The Analysis and Design of Saturators for Power Generation Cycles: Part 2 — Heat and Mass Transfer

The introduction of water into a gas turbine circuit is known to provide benefits of increased efficiency and specific work output. There are several methods of humidification but the use of a pressurised saturator in an evaporative cycle offers many attractive features. This two-part paper presents an in-depth study of the saturator, in Part 1 via a thermodynamic approach and in Part 2 via a detailed heat and mass transfer analysis. The analysis in Part 2 starts from a rigorous control volume formulation and includes a derivation of the entropy production equation in keeping with the theory of irreversible thermodynamics. The specification of the heat and mass transfer coefficients is discussed and an analytical expression derived for the liquid-gas interface temperature. A novel numerical procedure for solving the equations is then presented which provides more flexibility than the traditional method of solution. After a discussion on the possibility of occurrence of supersaturated vapour states, results are presented showing the effects of variation of saturator height. The limiting condition when the water exit temperature falls to the inlet vapour adiabatic saturation temperature is also discussed.