- Email: [email protected]
Multi-parameter influence on combined-hole film cooling system
International Journal of Heat and Mass Transfer 55 (2012) 4232–4240
Contents lists available at SciVerse ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Multi-parameter influence on combined-hole film cooling system Han Chang ⇑, Ren Jing, Jiang Hong-de Gas Turbine Institute, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 14 December 2011 Received in revised form 1 March 2012 Accepted 26 March 2012 Available online 18 April 2012 Keywords: Film cooling Double-jet Density ratio Blowing ratio Pressure sensitive paint (PSP)
a b s t r a c t Combined-hole film cooling system is a promising way to improve cooling efficiency of gas turbine. New combined-hole systems, such as double-jet [14], anti-vortex design [16] and NIKOMIMI [15] are developed to overcome the kidney vortexes. Round-hole and double-jet, which are typical in design of combined-hole film cooling system, are investigated in this study experimentally and numerically. It is observed that the anti-kidney vortexes produced in double-jet film cooling system are controlled by the pitch and compound angle of the two holes. The pitch of double-jet unit determines the formation of anti-kidney vortexes led by the interaction between two vortexes, while the compound angle affects the strength of each branch of anti-kidney vortexes. Being two of the most important factors for film cooling of gas turbine, blowing ratio and density ratio, vary in degree of influence on film cooling effectiveness, and the key point is whether the cooling gas has blown off from the surface. It is possible to achieve high film cooling effectiveness with a combined-hole system when the geometry parameters, (compound angle, pitch, etc.), and aerodynamic parameters, (blowing ratio, density ratio, etc.) match well on local flow structure. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In order to improve output and efficiency, the inlet temperature of modern gas turbine has been greatly raised, which is much higher than material limit of components. Film cooling is widely used to protect the components from being destroyed by hot mainstream. There are many factors influencing film cooling, such as geometry of film cooling hole, turbulence and Re-number of mainstream, momentum ratio, density ratio and blowing ratio between cooling gas and mainstream, etc. Blowing ratio, representing dimensionless mass flux rate of cooling gas, and density ratio, deciding dimensionless temperature of cooling gas, are the most important factors on film cooling, for they together determine the specific heat capacity of cooling gas, which is the potential capacity of protecting hot components. The density ratio between cooling air and hot mainstream is about 1.8–2.5 in the first stage blades of modern gas turbine because of the great difference of temperature between them. Researchers have studied the influence of blowing ratio and other parameters on film cooling for a long time, and Godstein et al. [1,2], Thole et al. [3], Bogard et al. [4], and Bell et al. [5] are mentioned as examples. However, it is difficult and expensive to test in real operation conditions of high density ratio. Researchers try to simulate density ratio in tests in different ways. Burns et al. [6] and Teekaram et al. [7] measured film cooling effectiveness in high density ⇑ Corresponding author. E-mail address: [email protected] (C. Han). 0017-9310/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2012.03.064
ratio condition using foreign gas as cooling gas. It was shown that there is close agreement between film cooling characteristic obtained where the density ratio is achieved by either changing ejection as well as mainstream temperature or using a foreign gas. Bazdidi-Tehrani et al. [8,9] studied film cooling characteristic at the density ratio between 1.0 and 3.2 by changing the temperature of mainstream gas from 300 to 930 K. Wright et al. [10] studied the effect of density ratio on plate film cooling with shaped holes using pressure sensitive paint (PSP) technology. They showed that high density ratio improves film cooling effectiveness at high blowing ratio. The classical kidney vortexes produced in round-hole system are the most important factor adversely influencing film cooling effectiveness. It is a good way to overcome kidney vortexes and improve film cooling effect by optimizing film cooling hole, including hole-shape and hole-combination. Leylek and his group [11– 13] analyzed the film cooling mechanism of round-hole, shapedhole and compound-angle-hole system in detail. Results showed that the characteristic of film cooling is dominated by flow structure. Kidney vortexes in the flow field of round-hole system are the main reason of cooling gas blowing off from the wall, but shaped-hole and compound-angle-hole system change the ejection exit flow structure and pressure distribution, so as to weaken the kidney vortexes and improve film cooling effect. Some researchers tried to improve film cooling effect by combining two or more film cooling holes together as a unit. Kusterer et al. [14] combined two round holes with different compound angles, named doublejet, and predicted its film cooling characteristic by numerical
C. Han et al. / International Journal of Heat and Mass Transfer 55 (2012) 4232–4240
simulation. Later, he designed another combined-hole system, named NEKOMIMI [15], for high film cooling effectiveness. Heidmann [16] combined three round holes together, the biggest one in the middle and the other two with compound angle on both sides, named anti-vortex design for high efficient film cooling. The combined-hole units are shown in Fig. 1. They all tried to design the specific flow structure of film cooling system, as to weaken kidney vortexes influence and improve film cooling effect. As shown in Fig. 2, the anti-kidney vortexes produced in double-jet film cooling system rotate oppositely to kidney vortexes, which are the exemplary flow structure of combined-hole film cooling system. The authors [17] firstly measured the film cooling effectiveness of double-jet using pressure sensitive paint (PSP), while round-hole and shaped-hole were also implemented as base cases. In order to discuss multi-parameter influence on the film cooling characteristic of combined-hole system, a typical style of combined-hole system, double-jet and round-hole are measured using PSP in different conditions of blowing ratio and density ratio. It is aimed to find a way to optimize the combined-hole system for high film cooling effectiveness, and provide experimental database for its industrial usage. Besides, numerical simulation is also conducted for analysis in this work.
4233
Fig. 2. Ideal anti-kidney vortexes [14].
2. Plate film cooling test rig and measurement technique 2.1. Plate film cooling test rig
Fig. 3. Schematic of plate film cooling test rig.
Film cooling experiments are conducted on the plate film cooling test rig built in Tsinghua University. The schematic arrangement of the rig is illustrated in Fig. 3, and the test section with the light source is shown in Fig. 4. Mainstream velocity is controlled by the valve and measured by U-tube. Cooling ejection, including two kinds of gases, is controlled and measured by flow meter. Pressure sensitive paint (Uni-FIB PSP, UF470-750, supplied by Innovative Scientific Solution, Inc. (ISSI)) is the main measuring technology to obtain film cooling effectiveness, and experimental data is collected by CCD-camera and dealt with by computer. The length of crossing square of main duct in the test section is 160 mm, and it is 100 mm for the ejection duct. Re-number of mainstream flow based on cooling hole diameter and mainstream velocity equals 4500, and that of cooling flow changes according to blowing ratio, from 2200 to 8000. 2.2. Pressure sensitive paint (PSP) technology PSP was used to measure surface pressure distribution in aerospace industry in the beginning [18], then it was used by Zhang
Fig. 4. Picture of test section with light source.
et al. [19] to measure film cooling effectiveness through heat/mass analogy. PSP can catch oxygen partial pressure on the surface coated with it. In film cooling experiments, mainstream gas is air, and cooling ejection gas is non-oxygen. Cooling gas is convected and mixed with mainstream gas, which changes the oxygen concentration that decides the oxygen partial pressure on test surface. So film cooling effectiveness is obtained as the expression
Fig. 1. Schematic of combined-hole unit [14–16].
4234
C. Han et al. / International Journal of Heat and Mass Transfer 55 (2012) 4232–4240 Table 1 Experimental cases. Cases
M [–]
DR [–]
T/K
Round-hole Double-jet
0.5/0.8/1.0/1.2/1.5/1.78 0.5/0.8/1.0/1.2/1.5/1.78
0.98/1.52 0.98/1.52
300 300
Fig. 5. Schematic of PSP calibration system.
Fig. 9. Double-jet computation domain.
Fig. 6. Picture of PSP calibration system.
Fig. 10. Double-jet computation grid.
Fig. 7. PSP calibration curves.
Fig. 11. Grid-independency study.
Fig. 8. Schematic of round and double-jet.
below through heat/mass analogy from the oxygen partial pressure information PSP gets.
g¼
C 1 C mix C 1 C eje
)
g¼
T 1 T aw T 1 T eje
ð1Þ
C. Han et al. / International Journal of Heat and Mass Transfer 55 (2012) 4232–4240
4235
Fig. 12. Experimental result: film cooling effectiveness distribution of double-jet and round.
picture of PSP calibration system with light source. Fig. 7 shows the calibration curves, which are almost coincident at different temperature, implying that PSP is more sensitive with pressure change than temperature change. Therefore, PSP can keep repeatable when atmosphere temperature changes. Based on 95% confidence interval the uncertainty of the film cooling effectiveness is estimated as 3% at a typical value of 0.5. However, the uncertainty rises with the effectiveness approaching zero, resulting in an uncertainty of approximately 8% when the value is 0.05. More details about PSP measuring film cooling effectiveness can be seen in paper [20]. In recent years, PSP has been widely used for film cooling study by many scholars, such as Wright et al. [21], Zhang et al. [19,22], and Je-chin Han et al. [23]. It has been proved that the technology has high precision, stability and repeatability. 2.3. Experimental condition Fig. 13. Validation of experimental results.
where C1 is oxygen mass concentration of mainstream gas, C eje is oxygen mass concentration of cooling ejection gas, and C mix is oxygen mass concentration of mixture gas on test surface. Oxygen mass concentration is alternated with oxygen partial pressure and the expression changes to
g ¼ 1 ðPO
1 Þ 2 air
ðPO Þmix 2
1
Meje M air
þ1
ð2Þ
where ðP O2 Þair is oxygen partial pressure of mainstream; ðPO2 Þmix is oxygen partial pressure on test surface gained by PSP; Mair and Meje is the molecular weight of air and cooling gas. PSP should be calibrated to obtain the relationship between image intensity and dimensionless pressure before experiments. The schematic of PSP calibration system is shown in Figs. 5 and 6 is the
The schematic of round-hole and double-jet unit is shown in Fig. 8. Double-jet unit consists of two round holes with compound angle, ±45°. The diameter of double-jet hole is D = 3 mm while the pffiffiffi diameter of round hole equals 2D, for the coincidence of flowing area of the two ejection hole-unit. Dimensionless lateral pitch is P/ D = 3.75 and dimensionless length of hole is L/D = 6. The experimental cases including different blowing ratios and different density ratios, are shown in Table 1. Here blowing ratio, density ratio and momentum ratio are defined as M, DR and I.
qc v c q1 v 1 q DR ¼ c q1 q v 2 M2 I ¼ c c2 ¼ q1 v 1 DR
M¼
ð3Þ ð4Þ ð5Þ
4236
C. Han et al. / International Journal of Heat and Mass Transfer 55 (2012) 4232–4240
Fig. 14. Experimental result: centerline film cooling effectiveness at two density ratios.
Fig. 15. Experimental result: lateral average film cooling effectiveness at two density ratio.
3. Numerical method
Fig. 16. Experimental result: area-average film cooling effectiveness in different conditions.
where qc, vc is the velocity and density of cooling gas, and q1, v1 is the velocity and density of mainstream gas.
In order to gain more understanding about film cooling characteristics of double-jet and round-hole, numerical simulation based on the experiments is also conducted in this work. RANS algorithm based on finite-volume method is applied in the numerical simulation. The whole computation is carried out on steady Reynolds–Average–Navier–Stock equations with k-e turbulence model. All geometry and boundary conditions are the same as that for the experiments, except for the symmetry condition in lateral direction to decrease domain grid number. As mentioned above, PSP measures film cooling effectiveness through heat/mass transfer analogy, so does it in numerical simulation to take the coincidence with experiments. The mass transfer coefficient of the components of mainstream and cooling gas is calculated by kinetic theory in Ansys code Fluent. Computation domain and grid are shown in Figs. 9 and 10. In all cases, y+ of the first point near wall is predominantly the order of 1, except a small region around the exit of ejection hole. Based on the grid-independency study as shown in Fig. 11, it is resulted that the suitable node number used in this work is about 1.3 million for round-hole, and about 1.7 million for double-jet which has one more cooling duct than round-hole.
C. Han et al. / International Journal of Heat and Mass Transfer 55 (2012) 4232–4240
4237
Fig. 17. Experimental result: centerline film cooling effectiveness.
Fig. 18. Experimental result: lateral average film cooling effectiveness.
4. Aerodynamic parameter influence on film cooling effectiveness Experimental results of film cooling effectiveness distribution contour of double-jet and round-hole film cooling system is shown in Fig. 12. At a low blowing ratio, there is uncovered area of film cooling between two units in cases of double-jet, but the film effectiveness increases significantly when blowing ratio increases. For round-hole, there is a bigger area without film coverage between two units, especially downstream near the holes, but it is a little better on downstream far away from the holes where the cooling air reattach the wall. The higher the blowing ratio is, the bigger the uncovered area is in the cases of round-hole, but it is a little more optimal at high density ratio. The film cooling effectiveness of double-jet is better than that of round-hole in the same condition. As blowing ratio increases, film cooling effectiveness of double-jet increases at both density ratios, but it decreases in the cases of round-hole. High density ratio increases the film cooling effectiveness of round-hole at high blowing ratio and puts off the transient blowing ratio, from which film cooling effectiveness begins to decrease as blowing ratio increases. Blowing ratio and density ratio have relative influence on the film cooling effective-
ness of double-jet and round-hole. The influence of blowing ratio and density ratio on the film cooling effectiveness is analyzed and discussed quantitatively below. Before going ahead, validation for experimental results is implemented by comparing the film cooling effectiveness of round-hole system with previous studies [24,25], as shown in Fig. 13. Considering the difference of round hole diameter, pitch and mainstream parameter, only centerline film cooling effectiveness is carried out. Results of double-jet are not validated directly due to lack of experimental data in published literatures so far, but they are supposed to be reliable since they come from the same experimental system with round-hole. In general, the experimental results in this work are coincident with the previous studies [24,25], except for the area near hole exit (less than X/D = 5), where it is more sensitive to the difference of condition. 4.1. Blowing ratio influence Figs. 14 and 15 show centerline and lateral average film cooling effectiveness of double-jet and round-hole at different blowing ratios. As the blowing ratio increases, the exit momentum of cooling ejection increases. The film cooling effectiveness of round-hole
4238
C. Han et al. / International Journal of Heat and Mass Transfer 55 (2012) 4232–4240
decreases since the cooling jet lifts off from the wall because of kidney vortexes; however, double-jet is kept high film cooling effect at a high blowing ratio because of combining holes to product antikidney vortexes, which constrain the lift-off of cooling ejection. The film cooling effect of double-jet is better than that of round-hole, and the higher the blowing ratio is, the bigger the difference is, except at the lowest blowing ratio. The centerline film cooling effectiveness of double-jet and round-hole is equivalent at the lowest blowing ratio because of the attachment of cooling ejection in both cases. Area-average film cooling effectiveness of double-jet and roundhole in different conditions is shown in Fig. 16. At a low density ratio, M = 0.8 is the transient blowing ratio in the case of round-hole, but high density ratio puts off the transient blowing ratio to M = 1.0, since it decreases the exit momentum of cooling ejection so as to delay the lift-off of cooling ejection. For double-jet, area-average film cooling effectiveness increases as the blowing ratio increases in both cases of different density ratios except the highest blowing ratio at a low density ratio, which implies that the transient blowing ratio of area-average film cooling effectiveness is higher than M = 1.78 at a high density ratio, and between M = 1.5 and M = 1.78 at a low density ratio. In fact, it can also be observed from Figs. 14 and 15. 4.2. Density ratio influence Experimental results of centerline film cooling effectiveness of double-jet and round-hole at different density ratios are shown
in Figs. 17 and 18 shows the results of lateral average film cooling effectiveness. Density ratio has relatively strong influence on film cooling effectiveness, depending on hole-shape and blowing ratio. High density ratio improves the film cooling effectiveness of round-hole injection, especially on the zone less than X/D = 15 at a high blowing ratio, but density ratio insignificantly influences the centerline film cooling effectiveness distribution of doublejet. The key point of density ratio’s role in film cooling is whether cooling ejection has blown off from the wall. When the cooling ejection has blown off, exit momentum of cooling ejection decreases as density ratio increases, constraining the lift-off of cooling gas and improving film cooling effect, such as the case of round-hole at a high blowing ratio. When the cooling jet stays near the wall without blowing off, however, mass magnitude of cooling air covering on the wall is the same for the same blowing ratio. Therefore, blowing ratio is the controlling factor for film cooling effect and density ratio hardly influences the film cooling effectiveness, such as the cases of double-jet. 5. Formation of anti-kidney vortexes for combined-hole system Flow structure of film cooling system dominates the distribution of cooling air near the wall, so as to determine the film cooling effectiveness. Fig. 19 shows the velocity vector obtained from the simulation in the crossing plan at X/D = 3 of downstream, and the contour color expresses oxygen mass concentration. Cooling
Fig. 19. Velocity vector and cooling gas concentration in the crossing plane.
C. Han et al. / International Journal of Heat and Mass Transfer 55 (2012) 4232–4240
4239
Fig. 20. Formation of anti-kidney vortexes.
gas is non-oxygen in PSP experiments, thus oxygen mass concentration represents cooling gas distribution. High oxygen mass concentration means low film cooling effectiveness, whereas low oxygen mass concentration means high film cooling effectiveness. The classical kidney vortexes are produced in the round-hole film cooling system, which attract hot mainstream gas to the bottom of cooling gas from both sides and lift off the cooling gas from the wall. Anti-kidney vortexes are produced in the cases of doublejet, which rotate oppositely to kidney vortexes. In fact, anti-kidney vortexes consist of two pairs of vortexes produced in the roundhole with compound angle film cooling system. The formatting process of anti-kidney vortexes is illustrated in Fig. 20. The classical kidney vortexes are produced in round-hole system, as shown in Fig. 20(a) and (b). They change to one weak branch and one strong branch in the round-hole with compound angle system, and the bigger the compound angle is, the stronger the branch is. Anti-kidney vortexes are produced by combining two pairs of asymmetrical vortexes in round-hole with opposite compound angle together with a correct pitch, making sure each vortex is pushed to the wall by shear stress led by the rotation of another vortex. If the pitch of the two holes is too big, the two strong
branches of vortexes can not format anti-kidney vortexes, as they do not have interactive influence with each other and put each other back to the wall. If the pitch is too small, the two strong branches will become blockage to each other and lose the function of putting each other back to the wall. Comparing to the ideal antikidney vortexes shown in Fig. 2, the anti-kidney vortexes in the cases of this work are asymmetrical and do not cover the wall parallel. That is because the two holes of double-jet unit in this work are arranged as upside-down, and mainstream distance between them S = 3D, lateral distance P = 0, as shown in Fig. 8. As a result, one is stronger than the other of anti-kidney vortexes in the crossing plan, while the cooling ejection from the down-hole becomes blockage to the one from the up-hole and pushes the cooling ejection from the up-hole away from the wall. The pitch of double-jet unit is less optimized in this work, so the advantage of anti-kidney vortexes is not taken fully of here. Anti-kidney vortexes are produced by shear stress, as well as momentum exchange between the mainstream flow and cooling jets, so the authors believe the optimal arrangement of doublejet unit depends on the local flow characteristic. When the aerodynamic parameters, (density ratio, blowing ratio, etc.) and geometry
4240
C. Han et al. / International Journal of Heat and Mass Transfer 55 (2012) 4232–4240
parameters, (compound angle, pitch, etc.) are matched well, more efficient anti-kidney vortexes will be gained in the combined-hole film cooling system.
210100) and National Natural Science Foundation of China Project (51076076). References
6. Conclusion Film cooling characteristics of the combined-hole film cooling system, double-jet are studied in multi-parameter with the PSP technique. Numerical simulation is also conducted to obtain more detailed information and understanding, especially on flow structure. Film cooling characteristics and mechanism of the doublejet and round-hole are analyzed and compared by experimental results and flow structure. Film cooling characteristics are dominated by the flow structure of film cooling system. Kidney vortexes are produced in the cases of round-hole, while anti-kidney vortexes are produced in the cases of double-jet. Anti-kidney vortexes constrain the lift-off of cooling jets and make the coverage area of cooling air more uniform and broad in lateral direction. The film cooling effectiveness of double-jet is in general higher than that of round-hole in the same condition, and the higher the blowing ratio is, the bigger the difference is. Density ratio and blowing ratio vary in degree of influence on film cooling effectiveness, and the key point is whether the cooling jets have blown off from the surface. When cooling ejection has blown off, high density ratio constrains the lift-off as decreasing the exit momentum of cooling ejection; however, when cooling jets have not blown off, density ratio has less influence on cooling gas distribution on the wall because the film cooling effectiveness is dominated by the blowing ratio. Anti-kidney vortexes consist of two asymmetrical kidney vortexes. The formatting process is controlled by the compound angle and pitch. The first factor determines the strength of each branch and the interaction between the two branches is dominated by the second factor. Vortexes are produced by shear stress, as well as momentum exchange of cooling ejection and mainstream flow. When the geometry arrangement of combined-hole system is matched well with local flow structure, the detrimental vortexes structure for film cooling is constrained or even eliminated; while the profitable vortexes structure for film cooling is strengthened. Therefore, Authors believe that optimizing the arrangement of combinedhole system should base on local flow structure, and more efficient anti-kidney vortexes will be obtained when the geometry parameters and aerodynamic parameters match well in double-jet film cooling system. Acknowledgments The authors would like to acknowledge the financial support from the National Basic Research Program of China (2007CB
[1] R.J. Goldstein et al., Heat transfer and film cooling following injection through inclined circular tubes, ASME J. Heat Transfer (1974) (74-HT-V). [2] R.J. Goldstein et al., Effects of hole geometry and density on three-dimensional film cooling, Int. J. Heat Mass Transfer 17 (5) (1974) 595–607. [3] K.A. Thole et al., Heat transfer and film-cooling for the endwall of a first stage turbine vane, Int. J. Heat Mass Transfer 48 (25–26) (2005) 5255–5269. [4] D. Bogard et al., A film-cooling correlation for shaped holes on a flat-plate surface, J. Turbomach. 133 (1) (2011) 011002. [5] C.M. Bell et al., Film cooling subject to bulk flow pulsations: effects of blowing ratio, freestream velocity, and pulsation frequency, Int. J. Heat Mass Transfer 42 (23) (1999) 4333–4344. [6] W.K. Burns et al., The influence of foreign gas injection and slot geometry on film cooling effectiveness, Int. J. Heat Mass Transfer 12 (8) (1969) 935–951. [7] A.J.H. Teekaram et al., Use of foreign gas to simulate the effects of density ratios in film cooling, J. Turbomach. 111 (1) (1989) 57–62. [8] F. Bazdidi-Tehrani et al., Full-coverage discrete hole film cooling: investigation of the effect of variable density ratio, J. Eng. Gas Turbines Power 116 (3) (1994) 587–596. [9] F. Bazdidi-Tehrani et al., Full coverage discrete hole film cooling: investigation of the effect of variable density ratio (Part II), in: Proceedings of the 1997 International Gas Turbine and Aeroengine Congress & Exposition, USA. [10] L.M. Wright et al., Effect of density ratio on flat plate film cooling with shaped holes using PSP, J. Turbomach. 133 (4) (2011) 041011. [11] J.H. Leylek et al., A detailed analysis of film-cooling physics: Part I streamwise injection with cylindrical holes, J. Turbomach. 122 (1) (2000) 102–112. [12] J.H. Leylek et al., A detailed analysis of film cooling physics: Part II compound-angle injection with cylindrical holes, J. Turbomach. 122 (1) (2000) 113–121. [13] J.H. Leylek et al., A detailed analysis of film cooling physics: Part III streamwise injection with shaped holes, J. Turbomach. 122 (1) (2000) 122–132. [14] K. Kusterer et al., Double-jet ejection of cooling air for improved film cooling, J. Turbomach. 129 (4) (2007) 809–815. [15] K. Kusterer et al., NEKOMIMI cooling technology: cooling holes with ears for high-efficient film cooling, in: Proceedings of ASME Turbo Expo 2011, Canada. [16] J.D. Heidmann, A Numerical Study of Anti-Vortex Film Cooling Designs at High Blowing Ratio, ASME Turbo Expo, Germany, 2008. [17] C. Han et al., Characteristic of double-jet film cooling tested by pressure sensitive paint, J. Aerosp. Power 25 (12) (2010) 2770–2778. [18] M.J. Morris et al., Aerodynamic applications of pressure sensitive paint, AIAA J. 31 (3) (1993) 419–425. [19] L.Z. Zhang et al., Turbine nozzle film cooling study using the pressure sensitive paint (PSP) technique, J. Turbomach. 123 (4) (2001) 730–738. [20] J. Li et al., Film cooling performance of the embedded holes in trenches with compound angles, in: 2008 ASME Turbo Expo, England. [21] L.M. Wright et al., Measurement of detailed heat transfer coefficient and film cooling effectiveness distributions using PSP and TSP, ASME Conf. Proc. 48845 (2009) 807–817. [22] L.Z. Zhang et al., Turbine nozzle endwall inlet film cooling – The effect of a back-facing step and velocity ratio, in: 2004 ASME International Mechanical Engineering Congress and Exposition, November 13–19, 2004, United States. [23] J.C. Han et al., Film-cooling effectiveness on a gas turbine blade tip using pressure–sensitive paint, J. Heat Transfer 127 (5) (2005) 521–530. [24] R.F. Martinez-Botas et al., Film cooling characteristics of rows of round holes at various streamwise angles in a crossflow: Part I. Effectiveness, Int. J. Heat Mass Transfer 48 (23–24) (2003) 4995–5016. [25] J.E. Mayhew, The effect of freestream turbulence on film cooling adiabatic effectiveness, Int. J. Heat Fluid Flow 24 (5) (2003) 669–679.
Contents lists available at SciVerse ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Multi-parameter influence on combined-hole film cooling system Han Chang ⇑, Ren Jing, Jiang Hong-de Gas Turbine Institute, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 14 December 2011 Received in revised form 1 March 2012 Accepted 26 March 2012 Available online 18 April 2012 Keywords: Film cooling Double-jet Density ratio Blowing ratio Pressure sensitive paint (PSP)
a b s t r a c t Combined-hole film cooling system is a promising way to improve cooling efficiency of gas turbine. New combined-hole systems, such as double-jet [14], anti-vortex design [16] and NIKOMIMI [15] are developed to overcome the kidney vortexes. Round-hole and double-jet, which are typical in design of combined-hole film cooling system, are investigated in this study experimentally and numerically. It is observed that the anti-kidney vortexes produced in double-jet film cooling system are controlled by the pitch and compound angle of the two holes. The pitch of double-jet unit determines the formation of anti-kidney vortexes led by the interaction between two vortexes, while the compound angle affects the strength of each branch of anti-kidney vortexes. Being two of the most important factors for film cooling of gas turbine, blowing ratio and density ratio, vary in degree of influence on film cooling effectiveness, and the key point is whether the cooling gas has blown off from the surface. It is possible to achieve high film cooling effectiveness with a combined-hole system when the geometry parameters, (compound angle, pitch, etc.), and aerodynamic parameters, (blowing ratio, density ratio, etc.) match well on local flow structure. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In order to improve output and efficiency, the inlet temperature of modern gas turbine has been greatly raised, which is much higher than material limit of components. Film cooling is widely used to protect the components from being destroyed by hot mainstream. There are many factors influencing film cooling, such as geometry of film cooling hole, turbulence and Re-number of mainstream, momentum ratio, density ratio and blowing ratio between cooling gas and mainstream, etc. Blowing ratio, representing dimensionless mass flux rate of cooling gas, and density ratio, deciding dimensionless temperature of cooling gas, are the most important factors on film cooling, for they together determine the specific heat capacity of cooling gas, which is the potential capacity of protecting hot components. The density ratio between cooling air and hot mainstream is about 1.8–2.5 in the first stage blades of modern gas turbine because of the great difference of temperature between them. Researchers have studied the influence of blowing ratio and other parameters on film cooling for a long time, and Godstein et al. [1,2], Thole et al. [3], Bogard et al. [4], and Bell et al. [5] are mentioned as examples. However, it is difficult and expensive to test in real operation conditions of high density ratio. Researchers try to simulate density ratio in tests in different ways. Burns et al. [6] and Teekaram et al. [7] measured film cooling effectiveness in high density ⇑ Corresponding author. E-mail address: [email protected] (C. Han). 0017-9310/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2012.03.064
ratio condition using foreign gas as cooling gas. It was shown that there is close agreement between film cooling characteristic obtained where the density ratio is achieved by either changing ejection as well as mainstream temperature or using a foreign gas. Bazdidi-Tehrani et al. [8,9] studied film cooling characteristic at the density ratio between 1.0 and 3.2 by changing the temperature of mainstream gas from 300 to 930 K. Wright et al. [10] studied the effect of density ratio on plate film cooling with shaped holes using pressure sensitive paint (PSP) technology. They showed that high density ratio improves film cooling effectiveness at high blowing ratio. The classical kidney vortexes produced in round-hole system are the most important factor adversely influencing film cooling effectiveness. It is a good way to overcome kidney vortexes and improve film cooling effect by optimizing film cooling hole, including hole-shape and hole-combination. Leylek and his group [11– 13] analyzed the film cooling mechanism of round-hole, shapedhole and compound-angle-hole system in detail. Results showed that the characteristic of film cooling is dominated by flow structure. Kidney vortexes in the flow field of round-hole system are the main reason of cooling gas blowing off from the wall, but shaped-hole and compound-angle-hole system change the ejection exit flow structure and pressure distribution, so as to weaken the kidney vortexes and improve film cooling effect. Some researchers tried to improve film cooling effect by combining two or more film cooling holes together as a unit. Kusterer et al. [14] combined two round holes with different compound angles, named doublejet, and predicted its film cooling characteristic by numerical
C. Han et al. / International Journal of Heat and Mass Transfer 55 (2012) 4232–4240
simulation. Later, he designed another combined-hole system, named NEKOMIMI [15], for high film cooling effectiveness. Heidmann [16] combined three round holes together, the biggest one in the middle and the other two with compound angle on both sides, named anti-vortex design for high efficient film cooling. The combined-hole units are shown in Fig. 1. They all tried to design the specific flow structure of film cooling system, as to weaken kidney vortexes influence and improve film cooling effect. As shown in Fig. 2, the anti-kidney vortexes produced in double-jet film cooling system rotate oppositely to kidney vortexes, which are the exemplary flow structure of combined-hole film cooling system. The authors [17] firstly measured the film cooling effectiveness of double-jet using pressure sensitive paint (PSP), while round-hole and shaped-hole were also implemented as base cases. In order to discuss multi-parameter influence on the film cooling characteristic of combined-hole system, a typical style of combined-hole system, double-jet and round-hole are measured using PSP in different conditions of blowing ratio and density ratio. It is aimed to find a way to optimize the combined-hole system for high film cooling effectiveness, and provide experimental database for its industrial usage. Besides, numerical simulation is also conducted for analysis in this work.
4233
Fig. 2. Ideal anti-kidney vortexes [14].
2. Plate film cooling test rig and measurement technique 2.1. Plate film cooling test rig
Fig. 3. Schematic of plate film cooling test rig.
Film cooling experiments are conducted on the plate film cooling test rig built in Tsinghua University. The schematic arrangement of the rig is illustrated in Fig. 3, and the test section with the light source is shown in Fig. 4. Mainstream velocity is controlled by the valve and measured by U-tube. Cooling ejection, including two kinds of gases, is controlled and measured by flow meter. Pressure sensitive paint (Uni-FIB PSP, UF470-750, supplied by Innovative Scientific Solution, Inc. (ISSI)) is the main measuring technology to obtain film cooling effectiveness, and experimental data is collected by CCD-camera and dealt with by computer. The length of crossing square of main duct in the test section is 160 mm, and it is 100 mm for the ejection duct. Re-number of mainstream flow based on cooling hole diameter and mainstream velocity equals 4500, and that of cooling flow changes according to blowing ratio, from 2200 to 8000. 2.2. Pressure sensitive paint (PSP) technology PSP was used to measure surface pressure distribution in aerospace industry in the beginning [18], then it was used by Zhang
Fig. 4. Picture of test section with light source.
et al. [19] to measure film cooling effectiveness through heat/mass analogy. PSP can catch oxygen partial pressure on the surface coated with it. In film cooling experiments, mainstream gas is air, and cooling ejection gas is non-oxygen. Cooling gas is convected and mixed with mainstream gas, which changes the oxygen concentration that decides the oxygen partial pressure on test surface. So film cooling effectiveness is obtained as the expression
Fig. 1. Schematic of combined-hole unit [14–16].
4234
C. Han et al. / International Journal of Heat and Mass Transfer 55 (2012) 4232–4240 Table 1 Experimental cases. Cases
M [–]
DR [–]
T/K
Round-hole Double-jet
0.5/0.8/1.0/1.2/1.5/1.78 0.5/0.8/1.0/1.2/1.5/1.78
0.98/1.52 0.98/1.52
300 300
Fig. 5. Schematic of PSP calibration system.
Fig. 9. Double-jet computation domain.
Fig. 6. Picture of PSP calibration system.
Fig. 10. Double-jet computation grid.
Fig. 7. PSP calibration curves.
Fig. 11. Grid-independency study.
Fig. 8. Schematic of round and double-jet.
below through heat/mass analogy from the oxygen partial pressure information PSP gets.
g¼
C 1 C mix C 1 C eje
)
g¼
T 1 T aw T 1 T eje
ð1Þ
C. Han et al. / International Journal of Heat and Mass Transfer 55 (2012) 4232–4240
4235
Fig. 12. Experimental result: film cooling effectiveness distribution of double-jet and round.
picture of PSP calibration system with light source. Fig. 7 shows the calibration curves, which are almost coincident at different temperature, implying that PSP is more sensitive with pressure change than temperature change. Therefore, PSP can keep repeatable when atmosphere temperature changes. Based on 95% confidence interval the uncertainty of the film cooling effectiveness is estimated as 3% at a typical value of 0.5. However, the uncertainty rises with the effectiveness approaching zero, resulting in an uncertainty of approximately 8% when the value is 0.05. More details about PSP measuring film cooling effectiveness can be seen in paper [20]. In recent years, PSP has been widely used for film cooling study by many scholars, such as Wright et al. [21], Zhang et al. [19,22], and Je-chin Han et al. [23]. It has been proved that the technology has high precision, stability and repeatability. 2.3. Experimental condition Fig. 13. Validation of experimental results.
where C1 is oxygen mass concentration of mainstream gas, C eje is oxygen mass concentration of cooling ejection gas, and C mix is oxygen mass concentration of mixture gas on test surface. Oxygen mass concentration is alternated with oxygen partial pressure and the expression changes to
g ¼ 1 ðPO
1 Þ 2 air
ðPO Þmix 2
1
Meje M air
þ1
ð2Þ
where ðP O2 Þair is oxygen partial pressure of mainstream; ðPO2 Þmix is oxygen partial pressure on test surface gained by PSP; Mair and Meje is the molecular weight of air and cooling gas. PSP should be calibrated to obtain the relationship between image intensity and dimensionless pressure before experiments. The schematic of PSP calibration system is shown in Figs. 5 and 6 is the
The schematic of round-hole and double-jet unit is shown in Fig. 8. Double-jet unit consists of two round holes with compound angle, ±45°. The diameter of double-jet hole is D = 3 mm while the pffiffiffi diameter of round hole equals 2D, for the coincidence of flowing area of the two ejection hole-unit. Dimensionless lateral pitch is P/ D = 3.75 and dimensionless length of hole is L/D = 6. The experimental cases including different blowing ratios and different density ratios, are shown in Table 1. Here blowing ratio, density ratio and momentum ratio are defined as M, DR and I.
qc v c q1 v 1 q DR ¼ c q1 q v 2 M2 I ¼ c c2 ¼ q1 v 1 DR
M¼
ð3Þ ð4Þ ð5Þ
4236
C. Han et al. / International Journal of Heat and Mass Transfer 55 (2012) 4232–4240
Fig. 14. Experimental result: centerline film cooling effectiveness at two density ratios.
Fig. 15. Experimental result: lateral average film cooling effectiveness at two density ratio.
3. Numerical method
Fig. 16. Experimental result: area-average film cooling effectiveness in different conditions.
where qc, vc is the velocity and density of cooling gas, and q1, v1 is the velocity and density of mainstream gas.
In order to gain more understanding about film cooling characteristics of double-jet and round-hole, numerical simulation based on the experiments is also conducted in this work. RANS algorithm based on finite-volume method is applied in the numerical simulation. The whole computation is carried out on steady Reynolds–Average–Navier–Stock equations with k-e turbulence model. All geometry and boundary conditions are the same as that for the experiments, except for the symmetry condition in lateral direction to decrease domain grid number. As mentioned above, PSP measures film cooling effectiveness through heat/mass transfer analogy, so does it in numerical simulation to take the coincidence with experiments. The mass transfer coefficient of the components of mainstream and cooling gas is calculated by kinetic theory in Ansys code Fluent. Computation domain and grid are shown in Figs. 9 and 10. In all cases, y+ of the first point near wall is predominantly the order of 1, except a small region around the exit of ejection hole. Based on the grid-independency study as shown in Fig. 11, it is resulted that the suitable node number used in this work is about 1.3 million for round-hole, and about 1.7 million for double-jet which has one more cooling duct than round-hole.
C. Han et al. / International Journal of Heat and Mass Transfer 55 (2012) 4232–4240
4237
Fig. 17. Experimental result: centerline film cooling effectiveness.
Fig. 18. Experimental result: lateral average film cooling effectiveness.
4. Aerodynamic parameter influence on film cooling effectiveness Experimental results of film cooling effectiveness distribution contour of double-jet and round-hole film cooling system is shown in Fig. 12. At a low blowing ratio, there is uncovered area of film cooling between two units in cases of double-jet, but the film effectiveness increases significantly when blowing ratio increases. For round-hole, there is a bigger area without film coverage between two units, especially downstream near the holes, but it is a little better on downstream far away from the holes where the cooling air reattach the wall. The higher the blowing ratio is, the bigger the uncovered area is in the cases of round-hole, but it is a little more optimal at high density ratio. The film cooling effectiveness of double-jet is better than that of round-hole in the same condition. As blowing ratio increases, film cooling effectiveness of double-jet increases at both density ratios, but it decreases in the cases of round-hole. High density ratio increases the film cooling effectiveness of round-hole at high blowing ratio and puts off the transient blowing ratio, from which film cooling effectiveness begins to decrease as blowing ratio increases. Blowing ratio and density ratio have relative influence on the film cooling effective-
ness of double-jet and round-hole. The influence of blowing ratio and density ratio on the film cooling effectiveness is analyzed and discussed quantitatively below. Before going ahead, validation for experimental results is implemented by comparing the film cooling effectiveness of round-hole system with previous studies [24,25], as shown in Fig. 13. Considering the difference of round hole diameter, pitch and mainstream parameter, only centerline film cooling effectiveness is carried out. Results of double-jet are not validated directly due to lack of experimental data in published literatures so far, but they are supposed to be reliable since they come from the same experimental system with round-hole. In general, the experimental results in this work are coincident with the previous studies [24,25], except for the area near hole exit (less than X/D = 5), where it is more sensitive to the difference of condition. 4.1. Blowing ratio influence Figs. 14 and 15 show centerline and lateral average film cooling effectiveness of double-jet and round-hole at different blowing ratios. As the blowing ratio increases, the exit momentum of cooling ejection increases. The film cooling effectiveness of round-hole
4238
C. Han et al. / International Journal of Heat and Mass Transfer 55 (2012) 4232–4240
decreases since the cooling jet lifts off from the wall because of kidney vortexes; however, double-jet is kept high film cooling effect at a high blowing ratio because of combining holes to product antikidney vortexes, which constrain the lift-off of cooling ejection. The film cooling effect of double-jet is better than that of round-hole, and the higher the blowing ratio is, the bigger the difference is, except at the lowest blowing ratio. The centerline film cooling effectiveness of double-jet and round-hole is equivalent at the lowest blowing ratio because of the attachment of cooling ejection in both cases. Area-average film cooling effectiveness of double-jet and roundhole in different conditions is shown in Fig. 16. At a low density ratio, M = 0.8 is the transient blowing ratio in the case of round-hole, but high density ratio puts off the transient blowing ratio to M = 1.0, since it decreases the exit momentum of cooling ejection so as to delay the lift-off of cooling ejection. For double-jet, area-average film cooling effectiveness increases as the blowing ratio increases in both cases of different density ratios except the highest blowing ratio at a low density ratio, which implies that the transient blowing ratio of area-average film cooling effectiveness is higher than M = 1.78 at a high density ratio, and between M = 1.5 and M = 1.78 at a low density ratio. In fact, it can also be observed from Figs. 14 and 15. 4.2. Density ratio influence Experimental results of centerline film cooling effectiveness of double-jet and round-hole at different density ratios are shown
in Figs. 17 and 18 shows the results of lateral average film cooling effectiveness. Density ratio has relatively strong influence on film cooling effectiveness, depending on hole-shape and blowing ratio. High density ratio improves the film cooling effectiveness of round-hole injection, especially on the zone less than X/D = 15 at a high blowing ratio, but density ratio insignificantly influences the centerline film cooling effectiveness distribution of doublejet. The key point of density ratio’s role in film cooling is whether cooling ejection has blown off from the wall. When the cooling ejection has blown off, exit momentum of cooling ejection decreases as density ratio increases, constraining the lift-off of cooling gas and improving film cooling effect, such as the case of round-hole at a high blowing ratio. When the cooling jet stays near the wall without blowing off, however, mass magnitude of cooling air covering on the wall is the same for the same blowing ratio. Therefore, blowing ratio is the controlling factor for film cooling effect and density ratio hardly influences the film cooling effectiveness, such as the cases of double-jet. 5. Formation of anti-kidney vortexes for combined-hole system Flow structure of film cooling system dominates the distribution of cooling air near the wall, so as to determine the film cooling effectiveness. Fig. 19 shows the velocity vector obtained from the simulation in the crossing plan at X/D = 3 of downstream, and the contour color expresses oxygen mass concentration. Cooling
Fig. 19. Velocity vector and cooling gas concentration in the crossing plane.
C. Han et al. / International Journal of Heat and Mass Transfer 55 (2012) 4232–4240
4239
Fig. 20. Formation of anti-kidney vortexes.
gas is non-oxygen in PSP experiments, thus oxygen mass concentration represents cooling gas distribution. High oxygen mass concentration means low film cooling effectiveness, whereas low oxygen mass concentration means high film cooling effectiveness. The classical kidney vortexes are produced in the round-hole film cooling system, which attract hot mainstream gas to the bottom of cooling gas from both sides and lift off the cooling gas from the wall. Anti-kidney vortexes are produced in the cases of doublejet, which rotate oppositely to kidney vortexes. In fact, anti-kidney vortexes consist of two pairs of vortexes produced in the roundhole with compound angle film cooling system. The formatting process of anti-kidney vortexes is illustrated in Fig. 20. The classical kidney vortexes are produced in round-hole system, as shown in Fig. 20(a) and (b). They change to one weak branch and one strong branch in the round-hole with compound angle system, and the bigger the compound angle is, the stronger the branch is. Anti-kidney vortexes are produced by combining two pairs of asymmetrical vortexes in round-hole with opposite compound angle together with a correct pitch, making sure each vortex is pushed to the wall by shear stress led by the rotation of another vortex. If the pitch of the two holes is too big, the two strong
branches of vortexes can not format anti-kidney vortexes, as they do not have interactive influence with each other and put each other back to the wall. If the pitch is too small, the two strong branches will become blockage to each other and lose the function of putting each other back to the wall. Comparing to the ideal antikidney vortexes shown in Fig. 2, the anti-kidney vortexes in the cases of this work are asymmetrical and do not cover the wall parallel. That is because the two holes of double-jet unit in this work are arranged as upside-down, and mainstream distance between them S = 3D, lateral distance P = 0, as shown in Fig. 8. As a result, one is stronger than the other of anti-kidney vortexes in the crossing plan, while the cooling ejection from the down-hole becomes blockage to the one from the up-hole and pushes the cooling ejection from the up-hole away from the wall. The pitch of double-jet unit is less optimized in this work, so the advantage of anti-kidney vortexes is not taken fully of here. Anti-kidney vortexes are produced by shear stress, as well as momentum exchange between the mainstream flow and cooling jets, so the authors believe the optimal arrangement of doublejet unit depends on the local flow characteristic. When the aerodynamic parameters, (density ratio, blowing ratio, etc.) and geometry
4240
C. Han et al. / International Journal of Heat and Mass Transfer 55 (2012) 4232–4240
parameters, (compound angle, pitch, etc.) are matched well, more efficient anti-kidney vortexes will be gained in the combined-hole film cooling system.
210100) and National Natural Science Foundation of China Project (51076076). References
6. Conclusion Film cooling characteristics of the combined-hole film cooling system, double-jet are studied in multi-parameter with the PSP technique. Numerical simulation is also conducted to obtain more detailed information and understanding, especially on flow structure. Film cooling characteristics and mechanism of the doublejet and round-hole are analyzed and compared by experimental results and flow structure. Film cooling characteristics are dominated by the flow structure of film cooling system. Kidney vortexes are produced in the cases of round-hole, while anti-kidney vortexes are produced in the cases of double-jet. Anti-kidney vortexes constrain the lift-off of cooling jets and make the coverage area of cooling air more uniform and broad in lateral direction. The film cooling effectiveness of double-jet is in general higher than that of round-hole in the same condition, and the higher the blowing ratio is, the bigger the difference is. Density ratio and blowing ratio vary in degree of influence on film cooling effectiveness, and the key point is whether the cooling jets have blown off from the surface. When cooling ejection has blown off, high density ratio constrains the lift-off as decreasing the exit momentum of cooling ejection; however, when cooling jets have not blown off, density ratio has less influence on cooling gas distribution on the wall because the film cooling effectiveness is dominated by the blowing ratio. Anti-kidney vortexes consist of two asymmetrical kidney vortexes. The formatting process is controlled by the compound angle and pitch. The first factor determines the strength of each branch and the interaction between the two branches is dominated by the second factor. Vortexes are produced by shear stress, as well as momentum exchange of cooling ejection and mainstream flow. When the geometry arrangement of combined-hole system is matched well with local flow structure, the detrimental vortexes structure for film cooling is constrained or even eliminated; while the profitable vortexes structure for film cooling is strengthened. Therefore, Authors believe that optimizing the arrangement of combinedhole system should base on local flow structure, and more efficient anti-kidney vortexes will be obtained when the geometry parameters and aerodynamic parameters match well in double-jet film cooling system. Acknowledgments The authors would like to acknowledge the financial support from the National Basic Research Program of China (2007CB
[1] R.J. Goldstein et al., Heat transfer and film cooling following injection through inclined circular tubes, ASME J. Heat Transfer (1974) (74-HT-V). [2] R.J. Goldstein et al., Effects of hole geometry and density on three-dimensional film cooling, Int. J. Heat Mass Transfer 17 (5) (1974) 595–607. [3] K.A. Thole et al., Heat transfer and film-cooling for the endwall of a first stage turbine vane, Int. J. Heat Mass Transfer 48 (25–26) (2005) 5255–5269. [4] D. Bogard et al., A film-cooling correlation for shaped holes on a flat-plate surface, J. Turbomach. 133 (1) (2011) 011002. [5] C.M. Bell et al., Film cooling subject to bulk flow pulsations: effects of blowing ratio, freestream velocity, and pulsation frequency, Int. J. Heat Mass Transfer 42 (23) (1999) 4333–4344. [6] W.K. Burns et al., The influence of foreign gas injection and slot geometry on film cooling effectiveness, Int. J. Heat Mass Transfer 12 (8) (1969) 935–951. [7] A.J.H. Teekaram et al., Use of foreign gas to simulate the effects of density ratios in film cooling, J. Turbomach. 111 (1) (1989) 57–62. [8] F. Bazdidi-Tehrani et al., Full-coverage discrete hole film cooling: investigation of the effect of variable density ratio, J. Eng. Gas Turbines Power 116 (3) (1994) 587–596. [9] F. Bazdidi-Tehrani et al., Full coverage discrete hole film cooling: investigation of the effect of variable density ratio (Part II), in: Proceedings of the 1997 International Gas Turbine and Aeroengine Congress & Exposition, USA. [10] L.M. Wright et al., Effect of density ratio on flat plate film cooling with shaped holes using PSP, J. Turbomach. 133 (4) (2011) 041011. [11] J.H. Leylek et al., A detailed analysis of film-cooling physics: Part I streamwise injection with cylindrical holes, J. Turbomach. 122 (1) (2000) 102–112. [12] J.H. Leylek et al., A detailed analysis of film cooling physics: Part II compound-angle injection with cylindrical holes, J. Turbomach. 122 (1) (2000) 113–121. [13] J.H. Leylek et al., A detailed analysis of film cooling physics: Part III streamwise injection with shaped holes, J. Turbomach. 122 (1) (2000) 122–132. [14] K. Kusterer et al., Double-jet ejection of cooling air for improved film cooling, J. Turbomach. 129 (4) (2007) 809–815. [15] K. Kusterer et al., NEKOMIMI cooling technology: cooling holes with ears for high-efficient film cooling, in: Proceedings of ASME Turbo Expo 2011, Canada. [16] J.D. Heidmann, A Numerical Study of Anti-Vortex Film Cooling Designs at High Blowing Ratio, ASME Turbo Expo, Germany, 2008. [17] C. Han et al., Characteristic of double-jet film cooling tested by pressure sensitive paint, J. Aerosp. Power 25 (12) (2010) 2770–2778. [18] M.J. Morris et al., Aerodynamic applications of pressure sensitive paint, AIAA J. 31 (3) (1993) 419–425. [19] L.Z. Zhang et al., Turbine nozzle film cooling study using the pressure sensitive paint (PSP) technique, J. Turbomach. 123 (4) (2001) 730–738. [20] J. Li et al., Film cooling performance of the embedded holes in trenches with compound angles, in: 2008 ASME Turbo Expo, England. [21] L.M. Wright et al., Measurement of detailed heat transfer coefficient and film cooling effectiveness distributions using PSP and TSP, ASME Conf. Proc. 48845 (2009) 807–817. [22] L.Z. Zhang et al., Turbine nozzle endwall inlet film cooling – The effect of a back-facing step and velocity ratio, in: 2004 ASME International Mechanical Engineering Congress and Exposition, November 13–19, 2004, United States. [23] J.C. Han et al., Film-cooling effectiveness on a gas turbine blade tip using pressure–sensitive paint, J. Heat Transfer 127 (5) (2005) 521–530. [24] R.F. Martinez-Botas et al., Film cooling characteristics of rows of round holes at various streamwise angles in a crossflow: Part I. Effectiveness, Int. J. Heat Mass Transfer 48 (23–24) (2003) 4995–5016. [25] J.E. Mayhew, The effect of freestream turbulence on film cooling adiabatic effectiveness, Int. J. Heat Fluid Flow 24 (5) (2003) 669–679.