Numerical Investigation of Heat Transfer and Fluid Flow ...psrcentre.org/images/extraimages/1215021.pdf · With (P/e =12) Reynolds number range, the trapezoidal-shaped ribs have the

Abstract—Numerical simulation is one of finest alternative

technique to predict and investigate heat transfer and fluid flow

characteristics of roughened solar air heater which aims to cost

effective and time saving comparative to experimentation. This

numerical simulation approaches to above by using computational

fluid dynamics coded software (Fluent 6.3.26 Solver. Parameters are

Reynolds number ranges (3000 to 15000) and p/e (5 to 20) and k-ɛ

turbulence model fitted best by comparing the predictions of various

turbulence models. Enhancement in heat transfer is found in between

Re-10000 to 12000 at p/e 12 and at reattachment zone between ribs.

Chamfered ribs giving maximum heat transfer enhancement and

giving 3 to 4 times Nusselt Number enhancement and only 1.5 to 1.8

times friction increasing as compared to smooth duct flow.

Furthermore effects on Nusselt Number, friction factor and thermo

hydraulic performance also discussed in this analysis.

Keywords— Solar air heater, CFD, Artificial roughness, Heat

transfer, Fluid Flow.

I. INTRODUCTION

OLAR air heaters, because of their simplicity are cheap

and widely used as energy collection devices. Here an

effort has been made to increase the heat transfer through

absorber plate by using roughness. A various experimental

analysis in this area have been carried but only few

computational analysis and investigation have been done.

The presence of rib increases heat transfer because of

interruption of the viscous sub layer, which enhance flow

turbulence and reattachment results to a higher heat transfer.

In this work, an attempt is done to predict the velocity and

temperature which is responsible for heat transfer enhancement

by reattachment of flow between ribs is considerably

maximum.

II. DATA FORMULATION OF SOLUTION DOMAIN AND CFD ANALYSIS

Solution domain of solar air duct considered was having

inner cross-sectional dimensions of 50mm x 20mm as shown

in geometry Fig. 1. The flow system consists of 150mm (>

5√WH) long entry section, 500 mm long test section and 75

mm (> 2.5√WH) long exit section and selected test length or

J.L. Bhagoria, Prof. and head,Department of Mechanical Engineering,

Maulana Azad National Institute of Technology (MANIT) , Bhopal-462001,

India. Email id: [email protected]

Ajeet Kumar Giri (R/S), Department of Mechanical Engineering, Maulana

Azad National Institute of Technology (MANIT) , Bhopal-462001, India.

Email id: [email protected].

plate length 500 mm. The entry and exit length of the flow

have been kept as per consideration for fully developed flow

and as per recommendation provided in ASHRAE Standard

93-77. On and average constant heat flux of 800 W/m2 is

considered

Duct Height(H)=20mm

Duct Width (W) =50mm

Hydraulic mean diameter, ‘Dh’ = 28.5 mm

Duct aspect ratio, ‘W/H’ =2.5

Length of Test Section=500mm

Minimum Inlet Length for fully developed flow =150mm

Outlet length= 75mm

Rib height height, ’e’ = 2 mm

Reynolds number, ‘Re’ = 3000-15000

p/e range= 5 to 20

Uniform Heat at bottom Surface=800 W/m2

Inlet Length for fully developed flow =150mm

Fig. 1 shows two dimensional view of problem in CFD, since

3D model increases time and computational complexities so

2D model is selected (Fig. 2) for analysis. In the present

study, FLUENT Version 6.3.2 was used for analysis.

The assumptions for mathematical model while CFD

analyses are

i. The flow is fully developed, steady, turbulent and three

dimensional.

ii. The thermal conductivity is not changing with

temperature.

iii. The working fluid is assumed incompressible

iv. This was assumed in respect of experimentation of solar

air heaters by various investigators.

III. GEOMETRY AND GRID INDEPENDENT TEST

A mesh model is created in GAMBIT with FLUENT 5/6 on

the rectangular face with .1 and .2 and .3 interval size

according to roughness height 2mm and interval size of .2 in

the vertical direction and .1 interval sizes in the horizontal

direction. In creating this mesh, it is desirable to have more

cells near the roughness because we want to resolve the

turbulent boundary layer, which is very thin compared to the

height of the flow field.

Numerical Investigation of Heat Transfer and

Fluid Flow Characteristics of Roughened Solar

Air Heater Duct

J.L. Bhagoria, and Ajeet Kumar Giri

S

4th International Conference on Mechanical, Electronics and Mechatronics Engineering (ICMEME'2015) Dec. 15-16, 2015 Pattaya (Thailand)

6

mailto:[email protected]

Fig. 1 Geometry of Solar Air Heater with square ribs in Gambit

Window

Boundary Types selected in GAMBIT (for Fluent 5/6) is

given below:

TABLE I

Edge

Position

Name Type

Left Duct Inlet VELOCITY_INLET

Right Duct Outlet PRESSURE_OUTLET

Top Top Surface WALL

Bottom Inlet Length WALL

TABLE II

VARIOUS GEOMETRIES CREATED FOR ANALYSIS IN GAMBIT

S.N

o.

Geome

try

Numb

er

Type of rib Pitc

h (p)

mm

p/

e

Roughnes

s height

(e) mm

1. 1 square 10 5 2

2. 2 square 20 10 2

3. 3 square 24 12 2

4. 4 square 30 15 2

5. 5 square 36 18 2

6. 6 square 40 20 2

7. 7 semicircular 10 5 2






13 13 chamfered 10 5 2

14 14 chamfered 20 10 2

15 15 chamfered 24 12 2

16 16 chamfered 30 15 2

17 17 chamfered 36 18 2

18 18 chamfered 40 20 2

19 19 triangular 10 5 2






For grid independence test, the number of cells taken from

50,567 to 629400 in various steps and it was viewed that after

420,100 cells, further increase in cells has negligible effect on

the results.

IV. RESULTS AND DISCUSSION

4.1 Velocity profile for rib

Fig 4.1 shows the velocity vectors for the square shape of

ribs inserted in a solar air heater duct other four cases have

also the similar velocity profiles, the flow over the triangular

ribs appears to be the most complex this is because the rib face

is perpendicular to the flow direction so triangular shape rib

should not be preferable.

Fig.2 Velocity profile for square ribs

4.2 Heat Transfer in Roughened Duct.

The heat transfer and flow gets affected because of ribs in

the solar air heater. Nusselt number just at the vicinity of rib

has been found to be low. This may be because that heat

transfer takes place at that rib area isdue to conduction only,

While at point where flow reattaches Nusselt Number is quite

high. The increase in Nusselt is due to the variation in flow

pattern downstream of the rib. Temperature profile is shown in

the figure given below.

Fig. 4 Temperature profile for chamfered ribs


7

Fig. 3 Variation in heat transfer coefficient along the Solar plate, With

Re=10000, I=800 W/m2

4.3 Comparison between various ribs at different p/e

CFD analysis is predicting performance of various ribs at

different p/e which is shown in figure given below and it is

seen that with particular range of p/e heat transfer is maximum

and while shifting both sides of p/e performance decreases. To

justify the heat transfer features for the investigated rib shapes,

the Nusselt number ratios along the bottom wall between two

successive ribs are plotted.

for square rib

05

1015202530354045

0 5000 10000 15000 20000

Renyolds number Re

Nu

sselt N

um

ber N

u for p/e 5for p/e 10for p/e 12for p/e 15for p/e 20for Smooth Duct

Fig.5 Variation of Nusselt number with Reynolds number for Squre

Ribs

For semicircular rib

05

1015202530354045

0 2000 4000 6000 8000 10000 12000 14000 16000

Reynolds number Re

Nu

sselt n

um

ber N

u

Smooth ductp/e 5p/e 10p/e 12p/e 15p/e 18p/e 20

Fig.6 Variation of Nusselt number with Reynolds number for

Semicircular Ribs

For triangular rib

05

101520253035404550

0 2000 4000 6000 8000 10000 12000 14000 16000

Reynolds number Re

Nu

sselt N

um

ber N

u


Fig. 9 Variation of Nusselt number with Reynolds number for

Triangular Ribs

For chamfered rib

0

5

10

15

20

25

30

35

40

0 2000 4000 6000 8000 10000 12000 14000 16000

Reynolds number Re

Nu

sselt

nu

mb

er

Nu


Fig.10 Variation of Nusselt number with Reynolds number for

Chamfered Ribs


8

It is also found that Chamfered ribs showing highest heat

transfer enhancement with minimum friction. Analysis also

shows that the lowest heat transfer rate is seen in case of duct

provided with semicircular ribs.

4.4 Comparison of average heat transfer and friction

characteristics

With (P/e =12) Reynolds number range, the trapezoidal-

shaped ribs have the highest friction loss; whereas, the

trapezoidal-shaped ribs have the lowest pressure drop.

Furthermore, the triangular-shaped ribs have higher friction

factor than that of square-shaped ribs, based on the law of the

wall similarity Tanda[12]. developed semi empirical formulas to

predict the heat transfer coefficient and friction factor in a

square duct roughened by the square-shaped ribs on one wall.

For square rib

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0 2000 4000 6000 8000 10000 12000 14000 16000

Reynolds number Re

Fric

tio

n f

acto

r F

Smooth Ductp/e 5p/e 10p/e 12p/e 15p/e 20

Fig.11 Variation of Friction factor with Reynolds number for Square

Ribs

For semicircular rib

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0 2000 4000 6000 8000 10000 12000 14000 16000

Reynolds number Re

Fri

cti

on

facto

r F

smooth ductp/e 5p/e 10p/e 12p/e 15p/e 18p/e 20

Fig.12 Variation of Friction factor with Reynolds number for

Semicircular ribs

For chamfered rib

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0 2000 4000 6000 8000 10000 12000 14000 16000

Reynolds number Re

Fric

tio

n f

acto

r F

Smooth Ductp/e 5p/e 10p/e 12p/e 15p/e 18p/e 20

Fig.13 Variation of Friction factor with Reynolds number for

Chamfered ribs

For triangular rib

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0 2000 4000 6000 8000 10000 12000 14000 16000

Reynolds number Re

Fri

cti

on

facto

r F

Smooth Ductp/e5p/e 10p/e 12p/e 15p/e 18p/e 20

Fig. 14 Variation of Friction factor with Reynolds number for

triangular rib

V. CONCLUSION

This investigation shows that Chamfered shape ribs giving

maximum heat transfer enhancement and minimum friction as

compared to other ribs geometries with only 1.5 to 1.8 times

friction increasing as compared to smooth duct flow, which is

very small and unaccountable. Maximum heat transfer is found

near reattachment zone. Experiments also confirm this. K-ɛ

turbulence model found good for close results on comparing

the predictions of various turbulence models during analysis.

Since 3D model requires much higher memory and

computational time compared to 2D ones and 2D model results

are found closer to the experimental ones so it is sufficient to

employ 2D model.

REFERENCES

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friction factor correlations for rectangular solar air heater duct having

transverse wedge shaped rib roughness on the absorber plate. Renewable

Energy 25 (2002) 341–369


9

[2] R.P. Saini , Jitendra Verma. Heat transfer and friction factor correlations

for a duct having dimple-shape artificial roughness for solar air heaters.

Energy 33 (2008) 1277– 1287

[3] Gupta Dhananjay, Solanki SC, Saini JS. Heat and fluid flow in

rectangular solar Air heater ducts having transverse rib roughness on

absorber plate. Solar Energy 1993; 51:31–7.

[4] J.C. Han, J.S. Park, C.K. Lei. Heat transfer enhancement in channels

with turbulence promoters. J. Eng. Gas Turb.Power 107 (1985) 628–

635.

[5] Y.M. Zhang, W.Z. Gu, J.C. Han, Heat transfer and friction in

Rectangular channel with ribbed or ribbed-grooved walls.

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ASME J. Turbomach.1998; 120, 564–570.

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shaped broken ribs. Int J Heat Mass Transfer 2004;47:229–43.

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[14] Apurba Layek, J.S. Saini, S.C. Solanki. Effect of chamfering on heat

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Numerical Investigation of Heat Transfer and Fluid Flow ...psrcentre.org/images/extraimages/1215021.pdf · With (P/e =12) Reynolds number range, the trapezoidal-shaped ribs have the