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Now showing items 17 - 32 of 41

  • Effects on particle dispersion by turbulent transition in a jet RID A-8039-2012

    Luo, Kun   Klein, Markus   Fan, Jian-Ren   Cen, Ke-Fa  

    The effects on particle dispersion by turbulence transition in a three-dimensional plane jet are investigated by means of direct numerical simulation. The governing equations of fluid motion are solved by a finite volume method and a fractional-step projection scheme. The particles are traced in the Lagrangian framework. It is found that the transition phenomenon of particle dispersion occurs during turbulence transition for particles at certain Stokes numbers. For particles at the intermediate Stokes numbers of 1 and 10, the dispersion changes from non-uniform to uniform patterns. These transition behaviors of particle dispersion reflect the self-selective mechanism between multi-scale structures in turbulent flows. (c) 2006 Elsevier B.V. All rights reserved.
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  • A comparative study of the multi-objective optimization algorithms for coal-fired boilers

    Wu, Feng   Zhou, Hao   Zhao, Jia-Pei   Cen, Ke-Fa  

    Combustion optimization has been proved to be an effective way to reduce the NOx emissions and unburned carbon in fly ash by carefully setting the operational parameters of boilers. However, there is a trade-off relationship between NOx emissions and the boiler economy, which could be expressed by Pareto solutions. The aim of this work is to achieve multi-objective optimization of the coal-fired boiler to obtain well distributed Pareto solutions. In this study, support vector regression (SVR) was employed to build NOx emissions and carbon burnout models. Thereafter, the improved Strength Pareto Evolutionary Algorithm (SPEA2), the new Multi-Objective Particle Swarm Optimizer (OMOPSO), the Archive-Based hybrid Scatter Search method (AbYSS), and the cellular genetic algorithm for multi-objective optimization (MOCell) were used for this purpose. The results show that the hybrid algorithms by combining SVR can obtain well distributed Pareto solutions for multi-objective optimization of the boiler. Comparison of various algorithms shows MOCell overwhelms the others in terms of the quality of solutions and convergence rate. (C) 2010 Elsevier Ltd. All rights reserved.
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  • Perturbation solution to heat conduction in melting or solidification with heat generation

    Yu, Zi-Tao   Fan, Li-Wu   Hu, Ya-Cai   Cen, Ke-Fa  

    The Stefan problem involving a source term is considered in this technical note. As an example, planar solidification with time-dependent heat generation in a semi-infinite plane is solved by use of a perturbation technique. The perturbation solution is validated by reducing the problem to the case without heat generation whose exact solution is available. An application to the case with constant heat generation is presented, for which a closed-form solution is obtained. The effects of heat generation and Stefan number on the evolution of solidification are examined using the perturbation solution.
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  • Measurement and simulation of the contact drying of sewage sludge in a Nara-type paddle dryer

    Deng, Wen-Yi   Yan, Jian-Hua   Li, Xiao-Dong   Wang, Fei   Lu, Sheng-Yong   Chi, Yong   Cen, Ke-Fa  

    Contact drying experiments were carried out in a Nara-type paddle dryer to study the drying kinetics of sewage sludge in the presence of air. In order to have a better understanding of the sewage sludge drying mechanism, a penetration model developed by Tsotsas and Schlunder is used to simulate the drying kinetics of the pasty, lumpy and granular phase which the sludge experiences during the drying process. The pasty phase is assumed to be a saturated particulate phase, and the granular phase is considered as a mono-dispersed particulate phase whose diameter was experimentally determined. In the lumpy phase, a sludge wall sticking still to the paddle-shaft surface was formed, and the heat transfer resistance from the sludge wall is considered during the model calculation. The influence of the drying parameters (temperature, stirrer speed and air flowrate) on the drying process is investigated experimentally and theoretically. The result indicates that the penetration theory is able to describe the sludge drying kinetics of the three distinct phases. Experimental and calculated drying kinetics are in satisfactory agreement for different drying parameters. (C) 2009 Elsevier Ltd. All rights reserved.
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  • Photocatalytic decomposition of gaseous 1,2-dichlorobenzene on TiO2 films: Effect of ozone addition

    Lu, Sheng-yong   Wang, Qiu-lin   Buekens, Alfons G.   Yan, Jian-hua   Li, Xiao-dong   Cen, Ke-fa  

    Gaseous 1,2-dichlorobenzene (1,2-DCBz) was photocatalytically decomposed on a powdered TiO2 layer using UV-light, or the latter combined with ozone addition. The effect of various experimental parameters was studied, including inlet concentrations of ozone, UV irradiation or not, carrier gas (dry air or nitrogen) and treatment time period. A temperature of 150 degrees C is selected, to replicate flue gas conditions after bag house filter. An optimum ozone addition of ca. 165 ppm was established, higher concentrations barely improving photocatalytic removal. Possible pathways and mechanisms are discussed on the basis of the intermediate products identified, the chloride ions distribution in the effluent and catalyst surface, and a carbon conversion balance. (c) 2012 Elsevier B.V. All rights reserved.
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  • Laboratory-scale study of the suppression of PCDD/F emission during coal and MSW co-incineration.

    Lu, Sheng-yong   Yan, Jian-hua   Li, Xiao-dong   Ni, Ming-jiang   Cen, Ke-fa  

    The experimental test of co-incinerating Chinese raw municipal solid waste (MSW) and coal in a laboratory-scale tubular reactor was first reported in present study, and the emission of normal gas components and the effects of the S/C1 molar ratio or coal mixing percentages on polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs/Fs) emission were investigated and discussed. The results indicated that OCDD was the only PCDD homologues since others like TCDD-HpCDD was hardly detected, while as the categories of PCDF homologues were comparatively much more general. The amount of PCDD was much larger than that of PCDF in all operating conditions. Since sigma PCDF/sigma PCDD<<1, the dominant role of the precursor formation was proven in our experimental conductions. With increasing the coal addition to MSW (from 0 to 16%), PCDD and PCDF were reduced considerably. Coal and MSW may suppress the PCDD/F emissions efficiently (over 95%) during the MSW incineration process. The PCDD/F suppression results of the present study could be helpful guidance to the industrial application of Chinese MSW and auxiliary coal co-incineration processes. The PCDD/F stack emission data of two industrial incinerators using co-incineration technology in China seem to have a great positive reduction of PCDDs/Fs.
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  • Effect of oxidation treatment on the adsorption and the stability of mercury on activated carbon.

    Hu, Chang-xing   Zhou, Jin-song   Luo, Zhong-yang   He, Sheng   Wang, Guang-kai   Cen, Ke-fa  

    Oxidation treatment on the adsorption and the stability of Hg on activated carbon (AC) was inrestigated. Both MnO2-AC and FeCl3-AC were produced during oxidation treatment. The measurement of modified AC's mercury adsorption capacity was conducted in a simulated coal-fired flue gas by adsorbing test apparatus. TCLP and column leaching methods were used to test the stability of mercury adsorbed on ACs. The results indicate that the oxidation treatment changed the pore structure of the AC and modified the carbon surface by creating chemical components such as MnO4-, Mn4+, O, NO3-, Fe3+, Cl-, etc. The Hg sorption capacity on MnO2-AC or FeCl3-AC was about three times higher than that of untreated carbon. In addition, the mercury control cost of each of the formers was about the half cost of the untreated carbon. The stability of Hg absorption was studied, it found that mercury adsorbed on the oxidation treated AC was not better than that of untreated carbon. It could concluded that the insoluble form of Hg is very important to the stability of mercury adsorbed on AC. This study suggests that the FeCl3-AC is the best absorbent for Hg with high adsorption capacity, better Hg adsorption stability in leaching environment, and lower cost among the three ACs tested.
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  • Experimental study on the separation of CO2 from flue gas using hollow fiber membrane contactors without wetting

    Yan, Shui-ping   Fang, Meng-Xiang   Zhang, Wei-Feng   Wang, Shu-Yuan   Xu, Zhi-Kang   Luo, Zhong-Yang   Cen, Ke-Fa  

    Experiments on CO2 removal from flue gas using polypropylene (PP) hollow fiber membrane contactors were conducted in this study. Absorbents including aqueous potassium glycinate (PG) solution, aqueous solutions of monoethanolamine (MEA) and methyldiethanolamine (MDEA) were used to absorb CO2 in the experiments. Based on the wetting experimental results, aqueous PG solution can offer a higher surface tension than water, aqueous MEA and MDEA solutions. Aqueous PG solution has a lower potential of membrane wetting after a continuously steady operation for 40 h to maintain CO2 removal efficiency of about 90%. Under moderate operating conditions, effects of the temperature, flow rate, and concentration of absorbents, and the flow rate of flue gas as well as the volumetric concentration of carbon dioxide in the flue gas on the mass transfer rate of CO2 were studied on a pilot-scale test facility. Unlike conventional absorbents, the mass transfer decreases with an increasing liquid temperature when using aqueous PG solution. Results show that CO2 removal efficiency was above 90% and the mass transfer rate was above 2.0 mol(m(2) h) using the PG aqueous solution. It indicates that the hollow fiber membrane contactor has a great potential in the area of CO2 separation from flue gas when absorbent's concentration and liquid-gas pressure difference are designed elaborately. (c) 2007 Elsevier B.V. All rights reserved.
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  • Kinetics of NO(x) Absorption into (NH(4))(2)SO(3) Solution in an Ammonia-Based Wet Flue Gas Desulfurization Process

    Gao, Xiang   Du, Zhen   Ding, Hong-lei   Wu, Zu-liang   Lu, Hao   Luo, Zhong-yang   Cen, Ke-fa  

    The mass transfer and kinetics of NO(x) absorption into (NH(4))(2)SO(3) solution, the main compound of an ammonia-based wet flue gas desulfurization process, have been investigated in a double-stirred reactor Under the experimental conditions, the gas-liquid reaction between NO(x) and the (NH(4))(2)SO(3) solution without O(2) coexisting is controlled mainly by the gas film because the (NH(4))(2)SO(3) concentration is higher than 0 05 mol/L In this case, the absorption rate of NO(x) is found to be zero-order with respect to the (NH(4))(2)SO(3) concentration The inlet partial pressure and the oxidation degree (Phi = NO(2)/NO(x)) have an apparent effect on the absorption rate of NO(x). In this research, a simplified mathematical calculated model is applied to the simulation of the absorption process The experimental results demonstrate that the orders of the reaction with respect to the concentration of NO(x) (NO(2)* or NO*) in the gas phase and the reaction rate constants of NO(x) (NO(2)* or NO*) with (NH(4))(2)SO(3) are all a function of the oxidation degree A kinetic equation for total NO(x) absorption as a function of the oxidation degree can be obtained, and the calculated value fits the experimental data well
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  • Pool boiling heat transfer on a nanoscale roughness-enhanced superhydrophilic surface for accelerated quenching in water

    Fan, Li-Wu   Li, Jia-Qi   Zhang, Liang   Yu, Zi-Tao   Cen, Ke-Fa  

    A superhydrophilic surface was prepared on stainless steel spheres by spray coating of silica nanopartides to enhance the surface roughness at the nanoscale. The quenching performance of such surface was tested in both saturated and subcooled water. It was shown that quenching is accelerated on the super hydrophilic surface by a factor of 60%, due to the marked boiling heat transfer enhancement with the critical heat flux being increased by 78%. Under saturated condition, the initial temperature of 800 degrees C is not high enough to lead to stable film boiling on the superhydrophilic surface. Instead, quenching begins with an extended transitional film boiling sub-regime that is dominated by point-contact mode of liquid-solid contacts. The quenching time becomes shorter with increasing the subcooling degree, in agreement with the enhancement in quasi-film-boiling heat transfer and CHF. Bubble jetting phenomenon is observed at the liquid-vapor interface for the subcooled cases, which becomes more violent at a deeper subcooling. The quenching performance maintains upon consecutive tests for up to 100 times, because the superhydrophilicity is sustained on the surface. The spray coating method adopted in this work is not only fast and cost-effective, but is also scalable to large and curved metallic surfaces for a variety of applications in thermal engineering including quenching. (C) 2016 Elsevier Ltd. All rights reserved.
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  • Effect of water on catalyzed de novo formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans.

    Li, Xiao-dong   Zhang, Ji   Yan, Jian-hua   Chen, Tong   Lu, Sheng-yong   Cen, Ke-fa  

    The effect of water vapor on catalyzed de novo formation of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) was investigated through experiments conducted on a fixed-bed apparatus. The results indicated that water vapor could promote the formation of PCDD and PCDF. The dominant pathway was activating fly ash in this work, while water also reacted with chlorine to change the equilibrium of Deacon reaction, which influenced the final yield of PCDD/PCDF. Also a suppression effect of water on CuCl(2) was found according to the values of the catalysis indicator. It is possible that water reduced the catalysis of CuCl(2) or prevented its direct chlorination. But the overall effect of water on the formation of PCDD and PCDF was promotion rather than inhibition.
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  • Source identification of PCDD/Fs in agricultural soils near to a Chinese MSWI plant through isomer-specific data analysis

    Xu, Meng-xia   Yan, Jian-hua   Lu, Sheng-yong   Li, Xiao-dong   Chen, Tong   Ni, Mina-jiang   Dai, Hui-fen   Cen, Ke-fa  

    Isomer-specific data were investigated in order to identify the sources of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in agricultural soils, including Fluvo-aquic and paddy soils, in the vicinity of a Chinese municipal solid waste incineration (MSWI) plant. Homologue and isomer profiles of PCDD/Fs in soils were compared with those of potential sources, including combustion sources, i.e., MSWI flue gas and fly ash; and the impurities in agrochemicals, such as the pentachlorophenol (PCP), sodium pentachlorophenate (PCP - Na) and 1,3,5-trichloro-2-(4-nitrophenoxy) benzene (CNP). The results showed that the PCDD/F isomer profiles of combustion sources and agricultural soils were very similar, especially for PCDFs, although their homologue profiles varied, indicating that all the isomers within each homologue behave identically in the air and soil. Moreover, factor analysis of the isomer compositions among 33 soil samples revealed that the contamination of PCDD/Fs in agricultural soils near the MSWI plant were primarily influenced by the combustion sources, followed by the PCP/PCP - Na and CNP sources. This implication is consistent with our previous findings based on chemometric analysis of homologue profiles of soil and flue gas samples, and identifies PCP/PCP - Na as an additional important source of PCDD/Fs in the local area. This makes the similarities and differences of isomer profiles between Fluvo-aquic and paddy soils more explainable. It is, therefore, advisable to use isomer-specific data for PCDD/F source identifications where possible. (c) 2007 Published by Elsevier Ltd.
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  • Increased Thermal Conductivity of Eicosane-Based Composite Phase Change Materials in the Presence of Graphene Nanoplatelets

    Fang, Xin   Fan, Li-Wu   Ding, Qing   Wang, Xiao   Yao, Xiao-Li   Hou, Jian-Feng   Yu, Zi-Tao   Cheng, Guan-Hua   Hu, Ya-Cai   Cen, Ke-Fa  

    Alkanes and their mixtures (paraffins) have widely been used as phase change materials (PCMs) for low-to-medium temperature thermal energy storage. Among the various alkanes, eicosane, with a nominal melting temperature of 37 degrees C, has emerged in energy-storage-based passive thermal management technologies, for electronics for example. In an effort to increase the thermal conductivity of eicosane, the effect of adding graphene nanoplatelets (GNPs) as thermally conductive nanofillers was investigated experimentally. The composite PCM samples were prepared by dispersing GNPs in liquid eicosane at various loadings (0, 1, 2, 5, and 10 wt.%) without any surfactants. Thermal conductivity of the composite PCM samples in their solid phase was then measured by means of the transient plane source technique at elevated temperatures from 10 to 35 degrees C. Latent heat of fusion and melting point of the samples were also characterized using a differential scanning calorimeter. It was shown that for the highest loading examined (10 wt.%), the relative thermal conductivity enhancement is above 400% at 10 degrees C, indicating that the emerging GNPs have much better performance than the conventional nanofillers attempted in the available literature, such as metal/oxide nanoparticles and carbon nanotubes. Reduced thermal interface resistance, related to the unique two-dimensional planar morphology of GNPs, was interpreted to be responsible for their superior performance. The matrix/filler thermal interface resistance was estimated to range from 6 x 10(-9) to 9 x 10(-9) m(2) K/W. In contrast to the markedly increased thermal conductivity, however, the penalty of decrease in the energy storage capacity, caused by the presence of GNPs, was shown to be less significant.
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  • Transient natural convective heat transfer from a heated triangular cylinder to its air-filled coaxial cylindrical enclosure

    Yu, Zi-Tao   Xu, Xu   Hu, Ya-Cai   Fan, Li-Wu   Cen, Ke-Fa  

    A numerical study of transient natural convection inside an air-filled horizontal cylinder with a coaxial inner triangular cylinder is performed with the Prandtl number and aspect ratio being fixed at 0.72 and 2.0, respectively. The effects of two different positions of the inner triangular cylinder and Grashof number are examined. The predicted development of the convective flow and heat transfer is presented by means of snapshots of streamlines and isotherms. It is shown that the flow development can be representatively classified into several stages based on time histories of the average Nusselt number over the outer circular cylinder. The time-averaged Nusselt number is scaled with Grashof number of power 0.21-0.24 and time duration for attaining a steady/quasi-steady state is also correlated to Grashof number. (C) 2010 Elsevier Ltd. All rights reserved.
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  • Design and characteristics of refractive index sensor based on thinned and microstructure fiber Bragg grating RID B-2921-2009

    Huang, Xue-Feng   Chen, Zhe-Min   Shao, Li-Yang   Cen, Ke-Fa   Sheng, De-Ren   Chen, Jun   Zhou, Hao  

    A refractive index sensor based on the thinned and microstructure fiber Bragg grating (ThMs-FBG) was proposed and realized as a chemical sensing. The numerical simulation for the reflectance spectrum of the ThMs-FBG was calculated and the phase shift down-peak could be observed from the reflectance spectrum. Many factors influencing the reflectance spectrum were considered in detail for simulation, including the etched depth, length, and position. The sandwich-solution etching method was utilized to realize the microstructure of the ThMs-FBG, and the photographs of the microstructure were obtained. Experimental results demonstrated that the reflectance spectrum, phase shift down-peak wavelength, and reflected optical intensity of the ThMs-FBG all depended on the surrounding refractive index. However, only the down-peak wavelength of the ThMs-FBG changed with the surrounding temperature. Under the condition that the length and cladding diameter of the ThMs-FBG microstructure were 800 and 14 mu m, respectively, and the position of the microstructure of the ThMs-FBG is in the middle of grating region, the refractive index sensitivity of the ThMs-FBG was 0.79 nm/refractive index unit with the wide range of 1.33-1.457 and a high resolution of 1.2 x 10(-3). The temperature sensitivity was 0.0103 nm/degrees C, which was approximately equal to that of common FBG. (c) 2008 Optical Society of America.
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  • A numerical investigation of transient natural convection heat transfer of aqueous nanofluids in a horizontal concentric annulus

    Yu, Zi-Tao   Xu, Xu   Hu, Ya-Cai   Fan, Li-Wu   Cen, Ke-Fa  

    A numerical study of transient natural convection heat transfer of aqueous nanofluids in a horizontal annulus between two coaxial cylinders is presented. The effective thermophysical properties of water in the presence of copper oxide nanoparticles with four different volume fractions are predicted using existing models, in which the effects of the Brownian motion of nanoparticles are taken into consideration. The predicted development of convective flow and heat transfer of nanofluids is presented by means of the average Nusselt number over the outer cylinder. The flow development time towards a steady state and the time-averaged Nusselt number are predicted and scaled with Rayleigh number. It is shown that at constant Rayleigh numbers, the time-averaged Nusselt number is gradually lowered as the volume fraction of nanoparticles is increased. In addition, the time-averaged Nusselt number will be overestimated if the Brownian motion effects are not considered. (C) 2011 Elsevier Ltd. All rights reserved.
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