Wet Compression Analysis Including Velocity Slip Effects

Abstract

Injection of water droplets into industrial gas turbines in order to boost power output is now common practice. The intention is usually to saturate and cool the intake air, especially in hot and dry climates, but in many cases, droplets carry over into the compressor and continue to evaporate. Evaporation within the compressor itself (often referred to as overspray) is also central to several advanced wet cycles, including the moist air turbine and the so-called TOP Humidified Air Turbine (TOPHAT) cycle. The resulting wet compression process affords a number of thermodynamic advantages, such as reduced compression work and increased mass flow rate and specific heat capacity of the turbine flow. Against these benefits, many of the compressor stages will operate at significantly off-design flow angles, thereby compromising aerodynamic performance. The calculations presented here entail coupling a mean-line compressor calculation method with droplet evaporation routines and a numerical method for estimating radial and circumferential slip velocities. The impingement of droplets onto blades and the various associated processes (including film evaporation) are also taken into account. The calculations allow for a polydispersion of droplet sizes and droplet temperature relaxation effects (i.e., the full droplet energy equation is solved rather than assuming that droplets adopt the wet-bulb temperature). The method is applied to a generic single-shaft 12-stage compressor. Results are presented for computed droplet trajectories, the overall effect on compressor characteristics (and how this depends on droplet size), and the effects of deposition and subsequent film evaporation. As with previously published wet compression calculations (with no velocity slip), it is found that pressure rise characteristics shift to higher mass flow and pressure ratio with increasing water injection rate and that aerodynamic efficiency falls due to the stages moving away from their design point. For droplet sizes typical of fog boosting, the overall effect of slip is to slightly increase the evaporative cooling effect through the enhanced heat and mass transfer rates.