Effects Of Internal Irreversibility Research Articles

Efficient Power Characteristic Analysis and Multi-Objective Optimization for an Irreversible Simple Closed Gas Turbine Cycle.

On the basis of the established irreversible simple closed gas turbine cycle model, this paper optimizes cycle performance further by applying the theory of finite-time thermodynamics. Dimensionless efficient power expression of the cycle is derived. Effects of internal irreversibility (turbine and compressor efficiencies) and heat reservoir temperature ratio on dimensionless efficient power are analyzed. When total heat conductance of two heat exchangers is constant, the double maximum dimensionless efficient power of a cycle can be obtained by optimizing heat-conductance distribution and cycle pressure-ratio. Through the NSGA-II algorithm, multi-objective optimizations are performed on the irreversible closed gas turbine cycle by taking five performance indicators, dimensionless power density, dimensionless ecological function, thermal efficiency, dimensionless efficient power and dimensionless power output, as objective functions, and taking pressure ratio and heat conductance distribution as optimization variables. The Pareto frontiers with the optimal solution set are obtained. The results reflect that heat reservoir temperature ratio and compressor efficiency have greatest influences on dimensionless efficient power, and the deviation indexes obtained by TOPSIS, LINMAP and Shannon Entropy decision-making methods are 0.2921, 0.2921, 0.2284, respectively, for five-objective optimization. The deviation index obtained by Shannon Entropy decision-making method is smaller than other decision-making methods and its result is more ideal.

Exergetic optimisation of a four-temperature-level absorption heat pump based on finite-time thermodynamics

ABSTRACT A finite-time thermodynamics study and an exergetic optimisation for a four-temperature-level absorption heat pump have been performed. The maximum of the exergetic-performance coefficient (EPC) and the corresponding optimal ecological coefficient of performance (ECOP), ecological objective function (E), exergetic efficiency (ex), coefficient of performance (COP), specific entropy generation rate and heating load have been obtained. The effects of internal irreversibilities, external irreversibilities, heat leakages, ratio of the rejected heat of the absorber to the condenser and the heat-transfer coefficients of different components on the system performance have been analysed and discussed. It was noticed a greater harmful impact of heat losses in the generator-absorber assembly. Comparisons of EPC criterion with ECOP, COP, E and ex criteria have been provided. The results demonstrate that the four-temperature-level absorption heat pump working at maximum EPC conditions has a significant advantage in terms of entropy generation rate and coefficient of performance.

COMPARISON OF EFFICIENCY OF A SOLAR DRIVEN CARNOT ENGINE UNDER MAXIMUM POWER AND POWER DENSITY CONDITIONS

A comparative analysis on thermodynamic efficiency based on maximum power & power density conditions have been performed for a solar-driven Carnot heat engine with internal irreversibility. In this analysis, the heat transfer from the hot reservoir is to be in the radiation mode and the heat transfer to the cold reservoir is to be in the convection mode. The thermodynamic efficiency function, power & power density functions have been derived and maximization of the power functions have been performed for various design parameters. From the optimum conditions, the thermal efficiencies at maximum power and power densities have been obtained. The effects of internal irreversibility, extreme temperature ratios & specific engine size in area ratio between the hot & cold reservoirs as various design parameters on thermodynamic efficiencies have been investigated for both the conditions. The efficiencies have been compared with Curzon- Ahlborn & Carnot efficiencies respectively. The analysis showed that the efficiency at maximum power output is greater than the efficiency at maximum power density. And the efficiencies can be greater than the Curzon- Ahlborn`s efficiency only for low values of design parameters.

COMPARISON OF EFFICIENCY OF A SOLAR DRIVEN CARNOT ENGINE UNDER MAXIMUM POWER AND POWER DENSITY CONDITIONS

A comparative analysis on thermodynamic efficiency based on maximum power & power density conditions have been performed for a solar-driven Carnot heat engine with internal irreversibility. In this analysis, the heat transfer from the hot reservoir is to be in the radiation mode and the heat transfer to the cold reservoir is to be in the convection mode. The thermodynamic efficiency function, power & power density functions have been derived and maximization of the power functions have been performed for various design parameters. From the optimum conditions, the thermal efficiencies at maximum power and power densities have been obtained. The effects of internal irreversibility, extreme temperature ratios & specific engine size in area ratio between the hot & cold reservoirs as various design parameters on thermodynamic efficiencies have been investigated for both the conditions. The efficiencies have been compared with Curzon- Ahlborn & Carnot efficiencies respectively. The analysis showed that the efficiency at maximum power output is greater than the efficiency at maximum power density. And the efficiencies can be greater than the Curzon- Ahlborn`s efficiency only for low values of design parameters.

COMPARISON OF EFFICIENCY OF A SOLAR DRIVEN CARNOT ENGINE UNDER MAXIMUM POWER AND POWER DENSITY CONDITIONS

A comparative analysis on thermodynamic efficiency based on max imum power & power density conditions have been performed for a solar-driven Carnot heat engine with internal irreversibility. In this analysis, the heat transfer from the hot reservoir is to be in the radiation mode and the heat transfer to the cold rese rvoir is to be in the convection mode. The thermodynamic efficiency function, power & power density functions have been derived and maximization of the power functions have been performed for various design parameters. From the optimum conditions, the thermal efficiencies at maximum power and power densities have been obtained. The effects of internal irreversibility, extreme temperature ratios & specific engine size in area ratio between the hot & cold reservoirs as various design parameters on thermodynamic efficiencies have been investigated for both the conditions. The efficiencies have been compared with Curzon-Ahlborn & Carnot efficiencies respectively. The analysis showed that the efficiency at maximum power output is greater than the efficiency at maximum power density. And the efficiencies can be greater than the Curzon- Ahlborns efficiency only for low values of design parameters.

Ecological optimisation of irreversible harmonic oscillator Carnot refrigerator

A model of a quantum harmonic oscillator Carnot refrigerator with heat resistance, internal irreversibility and heat leakage is established in this paper. Based on the quantum master equation and semigroup approach, equations of some important performance parameters, such as cooling load, coefficient of performance (COP), exergy output rate, entropy generation rate and ecological function (a criterion E = A/τ−T0σ representing the optimal compromise between exergy output rate A/τ and exergy loss rate T0σ), for the irreversible quantum Carnot refrigerator are derived. The optimal ecological performance of the refrigerator at high temperature limit is analysed using numerical examples. The effects of internal irreversibility and heat leakage on the optimal ecological performance are discussed. Performance comparisons of the quantum refrigerator under maximum ecological function, maximum COP and maximum cooling load conditions are also performed. Some special cases, i.e. endoreversible, frictionless and without heat leakage, are discussed. Taking ecological optimisation criterion as the objective makes larger improvement of the COP and the reduction in the exergy loss rate at the cost of smaller cooling load drop.

Ecological Optimization of an Irreversible Otto Cycle

The optimal ecological performance of an irreversible air-standard Otto cycle with heat transfer, friction and internal irreversibility is analyzed by using finite-time thermodynamics. The relations between the ecological function and the power output as well as between the ecological function and the efficiency of the cycle are derived by detailed numerical examples. Moreover, the effects of internal irreversibility, heat transfer loss and friction loss on the cycle performance are analyzed. The results indicate that the optimization of the exergy-based ecological function not only represents a compromise between the power output and the rate of entropy generation but also represents a compromise between the power output and the thermal efficiency. This work can provide guidelines for the design of practical internal combustion engines.

Finite-time thermodynamic modelling and analysis for an irreversible Miller cycle

The performance of an air-standard Miller cycle is analysed using finite-time thermodynamics. An irreversible Miller cycle model which is more close to practice is founded. In the model, the non-linear relation between the specific heats of working fluid and its temperature, the friction loss computed according to the mean velocity of the piston, the internal irreversibility described using the compression and expansion efficiencies, and heat transfer loss are considered. The relations between the power output and the compression ratio, between the thermal efficiency and the compression ratio, and the optimal relation between power output and the efficiency of the Miller cycle are derived by detailed numerical examples. Moreover, the effects of internal irreversibility, heat transfer loss, friction loss and pressure ratio on the cycle performance are analysed. The power output versus compression ratio and the efficiency versus compression ratio curves of Atkinson and Otto cycles will be maximum and minimum envelope lines of the Miller cycle, respectively. The results obtained herein may provide guidelines for the design of practical internal combustion engines.

Ecological optimization of an irreversible quantum Carnot heat engine with spin-1/2 systems

A model of a quantum heat engine with heat resistance, internal irreversibility and heat leakage and many non-interacting spin-1/2 systems is established in this paper. The quantum heat engine cycle is composed of two isothermal processes and two irreversible adiabatic processes and is referred to as a spin quantum Carnot heat engine. Based on the quantum master equation and the semi-group approach, equations of some important performance parameters, such as power output, efficiency, entropy generation rate and ecological function (a criterion representing the optimal compromise between exergy output rate and exergy loss rate), for the irreversible spin quantum Carnot heat engine are derived. The optimal ecological performance of the heat engine in the classical limit is analyzed with numerical examples. The effects of internal irreversibility and heat leakage on ecological performance are discussed in detail.

Finite time thermodynamic modeling and analysis for an irreversible Atkinson cycle

Performance of an air-standard Atkinson cycle is analyzed by using finite-time thermodynamics. The irreversible cycle model which is more close to practice is founded. In this model, the nonlinear relation between the specific heats of 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 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 power output and the efficiency of the cycle are derived by detailed 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 guidelines for the design of practical internal combustion engines.

Ecological optimization of an irreversible harmonic oscillators Carnot heat engine

A model of an irreversible quantum Carnot heat engine with heat resistance, internal irreversibility and heat leakage and many non-interacting harmonic oscillators is established in this paper. Based on the quantum master equation and semi-group approach, equations of some important performance parameters, such as power output, efficiency, exergy loss rate and ecological function for the irreversible quantum Carnot heat engine are derived. The optimal ecological performance of the heat engine in the classical limit is analyzed with numerical examples. Effects of internal irreversibility and heat leakage on the ecological performance are discussed. A performance comparison of the quantum heat engine under maximum ecological function and maximum power conditions is also performed.

Finite-time thermodynamic modeling and analysis for an irreversible Dual cycle

The performance of an air standard Dual cycle is analyzed by using finite-time thermodynamics. An irreversible Dual cycle model which is more close to practice is established. In the model, the nonlinear relation between the specific heats of working fluid and its temperature, the frictional loss computed according to the mean velocity of the piston, the internal irreversibility described by using the compression and expansion efficiencies, and heat transfer loss are considered. The relations between the power output and the compression ratio, between the thermal efficiency and the compression ratio, and the optimal relation between power output and the efficiency of the Dual cycle are derived by detailed numerical examples. Moreover, the effects of internal irreversibility, heat transfer loss, frictional loss and pressure ratio on the cycle performance are analyzed. The power output versus compression ratio and efficiency versus compression ratio curves of the Diesel and Otto cycles are the maximum and minimum envelope lines of the performance of the Dual cycle, respectively. The results obtained herein may provide guidelines for the design of practical internal combustion engines.

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

The performance of an air-standard diesel cycle is analysed using finite-time thermodynamics. An irreversible cycle model which is closer to practice is established. In the model, the non-linear relation between the specific heats 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 optimum relation between the power output and the efficiency of the cycle are derived by detailed numerical examples. Moreover, the effects of internal irreversibility, heat transfer loss, and friction loss on the cycle performance are analysed. The results obtained herein may provide guidance for the design of practical internal combustion diesel engine.

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.

An ecological optimisation of an irreversible radiative type heat engine

SYNOPSIS Finite time thermodynamics with an ecological criterion is applied to an irreversible radiative type heat engine with losses due to heat transfer across finite temperature differences and internal irreversibilities. The ecological function, defined as the power output minus the lost power or the product of environmental temperature and the entropy production rate, is optimised with respect to the cycle temperature ratio. The maximum of the ecological function and the corresponding optimal conditions have been derived analytically. The effects of internal irreversibilities and the extreme temperature ratio on the global and optimal performances have been investigated. The obtained results are compared with those obtained by using the maximum power criterion. The results show that the design parameters at the maximum ecological function lead to more efficient heat engines and are a compromise between the power output and the thermal efficiency or entropy generation rate. Moreover, from the viewpoint of the ecological function, the con-duction allocation parameter and extreme temperature ratio should be on the lower side of unity.

Ecological coefficient of performance (ECOP) optimization for generalized irreversible Carnot heat engines

A performance analysis and optimization based on a new thermoecological criterion has been performed for a generalized irreversible Carnot heat engine with losses due to heat transfer across finite temperature differences, heat leak and internal irreversibilities. The ratio of the power produced by the engine to the loss rate of availability, which is named ecological coefficient of performance (ECOP), has been taken as an objective function for the optimization. The maximum of the ecological coefficient of performance function and the corresponding optimal conditions have been derived analytically. The effects of internal irreversibilities, heat leakage and source temperature ratio on the global and optimal performances have been investigated. The results are compared with those of the ecological performance criterion in the literature.

Optimal design of the regenerative gas turbine engine with isothermal heat addition

A regenerative gas turbine engine, with isothermal heat addition, working under the frame of a Brayton cycle has been analyzed. With the purpose of having a more efficient small-sized gas turbine engine, the optimization has been carried out numerically using the maximum power (MP) and maximum power density (MPD) method. The effects of internal irreversibilities have been considered in terms of the isentropic efficiencies of the turbine and compressor and of the regenerator efficiency. The results summarized by figures show that the regenerative gas turbine engine, with isothermal heat addition, designed according to the maximum power density condition gives the best performance and exhibits highest cycle efficiencies.

00/01077 Effects of internal irreversibility and heat leakage on the finite time thermoeconomic performance of refrigerators and heat pumps

00/01077 Effects of internal irreversibility and heat leakage on the finite time thermoeconomic performance of refrigerators and heat pumps

Effects of internal irreversibility and heat leakage on the finite time thermoeconomic performance of refrigerators and heat pumps

By using finite time thermodynamic theory, a performance analysis based on the thermoeconomic criterion has been performed for an irreversible refrigerator and a heat pump. The thermoeconomic objective function is defined as the cooling load for the refrigerator and the heating load for the heat pump per unit total cost. The optimal performances and design parameters which maximize the objective functions are investigated. The design parameters under the optimal conditions have been derived analytically, and then, the effects of internal irreversibility, heat leakage and the economical parameter on the global and optimal performances have been discussed.

Analysis of an irreversible Ericsson engine with a realistic regenerator

An internally irreversible Ericsson engine, with a realistic regenerator, has been analyzed. The study considers internal irreversibilities with the introduction of turbine and compressor thermal-efficiencies and pressure-drops present in realistic regenerators. The effects of internal irreversibilities on the power output and thermal efficiency of the cycle have been determined using the finite-time thermodynamics. Maximum power-density, rather than maximum power, was used as the criterion for optimization, with the objective of having a more efficient small-sized engine.