Applicability of minimum entropy generation methodto optimizing thermodynamic cycles

Entropy generation is often used as a figure of merit in thermodynamic cycle optimizations. In this paper, it is shown that the applicability of the minimum entropy generation method to optimizing output power is conditional. The minimum entropy generation rate and the minimum entropy generation number do not correspond to the maximum output power when the total heat into the system of interest is not prescribed. For the cycles whose working medium is heated or cooled by streams with prescribed inlet temperatures and prescribed heat capacity flow rates, it is theoretically proved that both the minimum entropy generation rate and the minimum entropy generation number correspond to the maximum output power when the virtual entropy generation induced by dumping the used streams into the environment is considered. However, the minimum principle of entropy generation is not tenable in the case that the virtual entropy generation is not included, because the total heat into the system of interest is not fixed. An irreversible Carnot cycle and an irreversible Brayton cycle are analysed. The minimum entropy generation rate and the minimum entropy generation number do not correspond to the maximum output power if the heat into the system of interest is not prescribed.     

Applicability of the minimum entropy generation method for optimizing thermodynamic cycles

Entropy generation is often used as a figure of merit in thermodynamic cycle optimizations. In this paper, it is shown that the applicability of the minimum entropy generation method to optimizing output power is conditional. The minimum entropy generation rate and the minimum entropy generation number do not correspond to the maximum output power when the total heat into the system of interest is not prescribed. For the cycles whose working medium is heated or cooled by streams with prescribed inlet temperatures and prescribed heat capacity flow rates, it is theoretically proved that both the minimum entropy generation rate and the minimum entropy generation number correspond to the maximum output power when the virtual entropy generation induced by dumping the used streams into the environment is considered. However, the minimum principle of entropy generation is not tenable in the case that the virtual entropy generation is not included, because the total heat into the system of interest is not fixed. An irreversible Carnot cycle and an irreversible Brayton cycle are analysed. The minimum entropy generation rate and the minimum entropy generation number do not correspond to the maximum output power if the heat into the system of interest is not prescribed.     

双热源空调-热水器一体机冬季制热的实验研究

冬季室外温度低时,空气源热泵系统的蒸发器会结霜,使系统COP降低。所设计的空调-热水器一体机可以制冷、制热、一年四季提供生活用热水。冬季室外温度低时,用太阳能加热后的水作为热泵的低温热源,可以提高热泵的效率。分别用空气源蒸发器和水源蒸发器独立工作使系统给房间制热,实验结果发现水源蒸发器工作时系统的COP平均值为3.56,空气源蒸发器工作时的COP平均值为2.51。     

System analysis of dual purpose cycle for desalinated water and power production

A comprehensive study of the characteristics of a solar powered dual purpose plant for simultaneous production of fresh water and electric power was carried out. The dual purpose (hybrid) cycle consisted of an open water cycle and a Freon-12 closed power cycle interlinked in the heater and condenser of the power cycle. The hybrid system operating conditions and criteria for adjusting the ratio of fresh water production to power generation was described and their effects on the total system were evaluated.The results indicated an increase of both desalinated water and output work with the increase of hot water pressure and temperature. The increase of maximum pressure in the power cycle decreases the amount of desalination and increases the output work up to a certain limit. The inlet water temperature has no effect on the rate of fresh water production, but its increase results in a decrease in the output work. The system study showed the advantages of the proposed cycle as compared to other single purpose plants for desalination or power generation.     

NATURAL CONVECTION FROM DISCRETE HEAT SOURCES IN A VERTICALLY VENTED RECTANGULAR ENCLOSURE

Natural convection in a discretely heated, vertically vented enclosure has been investigated experimentally. A vertically vented enclosure is one whose top and bottom boundaries are partially open, allowing ambient fluid to be drawn in by buoyancy. Mach-Zehnder interferometry was used to visualize the temperature field within the enclosure and to determine the local and average heat transfer characteristics of the discrete heat sources. A smoke-generation technique was used to visualize the flow structure. The experimental parameters investigated include Grashof number, vent gap width, and heater locations for a dual heater configuration and a single-enclosure aspect ratio. The vent gap width was varied between a fully open condition (discretely healed parallel plates) and nearly closed. The Mach-Zehnder interferograms and local heat transfer results suggest a complex buoyancy-driven flow field that depends intimately on the rent gap width. For some geometric configurations, the heat transfer coefficient was found to be nearly uniform across the length of the heater, while for others traditional boundary-layer-type heat transfer characteristics were found. In general, the heat transfer coefficient increased with increased vent gap spacing. However, a maximum in average Nusselt number was observed for a dimensionless gap spacing of G/W – 0.67 for one of the two dual-heater placement configurations studied. The heat transfer characteristics were compared to those of a single isolated plate and unobstructed parallel-plate channels.     

Analyses of the endoreversible Carnot cycle with entropy theory and entransy theory

The endoreversible Carnot cycle is analyzed based on the concepts of entropy generation, entropy generation number, entransy loss, and entransy loss coefficient. The relationships of the cycle output power and heat-work conversion efficiency with these parameters are discussed. For the numerical examples discussed, the preconditions of the application for these concepts are derived. When the inlet temperatures and heat capacity flow rates of hot streams and environment temperature are prescribed, the results show that the concepts of entropy generation and entransy loss are applicable. However, in the presence of various inlet temperatures of streams, larger entransy loss rate still leads to larger output power, while smaller entropy generation rate does not. When the heat capacity flow rates of hot streams are various, neither larger entransy loss rate nor smaller entropy generation rate always leads to larger output power. Larger entransy loss coefficient always leads to larger heat-work conversion efficiency for the cases discussed, while smaller entropy generation number does not always.     

Thermoeconomic analysis of an irreversible Stirling heat pump cycle

In this paper an analysis of the Stirling cycle in thermoeconomic terms is developed using the entropy generation. In the thermoeconomic optimization of an irreversible Stirling heat pump cycle the F function has been introduced to evaluate the optimum for the higher and lower sources temperature ratio in the cycle: this ratio represents the value which optimizes the cycle itself. The variation of the function F is proportional to the variation of the entropy generation, the maxima and minima of F has been evaluated in a previous paper without giving the physical foundation of the method. We investigate the groundwork of this approach: to study the upper and lower limits of F function allows to determine the cycle stability and the optimization conditions. The optimization consists in the best COP at the least cost. The principle of maximum variation for the entropy generation becomes the analytic foundation of the optimization method in the thermoeconomic analysis for an irreversible Stirling heat pump cycle.     

Analyses of an air conditioning system with entropy generation minimization and entransy theory

In this paper,based on the generalized heat transfer law,an air conditioning system is analyzed with the entropy generation minimization and the entransy theory.Taking the coefficient of performance(denoted as COP) and heat flow rate Q~(out) which is released into the room as the optimization objectives,we discuss the applicabilities of the entropy generation minimization and entransy theory to the optimizations.Five numerical cases are presented.Combining the numerical results and theoretical analyses,we can conclude that the optimization applicabilities of the two theories are conditional.If Q~(out) is the optimization objective,larger entransy increase rate always leads to larger Q~(out),while smaller entropy generation rate does not.If we take COP as the optimization objective,neither the entropy generation minimization nor the concept of entransy increase is always applicable.Furthermore,we find that the concept of entransy dissipation is not applicable for the discussed cases.     

乳品行业低温余热回收系统性能分析

针对乳品行业排风风量大但品位低的特点,本文提出利用排风余热代替蒸汽来预热干燥空气的余热回收系统.在构建预热器、回热器和主加热器等部件热量流模型的基础上,建立了不包含中间节点参数的系统整体热量流模型.结合某奶粉厂运行数据对模型进行求解,结果表明,该系统将排风温度由90℃降低到42.58℃.在给定热负荷下,通过匹配中间回路...     

An open cycle absorption heat pump

The absorption cycle can be also of the open type. This concept has been utilized in developing solar cooling absorption systems. Another possibility not yet investigated is an open cycle absorption heat pump. The system rests upon the utilization of a packed tower operating with liquid desiccants. The tower dehumidifies both the exhausted air of heating plant and the exhaust of a natural gas boiler. A conventional heater heats up the regenerator of the sorbent and the inlet air. Simulations give a PER of the system (the open cycle heat pump) higher than 1.3 with respect to the Gross Calorific Value of natural gas. This value is difficult to obtain with the more complex closed cycle absorption or vapour compression cycle motor driven heat pump.     

换热器的效能与熵产分析

本文给出了以进口冷热流体温度、热容量流比和传热单元数表示的顺流、逆流及两边混合的叉流换热器熵产数的分析表达式，研究了这三类换热器熵产数随热容量流比及传热单元数变化的规律，给出了由于不平衡流动和传热面积有限产生的熵产数的表达式。分析表明，在进、出口温度均匀条件下换热器效能与熵产数之间存在一一对应的关系，效能与熵产数一样，所表征的是换热器热力学第二定律的完善程度。     

Transient Heat Transfer for Forced Convection Flow of Helium Gas Over a Horizontal Plate

Transient heat transfer coefficients for helium gas flowing over a horizontal plate (ribbon) were measured under wide experimental conditions. The platinum plate with a thickness of 0.1 mm was used as the test heater and heated by electric current. The heat generation rate was exponentially increased with a function of Q₀ exp(t/τ). The gas flow velocities ranged from 4 to 10 m/s, the gas temperatures ranged from 290 to 353 K, and the periods of heat generation rate, τ, ranged from 50 ms to 17 s. The surface superheat and heat flux increase exponentially as the heat generation rate increases with the exponential function. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period τ longer than about 1 s, and it becomes higher for the period shorter than around 1 s. Empirical correlation for transient heat transfer was also obtained based on the experimental data.     

Optimization of fin geometry in heat convection with entransy theory

The entransy theory developed in recent years is used to optimize the aspect ratio of a plate fin in heat convection.Based on a two-dimensional model,the theoretical analysis shows that the minimum thermal resistance defined with the concept of entransy dissipation corresponds to the maximum heat transfer rate when the temperature of the heating surface is fixed.On the other hand,when the heat flux of the heating surface is fixed,the minimum thermal resistance corresponds to the minimum average temperature of the heating surface.The entropy optimization is also given for the heat transfer processes.It is observed that the minimum entropy generation,the minimum entropy generation number,and the minimum revised entropy generation number do not always correspond to the best heat transfer performance.In addition,the influence factors on the optimized aspect ratio of the plate fin are also discussed.The optimized ratio decreases with the enhancement of heat convection,while it increases with fin thermal conductivity increasing.     

(火积)理论在热功转换过程中的应用探讨

分析和讨论了(火积)理论在热功转换过程的应用及其局限性.对Carnot循环的分析表明,Carnot循环中系统的(火积)是平衡的,但(火积)和熵之间不存在dG=T2dS这样的联系.对于一般热力学过程,分析表明,在热量传递到内可逆循环中间接对外做功时,现有的(火积)理论可用于系统的分析.讨论了热功转换过程分析中(火积)理论与熵理论的不同.分析表明,两个理论的分析角度及优化输出功的前提条件是不同的.熵产从可用能损失的角度分析热功转换过程,而(火积)理论则从热量势能消耗的角度.当输入系统的可用能给定或者输入系统的热量及热量进、出系统的热力学力给定时,熵产最小化对应于输出功最大;对于(火积)理论,则当输入系统的热量及热量进、出系统的温度给定时,最大(火积)损失对应于最大输出功.同时,它们各自均有局限性.当相应的前提条件不满足时,最大(火积)损失或最小熵产可能不与最大输出功相对应.     

吸收式制冷循环系统的热力学分析

从热力学观点讨论了工作温度对于制冷循环系统性能的影响。分析了与循环时间有关的温度效率和熵产数。对于一个相对较短的循环时间,吸收/解吸收热量转换器的温度效率在200秒后可以达到92%。熵产数Ns由在一个循环系统内生成的不可逆性参数和热量转换器流体有效性参数之间的比率决定。结果显示,在使用一个30℃冷源的情况下高级吸收式循环系统的熵产数Ns在热水温度是45℃至55℃之间时是相对较小的,而对于传统循环,在使用相同冷源温度的情况下,热水温度在65℃到75℃之间时,Ns是相对较小的。     

An Experimental Study of the Two-Phase Flow Adjacent to a Wire in Nucleate Boiling Under an Electric Field

An experimental study of the two-phase flow near a heated wire in nucleate pool boiling is presented. A nonuniform electric field of cylindrical geometry was imposed to the heater. The two-phase flow parameters were measured using specifically developed local phase-detection optical probes. The study includes experiments varying the vertical distance from the heater (z) and measurements of the void fraction and the impact rate in a plane perpendicular to the heater (y-z). The results show that the void fraction decreases with applied voltage and with z. The dependence of the void fraction and the impact rate with heat flux is complex, with a strong decrement in the impact rate and void fraction near the critical heat flux when voltage is applied. The maximum void fraction and impact rate move from the vertical centerline (z) to a position 3 heater radii away.     

Lattice Boltzmann simulation for hydrothermal analysis of free convection within dumbbell-shaped heat exchanger

The lattice Boltzmann simulation of nanofluid flow and heat transfer during natural convection within a dumbbell-shaped heat exchanger is carried out. The heat exchanger is filled with CuO–water. The KKL model is employed to predict the thermo-physical properties of nanofluid. In order to perform a comprehensive hydrothermal investigation, different post-processing approaches such as heatline visualization, total entropy generation, local entropy generation based on local fluid friction irreversibility and heat transfer irreversibility, average and local Nusselt variation are employed. In the present investigation, it is tried to present the impact of different influential parameters like Rayleigh number, solid volume fraction of nanofluid and thermal arrangement of internal fins-bodies on the fluid flow, heat transfer rate and entropy generation.     

New Pool Boiling Heat Transfer in the Presence of Low-Frequency Vibrations Into a Vertical Cylindrical Heat Source

A vertical cylinder was applied as a heat source into a water pool; the vibrations were imposed into the heater with different heat fluxes, and the frequencies were adjusted at 10, 15, 20, and 25 Hz. An imaging system was employed to observe the produced bubbles around the cylindrical heat source. The results showed that the boiling heat transfer was enhanced under the vibrations with a shorter transient process, and the wall temperature also decreased. The best enhancement ratio was achieved at the frequency of 25 Hz and a heat flux value of 30 kW/m² as a consequence of imposed vibrations.     

新型多能源互补系统的热力经济性分析

本文对SOLRGT系统（一种新型的中低温太阳能与化石燃料互补的动力系统）进行了热力经济性分析。该系统基于化学回热循环,通过中低温太阳能品位的间接提升、与先进燃气轮机的集成,实现中低温太阳能的高效、低成本热功转换以及和化石燃料的综合梯级利用。本文将其与同等输入下的两种单输入发电系统进行对比分析,研究表明：基本工况下,SO...     

Entropy resistance analyses of a two-stream parallel flow heat exchanger with viscous heating

Heat exchangers are widely used in industry, and analyses and optimizations of the performance of heat exchangers are important topics. In this paper, we define the concept of entropy resistance based on the entropy generation analyses of a one-dimensional heat transfer process. With this concept, a two-stream parallel flow heat exchanger with viscous heating is analyzed and discussed. It is found that the minimization of entropy resistance always leads to the maximum heat transfer rate for the discussed two-stream parallel flow heat exchanger, while the minimizations of entropy generation rate, entropy generation numbers, and revised entropy generation number do not always.