Dec 2009, Volume 3 Issue 4
    

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  • Research articles
    Numerical study on natural convection in a square enclosure containing a rectangular heated cylinder
    Jianhua LU, Zhaoli GUO, Zhenhua CHAI, Baochang SHI,
    2009, 3(4): 373-380. https://doi.org/10.1007/s11708-009-0078-x
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    In this paper, the natural convection in a square enclosure with a rectangular heated cylinder is investigated via the lattice Boltzmann method. A detailed study is conducted on the effect of the cylinder width and the Rayleigh number on the fluid flow and heat transfer. The flow structures and heat transfer patterns are classified into eight buoyant regimes, i.e., four steady regimes, two periodic regimes, one multiple periodic regime, and one chaos regime, two of which are reported for the first time.
  • Research articles
    Erosion-corrosion failure of REAC pipes under multiphase flow
    Ping TANG, Jian YANG, Jinyang ZHENG, Shizheng HE, Jianjun YE, Guofu OU, Ieong WONG, Yanbao MA,
    2009, 3(4): 389-395. https://doi.org/10.1007/s11708-009-0045-6
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    By using computational fluid dynamics (CFD) modeling and simulation, a predictive method was proposed to explore the erosion failure of reactor effluent air cooler (REAC) pipes with liner under multiphase flow. A theoretical model based on the erosion-corrosion effects of REAC on mixture turbulent flow was proposed for multiphase flow. Effects of various working conditions, liner shapes, and structures, as well as flow parameters on numerical simulations were investigated. Besides, the pipe’s erosion-corrosion rules under multiphase flow and the relationship between multiphase flow and erosion-corrosion under dangerous working conditions were studied. By CFD numerical simulations, the exact position where some typical pipes thinned and failed rapidly by erosion was found and the main factors causing erosion-corrosion failure were discussed. Finally, numerical results obtained by using the proposed method were compared with experimental results.
  • Research articles
    Numerical simulation of micro scale flowing and boiling
    Wen WANG, Rui ZHUAN,
    2009, 3(4): 396-401. https://doi.org/10.1007/s11708-009-0049-2
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    Numerical simulations of flowing and boiling in micro channels are presented, including the modeling of bubble dynamics of nucleate boiling, and a description of the interface of two phases with the volume-of-fluid (VOF). The two calculated cases are compared with related experimental data in literature. Some simulated results are found corresponding well to the experimental data. The simulated results also show the details of 3-dimensional heat transfer and the flow in micro channels, which are helpful to the investigation of the mechanism of two-phase heat transfer and flow in micro channels.
  • Research articles
    Application of entransy dissipation theory in heat convection
    Mingtian XU, Jiangfeng GUO, Lin CHENG,
    2009, 3(4): 402-405. https://doi.org/10.1007/s11708-009-0055-4
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    In the present work, formulas for calculating the rates of the local thermodynamic entransy dissipation in convective heat transfer in general, and the internal and external flows in particular, are established. Practically, these results may facilitate the application of entransy dissipation theory in thermal engineering. Theoretically they shed light on solving the contradiction of the minimum entropy production principle with balance equations in continuum mechanics.
  • Research articles
    Numerical investigation of the effectiveness of effusion cooling for plane multi-layer systems with different base-materials
    Dieter BOHN, Robert KREWINKEL
    2009, 3(4): 406-413. https://doi.org/10.1007/s11708-009-0041-x
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    Within Collaborative Research Center (SFB) 561 “Thermally Highly Loaded, Porous and Cooled Multi-Layer Systems for Combined Cycle Power Plants” at RWTH Aachen University, an effusion-cooled multi-layer plate configuration is investigated numerically by the application of a three-dimensional in-house fluid flow and heat transfer solver, CHTflow. CHTflow is a conjugate code, which yields information on the temperature distribution in the solid body. This enables a detailed discussion of the effects of a change in materials. The geometrical set-up and the fluid flow conditions derive from modern gas turbine combustion chambers and bladings. Within the SFB, two different multi-layer systems, one consisting of substrate made of CMSX-4 (a single-crystal super-alloy), an MCrAlY-bondoat and a ZrO2 thermal barrier coating (TBC), and the other consisting of a NiAl-alloy and a graded bondcoat/TBC, have been investigated. The grading will increase the life-span of the TBC as it can better compensate the different thermal expansion coefficients of different materials.
    The main focus in this study is on the different substrate materials, because the thermal conductivity of the NiAl is considerably higher than that of CMSX-4, which leads to different temperature profiles in the components.
    The numerical grid for the simulations contains the coolant supply (plenum), the solid body for the conjugate calculations, and the main flow area on the plate.
    The effusion-cooling is realized by finest drilled shaped holes with a diameter of 0.2mm. The investigation is concentrated on a cooling hole geometry with a laterally widened fan-shaped outlet, contoured throughout, and one without lateral widening that is only shaped in the TBC-region of the system. Two blowing ratios, M=0.28 and M=0.48, are investigated, both for a hot gas Mach number of 0.25.
    The results for the lower blowing ratio and the fully contoured hole are discussed as well as those of the higher blowing ratio and the non-laterally widened hole. These represent two characteristic cases.
  • Research articles
    Development of an axial-type fan with an optimization method
    Chong-hyun CHO, Soo-yong CHO, Chaesil KIM,
    2009, 3(4): 414-422. https://doi.org/10.1007/s11708-009-0038-5
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    An axial-type fan that operates at a relative total pressure of 671Pa and a static pressure of 560Pa with a flowrate of 416.6m3/min is developed using an optimization technique based on the gradient method. Prior to the optimization of the fan blade, a three-dimensional axial-type fan blade is designed based on the free-vortex method along the radial direction. Twelve design variables are applied to the optimization of the rotor blade, and one design variable is selected for optimizing a stator which is located behind the rotor to support a fan-driving motor. The total and static pressure are applied to the restriction condition with the operating flowrate on the design point, and the efficiency is chosen as the response variable to be maximized. Through these procedures, an initial axial-fan blade designed by the free vortex method is modified to increase the efficiency with a satisfactory operating condition. The optimized fan is tested and compared with the performance obtained with the same class fan to figure out the optimization effect. The test results show that the optimized fan not only satisfies the restriction conditions but also operates at the same efficiency even though the tip clearance of the optimized fan is greater than 30%. The experimental and numerical tests show that this optimization method can improve the efficiency and operating pressures on axial-type fans.
  • Research articles
    Calculation and analysis of sub/supercritical methanol preheating tube for continuous production of biodiesel via supercritical methanol transesterification
    Wen CHEN, Weiyong YING, Cunwen WANG, Weiguo WANG, Yuanxin WU, Junfeng ZHANG,
    2009, 3(4): 423-431. https://doi.org/10.1007/s11708-009-0075-0
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    Biodiesel is an important renewable energy. Supercritical methanol transesterification for biodiesel has recently been concerned because of its obvious advantages. The tubular reactor is an ideal reactor for continuous preparation of biodiesel via supercritical methanol transesterification. A methanol preheating tube is necessary for the tubular reaction system because the reaction temperature for supercritical methanol transesterification is usually 520―600K. Therefore, in the range of 298―600K, changes of the density, isobaric capacity, viscosity and thermal conductivity of sub/supercritical methanol with temperature are first discussed. Then on the basis of these thermophysical properties, an integration method is adopted for the design of sub/supercritical methanol preheating tube when methanol is preheated from 298K to 600K at 16MPa and the influencing factors on the length of the preheating tube are also studied. The computational results show that the Reynolds number Re and the local convection heat-transfer coefficient α of sub/supercritical methanol flowing in ф6mm×1.5mm preheating tube change drastically with temperature. For the local overall heat transfer coefficient K and the average overall heat transfer coefficient Km, temperature also has an important influence on them when the inlet velocity of methanol is lower than 0.5m/s. But when the inlet velocity of methanol is higher than 0.5m/s, K and Km almost keep invariable with temperature. Additionally, both the outlet temperature and the inlet velocity of methanol are the key affecting factors for the length of the preheating tube, especially when the outlet temperature is over the critical temperature of methanol. At the same time, the increase of tin bath’s temperature can shorten the required length of the preheating tube. At the inlet flow rate of 0.5m/s, the required length of the preheating tube is 2.0m when methanol is preheated from 298K to 590K at 16MPa with keeping the tin bath’s temperature 620K, which is in good agreement with the experimental results.
  • Research articles
    Numerical study of ignition mechanism of n-heptane direct injection compression-ignition engine
    Xiaoping GUO, Zhanjie WANG,
    2009, 3(4): 432-439. https://doi.org/10.1007/s11708-009-0050-9
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    A detailed chemical dynamical mechanism of oxidation of n-heptane was implemented into kiva-3 code to study the ignition mechanism of a high-temperature, high-pressure, three-dimensional-space, transient turbulent, non-homogeneous, mono-component fuel in the engine. By testing the quantity of the heat released by the chemical reaction within the cylinder cell, the elementary reaction showing an obvious increase in the cell temperature was defined as ignition reaction and the corresponding cell as ignition position. The main pathway of the ignition reaction was studied by using the reverse deducing method. The result shows that the ignition in the engine can be divided into low-temperature ignition and high-temperature ignition, both of which follow the same rule in releasing heat, called the impulse heat releasing feature. Low-temperature ignition reaction, whose ignition reaction is c5h9o1-4=ch3cho+c3h5-a, follows the oxidation mechanism, while high-temperature ignition reaction, whose ignition reaction is c2h3o1-2=ch3co, follows the decomposition mechanism. No matter which ignition it is in, the chemical reaction that restrains the ignition reaction from lasting is the deoxidization reaction of alkylperoxy radicals.
  • Research articles
    A reliable and practical reference objective for the deviation diagnosis of energy system parameters
    Liping LI, Zheng LI,
    2009, 3(4): 440-445. https://doi.org/10.1007/s11708-009-0051-8
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    The core objective to optimize a complex energy system is to set the reference target to guide the parameter adjustment of system operation. In this paper, a new case-based approach is proposed based on an online performance assessment program and its long-term operation data for a large power unit. The online model of a coal-fired power unit’s performance assessment is demonstrated, and the distribution pattern of the performance index is revealed by statistical analysis of the abundant data. The fundamental issues (representation of the similarity of two thermal processes, similarity measure, etc.) are tackled. The key sections and key parameters for the completion of similarity determination are proposed, which are essential to realize a case-based strategy. A full-scope simulator of power unit is used to test the availability of the method. The advantage of the case-based approach is the integrality of information over other methods.
  • Research articles
    Nonlinear design-point performance adaptation approaches and their comparisons for gas turbine applications
    Y. G. LI, P. PILIDIS,
    2009, 3(4): 446-455. https://doi.org/10.1007/s11708-009-0042-9
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    Accurate performance simulation and understanding of gas turbine engines is very useful for gas turbine manufacturers and users alike and such a simulation normally starts from its design point. When some of the engine component parameters for an existing engine are not available, they must be estimated in order that the performance analysis can be started. Therefore, the simulated design point performance of an engine may be slightly different from its actual performance. In this paper, two nonlinear gas turbine design-point performance adaptation approaches have been presented to best estimate the unknown component parameters and match available design point engine performance, one using a nonlinear matrix inverse adaptation method and the other using a Genetic Algorithm-based adaptation approach. The advantages and disadvantages of the two adaptation methods have been compared with each other. In the approaches, the component parameters may be compressor pressure ratios and efficiencies, turbine entry temperature, turbine efficiencies, engine mass flow rate, cooling flows, and by-pass ratio, etc. The engine performance parameters may be thrust and SFC for aero engines, shaft power, and thermal efficiency for industrial engines, gas path pressures, temperatures, etc. To select the most appropriate to-be-adapted component parameters, a sensitivity bar chart is used to analyze the sensitivity of all potential component parameters against the engine performance parameters. The two adaptation approaches have been applied to a model gas turbine engine. The application shows that the sensitivity bar chart is very useful in the selection of the to-be-adapted component parameters, and both adaptation approaches are able to produce good quality engine models at design point. The comparison of the two adaptation methods shows that the nonlinear matrix inverse method is faster and more accurate, while the genetic algorithm-based adaptation method is more robust but slower. Theoretically, both adaptation methods can be extended to other gas turbine engine performance modelling applications.
  • Research articles
    Hydrogen production by biomass gasification in supercritical or subcritical water with Raney-Ni and other catalysts
    Aixia PEI, Lisheng ZHANG, Bizheng JIANG, Liejin GUO, Ximin ZHANG, Youjun LV, Hui JIN,
    2009, 3(4): 456-464. https://doi.org/10.1007/s11708-009-0069-y
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    Gasification of peanut shell, sawdust and straw in supercritical or subcritical water has been studied in a batch reactor with the presence of a series of Raney-Ni and its mixture with ZnCl2 or Ca(OH)2. The main gas products were hydrogen, methane, carbon dioxide, and a small amount of carbon monoxide. Different types of Raney-Ni, containing different metal components such as Fe, Mo or Cr, have different influences on the gasification yield and hydrogen selectivity. The catalysis effect can be improved obviously by adding ZnCl2 or Ca(OH)2. Increasing the reaction temperature or adding ZnCl2 and Ca(OH)2 could improve the mass of H2 in gas products and reduce the mass of CH4 and CO2 at the same time. The possible mechanism is that ZnCl2 can decompose the biomass particle by accelerating cellulose hydrolyzation in high-temperature water, increasing more specific surface to admit catalysts, while Ca(OH)2 can absorb CO2 to produce CaCO3 deposit, which can drop out from the reactant system, and which will drive the reaction to get more hydrogen. With respect to the biomass conversion to gas product and selectivity of H2 at low temperature, the series of Raney-Ni has shown many advantages over other catalysts; thus, this kind of catalyst has great potential to be utilized in the hydrogen industry for the gasification of biomass.
  • Research articles
    A method of analysis for modal aero-damping of vibrating blade in incompressible flow
    Jun XING, Lin LI,
    2009, 3(4): 465-471. https://doi.org/10.1007/s11708-009-0037-6
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    By establishing a research model for aero-damping in a uniform incoming flow, the three-dimension analytical algorithm of the modal aero-damping based on the equivalent viscous damping principle was established. The algorithm, which is suited for low-speed flow, to compute aero-damping is based on classical lift line theory in aerodynamics, according to the analytical solution of unsteady aerodynamic force. According to the analytical equations, the modal aero-damping ratio for different nature frequencies of blades in various fluid field conditions can be solved, and the factors to affect the aero-damping can be determined.
    The first bend and the first torsion’s modal aero-damping ratios of a compressor blade are calculated by using the above algorithms. The results show that the aero-damping cannot be neglected comparing with the structure damping. It reflects the coupling degree between the flow field and the structure field, so the aero-damping is significant to compute the stress and life of blade.
  • Research articles
    Effects of thermocline on performance of underwater glider’s power system propelled by ocean thermal energy
    Hai YANG, Jie MA,
    2009, 3(4): 472-479. https://doi.org/10.1007/s11708-009-0052-7
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    The thermal glider’s changeable volume produces propelling force to power the glider’s descending and ascending through the thermocline. The different depth, thickness, and intensity of the thermocline at different seasons and locations affect the working processes of the glider’s power system. Based on the enthalpy method, a mathematical model of the underwater glider’s power system was established and the time efficiency of operation was introduced, so that the effects of different thermoclines on the underwater glider’s power system were analyzed theoretically. The simulation result shows that the thermocline affects the transition time of the phase change processes of working fluids within the thermal engine tubes. There exist the threshold values of the thermocline’s depth and upper thickness for the power system’s operation. A depth or upper thickness of the thermocline less than the corresponding threshold leads the power system to work abnormally. To keep the power system working efficiently, a glider must be kept in warm surface water for a certain period before it moves through cold water, so that the time efficiency of operation is reduced. A less time efficiency of operation is unfavorable to the thermal glider to penetrate through the ocean currents.
  • Research articles
    Model validation for structural dynamics in the aero-engine design process
    Chaoping ZANG, D. J. EWINS,
    2009, 3(4): 480-488. https://doi.org/10.1007/s11708-009-0043-8
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    A model validation technique in structural dynamics and its application in aero-engine development is introduced. The concept and the approaches of model validation based on reference data supplied from experimental tests or from supermodel simulation are discussed in detail. An aero-engine component is used as an example to demonstrate the validation using the experimental test and supermodel information, respectively. A satisfactory agreement with both approaches is achieved, and finally, a strategy of model validation for the whole engine model is introduced.
  • Research articles
    How far have we been? ―Summary of investigations on rotating cavity at IDG, RWTH Aachen University
    Dieter BOHN, Jing REN,
    2009, 3(4): 489-497. https://doi.org/10.1007/s11708-009-0040-y
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    Annular cavities are found inside rotor shafts of turbomachines with an axial or radial throughflow of cooling air, which influences the thermal efficiency and system reliability of the gas turbines. The flow and heat transfer phenomena in those cavities should be investigated in order to minimize the thermal load and guarantee the system reliability. An experimental rig is set up in the Institute of Steam and Gas Turbines, RWTH Aachen University, to analyze the flow structure inside the rotating cavity with an axial throughflow of cooling air. The corresponding 3D numerical investigation is conducted with the in-house flow solver CHTflow, in which the Coriolis force and the buoyancy force are implemented in the time-dependent Navier-Stokes equations. Both the experimental and numerical results show that the whole flow structure rotating slower than the cavity rotating speed. The flow passing the observation windows in the experimental and numerical results indicates the quite similar trajectories. The computed sequences and periods of the vortex flow structure correspond closely with those observed in the experiment. Furthermore, the numerical analysis reveals a flow pattern changing between single pair, double pair, and triple pair vortices. It is suggested that the vortices inside the cavity are created by the gravitational buoyancy force in the investigated case, while the number and strength of the vortices are controlled mainly by the Coriolis force.