Discussion of Some Myths/features Associated with Gas Turbine Inlet Fogging and Wet Compression

Gas turbine inlet fogging and overspray (high-fogging) have been considered the most cost-effective means of boosting a gas turbine’s total power output, especially under hot or dry weather conditions. The result of employing fogging or overspray is indisputably clear − total power output is increased; however, development of the theory and explanation of the phenomena associated with fogging and overspray are not always consistent and are sometimes misleading and incorrect. This paper focuses on reviewing several interesting features and commonly discussed topics, including (a) entropy production of water evaporation, (b) the effect of centrifugal force on water droplets, and (c) whether water droplets can survive the journey in the compressor and enter the combustor. Furthermore, three turbine myths: that fogging/overspray increases the air density in the compressor, reduces the compressor power consumption, and noticeably enhances the gas turbine efficiency, are examined and discussed. Some common mistakes in describing the compressor work are identified and corrected. A newly constructed multiphase T-S diagram is used to explain the physics of water droplet evaporation process and corresponding entropy production during wet compression. NOMENCLATURE CET Compressor exit temperature (K) CIT Compressor inlet temperature (K) d Droplet diameter (m) D Mean diameter of compressor shaft (m) Cp Specific heat (J/kg-K) f Mass fraction of liquid water F Force (N) g Gravitational acceleration (9.81 m/s2) GT Gas Turbine hfg Latent heat of vaporization (kJ/kg) N RPM OS Overspray q Heat transfer (kJ/kg) RH Relative humidity (%) s Specific entropy (kJ/kg.K) T Temperature (K) TIT Turbine inlet temperature (K) U Translational velocity of blade (m/s) V Axial velocity (m/s) v Mass fraction of water vapor w Specific work (kJ/kg)