CARNOT CYCLE WITH EXTERNAL AND INTERNAL IRREVERSIBILITIES ANALYZED IN THERMODYNAMICS WITH FINITE SPEED WITH THE DIRECT METHOD

Autor/autori: Stoian PETRESCU, Charles HARMAN (SUA), Adrian BEJAN (SUA), Monica COSTEA, Catalina DOBRE

Rezumat: Se analizeaza un ciclu Carnot direct cu ireversibilitati externe (datorate diferentelor de temperaturi la surse), cat si cu ireversibilitati interne (datorate vitezei finite). Ciclul ireversibil Carnot este reprezentat intr-o diagrama T-S originala, unde se evidentiaza lucrul mecanic pierdut (exergia) din cauza diverselor ireversibilitati, generate in timpul functionarii masinii cu viteza finita. Se dezvolta o metoda de calcul analitic (numita Metoda Directa din Termodinamica cu Viteza Finita) cu ajutorul careia se evidentiaza efectul pe care il are viteza pistonului in cadrul ireversibilitatilor interne, asupra performantelor masinii (randament si putere, exprimate analitic ca functie de viteza). Se prezinta un exemplu de rezultate pentru o serie de valori ale vitezei unui ciclu Carnot, caracterizat de o temperatura ridicata, la sursa calda (2000 K). Se determina expresia analitica a temperaturii optime necesare obtinerii unui randament maxim al ciclului Carnot, in functie de viteza pistonului. Exemplul este dezvoltat si in scopul determinarii unei temperaturi optime, necesare obtinerii unei puteri maxime a ciclului Carnot, intr-un anumit interval de valori ale vitezei pistonului.

Cuvinte cheie: ciclu Carnot ireversibil, ireversibilitati interne si externe, Termodicnamica cu Viteza Finita, optimizarea temperaturii, optimizarea vitezei, generarea entropiei, randamentul ireversibilitatii, Metoda Directa.


Abstract: A Carnot engine operating on a closed cycle and having both external and internal irreversibilities is analyzed. The internal irreversibilities are caused by losses generate by finite piston speed and the external irreversibilities are caused by heat transfer through a temperature difference. The irreversible Carnot cycle is displayed on a original T-S property coordinates in a manner that accurately illustrates the lost work (Exergy losses) due to the irreversibilities (internal and external). A method for calculating the effect of the piston speed on the internal irreversibilities of Carnot cycle is developed and an example of the results is shown for a range of values of cycle high temperature (2000 K). Using the results of this example, the optimal Carnot engine efficiency is determined as a function of piston speed. The example is extended to include the determination of the optimal system temperature for Maximum Carnot engine power over a range of piston speeds.

Keywords: irreversible Carnot cycle, internal and external irreversibilities, Finite Speed Thermodynamics, temperature optimization, speed optimization, entropy generation, Second Law Efficiency, Direct Method

 

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