Performance of Miller cycle with heat transfer, friction and variable specific heats of working fluid

Autor/autori: Lingen Chen (P. R. China), Yanlin Ge (P. R. China) and Fengrui Sun (P. R. China)

Rezumat: In lucrarea este prezentata analiza termodinamica in timp finit a performantelor unui ciclu Miller standard cu aer cu pierderi datorate transferului de caldura, frecarii si variatiei caldurii specifice a fluidului de lucru. Relatiile dintre puterea mecanica produsa si raportul de comprimare si randamentul termic in functie de raportul de comprimare al ciclului sunt obtinute prin exemple numerice detaliate. De asemenea, in lucrare este analizata influenta variatiei caldurii specifice a fluidului de lucru si a termenului ce tine seama de pierderile prin frecare asupra performantei ciclului ireversibil. Se considera o variatie liniara a caldurii specifice numai cu temperatura de forma Cpm= ap+k1T si Cpm= bv+k1T. Rezultatele arata ca puterea maxima produsa si domeniul de valori al raportului de comprimare, pentru care ciclul termodinamic poate lucra in conditii normale, vor creste odata cu cresterea lui ap si bv; randamentul termic al ciclului va scadea odata cu cresterea lui ap si bv; când se tine cont de variatia caldurilor specifice cu temperatura, puterea mecanica produsa si randamentul termic scad, domeniul de valori al raportului de comprimare, pentru care ciclul termodinamic poate lucra in conditii normale, va creste; puterea mecanica maxima produsa, randamentul termic maxim ca si raportul de comprimare si randamentul corespunzator puterii maxime vor scade cu cresterea frecarii. Sunt prezentate graficele de variatie ale puterii mecanice produse si ale randamentului termic al cilului in functie de raportul de comprimare pentru ciclurile Atkinson si Otto, precum si locurile geometrice ale valorilor de maxim si minim corespunzator ciclului Miller.

Cuvinte cheie: termodinamica in timp finit; ciclul Miller; rezistenta termica; optimizarea performantei; calduri specifice variabile ale fluidului de lucru.

Abstract: Performance of an air-standard Miller cycle with heat transfer loss, friction-like term loss and variable specific heats of working fluid is analyzed by using finite-time thermodynamics. The relations between the power output and the compression ratio, as well as between the thermal efficiency and the compression ratio of the cycle are derived by detailed numerical examples. Moreover, the effects of variable specific heats of working fluid and the friction-like term loss on the irreversible cycle performance are analyzed. It is supposed that the specific heats of the working fluid are linearly dependent on temperature alone, that is, and . The results show that the maximum power output and the range of compression ratio in which the cycle can work normally will increase with the increases of and ; the efficiency of the cycle will decrease with the increases of and ; when the variation degree of the specific heat with temperature is acutely, the power output and the efficiency of the cycle will decrease, the range of compression ratio in which the cycle can work normally will increase; and the maximum power output, the maximum efficiency as well as the compression ratio and efficiency at the maximum power output point will decrease with the increase of friction. The power output versus compression ratio curves and the efficiency versus compression ratio curves of Atkinson and Otto cycle are the maximum and minimum envelope lines of Miller cycle, respectively.

Keywords: finite-time thermodynamics; Miller cycle; heat resistance; friction loss; performance optimization; variable specific heats of working fluid.

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