Turbine Fundamentals and Thermodynamics
Core concepts
A turbine converts enthalpy drop of a working fluid into shaft work. Candidates must be fluent with the steady-flow energy equation, the Rankine and Brayton cycles, isentropic versus actual expansion, and the definitions of stage, pressure ratio, and reheat/regeneration. Understand nozzle behavior: subsonic and supersonic expansion, choking at the throat, and why the convergent-divergent nozzle appears in high-pressure stages. Know velocity triangles, degree of reaction, impulse versus reaction blading, and how leaving loss and tip leakage erode stage efficiency. For steam units, distinguish saturation temperature from superheat and know that wetness above roughly 12–14% causes blade erosion.
Traps
Questions often hide the difference between isentropic and actual enthalpy drop: always compute isentropic efficiency as actual work divided by isentropic work, never the reverse. Another trap is confusing pressure ratio with expansion ratio, or applying the ideal-gas relations to wet steam. Watch sign conventions in the energy equation, and remember that condenser pressure, not boiler pressure, sets most of the available energy in a condensing steam cycle.
Worked rule of thumb
For a steam turbine with inlet enthalpy 3400 kJ/kg, isentropic outlet 2200 kJ/kg, and actual outlet 2350 kJ/kg: isentropic drop = 1200 kJ/kg, actual drop = 1050 kJ/kg, so stage efficiency ≈ 1050/1200 = 87.5%. A quick sanity check: raising condenser vacuum by 1 kPa typically gains far more output than the same pressure change at the inlet, because the low-pressure stages handle large specific volumes.
Exam focus
Be ready to rank efficiency losses (leaving loss, leakage, moisture, partial admission) and to explain why reheat raises cycle efficiency without raising metal temperatures beyond limits.