2013-3-6 Irreversibilities One of the important uses of the second law of thermodynamics in engineering is to determine the best theoretical performance of systems. By comparing actual performance with best theoretical performance,insights often can be had about the potential for improved performance. Best theoretical performance is evaluated in terms of idealized processes. Actual processes are distinguishable from such idealized processes by the presence of non- idealities-called irreversibilities. Irreversibilities Commonly Encountered in Engineering Practice Heat transfer through a finite temperature difference Unrestrained expansion of a gas or liquid to a lower pressure Spontaneous chemical reaction Spontaneous mixing of matter at different compositions or states Friction-sliding friction as well as friction in the flow of fluids 2013-3-6 7 Irreversibilities ►One of the important uses of the second law of thermodynamics in engineering is to determine the best theoretical performance of systems. ►By comparing actual performance with best theoretical performance, insights often can be had about the potential for improved performance. ►Best theoretical performance is evaluated in terms of idealized processes. ►Actual processes are distinguishable from such idealized processes by the presence of non￾idealities – called irreversibilities. Irreversibilities Commonly Encountered in Engineering Practice ►Heat transfer through a finite temperature difference ►Unrestrained expansion of a gas or liquid to a lower pressure ►Spontaneous chemical reaction ►Spontaneous mixing of matter at different compositions or states ►Friction – sliding friction as well as friction in the flow of fluids