In a turbine, why is the final enthalpy (of the working substance) in an actual process greater than that of an isentropic process?

In a turbine, the final enthalpy of the working substance in an actual process is greater than that of an isentropic process due to irreversibilities and inefficiencies inherent in real-world systems. An isentropic process is idealized, meaning it is both adiabatic (no heat transfer) and reversible (no friction or other losses), resulting in no change in entropy (( \Delta S = 0 )) 3. However, actual processes are irreversible, involving friction, heat transfer over finite temperature differences, and other losses, which increase entropy and lead to inefficiencies 23.

In the case of a turbine, these inefficiencies mean that not all the potential energy available in the working fluid is converted into useful work. Instead, some energy is lost as heat or due to friction, resulting in a higher final enthalpy compared to the ideal isentropic case. This higher final enthalpy indicates that the turbine has not extracted as much work as it could have under ideal conditions, leading to a lower actual work output compared to the isentropic work output 23.

The isentropic efficiency of a turbine, which is the ratio of actual work to isentropic work, quantifies how closely the actual process approaches the ideal isentropic process. A higher isentropic efficiency indicates a better-designed turbine that more closely approximates the ideal conditions 23. However, even with high efficiency, real turbines cannot achieve the perfect conditions of an isentropic process, resulting in a higher final enthalpy than the isentropic value.