Film cooling is a cooling technique widely used in high-performance gas turbines
to protect turbine airfoils from being damaged by hot flue gases. Film injection holes are
placed in the body of the airfoil to allow coolant to pass from the internal cavity to the
external surface. The ejection of coolant gas results in a layer or “film” of coolant gas
flowing along the external surface of the airfoil.
In this study, a new cooling scheme, mist/air film cooling is proposed and
investigated through experiments. Small amount of tiny water droplets with an average
diameter about 7 μm (mist) is injected into the cooling air to enhance the cooling
performance. A wind tunnel system and test facilities were build. A Phase Doppler
Particle Analyzer (PDPA) system is employed to measure droplet size, velocity and
turbulence. Infrared camera and thermocouples are both used for temperature
measurements.
Mist film cooling performance is evaluated and compared against air-only film
cooling in terms of adiabatic film cooling effectiveness and film coverage. Experimental
results show that for blowing ratio M=0.6, net enhancement in adiabatic cooling
effectiveness can reach 190% locally and 128% overall along the centerline. The general
pattern of adiabatic cooling effectiveness distribution of the mist case is similar to that of
the air-only case with the peak at about the same location.
The concept of Film Decay Length (FDL) is proposed to quantitatively evaluate
how well the coolant film covers the blade surface. Application of mist in the M=0.6
condition is apparently superior to the M=1.0 and 1.4 cases due to the higher overall
cooling enhancement, the much longer FDL, and wider and longer film cooling coverage
area.
Based on droplet measurements through PDPA, a profile describing how the airmist
coolant jet flow spreads and eventually blends into the hot main flow is proposed. A
sketch based on the proposed profile is provided. This profile is found to be well
supported by the measurement results of Turbulent Reynolds Stress. The location where
a higher magnitude of Turbulent Reynolds Stress exists, which indicates higher strength
of turbulent mixing effect, is found to be in the close neighborhood of the edge of the
coolant film envelope. Also the separation between the mist droplets layer and the
coolant air film is identified through the measurements. In other words, large droplets
penetrate through the air coolant film layer and travel further over into the main flow.
Based on the proposed air-mist film profile, the heat transfer results are reexamined.
It is found that the location of optimum cooling effect is coincident with the
starting point where the air-mist coolant starts to bend towards the surface. Thus the data
suggests that the “bending back” film pattern is critical in keeping the mist droplets close
to the surface which improves the cooling effectiveness for mist cooling.
| Identifer | oai:union.ndltd.org:uno.edu/oai:scholarworks.uno.edu:td-2494 |
| Date | 18 May 2012 |
| Creators | zhao, lei |
| Publisher | ScholarWorks@UNO |
| Source Sets | University of New Orleans |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | University of New Orleans Theses and Dissertations |
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