Isentropic Efficiency of Turbines

The isentropic efficiency of a turbine is the ratio of actual work output of the turbine to the work output that would be achieved if the process between the inlet state and the exit pressure were isentropic.

$${\eta }_T=\frac{\text{Actual turbine work}}{\text{Isentropic turbine work}}=\frac{w_a}{w_s}$$

which can be calculated as:

$$\begin{array}{rl}{\eta }_T& \approx \frac{\dot{m}(h_1-h_{2a})}{\dot{m}(h_1-h_{2s})}\\ & \approx \frac{h_1-h_{2a}}{h_1-h_{2s}}\end{array}$$

where:

• $h_1$ is the starting enthalpy
• $h_{2s}$ is the final enthalpy for the ideal, isentropic process
• $h_{2a}$ is the final enthalpy for the actual, non-isentropic process

Example