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辐射换热是大型锅炉炉膛内的主要换热形式,准确的计算炉膛内的辐射换热量对大型锅炉设计和优化有重要意义。本文将有限体积法推广用于求解和分析大型电站锅炉炉膛内的辐射换热。给出了有限体积法对辐射传递方程进行离散和求解的基本过程。评估了有限体积法求解大型电站锅炉炉膛辐射换热的可靠性。将有限体积法用于分析某电厂600MW锅炉炉膛内的辐射换热,结果表明有限体积法可以有效的求解大型电站锅炉炉膛内的复杂辐射换热过程。 相似文献
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An important issue arising in supercritical steam boilers is to avoid the tube wall overheating due to high heat fluxes transferred from flue gases to the fluid. The paper presents a new hybrid one/two-dimensional model of the fluid heating in waterwall tubes in the combustion chambers of steam boilers for supercritical steam parameters. The model is based on distributed parameters. The analysis concerns tubes with externally finned surfaces. Using the proposed model, it is possible to estimate zones and locations where the tube wall overheating may occur. One-dimensional equations describing the mass, momentum and energy conservation are formulated and solved for the fluid domain. Each analyzed cross section of the finned waterwall tube is divided into 20 control volumes for which energy balance equations are solved in a two-dimensional space. In order to analyses the conjugate heat transfer between the waterwall tube and the fluid, the heat transfer coefficient is computed using the Kitoh correlation. The computations assume a variable heat flux along the combustion chamber height. Also, the heat flux variation on the waterwall tube circumference is incorporated within the model. The reduction in dimensionality in both the fluid and the solid domains leads to an improvement in the computational performance compared to complex three-dimensional computational fluid dynamics simulations. The paper presents an application of the proposed hybrid model to simulate heat and flow processes occurring in waterwall tubes of a supercritical boiler operating in one of the Polish power plants. The results of the simulations are compared with the data obtained from measurements and good agreement is obtained. Therefore, the developed model can be successfully applied, e.g. in simulators of the supercritical power boiler operation. 相似文献
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目前,燃煤锅炉三维CFD数值模拟中对炉膛水冷壁传热分布的预测大都基于给定的壁面温度边界条件。然而,此方法无法体现锅炉运行状态对壁面传热与壁温分布的影响。提出了一种基于锅炉烟气侧放热与汽水侧吸热间热平衡关系的壁面传热计算方法,并重点讨论了壁面传热系数的物理意义及取值方法。研究发现,壁面传热系数基本由壁面结渣状态决定,因此可根据壁面渣层的传热系数确定。本文方法将影响壁面传热的关键因素合理地体现在计算过程中,同时在模型复杂性与工程适用性之间保持了合理的平衡。采用此方法对一台320 MW锅炉的燃烧与传热分布进行了数值模拟,水冷壁吸热量的预测结果与锅炉运行数据吻合良好。 相似文献
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This paper describes a model of heat transfer for the convection section of a biomass boiler. The predictions obtained with the model are compared to the measurement results from two boilers, a 50 kWth pellet boiler and a 4000 kWth wood chips boiler. An adequate accuracy was achieved on the wood chips boiler. As for the pellet boiler, the calculated and measured heat transfer rates differed more than expected on the basis of the inaccuracies in correlation reported in the literature. The most uncertain aspect of the model was assumed to be the correlation equation of the entrance region. Hence, the model was adjusted to improve the correlation. As a result of this, a high degree of accuracy was also obtained with the pellet boiler. The next step was to analyse the effect of design and the operating parameters on the pellet boiler. Firstly, the portion of radiation was established at 3–13 per cent, and the portion of entrance region at 39–52 per cent of the entire heat transfer rate under typical operating conditions. The effect of natural convection was small. Secondly, the heat transfer rate seemed to increase when dividing the convection section into more passes, even when the heat transfer surface area remained constant. This is because the effect of the entrance region is recurrent. Thirdly, when using smaller tube diameters the heat transfer area is more energy‐efficient, even when the bulk velocity of the flow remains constant. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
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This article describes an on-line heat transfer simulation for the convection passes of a typical pulverized coal boiler (PCB) power plant that accounts for fouling. Performance analysis of heat exchanger assemblies employed in pulverized coal boilers was characterized using the effectiveness–number of transfer units (NTU) method. The model calculates instantaneous heat rates in different sections of the boiler so as to determine a local cleanliness factor. The effects of changing plant load are fully accounted for in the model. Generally, a close correlation between calculated cleanliness factors and normalized strain gage measurements of pendant section weight variations due to accumulated fouling was obtained. Furnace exit gas temperatures calculated by the model agreed reasonably well with measurements available in the literature for a similar design of PCB power plant. 相似文献
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P.T. Tsilingiris 《Solar Energy》2011,85(11):2561-2570
The aim of the present investigation was the derivation of validating field measurements from an experimental passive solar still model suitable for the evaluation of the existing theory, on the internal heat and mass transfer processes in solar distillation systems. A very good agreement was found between measurements and predictions from the basic model which was originally developed by Dunkle. However, it was surprising that although this model, with its simplified assumptions relaxed, was successfully validated against measurements at least as far as lower and intermediate temperatures and mass flow rates as high as about 0.1 g/m2 s is concerned, its prediction accuracy was found to degrade for higher, up to 0.2 g/m2 s measured yields, corresponding to modified Ra numbers up to about 6 × 106, while it was also derived that the modified Ra number is clearly a crucial dimensionless quantity which determines the heat and mass transfer processes in solar distillation systems. 相似文献
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Ricardo Vasquez Padilla Gokmen Demirkaya D. Yogi Goswami Elias Stefanakos Muhammad M. Rahman 《Applied Energy》2011
Solar Parabolic Trough Collectors (PTCs) are currently used for the production of electricity and applications with relatively higher temperatures. A heat transfer fluid circulates through a metal tube (receiver) with an external selective surface that absorbs solar radiation reflected from the mirror surfaces of the PTC. In order to reduce the heat losses, the receiver is covered by an envelope and the enclosure is usually kept under vacuum pressure. The heat transfer and optical analysis of the PTC is essential to optimize and understand its performance under different operating conditions. In this paper a detailed one dimensional numerical heat transfer analysis of a PTC is performed. The receiver and envelope were divided into several segments and mass and energy balance were applied in each segment. Improvements either in the heat transfer correlations or radiative heat transfer analysis are presented as well. The partial differential equations were discretized and the nonlinear algebraic equations were solved simultaneously. Finally, to validate the numerical results, the model was compared with experimental data obtained from Sandia National Laboratory (SNL) and other one dimensional heat transfer models. Our results showed a better agreement with experimental data compared to other models. 相似文献
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由于增压锅炉燃烧压力的提高,强化了对流传热,如果仍按常压燃烧锅炉炉膛热力计算,忽略对流传热,将直接影响增压锅炉炉膛热力计算的准确性。文中对增压锅炉与常压燃烧锅炉炉膛特性参数进行了比较,对前苏联增压锅炉的试验数据进行了分析探讨,指出了增压锅炉炉膛对流传热的影响因素。同时提出增压锅炉炉膛传热计算应将辐射与对流传热分开计算,给出了适用于增压锅炉炉膛对流传热的计算公式,并进行了实例计算分析,与前苏联增压锅炉试验的计算数据结果相近。对于完善增压锅炉的炉膛热力计算具有一定的理论和实际指导意义。 相似文献
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Jie Pan Dong Yang Hui Yu Qin-cheng Bi Hong-yuan Hua Feng Gao Zhong-ming Yang 《Applied Thermal Engineering》2009,29(11-12):2500-2507
Water wall design is a key issue for an ultra supercritical boiler. In order to increase the steam–water mixture turbulization and to prevent the burnout of tubes walls, vertical rifled tubes are applied in Yuhuan power plant boiler which is the first 1000 MW ultra supercritical boiler in China and began to operate in December 2006. Mathematical modeling and thermal-hydraulic analysis are key factors for the successful design and operation of water walls. The water wall system is treated in this paper as a network consisting of circuits, pressure grids and connecting tubes. The mathematical model for predicting the mass flux distribution and metal temperature in water wall is based on the mass, momentum and energy conservation equations. An experiment on the heat transfer characteristics of vertical rifled tube was conducted with the aim to obtain the heat transfer performance and corresponding empirical correlations. The fitting computational formulas are applied in the mathematical model. The presented modeling method is more accurate than the conventional graphic method and can be applied to complex circuit structures. The mass flux distribution and the metal temperature in the water wall are calculated at 35%, 50% and 100% of the boiler maximum continuous rating (BMCR). The results show a good agreement with the plant data. The maximum relative difference between the calculated mass flux and the plant data is 9.7% at 50% BMCR load. The metal temperature difference in the tip of fins in lower circuit 8 is about 3–7 °C at 35% BMCR load. The results show that the vertical water wall in the ultra supercritical boiler of Yuhuan power plant can operate safely. 相似文献
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近年来在水泥厂带补燃的余热电站设计中普遍地采用了循环流化床锅炉做为补燃锅炉。CPC循环流化床锅炉是其中应用较多的一种。针对这种锅炉在某水泥厂余热电站的实际运行中出现的结渣现象。文章分析了其结渣的原因及解决办法;对余热电站的设计及循环流化床锅炉的运行有参考意义。 相似文献
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《Applied Thermal Engineering》2014,62(2):545-552
Underground tunnels used in underground constructions serve as huge ventilation pipes that conduct outside fresh air into a cavern. Predicting the heat and mass transfer is critically important in order to exploit the relatively constant underground soil temperature for heat transfer, and to ensure sufficient ventilation for occupational safety. This paper presents a numerical model developed to describe the simultaneous heat transfer between air and the tunnel surface, taking into account the condensation phenomena inside the tunnel. The soil surrounding the tunnel is treated as an equivalent long annulus and divided into several cross-section slices. With appropriate assumptions, a set of discrete numerical equations and its solution is proposed. The developed model is validated against field measurements which showed good agreement between the simulated results and measurement data. The model is then applied to an underground tunnel operating for a ten-year-period. 相似文献
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A boiler plant is presented, in which the fuel is dried before combustion in a silo with air. The drying air is heated in a recuperative heat exchanger by the heat of flue gases. Hot air is then blown through the bed of fuel in the drying silo, while the fuel dries and the air cools down and becomes humidified. Heat of the moist exhaust air of the silo is recovered for the drying air and combustion air by a recuperative heat exchanger. Modelling of the thermal behaviour of the plant helps in understanding complex interdependencies of the two heat exchangers, the boiler and the dryer. The models of the heat exchangers and applications in analysing the boiler system are described in this paper. Calculating the combinations of extreme operational conditions gives the input data needed in comparing different types of heat exchangers, dimensioning the heat transfer area, choosing the control strategy and selecting the operating parameters and set‐values of the control system. Results of verification measurements and practical operation at a 40 kWth pilot plant and a 500 kWth demonstration plant are also discussed. Using engineering correlation formulas for heat and mass transfer, an adequate accuracy between the model and the measurements was achieved. Fouling was detected to be a major problem with the flue gas heat exchanger. However, in absence of condensation, the increase of a fouling layer with respect to time was observed to be low. Fouling was also a problem with the drying exhaust gas heat exchanger, but after the installation of a simple dust collector, a reasonable cleaning period was achieved. A mixed‐flow configuration was found to be the most appropriate for the flue gas heat exchanger. In order to avoid condensation of the flue gas the drying exhaust gas heat exchanger is indispensable in Finnish climate in the considered system. In addition to this, it decreases the need of fuel. A parallel‐flow type was found the most appropriate as the drying exhaust gas heat exchanger. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
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M. R. Golriz 《国际能源研究杂志》1996,20(7):569-579
An experimental investigation was carried out to study the effects of operating parameters on the local suspension-to-wall heat transfer in the combustor of a 12-MWth circulating fluidized-bed (CFB) boiler. The heat transfer coefficients were measured with a conduction-type heat flux meter at five different vertical levels. The measurements covered a range of superficial gas velocities from 4 to 6 m/s, a bulk bed temperature from 800 to 850 °C and a suspension density from 6 to 70 kg/m3 for 270-μm silica sand particles. The heat transfer coefficient for the membrane wall in the combustion chamber of the CFB boiler was in the range of 100 to 180 W/m2 K for the range of operating conditions employed in this work. The heat transfer coefficient decreased with increasing height and increased with increasing bulk bed temperature, superficial gas velocity and suspension density. Based on the experimental data, a simple correlation is proposed for predicting the suspension-to-membrane wall heat transfer coefficient. The results were analysed and compared with the experimental data of other workers. 相似文献
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Heat transfer improvement in a water wall tube with fins was investigated in a circulating fluidized bed (CFB) boiler. Experiments were first conducted in a 6 MWth CFB boiler then a model was developed to analyse and interpolate the results. Temperatures at some discrete points within the wall cross‐section of the tube were measured by burying 0.8 mm thermocouples within a tube. Experimental data showed an increase in heat absorption up to 45 per cent. A good agreement between measured and predicted values was noted. The distribution of temperature in the metal wall and of heat flux around the outer wall of a tube with longitudinal and lateral fins was analysed by numerical solution of a two‐dimensional heat conduction equation. Effects of bed‐to‐wall heat transfer coefficient, water‐to‐tube inside heat transfer coefficient, bed temperature, water temperature and thermal conductivity of the tube material on the heat flux around the water tube are discussed. The present work also examines the influence of the length of the longitudinal fin and the water tube thickness. Heat flux was highest at the tip of the longitudinal fin. It dropped, but increased again near the root of the lateral fin. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
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