A Comparison of Cylindrical and Row Trenched Cooling Holes with Alignment Angle of 90 Degree Near the Combustor Endwall
Ehsan Kianpour
1, Nor Azwadi C. Sidik
2, Syahrullail Samion
3Abstract – This study was carried out to find out the effects of cylindrical and row trenched cooling holes with alignment angle of 90 degree on the film cooling performance adjacent the end wall surface of a combustion simulator. In this research a linear representation of a true Pratt and Whitney gas turbine aero-engine was simulated and analyzed. The current study has been performed with Reynolds-averaged Navier-Stokes turbulence model (RANS) on internal cooling passage. This combustor simulator combined the interaction of two rows of dilution jets, which were staggered in the stream wise direction and aligned in the span wise direction. The entire findings of the study declared that when, the row trenched holes used the, near the enwall surface, film cooling effectiveness is almost two times more than film cooling performance of baseline case.
Copyright © 2013 Praise Worthy Prize S.r.l. - All rights reserved.
Keywords: Gas Turbine, Film Cooling, Thermal Contour, Cylindrical Holes, Trenched Holes
Nomenclature
k Turbulent kinetic energy U Free stream mean velocity μ Air viscosity
t Air turbulent viscosity
ε Turbulent energy dissipation rate ρ Density of air
I. Introduction
Using a Bryton cycle is a key to get higher engine efficiency and power to weight ratio. According to this cycle it is needed to increase outlet combustion temperature. This temperature increase creates harsh environment at the end of combustor and inlet of turbine vane and this is threatened the life of critical parts.
So, it is needed to design a design a cooling technique in this area. Film cooling is the traditional way which is used. In this system, a thin thermal boundary layer such as buffer zone is formed by cooling holes and attached on the protected surface.
Cylindrical and trenched cooling holes are two layouts of these holes. According to the importance of this research, a broad literature search was conducted to collect the information. Azzi and Jurban [1]tested several ways to study the film cooling thermal field. In this study, they used standard K-ε turbulence model to solve the Reynolds averaged Navier-Stokes equation. In concurred with Rozatiand Danesh Tafti [2], results show that the film cooling effectiveness was improved at low blowing ratios. Kianpour, Azwadi and Mirzabozorg [3], [4] in 2012, simulated the combustor end wall cooling holes with two different layouts and exit section area.
The results declared that while, the central part of the jets stayed nominally at the same temperature level for both configurations, the temperatures adjacent the wall and between the jets was a while cooler with less cooling holes. Aga and Abhari [5], I-Chien, Yun-Chung, Pei- Peiand Ping-Hei [6] studied the effects of different inclination and lateral angles of holes on film cooling performance.
The finding indicated that averaged adiabatic film cooling effectiveness was two times more than stream wise injection at high compound angles especially at high blowing ratios. Sundaram and Thole [7] and Lawson and Thole [8] studied the effects of trenched depth and width on film cooling performance at the vane- end Wall. Theresults showed that the maximum cooling effectiveness is obtained at the trench depth of0.80D.
However, Lawson and Thole stated that the trench depth of 0.8D has negative effect on the cooling performance downstream the cooling hole. Yiping, Dhungel, Ekkad and Bunker [9] tested the effects of depth and width of trenches on the film cooling under overall cooling effectiveness of ∅=0.6 as determined by Maikell, Bogard, Piggush and Kohli [10]. This constant value of overall cooling effectiveness declared that at the leading edge, the thermal barrier coating has moderate dependency to the stagnation line variation. Also it is figured out that the third (w=2.0D and d=0.75D) and fourth(w=3.0D and d=0.75D) case were more effective than other cases and base line case and it means the trench depth of 0.75D was the optimum one as well as approved by CFD studies.
According to the importance of this issue, more studies are required. There are many questions should be answered: How can trenched cooling holes improve the film cooling performance at the combustor end wall
compared to the cylindrical holes? This numerical study was conducted to optimize the combustor exit flow and cooling holes arrangement. In this study, the effects of various combustor holes configurations were analyzed.
Also in order to measure the validity of the results, a comparison between the data gained from this study and Vakiland Thole [11] project was made.
II. Methods and Materials
The combustor was a container with a width, height and length of 111.8cm and 99.1cm and 156.9cmrespectively.
The contraction angle was 15.8degrees and began at X=79.8cm. While the inlet cross-sectional area was 1.11 m2, the exit cross sectional area was 0.62 m2. The combustor simulator included four stream wise series of film cooling panels. The starting point of these panels was approximately at 1.6 m upstream of the turbine vanes.
The length of the first and second panels was39cm and 41cm respectively. However the third and fourth panels were 37 and 43 centimeter in length. The thickness of the panels was 1.27cm. The thermal conductivity (k= 0.037 W/mk) was such low that adiabatic surface temperature measurement was possible. There were two rows of dilution holes within the second and third panels of cooling panels.
These dilution holes were located at 0.67 m and0.90 m downstream of the beginning of the combustor liner panels. The diameter of the first row and second row of dilution holes was 8.5cm and11.9cm respectively. The centerline of the second row was staggered with respect to the first row of dilution holes.
To verify the purpose of this study, a three- dimensional presentation of a true Pratt and Whitney aero-engine which studied before in Virginia University was simulated [11], however, the present combustor simulator included two configurations of cooling holes.
The first arrangement was similar to the Vakil and Thole [11] model. For the second case, the cooling holes placed within a row trench with alignment angle of 90 deg.
The film-cooling holes were placed in equilateral triangles. The film-cooling holes diameter of both models was 0.76cm.
They drilled at an angle of 30degrees from the horizontal surface. For the baseline case, the length of each cooling hole was 2.5cm. Furthermore, the trench depth and width was 0.75 Dand 1.0D respectively. Also a global coordinate system (X, Y, and Z) was selected. The schematic view of the combustor simulator is shown in Fig. 1.
The film cooling effectiveness (η) distribution in the combustor simulator was measured along the specific measurement planes. These measurement planes are illustrated in Figs. 2.
In order to solve the combustor simulator and getting more accurate data and reasonable time consumption,
about 8×106 tetrahedral meshes were used and this is in concurred with Stitzel and Thole study [12].
Fig. 1. The 3-D view of the combustor simulator
Figs. 2. Location of the measurement planes (a) baseline (b) trenched case
The meshes were denser around the cooling and dilution holes and even wall surfaces. According to the specific flow ratio at the inlet of volume control, mass flow inlet and the condition was defined at the inlet of combustor simulator.
Wall boundary and slip less boundary conditions were applied to limit the interaction zone between fluid and solid layer. Also at the end of volume control the pressure outlet boundary condition was used. In addition, both cases were completely symmetric along the X-Y and X-Z planes. According to this issue, symmetry boundary condition 0 was applied. In addition these equations ere used as well.
Continuity equation:
(1) Momentum equation:
(2)
Energy equation:
(3)
and RNG K-ε equation:
(4)
(5)
To understand the thermal field results, the quantities should be defined. Film cooling effectiveness is defined as below:
(6) here, T is the local temperature; is main stream temperature and is the temperature of coolant.
According to the iteration chart of solution, convergence magnitude of this solution was about 10-4.
Also to solve this question, unsteady condition was applied. This condition led to different iterations.
However, the small variations were relating to the physics of the question and they continued to get the new time step. The combustor simulator was meshed by Gambit software and the model was simulated and analyzed by using Fluent software and the corresponding results were presented in the form of charts and different contours.
III. Findings and Discussion
Figs. 3(a) (Current Study) and 3b (Experimental) [11]
show the film cooling distribution of plane 0p.
A noticeable different between these two contours was the penetration depth of coolant, while for the current
study, the coolant is injected to Z/d=2.5, it was 7.4 for the experimental findings.
In addition the variation of film cooling effectiveness for both cases and at Y/W=0.45 is illustrated in Fig. 4.
According to this graph, it is declared that film cooling effectiveness for the experimental set was better. In addition the uncertainty prediction for these two cases was 46%as well. The distribution of film cooling effectiveness (η) for the plane 0p is shown in Fig. 5(a) (baseline) and 5(b)( trenched hole).
A noticeable difference between these figures is the thickness of film cooling layers.
For the baseline case, the thickness of film cooling layer reached to Z=2cm, which is about 2% of the combustor inlet height.
(a)
(b)
Figs. 3. Film cooling distribution of plane 0p (a)current study (b) experimental
Fig. 4. The variation of film cooling effectiveness of plane 0p at Y/W=0.45
For the trenched case, the maximum thickness of the film cooling layer is more than Z=8 cm and this is almost four times more than baseline. However, for this measurement plane, the thicker film cooling layer for the trenched hole does not automatically suggest that is desirable. At the position of 31cm<Y<36cm, the temperature level is higher for the trenched case.
However, the contours indicated that the film cooling effectiveness is higher with the usage of trenched cooling holes.
The distribution of film cooling effectiveness for plane 1p is shown in Fig. 6(a) (baseline) and 6(b) (trenched case). Note that, the film cooling was significantly increased between these two cases. Fig. 6(a) shows the high temperature near the end wall surface compared to the trenched case. However, no dominant cooling modifications are seen along the linear wall.
Just at the right side of Fig. 6(b) (50cm<Y<54cm) thermal field contours show that the film cooling is being entrained by the upward motion of the dilution jet.
Also, at the position of 18cm<Y<40cm and 8cm<Z<10cm is slightly hotter (0<η<0.05) for the trenched case as opposed to the baseline.
(a)
(b)
Figs. 5. Film cooling distribution for plane 0p (a) baseline (b) trenched case
The v and w velocity vectors of plane 1p are shown as well. From the middle of the temperature distribution contour, a significant movement of vortexes toward the left and right sides is found. This is the effect of dilution injection on the thermal behavior of flow. In addition, a counter rotating vortexes is seen at the right side of both contours.
Fig. 7 shows the changes of film cooling effectiveness along Z axis for plane 1p. It is found from this figure that, by the application of trenched cooling holes, the adiabatic film cooling effectiveness doubled compared to the baseline. Of course, along Z axis, this ratio is being reduced and reached to the value of film cooling effectiveness of baseline case at Z=8cm that is equal to η=0.17. At Z=10cm, this quantity is lesser than baseline case. Figs. 8(a) (baseline) and 8(b) (trenched case) show the stream wise film cooling distribution through a first row of dilution jet.
(a)
(b)
Figs. 6. The vectors of v and w with film cooling effectiveness contours for plane 1p(a) baseline (b) trenched case
Fig. 7. The changes of cooling effectivenessalong Z axis of plane 1p (a) baseline (b) trenched case
Note, at the position of 60cm<X<72cm, the dilution jet injected into the mainstream and the coolest region is created. Furthermore, upstream the dilution jet, the hot region is approximately disappeared for the trenched case and it is happened due to the effects of coolant penetration from a trenched cooling holes.
Fig. 9(a) (baseline) and 9(b) (trenched case) show the film cooling distribution for plane 2p. It is declared from the analysis of contour that, the rotating flow is seen on the left side of figure and it is entrained along the span wise direction. However, this hot temperature region is almost disappeared by the trenching of cooling holes.
Overwhelmingly apparent, however is the lack of uniformity within the combustor exiting profile at this point. Also, at the right side of Fig. 9(a), the hot gases covered more extended area compared to the trenched case. Although, in this region and from Y=52cm to Y=54cm, the temperature is higher than baseline. In addition at the middle of the contour, the cooling system performs better for the trenched case. Figs. 10(a) (baseline) and 10(b) (trenched case) indicated the film cooling effectiveness distribution of plane 3p. The results showed that film cooling effectiveness is higher for the trenched case.
(a)
(b)
Figs. 8. Film cooling distribution for plane 0s (a) baseline (b) trenched case
(a)
(b)
Figs. 9. Film cooling distribution for plane 2p (a) baseline (b) trenched case
As shown in the figures, a cooler area is spread better for the trenched case especially at the right side of the contour. At the middle, no significant different is seen between the contours and totally it is declared that the distribution of film cooling effectiveness is almost same at the end of combustor simulator. Lastly, this figure shows the v and w velocity vectors superimposed on the thermal field contours in plane 3p.
A dominant vortex, rotating in the counter-clockwise direction at 0<Y<30cm as well as the vortexes rotating in the clockwise direction at 30cm<Y<54cm is found.
However the rotating of the vortexes is more turbulent for the trenched case because the coolant is injected further and the interaction of the film-coolant flow is entrained by the shear forces created by the main flow.
(a)
(b)
Figs. 10. The vectors of v and w with film cooling effectiveness contours for plane 3p(a) baseline (b) trenched case
IV. Conclusion
The purpose of this study was to extend the database knowledge about the film cooling variation adjacent the combustor end wall. In this research a three dimensional representation of a true Pratt and Whitney aero-engine was simulated.
The combustor simulator was meshed by Gambit and simulated by Fluent software. To sum up, the penetration depth increase grew the thickness of the film-cooling layer. Also, at the further distance from the trenched case, the temperature near the wall and between the jets was slightly cooler with the usage of trenched holes especially for the plane 1p.
Comparison between the baseline and trenched cooling holes showed that the usage of trenched hole with alignment angle of 90 deg is highly recommended because of the importance of this ratio near the end wall surface. In addition, from the comparison of experimental and CFD prediction, it is found that from Z/d=5, the film cooling effectiveness ratio was almost equal for both data.
Acknowledgements
This work was supported by Universiti Teknologi Malaysia and Ministry of Higher Education of Malaysia.
This research was supported by Research University Grant 03H58.
References
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Authors’ information
Nor Azwadi C. Sidik was born in Kelantan on 23rd. September 1977. He received Ph.D degree (2010) from Keio University, Japan His current interest includes computational fluid dynamics, numerical methods and fluid structure interaction. Dr Nor Azwadi is a senior lecturer at Department of Thermo fluid, UniversitiTeknologi Malaysia, Malaysia.
Ehsan Kianpour was born in Iran on 22nd November 1980. He received Master degree (2008) from Malek Ashtar University of Technology, Iran. His current interest includes numerical heat transfer, combustion and thermodynamics. Mr Ehsan is a PhD candidate at the Department of Thermo fluid, Universiti Teknologi Malaysia, Malaysia.
Syahrullail Samion was born in Johorin 1976.
He received Ph.D degree (2010) from Kagoshima University, Japan His current interest includes experimental fluid dynamics, tribology and fluid structure interaction. Dr Syahrulail is a senior lecturer at Department of Thermofluid, Universiti Teknologi Malaysia, Malaysia.
