Performance of Gas Turbine Film Cooling with Backward Injection

doi:10.4236/epe.2013.54B025

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Energy and Power Engineering, 2013, 5, 132-137

doi:10.4236/epe.2013.54B025 Published Online July 2013 (http://www.scirp.org/journal/epe)

Performance of Gas Turbine Film Cooling with

Backward Injection

X. C. Li, G. Subbuswamy, J. Zhou

Department of Mechanical Engineering, Lamar University, Beaumont, Texas, USA

Email: [email protected]

Received March, 2013

ABSTRACT

Gas turbines have been widely used in power generation and aircraft propulsion. To improve the gas turbine perform-

ance, the turbine inlet temperature is usually elevated higher than the metal melting point. Therefore, cooling of gas

turbines becomes very critical for engines’ safety and lifetime. One of the effective methods is film cooling, in which

the coolant air from the discrete holes blankets the surface from the hot gas flow. The major issues related to film cool-

ing are its poor coverage, aerodynamic loss, and increase of heat transfer coefficient due to strong flow mixing. To im-

prove the cooling p erformance, this paper examined film cooling with b ackward injection. It is observed that film cool-

ing with backward injection can produce much more uniform cooling coverage under different conditions, which in-

clude cases on flat surface with low or high pressure and temperature. The backward injection also performs better in

the presence of blade curvature. The effect of other parameters on the film cooling is also reported. The numerical re-

sults are validated by simple experimental test in th is study.

Keywords: Film Cooling; Cooling Effectiven ess; Backward Injection

1. Introduction

Based on the principle of thermodynamics, a higher tur-

bine inlet temperature leads to a higher thermal effi-

ciency in gas turbine engines, which are widely used for

power generation and propulsion due to the compact

structure and ease of operation. As part of effort to in-

crease the engine efficiency, the operating temperatures

of a gas turbine can be elevated as high as 2000K, which

is much higher than the melting point of metal in use.

Next generation gas turbines are expected to operate at

even higher temperature. The operation of these engines

becomes impossible if the hot components are not pro-

vided with proper thermal protection. One of these com-

ponents is turbine blades. The turbine blade cooling is

especially difficult because of the space constraint and

aerodynamic requirement. Although there are few other

cooling techniques available, film cooling has been ex-

tensively studied and applied over years.

In film cooling, coolant air is drawn from compressor

and directed into the cooling channel of turbine blades

after bypassing the combustion chamber. It is then in-

jected through small holes onto the blade surface in a

proper angle to form a thin layer and blanket the surface

as shown in Figure 1. The thin film with relatively low

temperature is later deteriorated in the downstream be-

cause of the mixing of hot gas and coolant. The quality

of film cooling is generally measured by an adiabatic

film cooling effectiveness,



, which is defined as

awg







(1)

where Tg is hot gas temperature, Taw is adiabatic wall

temperature and Tc is the temperature of coolin g air. The

cooling effectiveness ranges between 0, where there is no

cooling, and 1, where the surface is perfectly protected.

The performance of film cooling is largely affected by

many parameters such as the flow Reynolds number,

blowing angle, blowing ratio, and the shape of the hole.

Significant studies have been done on these parameters.

Figure 1. Concept of film fooling of turbine blades.

X. C. LI ET AL. 133

Some of the earliest pioneers include Goldstein [1],

whose work provided the fundamental understanding of

film cooling. Although a cylindrical hole is simple and

easy to manufacture, jet holes with diffusive exit (called

shaped holes) have been proved to perform better [2-4].

Furthermore, the shaped hole combined with compound

jet angle gives some excellent cooling performance [5].

The blowing ratio, which represents the amount of cool-

ant air in use, can affect the cooling performance [3, 6,7].

If the blowing ratio is too low, it may not be able to pro-

vide the sufficient cooling, while a very high blowing

ratio can lead to jet lift off the cooling surface and un-

necessary aerodynamic loss. Further, the blowing angle

plays a very important role too in film cooling. When the

blowing angle is too big, the jet can easily separate from

the cooling surface. On the other hand, a small blowing

angle may limit the coverage region.

Though film cooling on flat surfaces was considered in

many studies, the surface curvature also influences the

performance of film cooling. Turbine blades typically

include both convex (suction) and concave (pressure)

profiles. The local pressure and velocity on the curved

surface make the cooling more complicated. Compared

to flat plate film cooling, it was shown that the adiabatic

effectiveness was increased on convex surface while de-

creased on concave surface [8].

Other techniques are also discussed in the literature to

enhance film cooling. For example, it is found that back-

ward ramps facing upstream can almost double the cool-

ing effectiveness [9]. Due to lateral spreading caused by

trenches, film cooling is greatly enhanced by using cy-

lindrical holes embedded in transverse trenches [10].

When tiny water droplets (mist) are injected into the

coolant flow, each droplet acts as a heat sink and it flies

to downstream before it completely vaporizes. Therefore,

mist injection improves the cooling performance [11,

12].

In conventional film cooling, the coolant is injected

generally in the same direction as the mainstream, which

can be termed forward injection. If the coolant jet is in

the opposite direction of mainstream, termed backward

injection, the strong interaction between coolant jet and

mainstream causes a significant je t momentum loss. Thus,

the jet spreads in the lateral direction, resulting in better

cooling along the span [13]. This paper presents the re-

search works on backward film cooling. It is observed that

the backward film cooling produces significantly uni-

form cooling coverage under different conditions, which

include cases on flat surface with low or high pressure

and temperature. The backward injection also performs

better in the presence of blade curvature.

2. Methodology

2.1. Numerical Simulation

Numerical method was first applied to simulate the flow

and heat transfer of film cooling at different conditions.

The commercial CFD software package, Fluent, was em-

ployed. The second-order upwind scheme is used for

spatial discretization. SIMPLE algorithm was chosen to

couple pressure and velocity. The convergence criteria of

a solution have been insured when the residual of all va-

riables is less than a specific value, 10-5 for continuity,

momentum, and turbulence, and 10-8 for ene rgy .

One problem associated with numerical simulation is

turbulence closure. In Fluent, a number of turbulence

models are available, but none of them is the best. For a

given problem some models work better than the others.

Therefore, it is important to choose the right turbulence

model. In this study, various models are tested and com-

pared with experimental results. The final selection is the

standard k- model with enhanced wall functions.

Since the standard k- model is only valid for fully tur-

bulent flow with high Reynolds number, in the region

close to the wall where the viscous force is dominant, the

flow needs to be modeled with wall functions.

Both structured and unstructured grids were used in

the computational domain for all the cases. The grids

near the cooling holes are denser when compared to

those in other regions. The boundary adaption is applied

on the cooling surface. The required number of grid

points, which is generally between 1 to 2 million, is eva-

luated through grid independence study.

2.2. Experimental Test

To validate numerical results of film cooling, an experi-

mental study was conducted by constructing a low-speed

wind tunnel (~10 m/s), which includes a driving unit,

diffuser, settling chamber, nozzle, test section, and the

exit diffuser. A Dayton blower (model no. 4TM03) is

used to feed air at atmospheric pressure and room tem-

perature into the wind tunnel. A laminator is added to

make the flow more uniform at the test section. Cooling

holes are drilled with an angle of 30 degrees to the main-

stream. The coolant flow is fed into the test section

through a compressor to the bottom of the test section

after passing a heat exchanger. The flow rate is metered

and regulated by using a flow meter (Dwyer RMC-108-

SSV). An infrared camera (FLIR 345001685) is used to

capture thermal images of test section surface. In addi-

tion, a total of 32 thermocouples thermocouples (Omega

GG-K-30-SLE) are installed on the test section surface to

measure the point temperatures. The flow distribution is

measured with a Pitot tube.

3. Results and Discussion

3.1. Cases at Laboratory Conditions

To explore the fundamentals of backward film cooling,

the first trial is only for a simple cylindrical hole with flat

X. C. LI ET AL.

134

surface under a typical laboratory condition featured with

low temperature, velocity and pressure. The jet has a

diameter (d) of 1mm and a backward blowing angle of

30 degrees. The main flow has a temperature of 400 K

and a velocity of 10 m/s, while the coolant velocity and

temperature are 10 m/s and 300 K, respectively. The op-

erating pressure is 1 atm. These conditions give a blow-

ing ratio (M) of 1.33. M is defined as (u)c/(u)g, where

 and u represent density and velocity, and c and g rep-

resent coolant and hot gas, respectively. To compare the

performance of backward injection with forward injec-

tion, the forward injection case is also simulated with

otherwise the same geometry.

The distribution in Figure 2 shows that the forward

blowing generates a very high effectiveness immediately

downstream the jet hole, while the cooling effectiveness

decreases sharply in both the lateral and mainstream di-

rections. However, the backward blowing generates a

much more uniform distribution although the cooling

effectiveness immediately downstream the cooling hole

is lower than the case with forward injection.

To further analyze the film cooling coverage, Figure 3

plots the cooling effectiveness at different locations in

the main flow direction (x). Except for the region very

close to the centerline (z = 0) and immediate downstream

of the jet hole (x/d~2), the backward blowing produces a

higher cooling effectiveness, and the difference between

backward and forward injections becomes even more

apparent in the far downstream (x/d= 10), where the per-

formance of film cooling with forward injection becomes

quite poor in general.

3.2. Cases at Gas Turbine Operating Conditions

Gas turbine operating conditions vary from one unit to

another. In this study, the operating pressure is taken as

15 atm. The main flow has a velocity of 128 m/s with a

temperature of 1561 K, while the coolant temperature is

644 K. To make a blowing ratio of 2 as referred in actual

operation, the velocity of the coolant flow is calculated to

be 106 m/s. Only one row of cylindrical holes on a flat

plate is considered. The hole has a diameter (d) of 1 mm

0.0

1.0

Backward blowing

Forward blowing

Figure 2. Film cooling effectiveness with backward and

forward blowing at laboratory conditions.

and is located at a distance of 10 jet diameters from the

main flow inlet. The blowing angle is 35 degrees. The total

size of the computational domain is 10 d × 40 d × 3 d.

Figures 4 and 5 present the cooling effectiveness, both

the overall coverage and the distribution in the spanwise

direction (z) at different downstream locations (x). The

centerline is indicated by z = 0. In the case of forward

injection, it is seen that the effectiveness is high along the

centerline (different x/d values) but decreases rapidly in

the spanwise direction. Thus it can be understood that the

cooling only performs well at the center. In the case of

backward jet, the effectiveness is high along the center-

line and reduces gradually outward only at planes very

Figure 3. Comparison of spanwise cooling effectiveness be-

tween two different injections at laboratory conditions.

Adiabatic film cooling effectiveness distribution

(b) Backward

injection

(a) Forward

injection

Figure 4. Film cooling effectiveness with backward and

forward blowing at gas turbine operating conditions.





010

101

Forw a rd

0.1

0.2

0.3

0.4

0.5

0.6

x/d= 3x/d = 8 x/d = 23

Forward

Backward

Figure 5. Comparison of spanwise cooling effectiveness be-

tween forward and backward injections at gas turbine op-

erating conditions.

X. C. LI ET AL. 135

close to cooling hole. Far in the downstream, cooling

with backward injection is not only more uniform but

also better than forward injection. On average, 61% en-

hancement on effectiveness is achieved just by changing

the direction of coolant inlet from forward to backward

scheme. Thus, it is concluded that backward injection

works better than the forward case.

3.3. Effect of Surface Curvature

In real applications the airfoil configuration, internal

channel, and jet holes are complicated. In this study, the

blade chord length is 226 mm and the maximum thick-

ness is about 14 mm. In addition, the blade cascade has

an inlet angle of 45 degrees and outlet angle of 68 de-

grees, and the distance between two blades is 225 mm.

The hole has a diameter (d) of 1 mm and the spanwise

pitch of the holes is 4 d. The jet hole is located at 29d

downstream from the leading edge on the suction side

and 42 d on the pressure side. The blowing angle is 35

degrees for both forward and backward coolant flows on

pressure and suction sides. The main flow has a velocity

of 128 m/s at a temperature of 1561 K and the coolant

has a velocity is 52.8 m/s at a temperature of 644 K. The

blowing ratio in this case is 1.0.

Figure 6 shows the cooling effectiveness in the span-

wise direction at different downstream locations (l) on

pressure and suction sides, respectively. The symbol “l”

is the distance from a given downstream location to the

cooling whole tip. On the pressure side, the cooling ef-

fectiveness at the center plane in the case of backward

injection is marginally less than the forward case. Along

the span, however, the backward injection produces

slightly higher and more uniform cooling. This is promi-

nent in far downstream regions. Note that on the pressure

side, the local main flow has a low velocity, which

means a “nominal” high blowing ratio since the coolant

velocity remains the same. The high blowing ratio can

result in a lifted jet. On the suction side, the centerline

effectiveness is high for forward injection. In the span-

wise direction, backward injection has slightly more

uniform cooling effectiveness. Different from the pres-

sure side, the local main flow velocity is higher than the

nominal main flow. The cooling performance depends on

whether there is flow separation from the suction surface.

3.4. Effect of Blowing Angles on Film Cooling

Although the blowing ratio is discu ssed in some prev ious

sections, the detailed impact has not been presented. Ba-

sically, a high blowing ratio means a strong jet, which

can penetrate into the main flow easily. However, if the

blowing ratio is low, there could be no enough coolant to

maintain the coo ling. It has been shown that the effect of

blowing ratio depends on other parameters such as the jet

angle and the surface curvature. Figure 7 gives the trend

for film cooling with a concave surface (pressure side).

The par ameters are other wise the same as in S ection 3.3.

It is observed that in this case lo wering the blowing ratio

can improve the cooling performance for both forward

and backward injection. Furthermore, the distribution

with different blowing ratios is similar, which means that

the advantage of backward injection stays the same.

3.5. Validation with Experimental Study

As mentioned earlier, an experiment was conducted to

validate numerical results of film cooling . Figure 8 gives

the infrared images of the cooling surface for both for-

ward and backward injections. It is clearly seen that the

cooling is highly concentrated along the center region of

the cooling surface for forward injection. In the case of

backward injection, the coo ling is very high near the hole

region, and also uniform along spanwise direction in the

Figure 6. Comparison of film cooling between forward and

backward injections on curved surfaces.

Figure 7. Effect of blowing angle on film cooling with for-

ward and backward injections on a concave surface.

X. C. LI ET AL.

136

downstream regions due to the strong interaction be-

tween the mainstream and coolant. The coolant with re-

duced velocity will demolish in the downstream after

mixing with the main flow. The phenomenon exposed

through the experiment agrees well with the results from

numerical study. Figure 9 compares the experimental

result to numerical simulation with various turbulence

models. It indicates that standard k- model and k- real-

izable models work well. Results from k- models are

too far away from experimental data. Thus, the k- model

with enhanced wall functions is adopted.

4. Conclusions

Based on the numerical simulation validated with test

data, the following conclusions can be reached.

 Backward injection can improve the film cooling

performance on flat surface at both laboratory and gas

turbine operating conditions. The interaction of the coolant

jet with main flow makes the cooling in the spanwise

direction much higher and more uniform when compared

to the forward injection case.

 For the cooling with curved surface, the perform-

ance of film cooling with backward injection decreases

along centerline on both concave and convex surfaces,

296

Temperature distribution

282

(a) For wa r d injection

(

)

Backwar d in

ection

Figure 8. Infrared images of temperature distribution of

film cooling with forward and backward injections.

Effectiveness 

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 102030 40

Standard k-

RNG

k- Realizable

Standard k-

k- SST

Experimental

x/d

Figure 9. Comparison of experimental result with numeri-

cal simulation with different turbulence models.

especially in the region close to the cooling hole. How-

ever, the span wise distribution becomes more uniform

due to the backward jet, and on the pressure side some

higher improvement is seen.

 The advantage of backward injection stays the same

when the blowing ratio varies. Results from cases with

different blowing angles also suggest that film cooling

with backward injection p erforms better than the forward

injection case.

 Experimental study can validate that the perform-

ance of film cooling with backward injection is better.

5. Acknowledgements

This research is supported by the US National Science

Foundation (Award #: 0927376).

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