A stage-by-stage wet-compression theory and algorithm have been developed for overspray and interstage fogging in the compressor. This theory and algorithm are used to calculate the performance of an 8-stage compressor under both dry and wet compressions. A 2D compressor airfoil geometry and stage setting at the mean radius are employed. Six different cases with and without overspray have been investigated and compared. The stage pressure ratio enhances during all fogging cases as does the overall pressure ratio, with saturated fogging (no overspray) achieving the highest pressure ratio. Saturated fogging reduces specific compressor work, but increases the total compressor power due to increased mass flow rate. The results of overspray and interstage spray unexpectedly show that both the specific and overall compressor power do not reduce but actually increase. Analysis shows this increased power is contributed by increased pressure ratio and, for interstage overspray, "recompression" contributes to more power consumption. Also it is unexpected to see that air density actually decreases, instead of increases, inside the compressor with overspray. Analysis shows that overspray induces an excessive reduction of temperature that leads to an appreciable reduction of pressure, so the increment of density due to reduced temperature is less than decrement of air density affected by reduced pressure as air follows the polytropic relationship. In contrast, saturated fogging results in increased density as expected.After the interstage spray, the local blade loading immediately showed a significantly increase. Fogging increases axial velocity, flow coefficient, blade inlet velocity, incidence angle, and tangential component of velocity. The analysis also assesses the use of an average shape factor in the generalized compressor stage performance curve when the compressor stage information and performance map are not available. The result indicates that using a constant shape factor might not be adequate because the compressor performance map may have changed with wet compression. The results of non-stagestacking simulation are shown to underpredict the compressor power by about 6% and net GT output by about 2% in the studied cases.

Compressor intercooling has traditionally been employed to reduce compressor work and augment gas turbine output power. Conventional intercooling schemes are usually applied through nonmixed heat exchangers between two compressor stages or by cooling the outside of the compressor casing. Any cooling schemes that may affect the flow field inside the compressors have not been favorably considered due to concerns of any disturbance that might adversely affect the compressor's performance stability. As the inlet fog cooling scheme has become popular as an economic and effective means to augment gas turbine output power on hot or dry days, consideration has been given to applying fog cooling inside the compressors by injecting fine water droplets between stages (i.e. interstage fogging). This paper focuses on developing a stage-bystage wet-compression theory for overspray and interstage fogging that includes the analysis and effect of pre-heating and pre-cooling at each small stage of the overall compressor performance. An algorithm has been developed to calculate the local velocity diagram and allow a stage-by-stage analysis of the fogging effect on airfoil aerodynamics and loading with known 2-D meanline rotor and stator geometries. Thermal equilibrium model for water droplet evaporation is adopted. The developed theory and algorithm are integrated into the system-wise FogGT program to calculate the overall gas turbine system performance.

Gas turbine inlet fog / overspray cooling is considered as a simple and effective method to increase power output. To help understand the water mist transport in the compressor flow passage, this study conducts a 3-D computational simulation of wet compression in a single rotor-stator compressor stage using the commercial code, Fluent. A sliding mesh scheme is used to simulate the stator-rotor interaction in a rotating frame. Eulerian-Lagrangian method is used to calculate the continuous phase and track the discrete (droplet) phase respectively. Models to simulate droplet breakup and coalescence are incorporated to take into consideration the effect of local acceleration and deceleration on water droplet dynamics. Analysis on droplet history (trajectory and size) with stochastic tracking is employed to interpret the mechanism of droplet dynamics under the influence of local turbulence, acceleration, diffusion, and body forces. An liquid-droplet erosion model is included. The sensitivity of turbulence models on the results is conducted by employing 6 different turbulence models and 4 different time constants. The result shows that the local thermal equilibrium is not always achieved due to short residence time and high value of latent heat of water. Local pressure gradients in both the rotor and stator flow passages drive up the droplet slip velocity during compression. The erosion model predicts that the most eroded area occurs in leading edge and one spot of trailing edge of the rotor suction side.

During the summer, power output and the efficiency of gas turbines deteriorate significantly. Gas turbine inlet air fog cooling is considered a simple and cost-effective method to increase power output as well as, sometimes, thermal efficiency. During fog cooling, water is atomized to micro-scaled droplets and introduced into the inlet airflow. In addition to cooling the inlet air, overspray can further enhance output power by intercooling the compressor. With continued increase of volatility of natural gas prices and concerns regarding national energy security, alternative fuels such as low calorific value (LCV) synthetic gases (syngas) derived from gasification of coal, petroleum coke, or biomass are considered as important common fuels in the future. The effect of fogging/overspray on LCV fuel fired gas turbine systems is not clear. This paper specifically investigates this issue by developing a wet compression thermodynamic model that considers additional water and LCV fuel mass flows, non-stoichiometric combustion, and the auxiliary fuel compressor power. An in-house computational program, FogGT, has been developed to study the theoretical gas turbine performance by fixing the pressure ratio and turbine inlet temperature (TIT) assuming the gas turbine has been designed or modified to take in the additional mass flow rates from overspray and LCV fuels. Two LCV fuels of approximately 8% and 15% of the NG heating values, are considered respectively. Parametric studies have been performed to consider different ambient conditions and various overspray ratios with fuels of different low heating values. The results show, when LCV fuels are burned, the fuel compressor consumes about 10–18% of the turbine output power in comparison with 2% when NG is burned. LCV fueled GT is about 10–16% less efficient than NG fueled GT and produces 10–24% of net output power even though LCV fuels significantly increase fuel compressor power. When LCV fuels are burned, saturated fogging can achieve a net output power increases approximately 1–2%, while 2% overspray can achieve 20% net output enhancement. As the ambient temperature or relative humidity increases, the net output power decreases. Fog/overspray could either slightly increase or decrease the thermal efficiency depending on the ambient conditions.

Gas turbine inlet fog / overspray cooling is considered as a simple and effective method to increase power output. To help understand the water mist transport in the compressor flow passage, this study conducts a computational simulation of wet compression in a single rotor-stator compressor stage using the commercial code, Fluent. A sliding mesh scheme is used to simulate the stator-rotor interaction in a rotating frame. The effect of different heat transfer models and forces (e.g. drags, thermophoretic, Brownian, Saffman’s lift force etc.) are investigated. Models to simulate droplet breakup and coalescence are incorporated to take into consideration the effect of local acceleration and deceleration on water droplet dynamics. Analysis on droplet history (trajectory and size) with stochastic tracking is employed to interpret the mechanism of droplet dynamics under influence of local turbulence, acceleration, diffusion, and body forces. An erosion model is also included. The results show that droplet local slip velocity is noticeably affected by local acceleration and deceleration of the compressor blade, and in turn, the heat transfer and water evaporation rate are affected. Due to the short droplet residence time in the compressor stage, local thermal equilibrium is not always achieved, and the air may not always reach saturation even sufficient amount of liquid mass is in the air. The results also show erosion occurs near the rotor fore-body on the suction side with the present erosion model, which is subject to continuous improvement and further verification. The transient results of different rotor/stator relative positions show low airflow blockage produces more effective compression and higher temperature rise. Different types of droplet boundary conditions show the effect is negligible.