The Effects of Wet Compression on Gas Turbine Engine Operating Performance

Water, in the liquid or vapor phase, injected at various locations into the gas turbine cycle has frequently been employed to improve engine performance. One such way to improve engine performance is by steam injection, of varied quantity, into the combustor section of the engine. Combustor steam injection increases turbine mass flow rate without increasing airflow rate and consequently increasing the specific power (power/lb_m of air). Another approach, receiving widespread acceptance in recent years, is to inject water droplets into the inlet duct upstream of the engine compressor inlet. As the droplets evaporate, prior to entering the compressor, the inlet air is cooled subsequently decreasing compressor power and thus increasing engine power output. The present paper examines the concept of injecting water droplets, termed fogging, in excess of the amount that can be evaporated before entering the engine compressor. This excess water, termed over-spray, is carried directly into the engine compressor. The computer simulated performance of a simple cycle gas turbine engine using evaporative cooling upstream of the compressor with over-spray is reported. The paper describes an improved simulation model developed to predict compressor performance as water is evaporated while passing through the stages of an axial flow compressor. The effects are similar to those of an intercooled compressor without the complications of additional piping, heat exchangers, and the requirement for a dual spool compressor. The results of a parametric study of the effects of evaporative cooling on engine operating characteristics are presented. These results include compressor performance characteristics modified for various inlet conditions (temperature, pressure, and humidity) and fogging conditions (flow rate, over-spray, and water temperature) as well as estimates of the reduced compression work and lowered compressor discharge temperatures. These modified compressor performance characteristics are used in the engine simulation to predict how an over-sprayed engine would perform under various operating conditions. Estimates of increased output power and increased specific power are presented.