Overspray and Interstage Fog Cooling in Compressor Using Stage-Stacking Scheme: Part 2—Case Study

Abstract

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.

Discussion of Some Myths/Features Associated With Gas Turbine Inlet Fogging and Wet Compression

“…Taking hot and 2% overspray cases from Wang and Khan [21,22]: The entropy change for the overspray case is 161% more than the hot case without fogging, which is significant. The increased entropy due to droplet evaporation would degrade the "equivalent" polytropic efficiency of aerodynamic performance, as proposed by White and Meacock [8] and is shown in Eq.…”

“…al. [11], Abdelwahab [15], Roumeliotis and Mathioudakis [18,23], and Wang and Khan [22] have all shown that the polytropic efficiency of the compressor during wet compression decreases. Roumeliotis and Mathioudakis [18] specifically showed that the compressor power increased by water injection and that the increased compressor power varied linearly with the quantity of water entering the stage.…”

“…3. Figures 4 show the real data for a wet compression with the ambient temperature of 300K, 60% relative humidity, and 2% overspray of water, where the data was taken from Wang and Khan [21,22]. Some quantitative data is presented below.…”

“…To examine the myths, the data from Wang and Khan [22] is used for illustrative purposes. In [22], a stage-stacking method was used to calculate the compressor performance under fogging/overspray condition of a compressor operated at constant RPM and a pressure ratio between 8-9.…”

“…In [22], a stage-stacking method was used to calculate the compressor performance under fogging/overspray condition of a compressor operated at constant RPM and a pressure ratio between 8-9. The water droplets were provided at 288K.…”