Experimental study on film cooling performance of imperfect holes

Ke’nn HUANG,Jingzhou ZHANG,b,*,Xioming TAN,Yong SHAN

aCollege of Energy and Power Engineering,Jiangsu Province Key Laboratory of Aerospace Power System,Nanjing University of Aeronautics and Astronautics,Nanjing 210016,China

bCollaborative Innovation Center for Advanced Aero-Engine,Beijing 100083,China

1.Introduction

Film cooling plays an important role on protecting the hotsection components from overheating.In the real applications,the initially designed perfect film holes may be partially obstructed by fine particulate matter due to foreign ingestion and combustion production.1–3In addition,the film-hole imperfections may also be resulted from thermal barrier coating spallation as well as imperfect manufacturing.4,5

It is well known that the geometric shape of film cooling holes is an important factor affecting film cooling behaviors.A lot of efforts have been devoted to the film cooling enhancement in past decades by actively optimizing the film-hole shape.6–12These shaped film-holes are properly designed to mitigate the detrimental effect of large-scale kidney vortices generated from a conventional film cooling hole.Differing from the shaped holes designed actively in the view of enhancing film cooling effectiveness,however,the alteration of initially designed film-hole caused by some uncontrolled reasons,such as particulate deposition,thermal barrier coatingspallation and manufacturing inaccuracy,etc.,is generally undesirable.In order to illustrate the influences of filmhole imperfection on the film cooling performance,some investigationswere carried outexperimentally4,5,13–16or numerically.17–19As the imperfections inside the film cooling holes are caused randomly with vast possibilities of blockage shapes as well as deposition orientations,some simplified configurations for simulating the in-hole blockage were presented,such as half torus,carving round rods and pyramid-shaped elements,etc.Of particular significance was that the affecting roles of film-hole imperfection on the film cooling behaviors illustrated by different researchers were found to be non-consistent,tightly dependent on the in-hole blockage orientations and blocking ratios.Due to the diversity of in-hole blockages and complexity of affecting roles,the knowledge about the film cooling performance of imperfect holes need further illustration.

To address this issue,a series of experiments are conducted in the present investigation to study the effects of in-hole blockages on a row of holes film cooling over a flat plate.

2.Experimental procedures

The experimental setup is schematically shown in Fig.1,the same as that used by Yang and Zhang11which consists basically of three main parts:the primary flow or mainstream supply passage,the secondary flow or coolant supply passage,and the test section.

The primary flow and secondary flow are supplied by two independent air compressors.Both flows are measured and adjusted by respective flow-meter and valve.In the primary flow supply passage,an electric heater is used for air heating.The test section has a constant rectangular cross-section(180 mm in width and 100 mm in height).Consequently,the primary inlet velocity(u∞)is controlled at 20 m/s approximately.The temperature of primary flow(T∞)is measured by a temperature probe.In the current tests,the temperature of primary flow is 85°C approximately.

Fig.1 Schematic of computational model.

The secondary flow plenum has a height of 18 mm.Its inlet is located at 90 mm ahead of the film-hole outlet and its end is located at 30 mm down the film-hole outlet.A row of perfect cylindrical holes with the same inclination angle(α)of 35°and diameter(d)of 6 mm is selected as the baseline case.Nine holes are involved in a single row with a fixed hole-to-hole spacing pitch of 3d.The temperature(Tc)and total pressure(p*c)of secondary flow are measured by a temperature probe and a total pressure probe respectively.Both probes are placed inside the coolant plenum.Besides,a static pressure probe is located immediately down the film hole to measure the exiting static pressure of coolant flow(pc).

Considering that a practical in-hole blockage is randomly distributed around the hole and the deposition generally has a bigger base and a smaller top,the specifically pyramidshaped element numerically investigated by Pan et al.19is selected as the in-hole obstruction in the present study.Six representative deposition locations are determined according to their orientations,as seen in Fig.2.The film-protected plate is made of a bakelite plate,which has a thermal conductivity of about 0.15 W/(m·K).All the pyramid-shaped elements occupy one-third of film-hole length.For the blockage deposited in vicinity of film-hole inlet or exit,the apex of in-hole blockage is located at the corresponding inlet plane or exit plane.While for the blockage deposited at the middle of film-hole,the apex of in-hole blockage is located at the middle plane.The in-hole blockage blocking ratio(B)is defined according to this specified cross-sectional plane where the apex of in-hole blockage is located,as seen in Eq.(1).

where Ab,sectionis the cross-sectional area of blockage at the cross-sectional plane where the apex is located.In this crosssectional plane,the obstruction of the in-hole blockage inside the film cooling hole is the maximum.

Fig.2 Schematic of in-hole blockage.

Two sets of blockage geometries are considered in the current experimental test.

(1)Varying the blockage orientation for a fixed blocking ratio of B=0.3.

(2)Varying the blocking ratio from 0.1 to 0.4 for a specific leading-exit blockage orientation.

The definitions of blowing ratio(M),cooling effectiveness(η)and discharge coefficient(Cd)are illustrated in Eqs.(2)–(4)in turns.

where ρ,u and T are density,velocity and temperature respectively;the subscripts ‘c”,‘∞”,‘w” and ‘inlet” denote the secondary flow,primary flow,wall and in let parameters respectively;mcis actual coolant mass flow rate,which is measured in the secondary flow supply passage;p*cand pcare total pressure at hole-inlet and static pressure at hole-outlet of the coolant flow respectively;Ac,inletis the total inlet area of a row of film cooling holes.

A special notice needs pointing out in the current definitions of blowing ratio(Eq.(2))and discharge coefficient(Eq.(4))is that the parameters of coolant flow are selected as the corresponding values at the film-hole inlet.Such selections ensure that both the imperfect hole and the perfect hole have the same coolant mass flow rate under a given blowing ratio.The coordinate origin is located at the trailing edge of central-hole exit.x-,y-,and z-directions are defined as the streamwise,normal and lateral directions respectively.

The wall temperature(Tw)is measured by an infrared camera.For ensuring a nearly perfect emissivity of tested surface,the surface treatment is made in advance by spraying a thin black paint uniformly.An infrared glass with a high transmissivity serves as the measuring window to the infrared camera.

Fig.3 Surface temperature distributions(M=0.5,B=0.3).

In addition,several flow field measurements are also conducted by using particle image velocimetry methodology.These measurements were performed on the film-hole centerline plane(z/d=0).

In the experiments,the individual measured temperature uncertainties of the primary flow,cooling air and wall surface are approximately ±1.0°C, ±0.5°C and ±1.0°C,respectively.According to the theory of error methodology,the maximum uncertainty for the cooling effectiveness measurement is estimated to be±7%approximately.The uncertainty in discharge coefficient measurements is estimated to be within of±4%.

3.Results and discussion

3.1.Detailed temperature and flow fields

Fig.3 presents some surface temperature distributions under a moderate blowing ratio of M=0.5.For the imperfect film cooling holes,the blocking ratio(B)is kept as 0.3.Comparing with the perfect hole or baseline case,it is found that the local blockages orientated at the film-hole exit have more obvious effect on the film cooling behaviors.When the local blockage is deposited at leading-exit of film-hole,the wall temperature on the film-cooling protected surface is decreased downstream the film-hole exit,indicating an improvement of film cooling is achieved.While for the trailing-exit blockage orientation,the wall temperature is increased drastically related to the baseline case,indicating a serious degradation of film cooling effectiveness occurs in this situation.For the other in-hole blockages orientated at inlet or mid positions,a negative influence on the film cooling is generally confirmed,but this influence seems relatively weaker than that of trailing-exit blockage orientation.

Fig.4 shows the measured velocity field on the film-hole centerline plane.It is confirmed that the in-hole blockages change the flow field near the film-hole exit significantly.Related to the baseline case(as seen in Fig.4(a)),the local blockage deposited at leading-exit of film-hole leads to a relatively weaker coolant penetration into the primary flow,as seen in Fig.4(b).This role was also proved by previous investigations11,19,20,which is helpful for film cooling enhancement.On the contrary,the local blockage deposited at trailing-exit of film-hole leads to a stronger coolant jet penetration into primary flow,as seen in Fig.4(c).As more coolant injection momentum is transferred along the normal direction,the coolant jet is more seriously lifted off the film-cooling protected surface,thus producing detrimental influence on film cooling effectiveness.

3.2.Film cooling effectiveness

Fig.4 Velocity vector at hole-centerline plane under M=1.0.

The influences of in-hole blockage orientation on the laterallyaveraged film cooling effectiveness distributions under a fixed blocking ratio of B=0.3 and a moderate blowing ratio of M=1.0 are demonstrated in Fig.5.In Fig.5(a),the experimental data presented by Lu et al.21are also displayed for comparison.It is seen that the current tested result for the cylindrical hole agrees well with that of Lu et al.21beyond x/d=7.Near the film-hole outlet,the current laterallyaveraged film cooling effectiveness varies more smoothly along the streamwise direction than that of Lu et al.21This is caused by the thermal conductive effect in the current test.It is found that the partial leading-exit blockage plays a role on enhancing the film cooling effectiveness.It leads to approximately 20%increase in the region between 2＜x/d＜12 related to the baseline case.While the other blockage orientations are shown to play roles on deteriorating the film cooling effectiveness,especially for the trailing-exit orientation.Approximately 50% reduction of the laterally-averaged film cooling effectiveness related to the perfect film-hole is found under the trailing-exit deposition case.From Fig.5,it is also found that the in-hole blockages deposited vicinity of film-hole inlet and the middle of film-hole play nearly the same influence on the film cooling effectiveness.

The influences of blowing ratio on laterally-averaged film cooling effectiveness distributions of imperfect film cooling holes under a fixed blocking ratio of 0.3 is shown in Fig.6.When the blowing ratio is far less than one,the laterallyaveraged film cooling effectiveness decreases along the streamwise direction monotonously despite the blockage deposition orientation.When the blowing ratio is bigger than one,the laterally-averaged film cooling effectiveness in the region immediately close to the film hole is weakened drastically.However,the film cooling far downstream from film-hole exit is sometimes enhanced,dependently on the blockage orienta-tion.It is also found that the influence of in-hole blockage orientation behaves more significantly under higher blowing ratios.In a common sense,the in-hole blockage induces the coolant flow distortion,which is the main due for creating the film cooling difference of the imperfect hole from the perfect case.Under a smaller blowing ratio,the coolant flow distortion is recovered more easily,thus showing a relatively weaker influence.

Fig.5 Effect of blockage position on laterally-averaged film cooling effectiveness(M=1.0,B=0.3).

The influences of blocking ratio on the laterally averaged film cooling effectiveness distributions for a specific leadingexit blockage orientation are illustrated in Fig.7.Under M=0.5,the leading-exit orientation seems to have a little influence.Under M=1.0,the positive role of leading-exit blockage on improving film cooling appears.Under M=1.5,the enhancement of film cooling effectiveness by the leading-exit blockage behaves more significantly.Also,it is found that the leading-exit blockage with B=0.3 produces the highest film cooling effectiveness.

3.3.Discharge coefficient

Fig.8 presents the influence of blockage orientation on filmhole discharge coefficient under a fixed blocking ratio of 0.3.Here the Reynolds number of coolant flow(Rec)is defined as

where μcis dynamic viscosity of the coolant.The parameters(such as ρcand uc)in this definition are selected as the corresponding values at the film-hole inlet.

Fig.6 Laterally-averaged film cooling effectiveness under different blowing ratios(B=0.3).

Fig.7 Laterally-averaged film cooling effectivenessunder different blocking ratios(leading-exit deposition).

Compared with the perfect film cooling hole,it is confirmed that the in-hole blockage makes the discharge coefficient reduction under the same coolant flow Reynolds number.According to the definition of discharge coefficient,as illustrated in Eq.(4),it is also concluded that the in-hole blockage creates a larger pressure drop across the film hole to under the same coolant flow Reynolds number related to the perfect filmhole,which is well satisfactory with the previous investigation.4,11Of particular is that the film-hole discharge coefficient is tightly associated with the blockage orientation.For examples,for the in-hole blockage deposited at leading edge of film hole,the leading-exit blockage seems to produce the most serious pressure drop.While for the in-hole blockage deposited at trailing edge of film hole,it is found the trailing-inlet deposition has the strongest influence on the film-hole discharge coefficient.In the situations of leading-exit and trailing-inlet blockage deposition,the reduction of film-hole discharge coefficient is approximately 50%related to the perfect film-hole,reflecting that the internal flows of coolant inside film hole are distorted more seriously.

Fig.8 Effect of blockage orientation on film-hole discharge coefficient(B=0.3).

Fig.9 shows the influence of in-hole blockage size on film hole discharge coefficient for a specific leading-exit blockage.It is seen that the discharge coefficient decreases with the increase of blocking ratio varying from 0.1 to 0.4,which means that a larger pressure ratio()is needed for the bigger blocking ratio case to achieve the same coolant mass- flow as that of perfect film-hole.

Fig.9 Effect of blocking ratio on film-hole discharge coefficient(leading-exit deposition).

4.Conclusions

The effects of in-hole blockage on the flat-plate film cooling behaviors are investigated experimentally.A specifically pyramid-shaped element is used for simulating the in-hole blockage.Six representative in-hole blockage orientations and four blocking ratios are taken into considerations.The main conclusions are summarized as the followings:

(1)In-hole blockage orientation shows complicated roles on the film cooling effectiveness.In general,the in-hole blockages except for the leading-exit blockage orientation produce detrimental influence on the film cooling effectiveness,especially for the trailing-exit blockage orientation.

(2)The influence of in-hole blockage on the film cooling effectiveness is significant under high blowing ratios.With regard to leading-exit blockage orientation,it is found that the leading-exit blockage with a blocking ratio of 0.3 produces the highest film cooling effectiveness.

(3)In-hole blockage makes discharge coefficient reduction under the same coolant Reynolds number.The leading-exit and trailing-inlet blockage orientations produce the most serious pressure drop across the film hole related to the perfect film-hole.As the blocking ratio increases,the discharge coefficient decreases rapidly.

Acknowledgements

The authors gratefully acknowledge the financial support for this project from the National Natural Science Foundation of China(Nos.51276090 and U1508212).

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