内可逆矩形循环有限时间热力学分析

应用有限时间热力学理论分析了空气标准矩形循环的性能,导出存在传热损失的空气标准矩形循环的功与膨胀比、效率与膨胀比以及功和效率的特性关系,同时分析了传热损失及循环各参数对循环性能的影响。     

Ecological performance analysis of an endoreversible regenerative Brayton heat-engine

A performance analysis based on an ecological performance criterion has been performed for an endoreversible regenerative Brayton heat-engine. In the model, the heat-transfer irreversibilities were considered and other irreversibilities were neglected. The ecologic objective-function, defined as the power output minus the loss rate of availability is taken as the optimization criterion. The optimum performance parameters that maximize the ecological objective function are investigated. The effect of the regenerator effectiveness on the global and optimal performance have been discussed. The results obtained are compared with those of the maximum-power criterion.     

Thermo-economic optimization of an endoreversible four-heat-reservoir absorption-refrigerator

Based on an endoreversible four-heat-reservoir absorption-refrigeration-cycle model, the optimal thermo-economic performance of an absorption-refrigerator is analyzed and optimized assuming a linear (Newtonian) heat-transfer law applies. The optimal relation between the thermo-economic criterion and the coefficient of performance (COP), the maximum thermo-economic criterion, and the COP and specific cooling load for the maximum thermo-economic criterion of the cycle are derived using finite-time thermodynamics. Moreover, the effects of the cycle parameters on the thermo-economic performance of the cycle are studied by numerical examples.     

Frequency-dependent performance of an endoreversible Carnot engine with a linear phenomenological heat-transfer law

On the basis of an endoreversible Carnot heat-engine model, the frequency-dependent performance of the engine is analyzed when the heat transfers between the working fluid and the heat reservoirs obey a linear phenomenological heat-transfer law, i.e., Q ∝ (ΔT⁻¹). The relations among average power-output, efficiency, available temperature-drop, cycle frequency and ratio of the heat-transfer times are derived. They are different from those obtained with Newton’s law. The results can provide guidance for selecting the appropriate working points of heat engines.     

Exergy optimization for an endoreversible cogeneration cycle

Exergy optimization has been carried out for an endoreversible cogeneration cycle using finite-time thermodynamics. The optimum values of the design parameters of the cogeneration cycle at maximum exergy output were determined. Our model is more general than the endoreversible power cycle found in the literature. The effects of design parameters on exergetic performance are investigated and the results discussed.     

Exergy-based ecological optimization of linear phenomenological heat-transfer law irreversible Carnot-engines

The optimal exergy-based ecological performance of a generalized irreversible Carnot-engine with losses due to heat-resistance, heat leakage and internal irreversibility, in which the heat-transfer between the working fluid and the heat reservoirs obeys a linear phenomenological heat-transfer law, is derived by taking an exergy-based ecological optimization criterion as the objective. This consists of maximizing a function representing the best compromise between the power output and entropy-production rate of the heat engine. A numerical example is given to show the effects of heat leakage and internal irreversibility on the optimal performance of the generalized irreversible heat-engine. The results provide theoretical guidance for the design of practical engines.     

Power optimization of an endoreversible Brayton gas heat engine

The power output of a simple endoreversible Brayton gas heat engine is analyzed and optimized. The endoreversible engine is defined as a power cycle in which the two processes of heat transfer from and to the surrounding heat reservoirs are the only irreversible processes in the Brayton cycle. A mathematical expression is derived for the power output of the irreversible heat engine. The power optimization provides the basis for designing a real gas heat engine and for a performance comparison with existing Brayton power plants.     

Power optimization of an endoreversible closed intercooled regenerated Brayton cycle

In this paper, power is optimized for an endoreversible closed intercooled regenerated Brayton cycle coupled to constant-temperature heat reservoirs in the viewpoint of finite-time thermodynamics (FTT) or entropy generation minimization (EGM). The effects of some design parameters, including the cycle heat reservoir temperature ratio and total heat exchanger inventory, on the maximum power and the corresponding efficiency are analyzed by numerical examples. The analysis shows that the cycle dimensionless power can be optimized by searching the optimum heat conductance distributions among the hot- and cold-side heat exchangers, the regenerator and the intercooler for fixed total heat exchanger inventory, and by searching the optimum intercooling pressure ratio. When the optimization is performed with respect to the total pressure ratio of the cycle, the maximum dimensionless power can be maximized again.     

Power,power density and efficiency optimization of an endoreversible Braysson cycle

The performance optimization of an endoreversible Braysson cycle with heat resistance losses in the hot- and cold-side heat exchangers is performed by using finite-time thermodynamics. The relations between the power output and the working fluid temperature ratio, between the power density and the working fluid temperature ratio, as well as between the efficiency and the working fluid temperature ratio of the cycle coupled to constant-temperature heat reservoirs are derived. Moreover, the optimum heat conductance distributions corresponding to the optimum dimensionless power output, the optimum dimensionless power density and the optimum thermal efficiency of the cycle, and the optimum working fluid temperature ratios corresponding to the optimum dimensionless power output and the optimum dimensionless power density are provided. The effects of various design parameters on those optimum values are studied by detailed numerical examples.     

内可逆空气制冷机性能的生态学优化

基于[火用]分析的观点，运用有限时间热力学方法对内可逆空气制冷机进行生态学优化，导出了换热器热导最优分配时的最佳制冷功率、熵产率以及生态学（E）目标函数的解析式，进一步求得最大E目标值时的工质等熵温比（压比）界限及相应的制冷系数、制冷功率和熵产率；采用数值计算分析了热源温比、换热器总热导以及高温热源温度和环境温度之比对该制冷机生态学最优性能的影响。结果表明：生态学目标函数不仅反映了[火用]输出率和熵产率之间的最佳折衷，而且也反映了制冷功率和制冷系数之间的最佳折衷。     

Potential of regenerative gas-turbine systems with high fogging compression

The study of evaporatively-cooled cycles is of interest because of the prospect of enhanced efficiencies and conceptual simplicity that can lead to low capital costs. This work focuses on a cycle that relies on continuous cooling of the air under compression, followed by recuperation of residual exhaust heat, combustion and expansion. Ideal gases are modeled, with realistic values of efficiencies, air-to-fuel ratios and turbine-inlet temperatures. As the amount of water injected in the compressor increases, the efficiency of the cycle peaks at progressively higher pressure-ratios. The pressure ratio and the recuperator effectiveness are important parameters for cycle efficiency. Compared to a dry cycle with no recuperation with a pressure ratio of 25, the efficiency can increase from 45% to 51.5% and the specific work from 410 kJ/kg to 680 kJ/kg when compression cooling and recuperation are implemented.     

Ecological performance analysis of an endoreversible modified Brayton cycle

An endoreversible closed modified simple Brayton cycle model with isothermal heat addition coupled to variable-temperature heat reservoirs is established using finite-time thermodynamics. Analytical expressions of dimensionless power output, thermal efficiency, dimensionless entropy generation rate and dimensionless ecological function are derived. Influences of cycle thermodynamic parameters on ecological performance and optimal compressor pressure ratio, optimal power output, optimal cycle thermal efficiency and optimal entropy generation rate corresponding to maximum ecological function are obtained and compared with those corresponding to maximum power output. The results show that cycle thermal efficiency improvement and entropy generation rate reduction are obtained at the expense of higher compressor pressure ratio and a little sacrifice of power output at maximum ecological function. The compromises between power output and entropy generation rate and between power output and cycle thermal efficiency, respectively, are achieved.     

Power optimization of an endoreversible closed intercooled regenerated Brayton-cycle coupled to variable-temperature heat-reservoirs

In this paper, in the viewpoint of finite-time thermodynamics and entropy-generation minimization are employed. The analytical formulae relating the power and pressure-ratio are derived assuming heat-resistance losses in the four heat-exchangers (hot- and cold-side heat exchangers, the intercooler and the regenerator), and the effect of the finite thermal-capacity rate of the heat reservoirs. The power optimization is performed by searching the optimum heat-conductance distributions among the four heat-exchangers for a fixed total heat-exchanger inventory, and by searching for the optimum intercooling pressure-ratio. When the optimization is performed with respect to the total pressure-ratio of the cycle, the maximum power is maximized twice and a ‘double-maximum’ power is obtained. When the optimization is performed with respect to the thermal capacitance rate ratio between the working fluid and the heat reservoir, the double-maximum power is maximized again and a thrice-maximum power is obtained. The effects of the heat reservoir’s inlet-temperature ratio and the total heat-exchanger inventory on the optimal performance of the cycle are analyzed by numerical examples.     

Power optimization of open-cycle regenerator gas-turbine power-plants

A performance analysis and optimization of a open-cycle regenerator gas-turbine power-plant is performed in this paper. The analytical formulae about the relation between power output and cycle overall pressure-ratio are derived taking into account the eight pressure-drop losses in the intake, compression, regeneration, combustion, expansion and discharge processes and flow process in the piping, the heat-transfer loss to the ambient environment, the irreversible compression and expansion losses in the compressor and the turbine, and the irreversible combustion loss in the combustion chamber. The power output is optimized by adjusting the mass-flow rate and the distribution of pressure losses along the flow path. Also, it is shown that the power output has a maximum with respect to the fuel-flow rate or any of the overall pressure-drops and the maximized power output has an additional maximum with respect to the overall pressure-ratio. The numerical example shows the effects of design parameters on the power output and heat-conversion efficiency.     

Finite-time thermodynamic modelling and analysis of an irreversible Otto-cycle

The performance of an air standard Otto-cycle is analyzed using finite-time thermodynamics. In the irreversible cycle model, the non-linear relation between the specific heat of the working fluid and its temperature, the friction loss computed according to the mean velocity of the piston, the internal irreversibility described by using the compression and expansion efficiencies, and the heat-transfer loss are considered. The relations between the power output and the compression ratio, between the thermal efficiency and the compression ratio, as well as the optimal relation between the power output and the efficiency of the cycle are indicated by numerical examples. Moreover, the effects of internal irreversibility, heat-transfer loss and friction loss on the cycle performance are analyzed. The results obtained in this paper may provide guidance for the design of practical internal-combustion engines.     

A dimensionless numerical analysis for the optimization of an active magnetic regenerative refrigerant cycle

Refrigeration by an active magnetic regenerative system (AMR) is potentially more attractive, as compared to conventional techniques. Indeed, devices based upon an AMR cycle are more efficient, compact, environment‐friendly and can operate over a broad range of temperatures. In this paper, attention is focused to the near room‐temperature range. On the other hand, however, the AMR cycle poses a variety of complex problems, in terms of fluid dynamics, heat transfer and magnetic field. In order to identify the optimal operational parameters, the design and optimization of a magnetic refrigeration system can be supported by modelling. In this paper, a dimensionless approach was adopted to simulate an AMR cycle following a Brayton regenerative cycle. In the simulation, the temperature range that has been explored is 260 – 280 K and 275 – 295 K. The heat transfer mediums are, respectively, water–glycol mixture (50% by weight) and pure water. The Gd_0.8Dy_0.2 alloy and pure Gd have been chosen as constituent material for the regenerator of the AMR cycle. With this model, the influence of the different parameters on cycle efficiency has been analysed. In particular, the study has been focused on the influence of the secondary fluid properties, magnetic material particle diameter, fluid blow time, secondary fluid mass flow rate, regenerator geometry and effect of axial thermal conduction. The model enables to find optimal dimensionless numbers in order to maximize the cycle performances. The results can be extended to widely different situations and therefore can be easily employed for the design and the optimization of new experimental prototypes. Copyright © 2012 John Wiley & Sons, Ltd.     

Analysis of an endoreversible stirling cooler

An internally reversible and externally irreversible Stirling refrigeration cycle which achieves cryogenic temperatures in a single stage is presented in this paper. The equations relating the maximum cooling load, working fluid temperatures and power input of the cryocooler are found. These relationships provide a base for practicing engineers to design a new cryocooler.     

Maximum useful energy-rate analysis of an endoreversible Joule–Brayton cogeneration cycle

Endoreversible Joule–Brayton cogeneration cycle has been optimized based on a new criterion, total useful energy-rate (including power output and useful heat output), and the efficiency at maximum total useful energy rate has also been determined. The effects of various cycle parameters on the maximum dimensionless total useful-energy rate and the efficiency at maximum total useful-energy rate have been assessed. Variations of dimensionless total useful-energy rate with respect to efficiency have also been analyzed. The reversible Joule–Brayton power cycle is a special case of the analyzed cycle.     

Finite-time optimization of a solar-driven heat engine

Maximum power and efficiency at the maximum power point of an internally and externally irreversible finite-size solar thermal power plant heat engine are treated. It was found that the thermal efficiency depends on the internal irreversibility resulting from the working fluid for a given value of reservoir temperatures ratio. It was also concluded that the heat-exchangers optimum size ratio must be less than one for maximum power output.