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  • 2016 Volume 36 Issue 4
    Published: 15 April 2016
      
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  • Effects of Operation Mode on Heat Load Distribution in a Supercritical Pressure W-flame Boiler
    ZHOU Wentai, YANG Yong
    2016, 36(4): 253-257.
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    The heat load distribution in a 600 MW supercritical pressure W-flame boiler was calculated by measuring the temperature of water wall by changing the operation conditions of the boiler, such as the combination mode of mills, opening degree of general air dampers on front and rear wall, fineness of pulverized coal and opening degree of tertiary air damper, etc. Results show that the fineness of pulverized coal has almost no effect on the heat load distribution. The opening degree of general air dampers on front and rear wall would affect the flame travel in the furnace, and it is proposed to adopt the "front wall air suppressing rear wall air" mode. Changing the opening degree of tertiary air damper would alter the heat load distribution in the upper and lower furnace. Low heat load would appear in the area where corresponding coal mill is out of service.
  • Micro-mechanism of Water Molecule Adsorption on Lignite Surfaces
    GAO Zhengyang, LÜ Shaokun, LI Jinda, YANG Pengfei, CHEN Chuanmin
    2016, 36(4): 258-264,293.
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    The micro-mechanism of water molecule adsorption on lignite surfaces was investigated by using density functional theory, following which different lignite surface models and coal-water adsorption models were obtained through calculation by Gaussian09 program at the level of B3LYP/6-311G+(d,p), while the interaction energies were corrected by complete counterpoise method considering the basis set superposition error (BSSE). Moreover, graphical analyses were also conducted for different lignite surface models using electrostatic potential diagram, scatter diagram and reduced density gradient (RDG) color-filled isosurface map with Multiwfn program and VMD software. Results show that the water adsorption on lignite surface is of the weak interaction physical adsorption kind, the major part of which is hydrogen bonding interaction while the rest is Van Edward interaction. The adsorbability of hydroxyl and carboxyl to water is the strongest, followed by other oxygen functional groups such as the ether bond and methoxy, etc., and that of benzene ring is the weakest. The interaction region formed through water adsorption does not simply concentrate on some groups or an atom but somewhat interacts with local lignite areas. Thus the water adsorption is reinforced and the structure of lignite is impacted obviously.
  • Numerical Simulation of Falling Film Flow on Horizontal Circular Tubes with Drainage Strips
    LI Huijun, WANG Jiong
    2016, 36(4): 265-270.
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    To study the effects of drainage strip on falling film flow and heat transfer of horizontal circular tubes, a physical model was established for the circular tube added with a drainage strip on the bottom area, based on which numerical simulations were conducted to the falling film flow using volume of fluid (VOF) function, and subsequently the simulation results were compared with literature experimental data. Results show that the average thickness of liquid film on horizontal pipes with drainage strips is thinner than that without drainage strips, and the local Nu of tube wall is relatively large, indicating that the drainage strip can accelerate the liquid drainage and reduce the film thickness, and therefore is beneficial to the enhancement of heat transfer. The larger the strip height is, the thinner the liquid film and the larger the local Nu will be at the same circumferential angle of tube wall. A drainage strip with too small or too large width would have no obvious effect on the enhancement of liquid drainage.
  • Effect of Hydrogen on Catalytic Combustion of Methane with Ultra-low Concentrations
    LU Chaohao, LÜ Xiaojing, WENG Yiwu
    2016, 36(4): 271-276.
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    Catalytic combustion experiments of methane were conducted in a self-developed test rig at constant inlet temperature and flow velocity of combustion chamber by changing the volum fraction of methane and hydrogen, so as to obtain the catalytic combustion characteristics of methane at different concentrations of hydrogen. Results show that when the inlet temperature of combustion chamber is kept at 520℃, adding low volume fraction of hydrogen could speed up the catalytic combustion reaction, reduce the ignition temperature and improve the conversion rate of methane. The higher the volume fractions of hydrogen and methane are, the more remarkable the combustion-supporting effect of hydrogen will be to the catalytic combustion of methane.
  • Transient Flow and Pressure Fluctuation in a Variable-pitch Axial-flow Fan
    YE Xuemin, DING Xueliang, LI Chunxi
    2016, 36(4): 277-285.
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    A variable-pitch axial-flow fan of OB-84 type was taken as the object of study to simulate the pressure fluctuation at monitoring points and the characteristics of transient flow field based on the flow field in steady state considered as the initial flow field of unsteady state. Results show that all the pressure signals appear in the pattern of periodic or quasi-periodic fluctuation, and the pulsation intensity decreases with increasing flow rate, while the maximum value is located at the tip clearance. The maximum amplitudes at different monitoring points are presented at blade passing frequency, and the variation of flow rate only affects the time-frequency distribution. With the rise of flow rate, the average turbulent kinetic energy on the section at 10% blade height improves, and the high-pressure region at outlet section of impeller moves to the hub. The turbulent kinetic energy is enhanced with duration at trailing edge of blades and in the passage between adjacent blades. The high-pressure region at outlet section of impeller reciprocates between the casing and the hub during the rotation of impeller.
  • Influence of Clearance Leakage Jet Flow in Wear-rings on Flow Characteristics of a Centrifugal Fan
    DONG Yunshan, YANG Ailing, CHEN Eryun, DAI Ren
    2016, 36(4): 286-293.
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    To study the effects of clearance leakage jet flow in wear-rings on the performance and internal flow characteristics of a centrifugal fan, numerical simulation was conducted on the three-dimensional flows in a high-pressure centrifugal fan with different relative lengths of wear-rings, and the Navier-Stokes equations were solved using RNG k-ε model with Fluent software. Results show that the leakage rate decreases first and increases later on as the relative length of wear-rings growing from 0.25 to 1.25, while the turbulent kinetic energy of jet flow increases first and decreases later on. There exists an optimal relative length of wear-rings, when the fan performance is least affected by the leakage jet flow, and the total pressure rise and aerodynamic efficiency reach the optimum. With different relative lengths of wear-rings, the leakage jet flow has different influences on cross-sectional parameters at entry of impeller and on the flow field in the impeller channel, which enhances both the flow turbulence near the shroud and the flow stability near the hub of centrifugal fan.
  • NOx Emission Characteristics and Nitrogen Conversion Mechanism in Algae Biomass Combustion Processes
    LIU Wang, ZHAO Bingtao, ZHANG Hang
    2016, 36(4): 294-299,325.
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    To explore the NOx emission characteristics in algae biomass combustion processes, experimental tests were conducted at different temperatures in a one-dimensional tube furnace to following typical algae biomass, such as the enteromorpha, sargasso and chlorella, etc., so as to obtain the law governing the NOx emission from pairwise algae combustion in same blending ratios. Results show that when the algae biomass is individually burned, the NOx emission of both chlorella and enteromorpha exhibits in unimodal distribution, whereas at the temperature above 600℃, the NOx emission of sargasso shows bimodal distribution, and all above NOx emission peaks increase with the rise of temperature. The NOx emission and nitrogen conversion rate of different algae biomass reach the minimum at 600℃. When above algaes are individually combusted from 700℃ to 900℃, the NOx emission would basically keep constant, but the conversion path of nitrogen would have great changes. At the temperature of 800℃, NOx emission of both the chlorella-enteromorpha mixture and chlorella-sargasso mixture lies in the range of their individual emission values, indicating no obvious interaction between them, however, the NOx emission of enteromorpha-sargasso mixture is higher than the sum of their individual values, indicating promotion effects between them.
  • Economical Optimization of a Boiler Denitration System Based on GA and Fuzzy Association Rules
    CUI Chao, LIU Jizhen, YANG Tingting
    2016, 36(4): 300-306.
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    A prediction model of denitrification cost was established using least squares support vector machines (LSSVM) according to historical data taken from the supervisory information system of a 660 MW coal-fired boiler. Based on the model, an optimal expert database was built up for off-line optimization of the frequently-operating load points by genetic algorithm, from which the associations between unit load and adjustment variables were extracted using fuzzy association rule mining (FARM) algorithm, so as to achieve online adjustment and optimization of various parameters under power grid dispatching conditions. Results show that the algorithm proposed has a close optimization effect and a shorter simulation time on denitrification cost when compared with the genetic algorithm, which therefore is suitable for online optimization and control of thermal power units.
  • Discussion on Waste Heat Recovery Technology by Separating the Water Chamber of Condensers
    WANG Guoshan
    2016, 36(4): 307-312,319.
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    The thermal performance of two condensers in 2×350 MW power units was numerically studied using self-developed simulation software under rated winter extraction heating mode. By separating the water chamber of condensers, cooling water with higher outlet temperature was led into an absorption heat pump for waste heat recovery. Results show that both the heat-transfer coefficient and cooling water outlet temperature are quite different in different condenser cooling regions. Under rated winter extraction heating mode, the temperature and exergy of waste water can be increased by separating the water chamber of condensers. The higher the waste water flow is, the lower the waste water temperature will be, resulting in increased quantity of waste heat. The higher the return water temperature is, the higher the waste water temperature will be, resulting in reduced quantity of waste heat. The division of water chamber hardly changes the thermal performance of condenser under summer operation conditions.
  • Analysis on Exergy Efficiency of a Compressed-air Energy Storage System Based on Orthogonal Design
    HE Qing, LIU Hui, ZHANG Junliang, CAI Youran, LIU Wenyi
    2016, 36(4): 313-319.
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    According to structural features of a compressed-air energy storage (CAES) system, six parameters of the system were experimentally designed and numerically simulated based on orthogonal method, such as the compressor adiabatic efficiency, inter-stage cooling temperature, minimum operating pressure of storage volume, regenerator effectiveness, expansion turbine adiabatic efficiency and combustion chamber efficiency etc., and subsequently an exergy analysis was conducted to the simulation results, while an analysis of variance was carried out to the experimental results to study the effects of above design parameters on the exergy efficiency of the system. Results show that the exergy efficiency of CAES system is mainly affected by the significant parameters, such as the compressor adiabatic efficiency, the inter-stage cooling temperature, the regenerator effectiveness, as well as the interactions between the compressor adiabatic efficiency and inter-stage cooling temperature, the inter-stage cooling temperature and regenerator effectiveness, and between the compressor adiabatic efficiency and expansion turbine adiabatic efficiency, etc. Under present condition of technology, the exergy efficiency of CAES system can be improved by reducing the inter-stage cooling temperature and increasing the regenerator effectiveness.
  • Preparation and Verification on New Zirconium Alloys with Improved Corrosion Resistance
    ZENG Qifeng, CHEN Lei, LU Junqiang, LIU Jiazheng
    2016, 36(4): 320-325.
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    To improve the corrosion resistance of zirconium alloys, a systematic study was carried out on the corrosion mechanism, and based on the free energy theory of surface oxide, a design idea was proposed subsequently. Referring to available test results, seven new zirconium alloys were designed, and for each alloy system, representative manufacturing process was worked out and validated by autoclave corrosion tests, after which two new zirconium alloys with improved corrosion resistance were obtained. The samples were corroded in pure water and lithiated water at 360℃/18.6 MPa for 200 days to make comparative analysis on the corrosion resistance. Results show that two zirconium alloys with additions of copper and silicon are found to be better than other five zirconium alloys. For zirconium alloys with 0.8%-1.2% Nb, conventional low temperature annealing process would be beneficial to the improvement of corrosion resistance, whereas for zirconium alloys with 0.2%-0.5% Nb, β water quenching process would be beneficial to the improvement of corrosion resistance instead of intermediate annealing.
  • Electrochemical Corrosion Property of Stainless Steel 347 with Ni-based Alloy 690 Surface Cladding
    CHEN Feng, LIU Guohui, LIN Qiaoli, CAO Rui, DONG Hao, CHE Hongyan
    2016, 36(4): 326-330.
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    Ni-based alloy 690 was deposited on the surface of stainless steel 347 by using TIG surfacing procedure, while electrochemical corrosion properties of the substrate stainless steel 347, surfacing layer alloy 690 and the welded joint were analyzed by electrochemical measurement technology in a 3.5% NaCl solution at room temperature. To study the corrosion mechanism of the welded joint, the microstructure and composition were analyzed by SEM and EDS, and the surface morphology after corrosion was characterized by optical microscope. Results show that the corrosion rate of the base metal and the welded joint is respectively 8.4 times and 11.8 times of the surfacing layer, indicating that the surfacing layer Ni-based alloy 690 has the strongest corrosion resistance, followed by the base metal stainless steel 347, and the welded joint has the poorest corrosion resistance. The dilution of Cr by surfacing layer in the base metal near fusion zone of welded joint is the main reason leading to the aggravated corrosion of the substrate stainless steel 347.
  • In Situ Observation and Related Research on Phase Transformation During Heating and Cooling Process of 00Cr13Ni5Mo Steel
    LIU Lu, YANG Zhigang, ZHANG Chi
    2016, 36(4): 331-336.
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    The phase transformation behaviors of 00Cr13Ni5Mo steel were studied in situ respectively at a heating rate of 100 K/min and a cooling rate of 200 K/min by using a confocal laser scanning microscope, while the crystallographic features of martensite were analyzed at room temperature after the cooling process by electron backscatter diffraction (EBSD). The mechnism of austenite memory was discussed. Results indicate that, acicular austenite would lead preferably to the recovery of the initial microstructure and exhibit austenite memory when heated to 1 250℃ at the heating rate of 100 K/min, where austenitic spontaneous recrystallization would not be observed. Lath martensite would be formed in the continuous cooling process as the temperature reach about 273℃. The increase of the amount of transformation-induced martensite in the cooling process mainly depends on the rise of batches of martensite.