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  • 2017 Volume 37 Issue 8
    Published: 15 August 2017
      
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  • Calculation Model for Circulation Ratio of Circulating Fluidized Bed Boilers
    LIU Jizhen, ZHANG Bao
    2017, 37(8): 597-602.
    Abstract ( ) Download PDF ( )   Knowledge map   Save
    A "four-circle two-iteration" calculation model was proposed for circulating fluidized bed (CFB) boilers based on the heat balance and material balance in the furnace, with which, not only the circulation ratio, but also some other key parameters can be calculated simultaneously, such as the combustion fraction in the separator, furnace and dense phase region as well as the excess air coefficient in the dense phase region. According to the operation data of a 300 MW CFB boiler, the effects of coal quality and primary to secondary air ratio on the circulation ratio were analyzed. Results show that the model can accurately calculate related parameters and reasonably reflect how above factors influencing the circulation ratio of CFB boilers, which therefore may serve as a reference for control and optimization of the circulation ratio in CFB boilers.
  • Effect of OFA Ratio on Outlet Gas Temperature of a Boiler
    CAO Chengque, DING Shifa, SHI Hongfei
    2017, 37(8): 603-607.
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    Taking the 1 000 MW ultra supercritical double-tangential firing boiler as an object of study, numerical calculation and analysis were conducted using Fluent software on the temperature of outlet gas from bituminous coal-fired furnace after air-staged combustion retrofit, and subsequently the calculation results were compared with actual operation data after retrofit. Results show that the flame center of boiler would move upward with the rise of OFA ratio, while the distance from SOFA to boiler outlet is shortened, resulting in increased temperature of outlet gas. By correcting the flame center coefficient M in the gas temperature calculation formula, the calculation results can then satisfy the requirements of actual engineering retrofit, which may serve as a reference for design and retrofit of air-staged combustion boilers.
  • Diagnosis Model of Additional Air Flow Through Gaps of Tangential Firing Oscillating Nozzles
    LIU Fuguo, CUI Fuxing, LIU Ke, WANG Haichao
    2017, 37(8): 608-614.
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    Based on resistance characteristics of secondary air baffle and boiler furnace measured under cold state conditions, considering the following character of pressure differential between wind box and furnace outlet with air/gas flow rate obtained according to such parameters as the actual pressure differential, the opening of secondary air baffle and the flow rate of pulverized coal-air mixture at different loads, an evaluation model was established for the mass flow rate of additional air entering into boiler furnace through gaps of tangential firing oscillating nozzles, following the principle of least sum of squares of deviations between calculated and measured data of the excess air coefficient at furnace outlet. Actual applications in a boiler indicate that the results calculated with the evaluation model agree well with both the design values and actual measurements, which therefore may be used to diagnose the running state of relevant nozzles.
  • Study on the Optimal Initial Pressure of a Steam Turbine Unit Based on Krill Herd Algorithm
    NIU Peifeng, CHEN Ke, LIU Aling, MA Yunpeng, ZHAO Zhen, LI Guoqiang
    2017, 37(8): 615-621.
    Abstract ( ) Download PDF ( )   Knowledge map   Save
    To obtain the optimal initial pressure of a steam turbine under variable working conditions, a heat rate prediction model was established using operating data of the turbine by improved krill herd algorithm (A-KH) and fast learning network (FLN), with which the optimal initial steam pressure corresponding to the lowest heat rate was searched in the range of premitted pressure by taking use of the global searching ability of A-KH algorithm, and the optimal initial pressure curve was subsequently compared with the design curve. Results show that via the optimal initial pressure curve, the heat rate could be lowered effectively, which may serve as a reference for safety and economic operation of steam turbines.
  • Effects of Steam Viscosity on Thermal Deformation of a Non-contacting Finger Seal Pad
    ZHU Dongxiao, WANG Weizhe
    2017, 37(8): 622-628.
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    Taking the non-contacting finger seal as an object of study, a conjugate heat transfer model was established using CFX, so as to calculate the temperature distribution of the pad, simulate the thermal deformation with ABAQUS software, and finally to analyze the effects of steam viscosity on the temperature distribution and thermal deformation of the pad. Results show that steam viscosity would cause the generation of friction heat at the coupling surface between pad bottom and highly rotating steam, resulting in non-uniform distribution of temperature in the pad and the increase of local temperature, which would lead to a significant thermal deformation of the non-contacting finger seal. In general, steam viscosity would enhance the thermal deformation of non-contacting finger seals.
  • Stress Analysis of a Coupling Under the Action of Torsional Vibration
    ZHAO Pengcheng, GU Yujiong, LIU Yang, JIN Tiezheng, YANG Kun
    2017, 37(8): 629-633.
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    A finite element model was established for the coupling to identify the hazardous location where the bolt hole wall squeezes with the bolt, then to calculate the maximum equivalent stress in the hazardous location when the coupling transmits torques of different values, so as to fit the relational expression between the torque and the maximum equivalent stress. Finally, a two-phase short circuit fault was simulated to get the electromagnetic torque response, based on which instantaneous local stress sustained by the coupling and bolts could be obtained with above relational expression. Results show that when torsional vibration appears on the turbo-generator shaft, the torque undertaken by the coupling fluctuates, where the stress would be in the alternating state, and fatique failure would occur in the parts bearing alternative stresses.
  • Study on the Control Strategy of a Boiler-Turbine Coordinated Control System After Low NOx Combustion Retrofit
    ZHANG Xuyan, YANG Jingqi, KANG Lei, ZHOU Wentai
    2017, 37(8): 634-639.
    Abstract ( ) Download PDF ( )   Knowledge map   Save
    To solve the problem of a power unit that can't response the requirement of AGC control after low NOx combustion retrofit on the boiler, a study was conducted on the combustion characteristics of the boiler, the regulating quality of the coordinated control system and the response ability of the AGC control, including an analysis on the effects of following factors on the NOx formation, such as the oxygen concentration in the furnace, the openings of SOFA damper, surrounding air damper and secondary air damper etc., while the influence of primary air combustion control on the unit response was investigated. On above basis, an optimized design and commissioning were carried out for the boiler-turbine coordinated control system. Results show that not only stable operation, but also high accuracy and quick response of the AGC control are achieved via the optimized coordinated control strategy, which has been successfully applied in low NOx combustion retrofit of several units, proving the control strategy to be extendable and replicable.
  • Intelligent Control Optimization for the Coordinated System of an Ultra-supercritical Power Unit Based on Neural Network Prediction Models and Condensate Throttling
    MA Liangyu, CHENG Lei, PENG Gang, YIN Zhe
    2017, 37(8): 640-648.
    Abstract ( ) Download PDF ( )   Knowledge map   Save
    Taking the 1 000 MW ultra-supercritical unit as an object of study, artificial neural network prediction models with high accuracy and good dynamic characteristics were established for the unit load and main steam pressure in consideration of its regenerative cycle system. Subsequently, an optimized intelligent predictive controller was proposed for the coordinate system, which was used to optimize the openings of deaerator water level control valve and steam turbine control valve, and to control the total fuel demand based on above prediction models under variable load conditions, so as to improve the coordinated control effect. Detail simulation tests were conducted on the optimized coordinated control with a full-scope simulator for the given 1 000 MW USC power unit. Results show that via the method, both the response speed of dynamic load and the load control accuracy can be effectively improved, with significant reduction in control deviation of the main steam pressure under varying load conditions, proving the method to have good practicability.
  • Influence of Air Inlet Angle on Flow Characteristics of a Finned Oval-tube Heat Exchanger
    TANG Linghong, DU Xueping, ZENG Min
    2017, 37(8): 649-654.
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    Numerical simulation was conducted on the flow characteristics of a finned heat exchanger with two rows of oval tube using porous media model at following air inlet angles, such as 30°, 45°, 60° and 90°, respectively, and subsequently the simulation results were compared with experimental data, so as to analyze the effects of air inlet angle on the distribution of exchanger face velocity. Results show that within the test range of air inlet angle, the flow resistance loss of heat exchanger increases with reducing air inlet angle; the error Δp between calculation results and experimental data is less than 7% at different inlet angles of air; the most uniform flow distribution can be achieved when the inlet angle between two neighboring oval tubes is set to be 90°.
  • Investigation on Flow Mechanism of a Wind Farm Based on Yawed Wind Turbine Using Wake Deflection Control Strategy
    MIAO Weipao, LI Chun, YANG Jun
    2017, 37(8): 655-662.
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    To study the flow mechanism of a wind farm based on yawed wind turbine using wake deflection control strategy by multi-scale simulation, two full-scale 5 MW wind turbines laid in line were simulated in the atmospheric boundary layer environment through unsteady CFD method. Results show that the total power generation of wind farm can be increased by intentionally yawing the upstream wind turbine. According to the analysis of velocity field, the wake of upstream wind turbine would deform when the turbine is placed yawingly, and according to the analysis of vorticity field, a counter-rotating vortex pair would be observed as the yawed wake propagates, which would lead to the macroscopic wake deflection. However, the wake deviation effect is significant just at the hub height. The distorted wake still spreads its low speed regions to other areas.
  • Comparative Analysis on Operation Strategies of a Gas-Steam Combined Cycle Unit Under Off-design Conditions
    BAI Ziwei, ZHANG Guoqiang, FU Xuchen, YANG Yongping, WANG Xiuyan
    2017, 37(8): 663-672.
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    For the existing combined cycle unit using PG3951FA gas turbine, off-design operation characteristics of the gas turbine cycle, steam turbine cycle and gas-steam combined cycle were studied under three different operation strategies. Results show that for the reference unit, IGV regulation is unfavorable for efficient operation of the gas turbine cycle, but is favorable for the combined cycle; the impact of operation strategy on the steam turbine cycle is more significant than on the gas turbine cycle, so the optimum strategy for highest efficiency of the combined cycle is to maintain T4 operating at its ultimate outlet temperature, when the combined cycle efficiency would be 2% higher than that of IGV T3-F strategy at low load rates. To obtain optimum operation performance of the unit under off-design conditions, IGV strategy is recommended, when the efficiency of steam turbine cycle should be kept at a high level.
  • Study on Reaction Kinetics of Soot Oxidation in O2/CO2 Atmosphere
    LI Shuaishuai, WANG Xuebin, LIU Zihan, WANG Zhao, LI Yan, TAN Houzhang
    2017, 37(8): 673-678.
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    To study the reaction kinetics of soot oxidation in O2/CO2 atmosphere, thermogravimetric analysis was conducted with focus on the influence of oxygen concentration (φ(O2)), so as to carry out a comparison with that in O2/N2 atmosphere, and to obtain the quantitative relations between φ(O2) and the kinetic parameters including apparent activation energy (E) and frequency factor (A) for soot oxidation under oxy-fuel combustion conditions. Results show that in O2/CO2 atmosphere, both the starting and ending temperatures of soot oxidation are higher, indicating that the reactivity is inhibited in contrast with that in O2/N2 atmosphere; with the rise of oxygen concentration in O2/CO2 atmosphere, the TG-DTG curve moves toward the lower temperature region, the maximum mass loss rate increases, and the reactivity improves significantly. Obvious kinetic compensation effect exists between A and E of soot oxidation. When the value of φ(O2) is lower than 20%, both E and ln A have positive correlations with φ(O2); whereas when the value of φ(O2) is higher than 20%, both E and ln A have nearly no correlation with φ(O2).
  • Numerical Simulation of the Flow Around a Square Cylinder at Higher Reynolds Numbers
    XIAO Yao, QIN Hao, SONG Wenwu, HU Shuai
    2017, 37(8): 679-684.
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    Based on the k-ω SST model and γ-Reθ transition model, a study was conducted on the flow characteristics around a square cylinder respectively at the Reynolds number of 1.4×104 and 2.2×104, so as to calculate the drag coefficient and Strouhal number of the square cylinder, analyze its average velocity profiles, and finally to find out the calculation cost and precision of different turbulent models. Results show that compared with k-ω SST model, the γ-Reθ transition model is more superior in calculation precision and cost, which is more effective in predicting the position and magnitude of the recirculation region behind the square cylinder, and in analyzing the reattachment of separated flows on side walls near the rear corner, and therefore is a practical model for engineering applications at high Reynolds numbers.