cfd analysis and enhancement of heat transfer in ...€¦ · geometries and altair hypermesh used...

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International Journal of Mechanical Engineering and Technology (IJMET)

Volume 6, Issue 7, Jul 2015, pp. 40-52, Article ID: IJMET_06_07_006

Available online at

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=6&IType=7

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

© IAEME Publication

___________________________________________________________________________

CFD ANALYSIS AND ENHANCEMENT OF

HEAT TRANSFER IN RECTANGULAR

CHANNEL USING BLOCKAGE WITH

ELONGATED HOLE

S. Kandwal

Assistant professor, Department of Mechanical Engineering,

Institute of Technology Gopeshwar, Kothiyalsen, Chamoli,

Uttarakhand, India, 246401

Rajeev Pandey

Assistant Professor,

Department of Mechanical Engineering

DIT University, Dehradun, Uttarakhand, India

Dr. S. Singh

Associate professor, Associate professor

Bipin Tripathi Kumaon Institute of Technology, Dwarahat, Almora,

Uttarakhand, India, 263653

ABSTRACT

A model of channel with blockage designed and meshed in ALTAIR®

HYPERMESH V.11 and used ALTAIR®

ACUSOLVE for CFD analysis. The

CFD analysis is validated by the experimental data. The rectangular channel

has a width-to-height ratio of 11:1. The blockages are subdivided into two

different cases using two different aspect ratios (hole-width-to-height ratio)

which are determined by the number of holes with four and six holes per

blockage. According to the results it is observed that at all Reynolds numbers,

the h and Nu increases with increase in pitch ratio up to the value of 6.0 and

then decreases with further increase in pitch ratio. This variation in h and Nu

is due to flow reattachment downstream of the blockage’s opening and then

redevelopment of boundary layer up to the succeeding next blockage’s

opening. The blockages with shorter holes enhance heat transfer better than

those with longer holes but they also yield significantly higher pressure drops

than blockages with longer holes.

Key words: Heat transfer coefficient and CFD analysis.

CFD Analysis and Enhancement of Heat Transfer in Rectangular Channel Using Blockage

with Elongated Hole


Cite this Article: Kandwal, S, Rajeev Pandey and Dr. Singh, S and. CFD

Analysis and Enhancement of Heat Transfer in Rectangular Channel Using

Blockage with Elongated Hole. International Journal of Mechanical

Engineering and Technology, 6(7), 2015, pp. 40-52.

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=6&IType=7

_____________________________________________________________________

1. INTRODUCTION

Gas turbine vanes and blades are exposed to high air temperature. And increasing

turbine rotor inlet temperature directly leads to a rise in thermal efficiency and output

power of gas turbines. The rotor inlet temperature is far higher than the melting point

of the blade and vane material. Hence, cooling technology of turbine blades has

become one of the most important key factors for improvement of gas turbine engine

efficiency since 1970s. According to Takeishi [1992] [4], cooling technology enables

turbine inlet temperature to increase by 25 °C per year but the achievement of super-

alloy development technology limits by 10 °C per year. Thus to study cooling

technology is more effective than to develop higher thermal resistance materials for

gas turbine improvement. Gas turbine blades are cooled by the air directly extracted

from the engine compressor. This extracted air causes a disadvantage of thermal

performance by incurring pressure drop. Hence an optimized cooling technique is

needed considering operating conditions. Internal cooling can be performed by

passing the extracted air from the compressor through several serpentine passages

inside the blades moving out the heat from the blades. For the gas turbine blades rib

turbulated cooling, pin-fin cooling, and impingement cooling are applied in the

blade’s internal coolant passage in order to remove heat from the blade inside. Figure

1(a) presents the commonly used turbine blade internal cooling techniques. Moon and

Lau [2003] [2] measured pressure drop and heat transfer coefficient by the liquid

crystal technique on the rectangular duct with perforated walls. They showed that the

number of walls and the configuration of holes did not affect the heat transfer level.

Their results also showed that the smaller holes could increase heat transfer

coefficient but pressure drop also greatly increased.

Figure 1 (a) External Film Cooling (b) Internal Convective Cooling

Lau et al. [2003] [5] examined the heat transfer and pressure drop on a rectangular

duct with perforated walls equipped with staggered holes. They showed that walls

S. Kandwal, Rajeev Pandey and Dr. S. Singh


with circular holes and square holes increased heat transfer but the increase in

pressure drop was much severe. They concluded that the shape and size of holes

should be optimized in order to get better thermal performance. In the early period of

gas turbine engines, only jet impingement cooling method was used for the leading

edge cooling of blade. Jet impingement cooling is to cool the blade by air

impingement on the surfaces of serpentine passages. Impinging was the most effective

cooling method for the leading edge making other methods unnecessary. But as gas

turbine technology has been developed, the rotor inlet temperature has been also

increased for turbine engine efficiency. Therefore, internal cooling technologies using

turbulators such as ribs and pin-fins have been developed [8].

1.1. Analytical Solution

The hydraulic diameter of test channel is calculated as:

The average Nusselt numbers for each of the three wall segments of the channel

Between two blockages was calculated as:

The average Nusselt numbers were normalized using reference Nusselt number for

fully developed turbulent flow in the channel with smooth walls. This reference

Nusselt number was defined as:

Nu0 = 0.023Re0.8

Pr0.4

Where the averaged heat transfer coefficients were defined as:

Where, Tb = Average bulk mean temperature =

Pressure drop across the blockages is calculated as:

∆P = g × y1 × (ρw ₋ ρa)

1.2. Modeling and Simulation

AcuSolve a computer program, based on finite Element Method (FEM) is one of the

powerful packages of existing commercial software for solving fluid flow and heat

transfer problems.

Figure 2 Design of Physical Model in ALTAIR® HYPERWORKS (11.0)


with Elongated Hole


The purpose of this study is to visualize the performance of air duct by providing

the artificial roughness in the form of blocks, as well as design of artificial roughness

geometry which will give optimum performance. CATIA V5 was used to create

geometries and ALTAIR HYPERMESH used to generate the unstructured mesh. A

schematic of the geometric model of the channel used in the study is shown in above

Figure.

Air flow inside a three-dimensional (3-D) channel object, with the Assumption

that the heat transfer to the atmosphere is negligible. The fluid properties were set to

be similar to that used in the experiment reported in. These blockages were installed

perpendicular to the direction of the main coolant flow in a wide rectangular channel.

Thus blockages had the same cross section as the rectangular channel

Figure 3 Geometry of channel with blockage of six elongated hole

1.3. Heat Transfer characteristics

Heat transfer enhancement effect occurs when the secondary flows mix with the main

flows. There are two different kinds of secondary flows between two staggeringly

arrayed consecutive blockages. One is formed when the coolant passes through the

elongated holes as in rib turbulators for instance [1]. The other is built when the

coolant impinges onto the solid part between the two consecutive holes of blockages

On the basis of flow structure between two consecutive blockages, five heat transfer

enhancement deciding factors can be speculated. The first factor is the number of

impingement region. For instance, the blockages with four holes have four

impingement regions and that with six holes have six impingement regions. The

second and the third are the total and partial areas of impingement region,

respectively. Here, “partial area” means the region between two consecutive holes.

The fourth and the fifth are the total and partial widths of reattachment region,

respectively. Here, “partial width” means the width of one hole. These five heat

transfer enhancement deciding factors are controlled by the relative pitch ratio (P/e)

between the blockages and hence decides the final heat transfer enhancement effect.

The impingement and reattachment regions are not formed when blockages come



closer to each other i.e., at lesser value of P/e, because the coolant in this case directly

passes to next blockage’s opening without giving any secondary flow [3, 6, 7].

1.4. Results and Discussion

Table 1 Heat transfer co-efficient Comparison between CFD Analysis and Experimental

Results for Blockages with four elongated holes:

P/e Re CFD

Value (h)

Exp.

Value (h) % error

Avg. %

Error

4.5

4347 36.96 35.62 3.761931

3.38 6148 42.08 40.73 3.31451

8133 55.61 55.86 3.132832

10195 69.23 67.01 3.312938

6.0

4347 41.02 39.31 4.350038

3.56 6148 45.96 44.61 3.026227

8133 62.14 59.92 3.70494

10195 76.39 74.05 3.160027

9.0

4347 36.72 35.12 4.555809

3.749 6148 41.33 40.04 3.221778

8133 55.32 53.58 3.24748

10195 69.10 66.46 3.972314

12.1

4347 31.18 30.00 3.933333

3.66 6148 35.601 34.31 3.762751

8133 46.76 45.20 3.451327

10195 58.66 56.68 3.493296

The plot shows Convective heat transfer coefficient variation with pitch distance.

Figure 4 Heat transfer coefficients CFD Analysis


with Elongated Hole


Figure 5 Heat transfer coefficients Experimental Analysis

As compared to blockages with four holes, the convective heat transfer

coefficients have superior values for blockages with six holes at all Reynolds number.

Table 2 Heat transfer co-efficient Comparison between CFD Analysis and Experimental

Results for Blockages with six elongated holes:

P/e Re CFD

Value (h)

Exp.

Value (h) % error

Avg. %

Error

4.5

4347 42.1 38.7 8.78553

7.871 6148 50.71 47.03 8.074685

8133 64.8 60.35 5.3644

10195 78.96 76.24 6.622951

6.0

4347 44.57 41.24 7.824793

7.612 6148 53.44 49.65 7.633434

8133 68.79 63.53 6.292184

10195 83.52 78.45 5.13321

9.0

4347 38.89 36.92 7.373654

5.998 6148 48.82 45.93 8.279553

8133 61.67 58.53 5.36477

10195 76.43 71.43 6.795741

12.1

4347 32.52 30.5 7.502623

5.882 6148 43.08 40.91 6.462715

8133 55.16 51.61 6.99986

10195 65.82 62.85 4.725537

The reason is that increased total widths of reattachment region (23.1 cm for six

holes as compared to 22.4 cm in four holes) and impingement region area are

dominant factors of heat transfer enhancement in case low hole aspect ratio (a/h) or

for blockage with six holes.



Figure 6 Heat transfer coefficients by CFD Analysis

As the hole aspect ratio increases both partial width of reattachment region and

partial impingement area between two consecutive blockages also increases but the

number of impingement regions decreases from six to four. This causes low value of

h at all Reynolds number for large aspect ratio hole i.e., blockage with four holes.

Figure 7 Heat transfer coefficients by Experimental Analysis

As the air flow rate increases, the heat transfer enhancement first decreases and

then again increases. Though average Nusselt number was found to increase with

increase in Reynolds number, the Nusselt number ratio decreases. The increase in

average Nusselt number is due to better turbulence mixing at increased flow rate. The

increase in Reynolds number also causes an increase in the unsteady reverse flow just

behind the downstream blockages which helps in better mixing. However, the Nusselt

number ratio decreases due to the fact that as Reynolds number increases the flow

tends to reattach quickly and so the effect on the heat transfer coefficient is

considerably reduced. These results also confirm that even though average convective


with Elongated Hole


heat transfer coefficient for forced convection increases proportionally to Reynolds

number, the increase rate of reference Nusselt number (Nu0) surpasses it. Table shows

that the average Nusselt number ratios of the blockages with four or six holes in case

of Reynolds number of 8133 and 10195 are superior to the Reynolds number value of

6148.

Figure 8 Nusselt number variation with pitch distance for four elongated holes

Figure 9 Nusselt number variation with pitch distance for six elongated holes

This is due to the enhancement in impinging and turbulent mixing effect of

blockages with increase in flow rate and therefore for higher value of Reynolds

number, the average Nusselt number ratios again increases. With an increase of the

aspect ratio of the holes, the corresponding Nusselt number ratios also significantly

increases.



Table 3 Pressure Drop Comparison between CFD Analysis and Experimental Results for

Blockages with four elongated holes

Figure 10 Pressure drop plot CFD analysis

Figure 11 Pressure drop plot Experimental analysis

Re

CFD

ΔP

(N/m2)

Experimental ΔP

(N/m2) % Error

4.5

4347 57.87 50.62 14.32

6148 109.36 97.98 11.61

8133 224.12 199.24 12.48

10195 346.53 303.76 14.08

6.0

4347 54.82 47.68 14.97

6148 106.76 94.72 12.71

8133 181.81 160.04 13.60

10195 269.88 238.43 13.19

9.0

4347 53.66 46.38 15.69

6148 95.17 83.28 14.27

8133 161 140.44 14.63

10195 219.15 195.97 11.82

12.1

.

4347 50.96 44.10 15.55

6148 81.17 73.49 10.45

8133 133.33 117.58 13.39

10195 185.24 163.31 13.42


with Elongated Hole


1.5. Pressure contour plot for four elongated holes

Figure 12 50.8 mm pitch at Re 4347

Table 4 Pressure Drop Comparison between CFD Analysis and Experimental Results for

Blockages with six elongated holes

P/e Re CFD ΔP

(N/m2)

Experimental ΔP

(N/m2) % Error

4.5

4347 68.262 58.795 16.10171

6148 135.847 117.581 15.53482

8133 239.889 210.673 13.86794

10195 351.133 310.291 13.16248

6.0

4347 61.924 55.526 11.52253

6148 124.781 113.665 9.779616

8133 210.237 202.507 3.817152

10195 270.642 244.96 10.48416

9.0

4347 53.984 48.994 10.18492

6148 119.714 109.419 9.408786

8133 198.738 192.708 10.39396

10195 252.985 232.14 8.979495

12.1.

4347 45.605 42.461 7.404442

6148 108.46 99.62 8.87372

8133 152.691 127.91 19.37378

10195 240.177 214.21 12.12222

Figure 13 Pressure drop plot by CFD analysis



Figure 14 Pressure drop plot by Experimental analysis

Pressure contour plot for six elongated holes

Figure 15 50.8 mm pitch at Re 4347

2. CONCLUSIONS

For the CFD analysis performed to study average heat transfer enhancement on

rectangular channel wall, the conclusions are as follows:

1. The low aspect ratio case (Blockages having six holes) showed the effective heat

transfer enhancement as compared to high aspect ratio case (Blockages having four

holes).

2. For the blockages, as hole aspect ratio increases, the heat transfer on the wall

segments is enhanced due to the increase in the reattached region and partial

impingement area. In this case, the flow reattachment and partial impingement area

are the dominant factors determining the heat transfer on the channel wall.

3. In the CFD analysis we have assumed that the duct wall as prefect adiabatic wall as

wall boundary condition which is not true in real case, no object is perfectly acts as

insulator or adiabatic i.e. heat transfer through the duct walls are assumed to be zero

hence heat transfer to atmosphere is zero. There is complete heat transfer from heated

wall to air is happening hence total heat transfer to air more than that of experimental

results.

4. Comparison of the computationally derived results with that of the experiments

shows a good correlation. Hence the proposed analysis method has demonstrated a


with Elongated Hole


workable alternative to obtain heat transfer enhancement and thermal performance by

manipulating the results from ACU Solve simulation.

In this study, analysis to measure average heat transfer enhancement were

performed for blockages with only elongated holes for the purpose of application of

gas turbine trailing edge or middle portion internal cooling. Based on the results, P/e

ratio of 6.0 and Case 1(blockages with four holes) is the best option based on TP

value, but Case 2 (blockages with four holes) can be accepted if the level of penalty

from pressure drop is acceptable.

ACKNOWLEDGEMENTS

I would like to express my sincere gratitude to Dr. Satyendra Singh and Rajeev

Pandey for their guidance and assistance in this study work. The reality is that Dr.

Satyendra Singh and Rajeev Pandey were much more than an advisor for me. They

always helped me in all the technical and non-technical issues during the production

of this work. Their encouragement and efforts led this report to successful completion

in a timely fashion.

REFERENCES

[1] Pandey, R. Heat transfer enhancement through blockages with elongated holes in

a rectangular channel. Ph. D thesis, Mechanical Engineering Department, India,

DIT University, 2010.

[2] Moon, S. W. and Lau, S. C. Heat transfer between blockages with holes in a

rectangular channel. ASME J. Heat transfer, 125, 2003, pp. 587–594.

[3] Metzger, D. E., Fan, Z. X. and Shepard, W. B. Pressure loss and heat transfer

through multiple rows of short pin fins, in Heat Transfer, 3, Grigull U. et al., eds.,

Washington, DC: Hemisphere, 1982, pp. 137–142.

[4] Takeishi, K. Heat transfer research in high temperature industrial gas turbines, in

Proc. International Symposium on Heat Transfer in Turbomachinery, Marathon,

Greece, 1992.

[5] Lau, S. C., Cervantes, J., Han, J. C., Rudolph, R. J. and Flannery, K.

Measurements of wall heat(mass) transfer for flow through blockages with round

and square holes in a wide rectangular channel. Int J. Heat mass Transfer, 46,

2003, pp. 3991–4001.

[6] Lee, Y. Heat transfer enhancement for turbulent flow through blockages with

elongated holes in a rectangular channel, Ph. D thesis, Mechanical Engineering

Department, Texas: A & M University.

[7] Shin, S. and Kwak, J. S. Effect of hole shape on the heat transfer in a rectangular

duct with perforated blockage walls, 2008.

[8] Taslim, M. E. and Spring, S. D. Effects of turbulator profile and spacing on heat

transfer and friction in a channel. AIAA Journal of Thermodynamics and Heat

Transfer, 8, 1994, pp. 555–562.

cfd analysis and enhancement of heat transfer in ...€¦ · geometries and altair hypermesh used...

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