Numerical model is developed in order to characterize the water droplet coalescence at the air intake duct of a gas turbine assumed under operation of Inlet Fogging System. Water droplet size is treated as a randomly distributed parameter instead of a single size representative diameter. The numerical calculation is executed by ANSYS Fluent® v.14.5 in a simple rectangular duct and verified using experimental data. The calculated evaporation time of an isolated water droplet in an infinite room shows a good agreement with experimental data. With fogging amount below saturation condition, injected water droplets with size distributions take a longer time to evaporate and it leads to a lower cooling efficiency than the one calculated using a single representative diameter. Then, the model is used to investigate droplet coalescence phenomena in an L-shaped duct geometry. It is shown that the L-shaped duct enhances coalescence due to drift behind the corner of L-duct. This reproduces larger droplets, therefore lower evaporation efficiency and increase of the water drainage.