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Research Papers

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

[+] Author and Article Information
Ting Wang, Jobaidur R. Khan

Energy Conversion & Conservation Center, University of New Orleans, New Orleans, LA 70148-2220

J. Thermal Sci. Eng. Appl 2(3), 031002 (Nov 17, 2010) (10 pages) doi:10.1115/1.4002755 History: Received May 11, 2010; Revised September 21, 2010; Published November 17, 2010; Online November 17, 2010

A stage-by-stage wet compression theory and algorithm have been developed for overspray and interstage fogging in the compressor. These theory and algorithm are used to calculate the performance of an eight-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 are 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 that this increased power is contributed by the 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 in temperature that leads to an appreciable reduction in pressure, so the increment of density due to reduced temperature is less than the 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 significant 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 nonstage-stacking simulation are shown to underpredict the compressor power by about 6% and the net gas turbine output by about 2% in the studied cases.

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Copyright © 2010 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Tip and hub diameters along the compressor

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Figure 2

Velocity diagram for cases 1 and 2 in the second stage

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Figure 3

Static temperature variations: overspray induces an excessive reduction in static temperature

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Figure 4

Stage total pressure ratio variation: overspray results in a reduction in local pressure due to an excessive temperature drop, followed by a splurge of pressure rise

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Figure 5

Cumulative compressor total pressure ratio variation

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Figure 6

Density variation

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Figure 7

Inlet velocity (actual magnitude) variations at each stage

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Figure 8

Flow coefficient (ϕ) variations. Note the striking difference between saturated fogging (Case 2S) and overspray (cases 3–5).

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Figure 9

Variation in rotor work coefficient (ψ)

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Figure 10

(a) Variation in compressor stage power. (b) Variation in compressor integrated power.

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Figure 11

p-v diagrams for the Brayton cycle. (a) Inlet saturated cooling with a constant pressure ratio. (b) Theoretical representation of cases 2–5. The shaded area represents the double compression work due to the interstage spray.

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Figure 12

p-v diagram illustration of actual wet compression processes of cases 2 and 3

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Figure 13

p-v diagram illustration of actual compression processes of cases 2–4. (A portion of the double compression work is qualitatively shown in Fig. 1.)

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