Model Analysis of High-Fogging Effects on the Work of Compression

In recent years, high fogging has received increasing attention as a comparatively simple and cost-efficient means of gas turbine power augmentation. The effects of high fogging on the work of compression are studied in this paper from a fundamental perspective. Considered is a prototype configuration, namely the evaporation of droplets in an initially saturated mixture of air and water, which is exposed to a prescribed pressure rise. Two different approaches are applied: The first approach (‘ideal model’) assumes that thermodynamic equilibrium prevails. In the second approach (‘droplet model’) the finite time of evaporation is taken into account by introducing discrete droplets and modeling explicitly the heat and mass transfer between liquid and gaseous phase. For compression speeds representative of modern gas turbines, it is found that droplets of 1μm in diameter are small enough to allow for approximate equilibrium during compression. The influence of polytropic efficiency on the gains of high fogging is addressed, and it is shown that high fogging is more effective for compressors of lower efficiency.