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火电机组回热系统(火用)损矩阵方程的改进及应用 总被引:2,自引:2,他引:0
通过对(火用)损方程进行推导和改进,使其更加接近实际运行过程.根据汽水分布方程和损分布方程,建立起它们之间的联系,把汽水分布方程应用于损分布方程之中.以便在运行工况下使方程变得简单易解,利用这个改进后的损分布方程可以方便地得出不同工况下的损分布的规律,进而可以指导现实的节能分析.同时还可以方便地开发出应用于现场实时监测的回热系统的(火用)损分布的计算机程序,为降低机组的的能耗提供了很好的工具. 相似文献
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火电机组回热系统[火用]损分布的通用矩阵方程 总被引:2,自引:0,他引:2
根据[火用]平衡方程,首次导出了火电机组回热系统[火用]损分布的通用矩阵方程。利用这一方程可方便地得出不同机组回热系统的[火用]损分布规律,同时这一方程也为建立回热系统乃至整个机组与[火用]损分布通用矩阵方程相关的通用的[火用]分析模型、[火用]经济学分析模型、[火用]经济学优化模型和[火用]经济学故障诊断模型奠定了基础。利用这一方程还可以方便地开发出实时监测回热系统[火用]损分布的计算机程序,为降低机组能耗提供一个实用化的分析工具。图3表1参6 相似文献
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分析了2台背压汽轮机的回热加热系统用汽,由1.27MPa经减压阀降为0.3MPa后,由于节流阀前后焓相等,故可认为没有能量损失,但节流前后熵变量△S>0,说明是不可逆过程而损失了技术功.为避免上述损失,根据节流前后参数,按照热能转换为电能平衡方程式计算结果选用背压汽轮机代替减压阀,从而获得巨大的节能效益. 相似文献
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采用CFD与熵产理论相结合的手段,分析了不同偏流角对潮流能水轮机水动力特性的影响,发现偏流使无导管水轮机和导管水轮机输出功率及轴向推力均不同程度地降低,偏流20°时,两者功率最大下降率分别达到30.6%、16.8%,转子轴向推力最大下降率分别达到30.2%、19.8%.由于导管的聚流效应,不同偏流角下导管水轮机相比无导... 相似文献
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粘性流体在流动和传热过程中,由于粘性耗散和热传导的存在造成能量损失。为分析流体流动和传热过程的能量损失并得到冷板的最优流道形式,本文以某电子器件用S型流道液冷冷板为分析对象,通过数值模拟,得到S型流道液冷冷板的流体域熵产率随工质流量的变化规律,对流体域充分发展的直段和弯段内熵产率大小进行了比较,并在固定流量下,分析了熵产率大小沿工质流动方向上的变化情况。提出冷板流道优化方案,并从换热表现、压头损失和总能量损失三方面对不同流道形式的冷板进行了综合评价和比较,得到了冷板流道的最优形式,为工程实际提供参考。 相似文献
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An entropy‐based approach for flow loss characterization with computational fluid dynamics (CFD) is presented. Unlike past methods of global loss characterization, this article outlines a new approach for predicting local losses of available energy. The local entropy generation provides information regarding the spatial distribution of mechanical energy loss, which can be used to systematically optimize thermofluid systems. An application representing subsonic flow through a diffuser is investigated. The main parameter under consideration is the desired inlet expansion angle, which yields the minimum entropy generation at a specified Reynolds number and inlet flow condition. The numerical results indicate that the entropy‐based approach offers a new way of establishing the optimal diffuser configuration exhibiting minimal flow losses. By successfully predicting the local flow irreversibilities, re‐design efforts can be more carefully focused on specific regions of highest entropy production. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
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A critical factor in the design of combustion systems for optimum fuel economy and emission performance lies in adequately
predicting thermodynamic irreversibilities associated with transport and chemical processes. The objective of this study is
to map these irreversibilities in terms of entropy production for methane combustion. The numerical solution of the combustion
process is conducted with the help of a Fluent 6.1.22 computer code, and the volumetric entropy production rate due to chemical
reaction, viscous dissipation, and mass and heat transfer are calculated as post-processed quantities with the computed data
of the reaction rates, fluid velocity, temperature and radiative intensity. This paper shows that radiative heat transfer,
which is an important source of entropy production, cannot be omitted for combustion systems. The study is extended by conducting
a parametric investigation to include the effects of wall emissivity, optical thickness, swirl number, and Boltzmann number
on entropy production. Global entropy production rates decrease with the increase in swirl velocity, wall emissivity and optical
thickness. Introducing swirling air into the combustion system and operations with the appropriate Boltzmann number reduces
the irreversibility affected regions and improves energy utilization efficiency. 相似文献
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Two indicators are presented to compare the Second law performances of different design-variants of the same process unit. The first indicator relates the entropy production to quantities like the total transferred thermal energy and the total chemical conversion. This allows a useful comparison, even in the case of different inlets and outlets. An important aspect of the entropy production in a process unit is its distribution. An even distribution, also known as equipartition of entropy production (EoEP), is related to an optimal design. The second indicator is based on the coefficient of variation of a local entropy production profile and allows one to calculate and compare degrees of equipartition of different designs. Both indicators have been used in a study on the entropy production minimization of a plug-flow reactor. A comparison using the first indicator showed that the optimized reactors perform slightly better than a comparison based on the total entropy production alone would suggest. This shows that the total entropy production is not always a good indicator. The second indicator was found to provide an excellent numerical basis for comparing the degrees of EoEP of the different designs. 相似文献
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A critical factor in the design of combustion systems for optimum fuel economy and emission performance lies in adequately predicting thermodynamic irreversibilities associated with transport and chemical processes. The objective of this study is to map these irreversibilities in terms of entropy production for methane combustion. The numerical solution of the combustion process is conducted with the help of a Fluent 6.1.22 computer code, and the volumetric entropy production rate due to chemical reaction, viscous dissipation, and mass and heat transfer are calculated as post-processed quantities with the computed data of the reaction rates, fluid velocity, temperature and radiative intensity. This paper shows that radiative heat transfer, which is an important source of entropy production, cannot be omitted for combustion systems. The study is extended by conducting a parametric investigation to include the effects of wall emissivity, optical thickness, swirl number, and Boltzmann number on entropy production. Global entropy production rates decrease with the increase in swirl velocity, wall emissivity and optical thickness. Introducing swirling air into the combustion system and operations with the appropriate Boltzmann number reduces the irreversibility affected regions and improves energy utilization efficiency. 相似文献
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《热科学学报(英文版)》2019,(2)
The conventional method cannot explicitly confirm the location and type of the energy loss, therefore this paper employs the entropy production theory to systematically analyze the category, magnitude and location of hydraulic loss under different blade thickness distribution. Based on the analysis, the turbulent entropy and viscosity entropy produced by the separation of boundary layer at the trialing edge are major factors leading to the hydraulic loss. In addition, the separation of the boundary layer can not only cause the energy loss, but also block the passage of the impeller and reduce the expelling coefficient of the blade. Therefore, the hydraulic performance of the blades with increment thickness distribution is obviously better than the decrement one. Further, the flow rate has different influence on the three types of entropy production. Meanwhile, the pressure pulsation on the working surface was investigated. It was concluded that with flow rates increasing, the amplitude of pressure pulsation firstly decreases and then smoothly improves, and reaches the minimum under design flow rate. Finally, the optimal blade was obtained, and the relevant hydraulic performance test was performed to benchmark the simulation result. This research can provide the theoretical reference for designing the reasonable thickness distribution of the blades. 相似文献
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We minimize the total entropy production of a process designed for dehydrogenation of propane. The process consists of 21 units, including a plug-flow reactor, a partial condenser, two tray distillation columns and a handful of heat exchangers and compressors. The units were modeled in a manner that made them relatively insensitive to changes in the molar flow rates, to make the optimization more flexible. The operating conditions, as well as to some degree the design of selected units, which minimized the total entropy production of the process, were found. The most important variables were the amount of recycled propane and propylene, conversion and selectivity in the reactor, as well as the number of tubes in the reactor. The optimal conversion, selectivity and recycle flows were results of a very clear trade-off among the entropy produced in the reactor, the partial condenser and the two distillation columns. Although several simplifying assumptions were made for computational reasons, this shows for the first time that it is also meaningful to use the entropy production as an objective function in chemical engineering process optimization studies. 相似文献
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《International Journal of Hydrogen Energy》2019,44(29):15045-15055
Liquefaction of hydrogen is a promising technology for transporting large quantities of hydrogen across long distances. A key challenge is the high power consumption. In this work, we discuss refrigeration strategies that give minimum entropy production/exergy destruction in a plate-fin heat exchanger that cools the hydrogen from 47.8 K to 29.3 K. Two reference cases are studied; one where the feed stream enters at 20 bar, and one where it enters at 80 bar. Catalyst in the hot layers speeds up the conversion of ortho-to para-hydrogen. Optimal control theory is used to formulate a minimization problem where the objective function is the total entropy production, the control variable is the temperature of the refrigerant and the constrains are the balance equations for energy, mass and momentum in the hot layers. The optimal refrigeration strategies give a reduction of the total entropy production of 8.7% in the 20-bar case and 4.3% in the 80-bar case. The overall heat transfer coefficient and duty is higher in the 20 bar case, which compensates for the increase in entropy production due to a thermal mismatch that is avoided in the 80 bar case. This leads the second law efficiency of the 20 bar case (91%) to be similar to the 80 bar case (89%). We demonstrate that equipartition of the entropy production and equipartition of the thermal driving force are both excellent design principles for the process unit considered, with total entropy productions deviating only 0.2% and 0.5% from the state of minimum entropy production. Equipartition of the thermal driving force i.e. a constant difference between the inverse temperatures of the hot and cold layers represents a particularly simple guideline that works remarkably well. We find that both heat transfer and the spin-isomer reaction contribute significantly to the entropy production throughout the length of the process unit. Unlike previous examples in the literature, the process unit considered in this work is not characterized by a “reaction mode” at the inlet followed by a “heat transfer mode”. Therefore, it does not follow a highway in state space, i.e. a band that is particularly dense with energy efficient solutions. By artificially increasing the spin-isomer conversion rate, the highway appears when the conversion rate becomes sufficiently high. 相似文献
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Gregory P. Thiel John H. Lienhard 《International Journal of Heat and Mass Transfer》2012,55(19-20):5133-5147
The physical mechanisms of entropy generation in a condenser with high fractions of noncondensable gases are examined using scaling and boundary layer techniques, with the aim of defining a criterion for minimum entropy generation rate that is useful in engineering analyses. This process is particularly relevant in humidification-dehumidification desalination systems, where minimizing entropy generation per unit water produced is critical to maximizing system performance. The process is modeled by a consideration of the vapor/gas boundary layer alone, as it is the dominant thermal resistance and, consequently, the largest source of entropy production in many practical condensers with high fractions of noncondensable gases. Most previous studies of condensation have been restricted to a constant wall temperature, but it is shown here that for high concentrations of noncondensable gases, a varying wall temperature greatly reduces total entropy generation rate. Further, it is found that the diffusion of the condensing vapor through the vapor/noncondensable mixture boundary layer is the larger and often dominant mechanism of entropy production in such a condenser. As a result, when seeking to design a unit of desired heat transfer and condensation rates for minimum entropy generation, minimizing the variance in the driving force associated with diffusion yields a closer approximation to the minimum overall entropy generation rate than does equipartition of temperature difference. 相似文献