Jjmjjgmhggjum Essay

1389 words - 6 pages

CICLO DI CARNOTIl ciclo di Carnot è un ciclo termico ideale e reversibile che ha due sorgenti a temperature diverse (T1 e T2).Il ciclo prevede scambi di calore con trasformazioni isoterme ed adiabatiche che consentono di tornare alle condizioni iniziali e generare lavoro meccanico dal calore.MACCHINE TERMICHESe in un motore s'incrementa il rapporto di compressione, ci si accorge che a parità di combustibile bruciato, si avrà un incremento dell'aria del ciclo termico, un incremento dell'energia ottenuto e quindi un incremento del rendimento della macchina.In altre parole si può definire un motore come un convertitore d'energia, capace di trasformare l'energia termica in lavoro meccanico, tramite la realizzazione di un ciclo termico il cui rendimento è strettamente legato al rapporto di compressione. Se non realizziamo le condizioni in alta pressione in cui avviene la combustione, otterremo solo calore ma non lavoro meccanico.Macchina a vaporeLa macchina a vapore è un sistema per produrre lavoro meccanico a spese dell'energia termica.Anche se le attuali macchine a vapore sono molto perfezionate, lo schema di base è sempre lo stesso: una sorgente di calore porta all'ebollizione una certa quantità di acqua producendo vapore che, espandendosi, preme sulle pareti del contenitore, mettendo in movimento un pistone. Usata per almeno due secoli come base per la costruzione di macchine industriali, per far viaggiare locomotive, navi e anche le prime automobili ed autocarri, la macchina a vapore, nella versione a turbina, trova ancora un uso importante nella produzione di energia nelle centrali nucleari o tradizionali e nelle grandi navi militari.Una macchina a vapore converte l'energia termica del vapore in lavoro meccanico. Questa macchina è costituita da un contenitore, il bollitore o caldaia, al cui interno, grazie all'azione di una fonte di calore, viene riscaldata acqua fino a temperature vicine all'ebollizione. In questo modo si ottiene vapore ad ad alta temperatura, che tende a espandersi in tutto il bollitore, o in un secondo contenitore in cui si riversa, esercitando una pressione sulle pareti tanto maggiore quanto più alta è la concentrazione del vapore stesso.Il vapore può venire convogliato, con tecniche molto diverse, su un pistone o in una turbina: sia il pistone, sia la turbina si mettono in moto per la pressione che ricevono, producendo lavoro meccanico. Ovviamente questo schema è molto semplificato, ma rende bene l'idea di come lavora una macchina di questo genere. Nel corso degli ultimi tre secoli ne sono state costruite di tipi diversi, nell'intento di migliorarne soprattutto l'efficienza, vale a dire la quantità di energia termica effettivamente trasformata in lavoro meccanico, che in queste macchine è particolarmente bassa, specie in quelle a pistone.Uno dei vantaggi della macchina a vapore risiede nel fatto che è possibile...

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