Parametric Study of Installed Fogging Systems Using CFD Model

Abstract

Water fogging of inlet air has become widely acceptable for gas turbine power augmentation. Fogging systems are generally designed for a specific set of conditions called the design point. As a rule, the design point is specified to provide the maximum capacity of the fogging system at extreme ambient temperatures, when gas turbine power output is maximally decreased. Since the extreme ambient conditions rarely occur, the fogging system primarily operates at partial loads, when only a part of the installed cooling stages is in operation. At partial loads the fogging system does not provide uniform fogging of the inlet air, which could result in insufficient fog evaporation, less air-cooling and entrainment of non-evaporated water into the compressor. Prolonged operation at such off-design conditions requires careful tuning of the system in order to maximize cooling efficiency and eliminate a possible impact on gas turbine maintenance. This requires clear understanding of conceptual features of thin droplets evaporation and fog cloud behavior inside gas turbine inlet ducts. To this purpose, computational fluid dynamics (CFD) analysis was used. A CFD model is described, which comprises a straight duct equipped with fog nozzles operating in conditions similar to field conditions at design point, as well as, at partial load operation of the fogging system. The study focused on problems, which are critical in design and operation of real fogging systems, namely: single droplet, mono- and poly-fraction fog evaporation; influence of flow turbulent intensity; fog cloud shape and dimensions; polyfraction fog evaporation in the wide range of ambient conditions; over-spray and under-spray operation of the fogging system. The study results were helpful in the tuning of the installed fogging systems to site specific ambient conditions that provided efficient cooling with safe compressor operation. These results should be useful to both designers and operators of the fogging systems.

Experimental investigation on spray cooling using saline water for natural draft dry cooling towers

“…Figure 19 shows that if the shortest wet length and the highest efficiency is desired, a spray system should be designed to operate in the top left of the graph with high mass flow rates and small initial droplet sizes. In a numerical study relating to gas turbines, Hain et al [29] found that attempting to reach 100% final air saturation would lead to incomplete evaporation. Their study identified that smaller droplets evaporated first and thus cooled the local ambient air and increased the relative humidity thereby reducing the potential for larger droplets to evaporate.…”