تجزیه و تحلیل عملکرد زیست محیطی یک احیا کننده برایتون موتور حرارتی

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.

In recent years, many optimization studies for heat engines based on the endoreversible and irreversible models have been carried out by considering finite-time and finite-size constraints under various heat-transfer modes, especially linear and nonlinear ones. The reader interested in these optimization studies could refer to literature surveys written by Bejan [1], Chen et al. [2] and Durmayaz et al. [3]. Usually in these studies, the power and the thermal efficiency were chosen according to the optimization criteria and the design parameters at maximum power (MP) and/or at maximum thermal efficiency were investigated. In the literature, many optimization studies have been completed for non-regenerative Brayton heat-engines. Most of these previous works have concentrated on power and power-density optimizations [4], [5], [6], [7], [8], [9], [10], [11] and [12]. Hernandez et al. [13] analysed the effect of a regenerative heat-exchanger in a gas turbine by using a regenerative Brayton-cycle model. In the model, the compressor and turbine isentropic efficiencies and all global irreversibilities in the heat exchanger were taken into account. They presented results for the behaviour at maximum efficiency, efficiency at maximum power, maximum power and power at maximum thermal efficiency in terms of the isentropic and regenerator efficiencies. Medina et al. [14] applied the power-density maximization method to the regenerative gas-turbine. They showed that the efficiency at maximum power-density is greater or smaller than the efficiency at maximum power depending on the value taken for the regenerator effectiveness, εR. There exists a critical point (εR,cr) which marks the difference between ηmp and ηmpd. In all cases, this critical value is about 0.3: ηmpd is greater than ηmp when εR is below εR,cr and it is smaller when εR exceeds εR,cr. Roco et al. [15] have also analysed the optimum performance of regenerative Brayton-cycle. Their model includes external and internal irreversibilities. They presented numerical results for the maximum efficiency, maximum-power output, efficiency at maximum-power output, power at maximum efficiency in terms of the parameters considering each type of irreversibility. Cheng and Chen [16] performed a power optimization for an endoreversible regenerative Brayton-cycle. They analysed the effect of regeneration on the thermal efficiency and power of endoreversible Brayton-cycle. They found that the maximum power and the corresponding thermal efficiency are decreased by the use of regenerators. Sahin et al. [17] studied the irreversible regenerative reheating Brayton heat-engine under maximum power density and maximum power conditions. They derived analytically the design parameters under the optimal conditions and discussed their effects on the engine's performance. Angulo-Brown [18] proposed an ecological optimization criterion for finite-time Carnot heat engines as , where Ẇ is the power output, is the entropy-generation rate and TL is the cold-reservoir temperature. He found that the thermal efficiency for the maximum ecological function is almost the average of the Carnot and the Curzon and Ahlborn efficiencies [10]. Yan [19] proposed that it is more reasonable to use when the cold-reservoir temperature is not equal to the environment temperature T0. Cheng and Chen [20] investigated the ecological optimization of an irreversible Carnot heat-engine. They found that the thermal capacity of the cold external-fluid should be larger than that of the hot external-fluid and the heat conductance of the hot-end heat-exchanger should be smaller than that of the cold-end heat-exchanger. Cheng and Chen [21] studied the ecological optimization of an endoreversible Brayton cycle, and found that the ratio of the ecologically optimum power to maximum power is independent of the number of transfer units of the hot-side and the cold-side heat-exchangers. The ecological optimization of the irreversible Brayton-cycle has been carried out by Cheng and Chen [22]. They optimized the ecological performance with respect to the adiabatic-temperature ratio and the thermal-conductance ratio and the optimal values of these parameters have been presented. They found that the thermal conductance of the cold-side heat-exchanger should be larger than that of the hot-side heat-exchanger to obtain a higher ecological performance. A performance analysis on the regenerative Brayton heat engines using ecological optimization technique does not appear to have been published. Therefore, we have performed an ecological optimization on the endoreversible regenerative Brayton heat-engine model. The effects of the regenerator effectiveness on the global and optimal performances have been investigated and the results obtained are compared with those for the maximum power criterion.

We have presented a thermo-ecological performance analysis in order to determine the optimal design conditions for an endoreversible regenerative Brayton heat-engine. The optimal values of the state-point temperatures, power output, entropy-generation rate and thermal efficiency at which the ecological objective-function attains its maximum are derived analytically. The effect of the regenerator effectiveness on the optimal performance for the maximum ecological function conditions is discussed in detail and the results obtained are compared with those for maximum power conditions. It is shown that a design based on the maximum ecological objective-function has the advantage of a lower entropy-generation rate and a higher thermal efficiency although it suffers the disadvantage of a power loss compared with that for a design at maximum power conditions. Moreover, the entropy-generation rate and thermal-efficiency advantages of the heat engine working under MEF conditions increase as the regenerator effectiveness increases.