Research Articles
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Using a discrete stochastic model to study the epidemic dynamics of COVID-19 in Hubei, China
- SHI Yaolin, CHENG Huihong, HUANG Luyuan, REN Tianxiang
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2020, 37 (2):
145-154.
DOI: 10.7523/j.issn.2095-6134.2020.02.001
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Abstract (
514 )
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The outbreak of New Coronary Pneumonia COVID-19 and its spread throughout the China and many foreign countries have produced a huge social impact. Studying the dynamic characteristics of epidemic transmission will help us better control and prevent the epidemic. We have developed a discrete stochastic method to simulate the evolvement of the epidemic in Hubei Province of China. Firstly, the daily confirmed number of patients was processed according to the Erlang probability distribution of the queuing theory, and the daily number of patients onset and infected were obtained. The results are compared with references recently published by Chinese CDC, validating the scientific credibility of the method. Then, the effective reproduction rates at different stages of the epidemic are inverted to fit the number of daily onsets, and predict the future trend of the epidemic. It was found that the basic reproduction number R0 decreased from 6.1 to 4.0 at the initial stage of the epidemic. After taking dramatic measures to close the Wuhan city, the effective R value decreased below 1, and gradually decreased to below 0.13. The peak of the onset of patients has already passed in early February. Although there small fluctuations in the epidemic are not ruled out, the overall trend will not change as long as strict quarantine measures are adhered to. The epidemic is expected to end around the end of March and the cumulative number of patients will reach around 71,000. Migrant workers and students returning after the Spring Festival are unlikely to induce a large epidemic rebound. However, some countries in the world are at the stage of possible outbreaks, and China should pay attention to inspecting and quarantine international travelers.
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Investigation on the 3D printing process of ceramic cores of blades based on MIP-SLA
- WANG Ruichang, JIANG Ruisong, MING Xinguo
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2020, 37 (2):
162-168.
DOI: 10.7523/j.issn.2095-6134.2020.02.003
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Abstract (
321 )
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Mask-image projection stereolithography (MIP-SLA) was studied to solve the manufacturing problem of ceramic cores of aero-engine hollow turbine blades using 3D printing. We built an MIP-SLA platform to study the preparation method of ceramic slurry and the parameter configuration during printing process. Effects of photosensitive resin type, solid content, size of ceramic particles, content of dispersant, and milling time on the properties of ceramic slurry were all analyzed using the ceramic paste which was prepared by mixing the photosensitive resin and the ceramic particles with the ball mill. The printing results under different configuration parameters were also compared and analyzed.
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Hyperspectral image classification based on convolutional neural network and active learning algorithm
- SONG Han, YANG Weitun, GENG Xiurui, ZHAO Yongchao
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2020, 37 (2):
169-176.
DOI: 10.7523/j.issn.2095-6134.2020.02.004
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Abstract (
623 )
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When we apply the active learning method to the hyperspectral image classification, there always exist problems that the spectral-spatial features cannot be effectively utilized and the samples need to be manually labeled. In this work, we propose an active learning method combined with the convolutional neural networks to solve the above problems. Firstly, the neighborhood pixels of each pixel are extracted to form training samples. Then the spatial and spectral features of the samples are trained by the convolutional neural network and the data are preliminarily classified. Then, based on the spatial similarity and spectral similarity of the hyperspectral image, some of the unlabeled samples are labeled and added to the training set, which will further increase the accuracy of the classifier. Validated by Salinas image, Pavia University image, and Indian Pines image, the proposed method effectively improves the classification accuracy with less labeled samples.
Columns of Multi-phase Flow
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Study on oil-water separation performance of copper-based special wettability meshes
- YUAN Jia, CUI Chenyi, ZHAO Long, QI Baojin, WEI Jinjia
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2020, 37 (2):
177-185.
DOI: 10.7523/j.issn.2095-6134.2020.02.005
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Abstract (
310 )
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In order to study the oil-water separation ability of meshes with special wettability, two copper-based superhydrophilic/underwater-superoleophobic meshes with micro-nano structures of Ag and Cu(OH)2 were prepared by in situ displacement and oxidative modification methods. The effects of microstructure, surface wettability, permeation pressure, and the pH value on the separation performance of meshes were experimentally studied. The results show that the efficiencies of the two meshes for separating oil-water mixtures with single oil (e.g., diesel or gasoline) and multi-oil (e.g., the mixture of diesel and gasoline) both reached more than 99%. Owning to the difference in microstructures, the separation efficiency of the mesh with Ag micro-nano structure was slightly higher than that of the Cu(OH)2 mesh. However, the mesh with Cu(OH)2 micro-nano structure withstood a higher permeation pressure and had a greater liquid flux. Moreover, both the meshes were excellent in acid and alkali resistance. They can be reused for more than 10 times after washing without significant efficiency reducing, and there was no obvious degeneration arising after long-term placement. In short, these two types of meshes showed distinct advantages in separation efficiency, medium adaptability, repeatability, and durability for oil-water separation, and they would have broad application prospects.
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Three-dimensional modeling of pebble bed type high temperature gas-cooled reactor core and steady-state thermal hydraulic analysis
- ZHANG Shuangbao, LI Liangxing, XIE Wei, WANG Kailin
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2020, 37 (2):
186-191.
DOI: 10.7523/j.issn.2095-6134.2020.02.006
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Abstract (
426 )
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Accurate and reliable reactor thermal hydraulic analysis has important significance for the design and safe operation of the pebble bed modular high temperature gas-cooled reactor. Based on the helium coolant flow and heat transfer process at the core of high temperature gas-cooled reactor, we establish a three-dimensional coupled model by using CFD (computational fluid dynamics) software to simulate the flow and heat transfer of helium coolant at the core. Then the model is verified by comparison of the calculation results with the published results using THERMIX software. The results show that the steady-state results using the three-dimensional model are in agreement with THERMIX calculation data under the rated working conditions, and the relative error is less than 1%. The three-dimensional model simulates the coolant temperature distribution at the core. The research results lay the foundation for the subsequent analysis of complex thermal hydraulic phenomena such as core bypass of high temperature gas-cooled reactor.
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Numerical simulation of two-phase blood flow after thoracic endovascular aortic repair with in situ fenestration
- QIAO Yonghui, LUO Kun, FAN Jianren, MAO Le, ZHU Ting
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2020, 37 (2):
192-197.
DOI: 10.7523/j.issn.2095-6134.2020.02.007
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Abstract (
288 )
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Thoracic aortic dissection is a cardiovascular disease which seriously endangers human health. The main clinical treatment is thoracic endovascular aortic repair (TEAVR). TEVAR with in situ fenestration technique (ISF-TEVAR) can be used to treat complex cases involving the aortic arch and its branches. In this study, aortic geometry model of a patient suffering from aortic dissection treated by ISF-TEVAR was reconstructed, and two-phase blood flow simulation was carried out. The efficacy of ISF-TEVAR was quantitatively explored by assessing the hemodynamics. Meanwhile, the protruding length of the fenestration stent in the aortic arch was shortened by virtual surgery, and the "half protrusion" and "no protrusion" postoperative models were compared to reveal the effect of the protruding length on postoperative blood flow. Results show that appropriately shortening the protrusion length of the stent-graft improves the efficacy of ISF-TEVAR.
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Effect of rolling friction between the particle and wall on particle flow characteristics in the 3D printer feeding device
- TAI Tong, WANG Biao, TANG Tianqi, HE Yurong
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2020, 37 (2):
198-203.
DOI: 10.7523/j.issn.2095-6134.2020.02.008
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Abstract (
298 )
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3D printing is one of the revolutionary production modes. In this work, a screw extrusion device for the fused deposition modeling printer was applied, and the transport processes of three kinds of particles (polylactic acid, polypropylene, and polyurethane) in the device were simulated by using the discrete element method. This work aims to study the influence of rolling friction between the particle and wall on the characteristics of outlet particles in the feeding section. The results show that, with the increase of rolling friction coefficient between the particle and wall, the outlet mass flow rate and the velocity uniformity for the three particles are reduced and the total torques for the three particles increase. The tendencies of friction torque and contact torque are identical to that of the total torque, with the increase of rolling friction coefficient. The research results have important implication for the optimization of 3D printer feeding methods and the selection of feeding particles.
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Experimental study on fluidization of nanoparticle agglomerates with the assistance of vibration
- ZHAO Zhiduan, LIU Daoyin, MA Jiliang, CHEN Xiaoping
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2020, 37 (2):
204-209.
DOI: 10.7523/j.issn.2095-6134.2020.02.009
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Abstract (
302 )
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The fluidization characteristics of SiO2, Al2O3, and TiO2 nanoparticles with the assistance of vibration at different frequencies and amplitudes are studied in a plexiglass fluidized bed with the inner diameter of 40 mm. The effects of vibration on fluidization of nanoparticles with different cohesiveness are compared. The results show that, without vibration, the SiO2 nanoparticle agglomerates are fluidized smoothly while the Al2O3 and TiO2 nanoparticle agglomerates are fluidized with segregation because there are large agglomerates at the bottom of the fluidized bed. When the vibration is introduced, the minimum fluidization velocity of SiO2 decreases,and the bed expansion ratio of SiO2 decreases with the increase of amplitude and frequency. For the Al2O3 and TiO2 nanoparticles, as the amplitude and frequency increase, the minimum fluidization velocity decreases, the bed expansion ratio increases, and the agglomerate size decreases at the bottom of the fluidized bed. However, the vibration makes no significant improvement in fluidization at low frequency and low amplitude for the Al2O3 and TiO2 nanoparticles. Vibration enhances the collision of nanoparticle agglomerates and has the dual effect of promoting the breakage and compaction of the agglomerates. The optimal vibration parameters need to be explored to achieve the optimal fluidization quality of different nanoparticles.
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Three-dimensional numerical simulation of fast pyrolysis of coal in pressurized spout-fluid bed
- ZHOU Guanwen, ZHONG Wenqi, YU Aibing
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2020, 37 (2):
210-219.
DOI: 10.7523/j.issn.2095-6134.2020.02.010
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Abstract (
339 )
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Numerical simulation of pressurized fast pyrolysis process of coal in a pilot-scale reactor with the capacity of 50 kg/h was carried out. Characteristics of structural parameters, such as reactor height, coal feeding inlet position, pyrolysis pressures (0.1, 0.3, and 0.5 MPa), ratios of spouted gas to fluidized gas (0.2, 0.3,and 0.4), and particle size (0-3 and 0-6 mm), were investigated. The results are shown as follows. With the increase of pressure, the mixing of coal and semi-coke became worse while the gas yield increased. Keeping the fluidized gas Qf unchanged, with the increase of spouted gas Qs, the gas-solid flow became more intense, the particle temperature distribution became more uniform, and the particle outflow at the volatile outlet increased. Although increasing particle size reduced the particle outflow at the volatile outlet, it might weaken the mixing of coal and semi-coke.
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Experimental investigation of electrostatic generation of coal granules
- FANG Jia, ZHAO Yanlin, YAO Jun, WANG Chi-Hwa
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2020, 37 (2):
220-227.
DOI: 10.7523/j.issn.2095-6134.2020.02.011
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Abstract (
303 )
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Coal granules are widely used in gas-solid two-phase transmission industries, such as mining and energy. In the industrial process, electrostatics is generated by the collision and friction between the coal granule and between the granule and the wall surface. If it is not discharged in time, it will gradually accumulate in the working system, thereby causing many problems for the safety production, such as granule clustering, blocking, sparking, explosion, etc. In addition, the charged granules in the gas-solid two-phase flow cause a series of measurement problems, which seriously affect the measurement accuracy and even damage the instruments. The main task of this experiment is to establish a measurement experiment system for electrostatic generation of single coal granules,and to study the effects of the physical factors, sliding velocity, environmental relative humidity, surface roughness, and granular materials on the generation of electrostatics. After comparing and analyzing the data, we found that these factors have greater impacts on electrostatics, and a comparative analysis was conducted to summarize the mechanism of electrostatic generation of single granules and the influences of various factors.
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Study on mixed flow and initial deposition characteristics of extra-high water-cut crude oil
- XING Xiaokai, Lü Zhaoxu, LIU Jiaquan
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2020, 37 (2):
228-233.
DOI: 10.7523/j.issn.2095-6134.2020.02.012
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Abstract (
271 )
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An experimental flow installation is designed and constructed,and it is used to study the flow characteristics and initial deposition process of extra high water-cut crude oil. The effects of the experimental temperature, mixing velocity, and volumetric water content on the initial deposition process of extra high water-cut oil are studied systematically. According to the movement patterns of crude oil and the initial deposition modes gained at different temperatures, the initial deposition process has two types, uniform deposition process and non-uniform deposition process. According to the sedimentary characteristics and the variation trends of the pressure drop curves, the initial deposition process can be divided into three stages:induction period of deposition, period of rapid deposition, and dynamic equilibrium period of deposition. On this basis, through the analysis of the carbon number distributions of the sediments, we find that there exist shear peeling, gelation, and molecular diffusion in the initial deposition process. The experimental temperature and volumetric water content affect the initial deposition process mainly by changing the gelation and molecular diffusion, and the mixing velocity affects the initial deposition process by changing the shear peeling.
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High resolution reconstruction of ultrasonic tomography based on Gaussian regression prediction
- LIU Hao, TAN Chao, DONG Feng
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2020, 37 (2):
234-241.
DOI: 10.7523/j.issn.2095-6134.2020.02.013
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Abstract (
278 )
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Visualized measurement of oil-water two-phase flow is of vital importance in multi-phase flow measurement and process industry. Ultrasonic tomography (UT) is widely applied in two-phase flow measurement due to its non-invasive, non-radiative, safe, and portable advantages. Aiming at UT reconstruction of oil-water two-phase medium with low acoustic impedance, we propose a high-resolution reconstruction algorithm based on Gaussian regression prediction. The iteratively simultaneously algebraic reconstruction technique is modified with high pass filter, and the truncated singular value decomposition is used to reduce dimensionality of the coefficient matrix. Gaussian process regression is applied to gain high-resolution image. Simulation results indicate that the proposed algorithm provides high accuracy and high-resolution reconstruction with fast computing speed.
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Sparse regularization algorithm with adaptive shrinkage thresholding operator for image reconstruction
- ZHANG Shengnan, XU Yanbin, DONG Feng
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2020, 37 (2):
242-247.
DOI: 10.7523/j.issn.2095-6134.2020.02.014
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Abstract (
351 )
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A sparse regularization algorithm with an adaptive shrinkage thresholding operator is proposed. In the operator the thresholding parameter can be updated according to the sparsity of the solution during iteration process, since the thresholding parameter is hard to select in sparse regularization algorithm for image reconstruction. In addition, a weight coefficient is introduced in the thresholding operator, and the effect of attenuation characteristics on image reconstruction quality is studied. The feasibility of this proposed algorithm is validated by image reconstruction of electrical tomography. The results show that the performance of images reconstructed using the sparse regularization algorithm with the proposed adaptive shrinkage thresholding operator is better than that using traditional shrinkage thresholding algorithm. When the distribution of the measured field is relatively simple, a larger weight coefficient is adopted. When the distribution of the measured field is relatively complicated, a smaller weight coefficient is used, which is beneficial to improve the quality of the reconstructed image.
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Lattice Boltzmann simulation of flow and mass transfer of CO2 adsorption in porous MgO particle
- LIU Bo, DING Yudong, LIAO Qiang, ZHU Xun, WANG Hong
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2020, 37 (2):
248-254.
DOI: 10.7523/j.issn.2095-6134.2020.02.015
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Abstract (
297 )
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The lattice-Boltzmann method was used to simulate the characteristics of flow and mass transfer of CO2 adsorption process in MgO particle. Effects of porosity and particle size of the MgO particle on the percolation velocity, CO2 concentration, CO2 adsorption rate, and adsorbent conversion rate were simulated based on the second-order reaction kinetic model. The results show that the CO2 concentration and adsorbent conversion rate decreased gradually along the gas flow direction in MgO particle. For the same adsorption time, the bigger the particle size was, the lower the percolation velocity, CO2 adsorption rate, and adsorbent conversion rate in particle were. For the same adsorption time and particle size, with the increase in the porosity of the particle, the absorbent conversion rate and CO2 concentration increased and the average adsorption rate also sped up.
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Dynamic behavior of droplet impact on the concave surface with liquid film
- ZHOU Xin, WANG Hong, ZHU Xun, CHEN Rong, LIAO Qiang, DING Yudong
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2020, 37 (2):
255-262.
DOI: 10.7523/j.issn.2095-6134.2020.02.016
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Abstract (
295 )
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In this paper we present results of a numerical investigation into a single drop impact on a rectangular grooved substrate with thin liquid film covering it. Particularly, effects of the height and width of the ridge on surface and dynamic behavior of droplet impact into thin liquid film were mainly studied. Simulations were performed using CLSVOF (coupled level-set and VOF) to track the air-water interface accurately and to keep conservation of mass at the same time. Results show that the structure of grooved surface has significant influence on the evolution of liquid film when droplet impacts on it. With the increase of rectangular groove height, the thickness of the coronal water becomes thin and the secondary droplets produce easily. Increase of the viscosity of liquid film inhibits coronal water flower formation and secondary droplet generation. When the surface tension of liquid film reduces, the jet is obvious and more secondary droplets produce. The splash phenomenon is advanced, and the disappearance of the crown water flower is delayed. Our research sheds some lights on the fundamental understanding of many industrial phenomena.
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Numerical simulation of the bubble detachment from the substrate
- XIAO Xuan, MA Yang, SHEN Yang, CHENG Yongpan, XU Jinliang
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2020, 37 (2):
263-267.
DOI: 10.7523/j.issn.2095-6134.2020.02.017
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Abstract (
292 )
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The detachment of bubbles from the substrate is of great significance for nucleate boiling and phase-change heat transfer. In this study, the two-dimensional axisymmetric model is built up using the Level Set method, and the bubble dynamics is obtained by considering buoyant force, viscous force, inertial force, and surface tension in terms of non-dimensional parameters. The effects of Reynolds numbers, Bond numbers, and surface wettability on the bubble detachment from the substrate are revealed, and the critical parameters for bubble detachment are also obtained. We find that, with the increase in Reynolds number, the inertial force increases, leading to easy detachment of bubbles from the substrate. With the increase in Bond numbers, the bubble transits from the complete detachment to the partial detachment. With the decreases in surface wettability, the contact area between the bubble and substrate increases, leading to large interactive forces of the bubble with the substrate and the liquid near the contact area. Thus the bubble is difficult to detach from the substrate. These studies may be quite useful for the study of nucleate boiling heat transfer.
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Investigation of the evaporation process of falling film outside a horizontal tube using diode laser absorption spectroscopy
- QIAO Wenyou, JIANG Yong, YANG Huinan, LI Ling, SU Mingxu, CAI Xiaoshu
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2020, 37 (2):
268-273.
DOI: 10.7523/j.issn.2095-6134.2020.02.018
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Abstract (
300 )
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Liquid film flow widely exists in various industrial processes. High-precision measurement of various parameters in the process of liquid film flow is crucial to understand relevant processes and heat transfer mechanisms. Diode laser absorption spectroscopy (DLAS) is used to determine liquid film thickness of the falling film outside horizontal tube. The heat transfer coefficient of the liquid film is calculated by using the tube-wall temperature and the liquid-film temperature. The relationships between liquid film thickness and heat transfer coefficient under different conditions, including spray density (0.16/0.24/0.32 kg (m·s)-1) and hot water temperature at the inlet of the tube (40/50/60℃), are analyzed. The results show that the average thickness of the liquid film increases with the spray density. The heat transfer coefficient of the liquid film increases with the spray density. When the spray density increases, the heat transfer coefficient of the liquid film increases with the increase of film thickness. When the temperature increases, the liquid film thickness remains constant while the heat transfer coefficient increases.
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Experimental study on the deformation and breakup characteristics of charged droplets in oil phase
- WANG Dongbao, WANG Junfeng, HUO Yuanping, LIU Hailong, WANG Xiaoying
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2020, 37 (2):
274-280.
DOI: 10.7523/j.issn.2095-6134.2020.02.019
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Abstract (
294 )
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The evolution process of charged droplet breakup in immiscible oil phase was visualized using micro-high-speed digital camera technology. The micro-morphology characteristics of charged droplets in oil phase under different electric field intensities were accurately captured. The deformation and breakup processes of charged droplets under different conditions were analyzed, and the formation mechanisms of charged droplets in different breakup modes were explored. The relationships of the charged droplet size with voltage and flow rate were quantitatively analyzed, while the cumulative distribution rules and Rosin-Rammler function of the droplet size distribution were obtained. The results indicate that the droplet size decreases rapidly with the electric field intensity while the uniformity of particle size distribution increases. Electric field strongly enhances the interphase interaction.
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Experimental and numerical simulation study of dynamics of nanofluid droplet impact on solid surfaces
- WANG Rui, SHEN Xuefeng, HUO Yuanping, WANG Junfeng, ZHENG Nuo, LIU Hailong
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2020, 37 (2):
281-287.
DOI: 10.7523/j.issn.2095-6134.2020.02.020
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Abstract (
380 )
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The phenomena of droplet impacting on solid surfaces widely occur in engineering fields such as the power machinery, spray cooling, and coating. The homogeneous and stable nanofluids were prepared by dispersing graphene and MWCNT to epoxy resin using ultrasound technique. The impacting process of droplet on solid surface was investigated by means of high-speed camera technique. Simulations of droplet impacting on surfaces were carried out by employing the finite element scheme. The level-set method was used to capture the interface movement and a modified power-law model was used to characterize the effect of nanoparticle additives. Simulations showed that the results were in reasonably good agreement with the experimental data. The experimental results showed that the nanoparticle additives brought shear-thinning properties to base fluids and suppressed the spreading behaviors of droplet. The simulation results revealed that with the decrease of the power law index m, the range of variation during the droplet spreading process became significantly large. With the increase of the surface tension, there was no significant change in the dimensionless diameter during the spreading phase, while during the receding phase the dimensionless diameter of droplet decreased gradually.
