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International Journal of Mechanical Engineering and Technology (IJMET) Volume 8, Issue 1, January 2017, pp. 304–316, Article ID: IJMET_08_01_033

Available online at http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=8&IType=1

ISSN Print: 0976-6340 and ISSN Online: 0976-6359

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NUMERICAL SIMULATION TO INVESTIGATE

EFFECT OF DOWNSTREAM GROOVES ON FILM

COOLING EFFECTIVENESS OF GAS TURBINE

BLADES

Yash Krishna Menon

PG Scholar, Mechanical Engineering Department,

Amrita Vishwa Vidyapeetham, Kollam, India

Dr. Jayakumar J. S.

Professor, Mechanical Engineering Department,

Amrita Vishwa Vidyapeetham, Kollam, India

ABSTRACT

This paper presents a new design concept to increase the adiabatic effectiveness of cylindrical

holes used for film cooling in gas turbine. Rectangular grooves are created at the downstream of

each cylindrical hole. The grooves are incorporated into the film cooling system to reduce the

adverse effects of the kidney vortices since the kidney vortices reduce the effectiveness of circular

cross-section film cooling holes at moderate to high blowing ratios by inducing jet lift-off. The

groove shape is defined by two geometric parameters viz. width and depth. A single row of five

discrete film cooling holes on a flat plate with an inclination angle of 30° along streamwise

direction and pitch to diameter ratio of 2 was chosen as the baseline test case. In this study, cooling

effectiveness curves obtained by four grooved plates having different groove configuration is

compared amongst themselves and with the simple plate. Numerical simulations have been

performed at three different blowing ratios of 0.5, 1.0 and 1.5 for each case of the rectangular

groove and simple plate by using CFD technique (ANSYS Fluent) and the flow field is solved by

using k-ε realizable turbulence model. The results showed that the lateral averaged cooling

effectiveness is increased remarkably when the downstream rectangular grooves are present. This

increase is because of the fact that, in grooved plates, majority volume of the coolant is flowing

within the grooves, so it is properly guided and protected by the grooves. This reduced the

turbulent mixing between mainstream and coolant flow. Another reason is that side walls of the

groove do not allow hot mainstream gasses to enter underneath coolant jet from the sideways. This

reduced the jet lift-off and improved the cooling effectiveness. Apart from this, the effects of each

geometrical parameter of the groove on the film cooling effectiveness were studied in detail and

observed that average cooling effectiveness distribution is higher for grooves with least aspect

ratio for low blowing ratio.

Key words: Film cooling, Adiabatic cooling effectiveness, Grooves, Numerical simulation.

Numerical Simulation to Investigate Effect of Downstream Grooves on Film Cooling Effectiveness of Gas

Turbine Blades


Cite this Article: Yash Krishna Menon and Dr. Jayakumar J. S., Numerical Simulation to

Investigate Effect of Downstream Grooves on Film Cooling Effectiveness of Gas Turbine Blades.

International Journal of Mechanical Engineering and Technology, 8(1), 2017, pp. 304–316.

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=8&IType=1

1. INTRODUCTION

Over the past sixty years, aircraft and power generation gas turbine designers have been trying hard to

increase the turbine inlet temperatures. The main reason behind this effort is, with higher turbine inlet

temperatures, improved efficiency and reduced fuel consumption can be achieved. Similarly, in aircraft

applications, the higher temperature leads to increased thrust power [1]. Unfortunately, these higher

temperatures have badly affected the integrity of the high-pressure turbine components and specifically the

turbine blades. Modern turbine inlet temperatures exceed the melting point temperatures of turbine blade

materials. To combat and prevent the failure of turbine blades and combustor liners in gas turbine engines

resulting from these excessive operating temperatures, various cooling techniques such as “film cooling”

have been invented and incorporated into combustor and blade designs [2]. A number of cooling

technologies are in use nowadays. A comprehensive summary of the various cooling technologies could be

found in Han et al.[1] and Metzger et al. [3]. Apart from using cooling techniques, manufacturers have

used advanced materials such as superalloys and thermal barrier coatings (TBC) to protect the key

components of the gas turbine. However, use of cooling techniques is quite cheaper and reliable compared

to other techniques. Modern gas turbine engines use both advanced materials as well as cooling techniques

to ensure maximum safety of the engine’s parts at high temperatures [3].

Film cooling is the most basic external cooling technique to protect key gas turbine engine components

during operation. In this technique, the cool air (secondary flow) bled from the compressor stage is ejected

through a series of discrete holes in a given component. Under the appropriate conditions, the flow from

the holes coalesces to form a blanket or film of cooler air that “insulates” the component from an

extremely hot mainstream flow [3]. The boundary layer of coolant air formed near the component surface

acts as a heat sink, which reduces heat transfer to the component’s surface. The purpose of film cooling is

not only to protect the surface in the immediate vicinity of the film cooling holes, but also the surface

downstream of the injection location [1-3].

For modern high efficiency and long-duration gas turbines, film cooling is one of the essential method

that allows the turbine to operate at extremely high inlet temperature. Film cooling is effective in reducing

the metal temperature and widely applied to the external surfaces of high-temperature nozzle vanes and

turbine blades, such as the leading edges, pressure and suction surfaces, blade tips, and the end walls.

A large number of studies on film cooling effectiveness for single and multiple rows of cylindrical

holes on a flat plate, a curved plate, and a cascade have been carried outboth experimentally and

numerically. The desirable and undesirable effects of various geometrical and aerodynamic parameters

such as hole inclination, hole length-to-diameter ratio, spacing between holes, blowing ratio, density ratio,

mainstream turbulence intensity on the cooling effectiveness were intensively investigated (see reviews by

Han et al. [1] and Goldstein [4]).

At present, the film cooling mechanism of the cylindrical hole is understood. The common conclusions

are that the cooling effectiveness of the simple cylindrical hole is acceptable at low blowing ratios and

drops rapidly with the increase of blowing ratio. It is well known that good film cooling performances can

be obtained if the cooling jets remain attached to the wall, without penetrating the main flow. The jets

should have a high enough mass flow rate to cover a large area, and a low turbulent mixing to avoid

dilution by the mainstream. However, research and development activities have proved that use of

cylindrical holes in film cooling had disadvantages in gas turbine applications due to the “jet lift-off” from

the surface, particularly at higher blowing ratios (~1 and above) leading to the deterioration of the film

cooling performance. The main reason behind the cooling jet lift-off is the generation of an up-wash pair of

Yash Krishna Menon and Dr. Jayakumar J. S.


vortices, sometimes referred as kidney vortices. This phenomenon has been extensively studied by Kelso

and Lim [5] and Haven et al. [6-7]. The up-wash vortex pair has been recognized as being detrimental to

the film cooling effectiveness. So it can be concluded that simple cylindrical hole is deficient in terms of

film cooling effectiveness and lateral film coverage. Therefore many methods were proposed to prevent

the formation of kidney vortices and to improve the film cooling effectiveness of the cylindrical hole.

The improving methods can be divided into two categories. In the first category, the area at the

cylindrical hole exit is enlarged [8,9], by means of lateral diffusion, forward diffusion, both lateral and

forward diffusion [10], and diffusion with compound angle [11,12]. Compared to a cylindrical hole, the

enlarged exit area of shaped-diffusion holes use the Coanda effect due to this the film cooling jet remained

attached to the curved surface and an expanding cross-sectional area near the hole exit decreases cooling-

jet speed and increases lateral spreading of coolant. Shaped holes provide 30–50% higher film

effectiveness and about 20% lower heat transfer coefficients on the surface compared to a typical

cylindrical hole at the same blowing ratio. Bunker [13] provides a comprehensive review of the research

on shaped holes.

In the second category, the configuration inside the holes and surrounding the holes is modified to

improve the film cooling effectiveness. Some researchers used vortex generators to change the vortical

structures. Haven and Kurosaka [14] investigated the effects of placing vanes inside film cooling holes that

produce vortices in the same sense as the anti-kidney vortices so that jet lift-off is reduced and

effectiveness is improved to a great extent. Zaman and Foss [15], Zaman [16], and Ekkad et al. [17]

investigated the effects of tabs placed at the film cooling hole exit. Tabbed holes produced significant

increase (nearly 200%) in the film cooling effectiveness. This improved performance is related to the

generation of vorticity by the tabs that counter the kidney-pair vortex of the coolant jets and reduces jet

penetration keeping the coolant jet closer to the blade surface. Ekkad et al. [17] investigated various tab

locations and the tab located at the upstream edge of the hole was shown to have the best performance.

Bunker [18] proposed creating a trench about a row of film cooling holes to modify the boundary-

layer/cooling jet interactions. The results showed that this novel surface geometry yields the best film

effectiveness compared with the simple cylindrical hole. Baheri et al. [19] also investigated the effects of

trenched hole using both cylindrical hole and forward diffusion hole. The results showed that the trench

could significantly affect the film cooling flow over the protected surface. Na and Shih [20] presented a

method in which a ramp is placed at the hole upstream. The approaching boundary-layer flow and its

interaction with the film cooling jets are modified by the ramp. The results showed that the lateral averaged

adiabatic effectiveness with a ramp could be two or more times higher than without the ramp by increasing

the upstream and lateral spreading of the coolant. Rigby and Heidmann [21] placed the vortex generator at

the downstream of each hole. The vortex generator is of a delta shape. The results demonstrated that the

delta vortex generator is able to vanish the up-wash vortex pair (kidney vortices) and produces a

downwash vortex pair (anti-kidney vortices) downstream. The resultant anti-kidney vortices cause coolant

to be pushed toward the wall and spread out along the wall and the film width increases considerably.

In this paper, a new design concept is presented to increase the adiabatic effectiveness of film cooling

from a row of discrete film cooling holes. Instead of shaping the geometry of each hole; placing tabs or

vortex generators in each hole; or creating a slot about a row of holes, this study proposes a geometry

modification downstream of the holes to modify the boundary-layer flow of the coolant and its interaction

with main-stream flow. So far, no one has studied or investigated the effect of grooves towards

downstream of a row of film cooling holes to improve the film cooling effectiveness. This paper presents

the numerical investigation of the effect of inclusion of grooves on film cooling effectiveness. This

numerical analysis was accomplished by using CFD software ANSYS Fluent 14.

As it has been seen that the shaped holes give better cooling effectiveness compared to simple

cylindrical holes [9-13]. However, manufacturing tiny shaped holes are quite complex and expensive. On

the other hand, making simple cylindrical holes is quite easy and cheap. Considering this advantage, this

study is focused on improvement of cooling effectiveness for simple cylindrical holes.


Turbine Blades


The main objective of providing grooves downstream of film cooling holes is to reduce the adverse

effect of kidney vortices, which tries to lift-off the cooling jet from the surface. During the jet lift-off, the

surrounding hot mainstream air occupies the space underneath cooling jet and pushes the cooling jet

further away from the blade surface. Eventually, it helps in increasing the jet lift-off which is highly

undesirable. When grooves are incorporated into this system, the first benefit is, it provides proper

guidance to the cooling jet flow throughout the plate length so that the flow does not spread randomly

everywhere. In other words, a groove provides guided flow and reduces turbulent mixing between coolant

jet and mainstream flow. Due to this coolant does not lose its coolness and retain its coolness throughout

the length of the plate. The second benefit is the side walls of grooves do not allow the hot mainstream

gasses to enter underneath cooling jet from sideways. This reduces the cooling jet lift-off and increases the

cooling effectiveness to a great extent.

1.1. Nomenclature

d diameter of the film cooling holes (mm)

p Pitch, inter-hole distance in transvers direction (mm)

α blowing angle (acute angle between the axis of hole and the plate surface)

T∞

mainstream temperature (K)

cT coolant temperature at entrance of film cooling holes (K)

wT wall temperature at the top plate surface (K)

ρ∞ , c

ρ density of mainstream and coolant flow (Kg/m3)

U∞ , c

U velocity of mainstream and coolant flow (m/s)

M blowing ratio = c cU

U

ρ

ρ∞ ∞

η cooling effectiveness = w

c

T T

T T

∞

∞

−

−

x, y, z streamwise, normal and transversal coordinate respectively

AR Aspect Ratio of grooves, AR=�� (�)

�� (�)

2. NUMERICAL COMPUTATION

2.1. Physical Model

To study the effect of downstream grooves on film cooling effectiveness, a simple flat plate model and

four grooved plate models were adopted as shown in Figure 1 and Figure 2. All these models are made

using SolidWorks 2014. Length, width and thickness of all the plates are set as 140mm, 20mm and 5mm

respectively. As shown in Figure 2, in grooved plates, grooves are fed by a single row of five discrete film

cooling holes with inclination angle α=30° along streamwise direction and pitch to diameter ratio of 2. The

diameter of the holes d is set as 2mm and pitch p is 4mm. All dimensions of all computational models are

measured in Cartesian coordinate system with its origin placed at the center of first film cooling hole. All

plates can be divided into two regions along x direction: (I) the upstream region, -20 ≤ x ≤ 0 in which only

hot mainstream gasses are flowing and (II) the downstream region, 0 ≤ x ≤120 in which rectangular



grooves are created and both coolant and hot mainstream gasses are mixing while flowing. Figure 3 shows

the direction of the flow of mainstream gasses and coolant air in a test plate.

In grooved plates, groove shape is defined by two geometrical parameters viz. width w and depth h. In

order to study the effect of groove width variation and groove depth variation on film cooling

effectiveness, four different cases of grooves are considered for investigation.

Figure 1 Schematic of Simple Plate

Figure 2 Schematic of grooved plate (Case 1 grooves) with magnified groove details

Figure 3 Side view of the test plate with flow directions

In the first case, width and depth of the groove are set as 1mm and 0.5mm respectively. In the second

case, the groove width is made larger (i.e. 2mm) and depth is kept constant (i.e. 0.5mm). This is done to

gauge the sensitivity of cooling effectiveness with width variation. In the third case, the groove depth is

made larger (i.e. 1mm) and width is kept constant (i.e. 1mm). This is done to gauge the sensitivity of

cooling effectiveness with depth variation and in the last case i.e. fourth case, both width and depth are


Turbine Blades


made larger (i.e. width becomes 2mm and depth becomes 1mm). Figure 4 represents these four cases of the

grooves.

Figure 4 Types of Grooves

Lateral averaged film cooling effectiveness curves were plotted for the simple plate and four grooved

plates at three different blowing ratios of 0.5, 1, and 1.5. These curves were compared amongst four groove

cases and also compared with the simple plate case.

The computation domain consist of the solid plate, flow domain inside the holes and the flow domain

above the plate which extends in the y direction to 30d. However, in this paper, we have considered

adiabatic heat transfer model while performing computations. Hence the only effect of convection heat

transfer on the top plate surface is taken into account for estimating lateral averaged cooling effectiveness,

the conduction heat transfer within the solid plate is not considered and hence solid plate is subtracted from

the computation domain using “Boolean” operation.

2.2. Grid Distribution

Using ANSYS ICEM-CFD, an unstructured tetrahedral grid is applied to the whole computation domain.

In the areas where the disparity in the flow parameter is high, the fine grids are used for accurate

simulation. For example in the region near the top surface of the plate the size of the grid is made very fine

in order to accurately model the process of the coolant mixing with the mainstream flow. The grid size is

then gradually increased along the y direction. The total number of grid cells in the whole computation

domain is about between 2.1×105 and 4.0×10

5 (It varies with different groove cases). The grids used in this

study has been tested which showed that computation results are independent of the number of grids.

Figure 5 and 6 shows the grid distribution near coolant holes and in the entire computation domain.



Figure 5 Grid distribution near coolant hole in Simple Plate and Grooved plate (Case 1 grooves)

Figure 6 Grid distribution in the entire computation domain

2.3. Grid Independence Test

When any real life problem is solved by using CFD technique, its solution should not be affected by the

size of the grid. In other words, grid independence test is done to ensure that the obtained solution is

independent of the grid size. In this test, same problem has been solved by using a coarse grid, medium

grid and fine grid and the variation in the final results were analyzed. After this, optimum number of grids

have been found out above which the solution is invariant. In this test, analysis started with 0.2 million grid

cells, then performed same analysis with 1.57 million grid cells and 11.8 million grid cells. All these

analyses have been carried out on a simple flat plate at M=0.5. Figure 8 represents the graph of lateral

averaged cooling effectiveness vs. x/d for these three grid sizes.

Figure 7 Grid Independence Test

0

0.1

0.2

0.3

0.4

0.5

0.6

0 10 20 30 40 50 60

η

x/d

0.2 million grids

1.57 million grids

11.8 million grids


Turbine Blades


By observing Figure 7 it can be concluded that, there is negligibly small variation in the results

obtained using 0.2 million grids and 1.57 million grids since both the curves almost overlaps on each other;

on the other hand, the cooling effectiveness curve obtained using 11.8 million grid cells is slightly higher

than former two curves in some regions. However, there is negligibly small overall variation in the

distribution of cooling effectiveness over the wide range of number of grids. Therefore considering

computational time, memory and cost, 0.2 million is set as an optimum number of grids and hence

numerical simulations are carried out for all the 15 cases with grids ranging from 0.2 million to 0.4 million.

2.4. Boundary Conditions and Computation Methods

In this paper, one simple plate and four grooved plates have been taken for numerical simulations. All the

numerical simulations are performed at three different blowing ratios M=0.5, 1, 1.5 for each plate so total

15 cases were studied.

The boundary conditions applied to the problem are extracted from several advanced gas turbine

engines to simulate the actual situation of turbine guide vane cooling.

In order to compare the computational results of each case with other cases, the main boundary

conditions for all the 15 cases are set to be the same. For the mainstream, the temperature and velocity at

the entrance of the fluid domain are assumed to be uniform and are set as T∞＝1600K and U∞=80m/s,

respectively. Coolant flow to mainstream flow density ratio is set as 2. For the coolant flow, the

temperature at the entrance of the film cooling holes is set as Tc＝800K and the velocity Uc at the entrance

of film cooling holes varies with the blowing ratio as shown in Table 1.

Table 1 Coolant velocity at various Blowing Ratios

M Uc (m/s)

0.5 20

1 40

1.5 60

The exit of the mixing flow, i.e. the downstream end face of the fluid domain is treated as a “pressure

outlet” with pressure set to atmospheric pressure.

The computational domain is solved using ANSYS Fluent 14 solver. The flow is assumed to be steady

and compressible. The pressure, density and temperature are related by using the ideal gas law. The

realizable k−ε turbulence model is applied to simulate the flow field. The near wall region is treated by

standard wall functions. The discretization formats of the flow and turbulence equations are all set to

second order upwind. Each equation is solved using SIMPLEC scheme with under-relaxation. The

convergence criterion is set in such a way that the residual of each calculated parameter is less than 10-4

.

3. RESULTS AND DISCUSSION

The objective of this investigation is to understand the usefulness of placing grooves downstream of a row

of film cooling holes in improving cooling effectiveness η.In this study, cooling effectiveness obtained by

four grooved plates having different groove configurations is compared amongst themselves and with the

simple plate. The main factors that influence the value of η are flow parameters such as blowing ratio M

and groove geometry parameters such as groove width w and depth h.

The adiabatic cooling effectiveness of the film cooled plate on the top hot side surface of the plate is

defined as

w

c

T T

T Tη ∞

∞

−=

− (1)



where T∞and Tc denote the entrance temperature of the hot mainstream and coolant flow respectively,

Tw is the wall temperature on the hot side surface averaged along transversal direction z for a given

location along the main flow direction x.

Figure 8 Distribution of η and temperature contours at M=0.5

Figure 9 Distribution of η and temperature contours at M=1

Figure 10 Distribution of η and temperature contours at M=1.5

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 10 20 30 40 50 60

η

x/d

Simple Plate

Case 1

Case 2

Case 3

Case 4

M=0.5

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 10 20 30 40 50 60

η

x/d

Simple Plate

Case 1

Case 2

Case 3

Case 4

M=1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 10 20 30 40 50 60

η

x/d

Simple Plate

Case 1

Case 2

Case 3

Case 4

M=1.5


Turbine Blades


3.1. Comparison between Grooved Plates and Simple Plate

Figure 8, 9 and 10 show the distribution of lateral averaged adiabatic cooling effectiveness η and the

temperature contours for simple plate and four grooved plates (having Case 1, Case 2, Case 3 and Case 4

grooves) at three blowing ratio M=0.5, 1 and 1.5 respectively. From all these figures it can be seen that

grooved plates showed improved cooling effectiveness compared to the simple plate at all blowing ratios.

The main reason behind this is “jet lift-off” phenomenon.

In case of simple plate, the cooling effectiveness is quite high near the hole region but as x/d increases,

rapid decrease in effectiveness has been observed compared to grooved plates. In other words, the rate of

decrease in effectiveness is quite high for simple plate compared to grooved plates. This is because of the

fact that, in simple plate when coolant jet comes out of the hole, a pair of kidney vortices set up in the jet

as shown in Figure 11. This pair of kidney vortices tries to lift-off the coolant jet from the surface. As jet

lift-off occurs, the surrounding hot mainstream air occupies the space underneath coolant jet and pushes

the coolant jet further away from the blade surface. Due to this coolant jet detaches from the surface and

cooling effectiveness rapidly decreases.

Figure 11 Kidney shaped vortices [5]

On the other hand, in grooved plates the film cooling effectiveness is found to be higher compared to a

simple plate. The main reason behind this is reduced jet lift-off. In grooved plates, jet lift-off is reduced to

the great extent because of the fact that, majority volume of the coolant is flowing within the grooves, so it

is properly guided and protected by the grooves. This reduced the turbulent mixing between the

mainstream and coolant flow. Due to this reason coolant jet did not lose its coolness (just like simple plate

case) and retained its coolness throughout the length of the plate. This is evident from the temperature

contours shown in Figure 8, 9 and 10. Another reason is that the side walls of the groove did not allow hot

mainstream gasses to enter underneath the coolant jet from sideways. This reduced the jet lift-off and

improved the cooling effectiveness of grooved plates. Compared to simple plate case, in grooved plates,

minimum 12% and maximum 60% improvement in film cooling effectiveness has been observed. Hence it

can be stated that “Incorporation of grooves into film cooling system is advantageous.”

3.2. Comparison amongst all Grooved Plates

In this subsection, the influence of various groove configurations on film cooling effectiveness is

described. Case 1 groove is set as the reference case. Effect of width variation, depth variation and both

width and depth variation on cooling effectiveness has been studied by comparing their cooling

effectiveness curves with Case 1 groove curves. In simple words, Case 2, Case 3 and Case 4 curves are

compared with Case 1 curves.



3.2.1. Cooling Effectiveness at M=0.5

With reference to Figure 8, when comparing lateral averaged cooling effectiveness curves amongst Case 1,

Case 2, Case3 and Case 4 grooves at M=0.5, it can be seen that cooling effectiveness obtained by Case 3

groove plate is higher over a wide range of plate length (8 ≤ x/d ≤ 60) compared to other cases. Even

though the highest effectiveness value is obtained for Case 2 grooves near the hole region, Case 3 grooves

showed better effectiveness over the wide range of plate. This is because of the fact that, in Case 2

grooves, there is a rapid decrease in effectiveness compared to Case 3 grooves. Compared to Case 1

grooves having AR=2, the effectiveness of Case 2 grooves having AR=4 is slightly decreased and Case 4

grooves having AR=2 is slightly increased. In other words, it can be stated that effectiveness values for

Case 2 and Case 4 grooves are nearly same as Case 1 grooves with ±15% variation. Hence it can be

concluded that for shallow grooves i.e. grooves having high AR value (high width and less depth), cooling

effectiveness distribution is poor across the plate length whereas for low AR value (less width and high

depth) i.e. Case 3 grooves, distribution of cooling effectiveness is better than others. From Figure 8 it has

been observed that there is a strong influence of depth variation on cooling effectiveness compared to

width variation. Since width variation does not show significant changes in effectiveness curves whereas

depth variation shows significant variation in cooling effectiveness curve.

Near the hole region, grooves with low AR i.e. Case 3 grooves showed poor effectiveness compared to

high AR grooves i.e. Case 1, Case 2 and Case 4 grooves. This is because of the fact that, in low AR

grooves, the lateral spreading of coolant jet is poor due to high depth of the groove whereas, in high AR

(i.e. shallow) grooves, the lateral spreading of coolant is quite high. However, in shallow grooves, the

degree of jet lift-off is higher compared to deep grooves. The main reason behind this phenomenon is as

follows:

In deep grooves (low AR) majority volume of coolant remain inside the grooves while flowing across

the length of the plate. Due to this, coolant jet retains its coolness throughout the plate length. As x/d

increases, some portion of the coolant jet comes out of the groove and spreads in lateral direction.

However, majority volume of coolant remains inside the groove, so it is unaffected by jet lift-off

phenomenon. On the other hand in shallow grooves, majority volume of coolant is above the grooves. This

volume of the coolant spreads randomly in the lateral direction and becomes victim to jet lift-off. As there

is a very little volume of coolant remain in the grooves, the coolness retained by this small volume is poor

compared to deep groove case and hence rapid decrease in cooling effectiveness has been observed for

Case1, Case2 and Case 4 grooves compared to Case 3 grooves.

3.2.2. Cooling Effectiveness at M=1 and 1.5

Since the nature of effectiveness curves obtained at M=1 and M=1.5 are quite similar, both the Figure 9

and 10 are discussed in this subsection. From Figure 9 and 10 it can be observed that, cooling effectiveness

distribution shown by Case 4 grooves is higher than all other cases at low x/d (i.e. near hole region). As x/d

increases, Case 3 grooves show better effectiveness over Case 4 grooves. However, the difference in the

effectiveness shown by Case 3 and Case 4 grooves is quite small (about 8% at M=1 and about 4% at

M=1.5) and taking into account the better effectiveness distribution shown by Case 4 grooves near hole

region, it can be concluded that on an average Case 4 grooves show best cooling effectiveness distribution

across the length of the plate. In addition to this, it can be seen that as blowing ratio increased from 0.5 to 1

and from 1 to 1.5, the cooling effectiveness also increased for grooved plates.

Case 3 grooves showed poor effectiveness at high blowing ratio whereas Case 4 grooves showed the

best effectiveness. This is because of the fact that, in Case 3 grooves, due to its low AR, the lateral

spreading of the coolant flow is reduced and that is why the cooling effectiveness near the hole region is

poor. With increase in x/d, the lateral spreading of the coolant increased and hence the effectiveness curve

started rising till x/d=10 as shown in Figure 10. Later on effectiveness decreased, but the rate of decrease is

minimum compared to other cases. The reason for this observation is already explained in the previous

section. On the other hand in case of Case 4 grooves, the effectiveness is found to be maximum near the


Turbine Blades


hole region this because of good lateral spreading of coolant due to more width of Case 4 compared to

Case 3 grooves. Along with good later spreading of coolant, good longitudinal spreading of coolant also

has been observed from the temperature contours shown in Figure 9 and 10. At low blowing ratio M=0.5

the longitudinal spread of the coolant till end of the plate was poor because of less mass flow rate of

coolant injection on the other hand at high blowing ratios due to more mass flow rate of coolant injection,

longitudinal spread of coolant has increased. That’s why Case 4 grooves showed overall better

performance compared to other cases.

4. CONCLUSION

The present numerical study has highlighted the potential of using longitudinal grooves fed by discrete,

angled film holes for cooling purposes in gas turbines. In this study, influence of groove geometry on film

cooling effectiveness has also been investigated by varying groove width and depth at three different

blowing ratios M = 0.5, 1 and 1.5. The lateral averaged cooling effectiveness curves obtained by four

different groove configurations were analyzed and compared with simple plate as well as with each other.

After analyzing all the results and graphs, the key conclusions drawn are as follows:

• Incorporation of grooves into film cooling system has improved the cooling effectiveness of simple

cylindrical holes remarkably at all conditions. Compared to simple plate case, in grooved plates,

minimum 12% and maximum 60% improvement in film cooling effectiveness has been observed.

• Influence of groove width variation on cooling effectiveness is less whereas influence of depth

variation is quite strong.

• The increase in the groove width decreases the cooling effectiveness whereas the increase in the

groove depth increases the cooling effectiveness.

• At low blowing ratio, M=0.5 grooves with least AR (i.e. Case 3) showed best overall cooling

effectiveness distribution across the length of the plate.

• Highest cooling effectiveness near the hole region is achieved by grooves having high AR values at

all blowing ratios However their effectiveness rapidly decreases with increase in x/d compared to

low AR grooves (Case 3).

• With increase in blowing ratio from 0.5 to 1.5, cooling effectiveness of grooved plate has also

increased.

From this conclusion it can be stated that, in order to get maximum cooling effectiveness distribution

across the length of the plate using grooves, grooves should have aspect ratio as minimum as possible at

low blowing ratios.

REFERENCES

[1] Han, J., Dutta, S., and Ekkad, S., “Gas Turbine Heat Transfer and Cooling Technology,” Taylor &

Francis, New York, 2000.

[2] Goldstein, R. J., “Film Cooling,” Advances in Heat Transfer, Academic, New York, Vol. 7, pp. 321–

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