Performance Modeling of a Power Generation Gas Turbine With Wet Compression

In the last 15 years more than 1000 power generation gas turbines have been modified with an OEM or aftermarket module to generate the wet compression phenomenon where “Hot Day” conditions are present on the site. This modification to the gas turbine increases power, but can produce performance problems including reduced compressor surge margin and possibly a shorter maintenance cycle because of resulting problems present in the compressor such as blade vibration and erosion with impingement of water droplets on the surface of the compressor blades[1]. In the last few years researchers in academia and the private sector have worked to understand the principles behind the wet compression process in order to know in depth how to use the application to best advantage with gas turbines. The main areas of the research on wet compression are thermodynamic analyses, computer fluid dynamic analysis, and the use of operational data. Because present technology is unable to obtain detailed operational data on the evaporation process within the compressor, researchers rely on computer simulations based upon aerothermodynamics and physical measurements of the gas turbines, and assumptions based upon available information. These computer simulations are typically aimed toward explaining the performance data from a specific gas turbine model. Most of these computer simulations are cycle analyses of the gas turbine [2–7], although a few are CFD analyses for a specific compressor using either in-house computer programs or commercial CFD software [8–10]. CFD analysis takes into account the fact that an evaporation model should be used in order to predict how the evaporation of the water droplets occurs through the stages of the compressor. Many of the CFD simulations that have been performed for wet compression assume that the mixture of air, liquid water, and water vapor is at equilibrium throughout the compressor. Also, a single water droplet size is sometimes used for the simulation instead of a size distribution for the droplets. These assumptions simplify the calculations for the software. The results of these simulations may over-forecast the effect of the wet compression and the power output of the gas turbine because of incorrect predictions of evaporation models, or because of the lack of a proper droplet size distribution affecting the calculation. An analysis that properly forecasts the power output of a gas turbine with wet compression is important for design, performance prediction, and operation. The intention of this paper is to show how performance predictions for a power generation gas turbine is affected by applying several evaporation models [2, 4, 5, 7] in a gas turbine model with a detailed, stage-by-stage compressor model. Model predictions are compared with available operational performance data. Conclusions are provided regarding the best evaporation model assumptions for accurate predictions of gas turbine performance with wet compression.