Gas turbine inlet air-cooling using a fogging system is accomplished by using an array of high-pressure nozzles that inject micron-sized droplets in air stream. These droplets evaporate and diffuse in the air stream resulting in cooling and humidification of air. The cooled and moist inlet air increases net turbine power output, improves heat rate and reduces Nitrogen Oxides formation (NOx). The evaporation and mass diffusion of these droplets are influenced, among other factors, by its surface area to volume ratio. Large surface area facilitates drop interfacial heat transfer and smaller volume or weight aids higher droplet residence times. A fogging nozzle’s atomizing performance can be evaluated from its spray properties that include a mean drop size, droplet distribution, numerical droplet density, spray cone angle, and spray penetration. The spray industry adopts various definitions of mean drop size that suits its application and objective. Mean drop sizes or more commonly droplet diameters used in the gas turbine inlet air fogging industry are 90% cumulative volume frequency, Dv0.90 and the Sauter Mean Diameter, D32. Two sprays having identical mean or representative diameter are not necessarily similar in performance. Further, a spray from nozzle ‘A’ having a Dv0.90 less than another nozzle ‘B’ does not necessarily imply that ‘A’ is superior to ‘B’. This paper explains why the use of one or both of the above characteristic diameters does not effectively reflect a fog nozzle’ atomizing performance. This paper also analyzes various characteristic diameters and their relevance to evaporative cooling using fog nozzles. In fogging applications, the smallest and/or the largest sized drops in a spray will have significant impact on performance and neither Dv0.90 nor D32 can independently provide this information. Therefore, at least one other parameter such as the droplet distribution must be known in order to qualitatively define a spray from a fogging nozzle. This paper also determines these parameters such as the Relative Span Factor and Dispersion Boundary Factor and analyzes their importance to fogging performance.

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