • Tidskriftsartikel (refereegranskat)abstract
    • The external forward shock models have been the standard paradigm to interpret the broadband afterglow data of gamma-ray bursts (GRBs). One prediction of the models is that some afterglow temporal breaks at different energy bands should be achromatic; that is, the break times should be the same in different frequencies. Multiwavelength observations in the Swift era have revealed chromatic afterglow behaviors at least in some GRBs, casting doubts on the external forward shock origin of GRB afterglows. In this paper, using a large sample of GRBs with both X-ray and optical afterglow data, we perform a systematic study to address the question: how bad or good are the external forward shock models? Our sample includes 85 GRBs up to 2014 March with well-monitored X-ray and optical light curves. Based on how well the data abide by the external forward shock models, we categorize them into five grades and three samples. The first two grades (Grade I and II) include 45 of 85 GRBs. They show evidence of, or are consistent with having, an achromatic break. The temporal and spectral behaviors in each afterglow segment are consistent with the predictions (the "closure relations") of the forward shock models. These GRBs are included in the Gold sample. The next two grades (Grade III and IV) include 37 of 85 GRBs. They are also consistent with having an achromatic break, even though one or more afterglow segments do not comply with the closure relations. These GRBs are included in the Silver sample. Finally, Grade V (3/85) shows direct evidence of chromatic behaviors, suggesting that the external shock models are inconsistent with the data. These are included in the Bad sample. We further perform statistical analyses of various observational properties (temporal index alpha, spectral index beta, break time t(b)) and model parameters (energy injection index q, electron spectral index p, jet opening angle theta(j), radiative efficiency eta(gamma), and so on) of the GRBs in the Gold sample, and derive constraints on the magnetization parameter epsilon(B) in the forward shock. Overall, we conclude that the simplest external forward shock models can account for the multiwavelength afterglow data of at least half of the GRBs. When more advanced modeling (e.g., long-lasting reverse shock, structured jets, arbitrary circumburst medium density profile) is invoked, up to >90% of the afterglows may be interpreted within the framework of the external shock models.
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8.
  • Xiang, Yan, et al. (författare)
  • An experimental investigation on debris bed formation from fuel coolant interactions of metallic and oxidic melts
  • 2023
  • Ingår i: Applied Thermal Engineering. - : Elsevier BV. - 1359-4311 .- 1873-5606. ; 233
  • Tidskriftsartikel (refereegranskat)abstract
    • During postulated severe accidents in a light water reactor (LWR), the core melt (corium) may relocate to the lower head and fail the reactor pressure vessel (RPV). The corium is expected to undergo fuel coolant interactions (FCI) if the reactor cavity is flooded with water. Both FCI energetics and resulting debris bed coolability are of paramount importance to reactor safety, since the ex-vessel corium poses a threat to the containment integrity if steam explosion occurs or the debris bed is uncoolable, leading to release of radioactive fission products to the environment. The present study is intended to quantify the characteristics of a debris bed resulting from FCI, which are crucial to debris bed coolability. Different from the previous studies with only oxidic materials, various materials, including metallic ones of Sn, Sn-Bi and Zn as well as oxidic one of Bi2O3-WO3, were employed as the simulants of corium (mixture of UO2/ZrO2/Zr/Fe) in the present study to investigate the effects of melt materials, melt superheat and coolant subcooling on debris bed formation in a water pool. High-speed photography was applied to visualize melt jet breakup, droplets fragmentation, as well as fragments sedimentation on the pool floor. Other obtained data are debris bed shape (profile) and porosity, as well as morphology and size distri-bution of debris particles. The comparative results of various tests provided insights toward filling the knowledge gap on debris bed characteristics under different melt materials and compositions.
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9.
  • Xiang, Yan, et al. (författare)
  • Experimental investigation on debris bed formation from metallic melt coolant interactions
  • 2023
  • Ingår i: International journal of thermal sciences. - : Elsevier BV. - 1290-0729 .- 1778-4166. ; 192, s. 108398-
  • Tidskriftsartikel (refereegranskat)abstract
    • Motivated by risk quantification of severe core meltdown accidents which may occur light water reactors, an extensive DEFOR (DEbris FORmation) test series with various molten oxidic materials was conducted previously at Royal Institute of Technology to investigate the characteristics of debris beds formed from oxidic melt-water interactions, which are important to the coolability of debris beds and thus the retention of core melt. Yet, little attention has been paid to metallic melt which exist in the core melt. The present study is intended to fill in the knowledge gap, i.e., to characterize metallic melt-water interactions when a melt jet falls into a deep water pool. Nine DEFOR-M tests are carried out to clarify the effects of melt jet diameter, free fall height and water pool depth on metallic debris formation characteristics. Molten tin of 20 kg is employed as the simulant of metallic melt. A melt sensor and a mass sensor are installed to detect the discharging period of coherent jet and the mass accumulation of debris bed, respectively. A high-speed camera is applied to record the process of jet breakup, debris fragmentation and solidification, and debris bed formation. The experimental results show that the water pool depth has a significant influence on steam explosion and debris bed formation characteristics. Moreover, the jet breakup length is sensitive to jet diameter and free fall height. Finally, the features of metallic debris beds are compared with those of oxidic ones in respects of debris bed's configuration and porosity, particles' morphology and size distribution.
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10.
  • Xiang, Yan, et al. (författare)
  • Experimental investigation on ex-vessel debris bed formation using low melting-point melt of binary metals
  • 2023
  • Ingår i: Progress in nuclear energy (New series). - : Elsevier BV. - 0149-1970 .- 1878-4224. ; 157
  • Tidskriftsartikel (refereegranskat)abstract
    • During severe accidents in a light water reactor, the core melt (corium) may relocate to the lower head and fail the reactor pressure vessel (RPV). The corium will be ejected to the reactor cavity upon the RPV failure and undergo melt coolant interactions (FCI) if the cavity is flooded with water. The FCI process does not only de-termines the characteristics of the resulting debris bed which are important to coolability, but also induces a steam explosion risk which may threaten containment integrity. The present study is concerned with charac-terization of debris bed formed from FCI of metal-rich corium failing into a deep water pool in the reactor cavity. Low melting-point metals Tin and Tin-Bismuth (20 kg) were employed as the simulant materials of metal-rich corium melt. Ten tests were carried out on the DEFOR-M test facility at KTH to investigate the effects of various parameters on debris bed formation, such as melt superheat, coolant subcooling, material. The melt jet fragmentation and fragments movement in the water pool as well as debris deposition on the pool floor were recorded by high-speed cameras. Melt sensors and weight sensors were installed to detect the period of melt jet discharge and the mass of forming debris bed. The porosity of debris bed was obtained through the debris bed volume measured by a three-dimensional laser scanner and the pore volume measured by water absorption. The final configuration of debris bed was also reconstructed through the laser scanner data, and the debris particles were sieved for their size distribution. The experimental results revealed the FCI phenomena and debris bed characteristics including configuration and porosity of debris bed as well as morphology and size distribution of debris particles under different melt superheats, coolant subcooling, materials.
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