For a turbine, which expression represents the shaft work in terms of enthalpy?

For a turbine, which expression represents the shaft work in terms of enthalpy?

• A. w = h1 - h2
• B. w = h2 - h1
• C. w = h1 + h2
• D. w = -(h1 - h2)

Answer: (A)

In a turbine, the shaft work is tied to the decrease in the fluid’s enthalpy. With the steady-flow energy balance and assuming negligible changes in kinetic and potential energy, the shaft work per unit mass equals the inlet enthalpy minus the outlet enthalpy: w_shaft = h1 − h2. Since the fluid expands and cools as it passes through the turbine, h1 is greater than h2, making the shaft work positive—the turbine delivers energy to the shaft. The other forms would either add enthalpies or use the opposite sign, which doesn’t represent the energy transfer to the shaft in this device.

In a turbine, the shaft work is tied to the decrease in the fluid’s enthalpy. With the steady-flow energy balance and assuming negligible changes in kinetic and potential energy, the shaft work per unit mass equals the inlet enthalpy minus the outlet enthalpy: w_shaft = h1 − h2. Since the fluid expands and cools as it passes through the turbine, h1 is greater than h2, making the shaft work positive—the turbine delivers energy to the shaft. The other forms would either add enthalpies or use the opposite sign, which doesn’t represent the energy transfer to the shaft in this device.