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
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
cg
awg
TT
TT


 
(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.
1
Adiabatic film cooling effectiveness distribution
(b) Backward
injection
(a) Forward
injection
0
Figure 4. Film cooling effectiveness with backward and
forward blowing at gas turbine operating conditions.


z
/d
010 101
Forw a rd
0
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
K
Temperature distribution
282
K
(a) For wa r d injection
(
b
)
Backwar d in
j
ection
Figure 8. Infrared images of temperature distribution of
film cooling with forward and backward injections.
Effectiveness 
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
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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