Lingen Chen

Effect of heat transfer law on the performance of a generalized irreversible Carnot engine

In a classical endoreversible Carnot engine model, irreversibility in the form of heat resistance between the reversible Carnot cycle and its heat reservoirs is taken into account. This paper presents a generalized irreversible Carnot engine model that incorporates several internal irreversibilities, such as heat leak, friction, turbulence etc. These added irreversibilities are characterized by a constant parameter and a constant coefficient. The relation between optimal power output and efficiency is derived based on a generalized heat transfer law . The effect of heat leakage, internal irreversibility and heat transfer law on the optimal performance of the generalized irreversible heat engine is investigated.

Heat transfer effects on the net work output and efficiency characteristics for an air-standard Otto cycle

Finite-time thermodynamic analysis of an air-standard Otto cycle is performed in this paper. The relation between net work output and efficiency of the cycle is derived. The maximum net work output and the corresponding efficiency bound of the cycle with heat transfer considerations are also found. Detailed numerical examples are given. The result obtained herein provides a guide to the performance evaluation and improvement for practical Otto engines.

Theoretical analysis of the performance of a regenerative closed Brayton cycle with internal irreversibilities

Abstract Using the technique of finite-time thermodynamic analysis, the performance of a closed Brayton cycle with regeneration is assessed. Specifically, analytical expressions for the power output and the thermal efficiency, as functions of the pressure ratio and the reservoir temperatures, are derived. The analysis also takes into account all the irreversibilities associated with finite-time heat transfer processes. The paper also shows that the power output can be maximized with judicious selection of parameters such as the heat exchanger surface areas and the heat conductances. Maximum power is obtained when the effectiveness of the hot- and cold-side heat exchangers are related in a specific manner, and this power output is a strong function of the regenerator effectiveness.

Effect of heat transfer on the performance of thermoelectric generators

The power output and efficiency expressions for thermoelectric (semiconductor) generators which is composed of multi-elements are derived with considerations of heat transfer irreversibility in the heat exchangers between the generator and the heat reservoirs. Numerical examples are provided. The effects of heat transfer and the number of elements on the performance are analyzed.

Ecological optimization for generalized irreversible Carnot engines

The optimal ecological performance of a Newtons law generalized irreversible Carnot engine with losses due to heat-resistance, heat leak and internal irreversibility is derived by taking an ecological optimization criterion as the objective. This consists of maximizing a function representing the best compromise between the power 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.

Efficiency of an Atkinson engine at maximum power density

In studies of finite-time thermodynamics, most performance analyses concern the maximum power output and the corresponding efficiency for heat engines. In this paper, instead of just maximizing power for a cycle, the power density (the ratio of the power to the maximum specific volume in the cycle) is maximized for an Atkinson engine. The results showed that the efficiency at maximum power density is always greater than that at maximum power, and the design parameters at maximum power density lead to smaller and more efficient Atkinson engines with larger pressure ratios.

Optimum performance of irreversible stirling engine with imperfect regeneration

Abstract An optimal performance analysis is performed for a Stirling engine with heat transfer and imperfect regeneration irreversibilities. The relationship between the net power output and thermal efficiency of the engine is derived. Detailed numerical examples are given. The results obtained inthis paper provide guidance to performance evaluation and design improvement for Stirling engines.

Performance of a regenerative Brayton heat engine

The performance of an endoreversible regenerative Brayton heat engine has been studied. The analysis is focused on minimizing irreversibilities associated with the hot and cold heat exchangers and regenerator.

Power density analysis and optimization of a regenerated closed variable-temperature heat reservoir Brayton cycle

In this paper, the power density, defined as the ratio of power output to the maximum specific volume in the cycle, is taken as the objective for performance analysis and optimization of an irreversible regenerated closed Brayton cycle coupled to variable-temperature heat reservoirs from the viewpoint of finite time thermodynamics (FTT) or entropy generation minimization (EGM). The analytical formulae about the relations between power density and pressure ratio are derived with the heat resistance losses in the hot- and cold-side heat exchangers and the regenerator, the irreversible compression and expansion losses in the compressor and turbine, the pressure drop losses at the heater, cooler and regenerator as well as in the piping, and the effect of the finite thermal capacity rate of the heat reservoirs. The obtained results are compared with those results obtained by using the maximum power criterion, and the advantages and disadvantages of maximum power density design are analysed. The maximum power density optimization is performed in two stages. The first is to search the optimum heat conductance distribution corresponding to the optimum power density among the hot- and cold-side heat exchangers and the regenerator for a fixed total heat exchanger inventory. The second is to search the optimum thermal capacitance rate matching corresponding to the optimum power density between the working fluid and the high-temperature heat source for a fixed ratio of the thermal capacitance rates of two heat reservoirs. The influences of some design parameters, including the effectiveness of the regenerator, the inlet temperature ratio of the heat reservoirs, the effectiveness of the heat exchangers between the working fluid and the heat reservoirs, the efficiencies of the compressor and the turbine, and the pressure recovery coefficient, on the optimum heat conductance distribution, the optimum thermal capacitance rate matching, and the maximum power density are provided by numerical examples. The power plant design with optimization leads to a smaller size including the compressor, turbine, and the hot- and cold-side heat exchangers and the regenerator. When the heat transfers between the working fluid and the heat reservoirs are carried out ideally, the pressure drop loss may be neglected, and the thermal capacity rates of the heat reservoirs are infinite, the results of this paper then replicate those obtained in recent literature.

Ecological optimization for generalized irreversible Carnot refrigerators

The optimal ecological performance of a Newtons law generalized irreversible Carnot refrigerator with the losses of heat resistance, heat leakage and internal irreversibility is derived by taking an ecological optimization criterion as the objective, which consists of maximizing a function representing the best compromise between the exergy output rate and exergy loss rate (entropy production rate) of the refrigerator. Numerical examples are given to show the effects of heat leakage and internal irreversibility on the optimal performance of generalized irreversible refrigerators.