Overspray Fog Cooling in Compressor Using Stage-Stacking Scheme With Non-Equilibrium Heat Transfer Model for Droplet Evaporation

Abstract

The inlet fog cooling scheme has been proven as an economic and effective means to augment gas turbine output power on hot or dry days. A previous paper developed a stage-by-stage wet-compression theory for overspray and interstage fogging using the equilibrium droplet evaporation model with given compressor and blade configurations. This paper extends the previous work by including the non-equilibrium droplet heat transfer model. An 8-stage, 2-D compressor airfoil geometry and stage settings at the mean radii are employed. Eight different cases including saturated fogging, overspray with different droplet sizes with both equilibrium and non-equilibrium heat transfer models have been investigated and compared. The results show saturated fogging increases the pressure ratio and reduces the compressor power consumption; however, overspray actually increases both the specific and total compressor power consumption. Non-equilibrium method differs from the equilibrium method due to the change of evaporation rate. Droplet size doesn’t play a role in equilibrium approach, but plays a major role in affecting the result in the non-equilibrium case. For small droplet size of 10 μm, the droplet evaporation rate is fast, so the non-equilibrium method predicts close results as the equilibrium method. Larger droplets lead to slower evaporation, reduction of pressure ratio, and less effective compressor performance than the smaller droplets. Equilibrium method predicts that wet compression increases axial velocity, blade inlet velocity, incidence angle, and tangential component of velocity. Non-equilibrium methods predict a similar trend except with lesser increments as the droplet size increases. In the present study, the equilibrium method predicts that all the water droplets evaporate completely at the end of stage 3, while the non-equilibrium approach predicts that the completion of evaporation delays, but all droplets completely evaporate in the compressor except the biggest droplets (30μm). Saturated fogging increases air density; however, both equilibrium and non-equilibrium methods predict that overspray wet compression actually reduces air density in the earlier 70% of the compressor. Non-equilibrium predicts small droplets relax the load in the earlier stages but increases the load in the later stages. Larger droplets show less load changes. Detailed stage-to-stage performance and property value changes are analyzed and discussed in this study.