NUMERICAL INVESTIGATION OF HEAT TRANSFER & FLUID FLOW … · the ribs to the passing air which...

© 2017 IJRTI | Volume 2, Issue 4 | ISSN: 2456-3315

IJRTI1704021 International Journal for Research Trends and Innovation (www.ijrti.org) 83

NUMERICAL INVESTIGATION OF HEAT TRANSFER &

FLUID FLOW IN A SOLAR AIR HEATER DUCT USING

GAP IN V-SHAPED RIBS ON ABSORBER PLATE

Jouhri Kumar Akale1, Dr. S.k. Nagpure

2

M.tech. Scholar1, HOD2 Department of Mechanical Engineering

Scope College of Engineering, Bhopal, India

Abstract — In a Computational Fluid Dynamics (CFD), the

trial has been conducted on the heat transfer and friction

factor, for a solar air heater with Circular ribs with

different relative gap width. In the CFD investigation it

was assumed that, the rib height is 2mm, the pitch of the

rib (P) is 20, the Relative Roughness pitch (P/e) is 10, and

the angle of attack (α) is 60°. This investigation was

carried out on a Reynolds Number ranging in between

3000-15000.

When the desired heat transfer was carried with smooth

and rough ribs with symmetrical gaps it was found that;

I. The smooth ribs could not transfer the desired

heat due to absence of friction; so, they are not preferred

practically.

II. The rough ribs were efficient enough to transfer

the desired heat, but they are not economical and are very

complex in design and construction. Whereas, the Circular

ribs with different relative gap width gives more area of

contact. So, there is enough time to transfer the heat from

the ribs to the passing air which touches the ribs and the

heat transfer takes place efficiently. Thus, the Circular

ribs with different relative gap width, when compared with

the smooth and rough ribs with symmetrical gaps, it was

concluded that the related Nusselt Number and Friction

Factor for Circular ribs with different relative gap width

was more efficient as well as economical, than that of

smooth and rough ribs with symmetrical gaps for flow

Reynolds Number.

Keywords— CFD analysis, FLUENT, UNIGRAPHICS,

Solar Duct, Friction factor, and Nusselt number.

I INTRODUCTION

Due to depletion of fossil fuels and the fuels like coal, crude

oil which produces petrol, LPG,coal tar etc. for energy

generation like electrical energy ,mechanical energy and in

aerospace applications preferably harms the environment and

ozone layer of our earth due to this hazardous effect takes

place in our environment and causes the effect of green house

,major issues like for ventilation in industries ,offices ,houses

basically an air conditioning system is preferred and other

criteria if we see in refrigeration system cloroflouro carbon is

used this substance is also used in aeroplanes and satellites launching systems this substance plays an important role in

these such applications as a major part of fuel combustion to

avoid these effects a solar energy plays an important role now

a day’s solar panel operated aeroplanes were manufactured to

conserve our fossil fuels and to protect our environment from

harmful fuel combustion fume in ventilation and air drying

process solar duct plays an important role the assembled view

of solar duct

1.1 About the solar duct

1.2.1 DUCT –Ducts are used in HVAC system and ventilation

process in industries ,offices ,houses they consists of an plate

assembled in circular rib with different relative gap width or

rectangular form with roughness for better conductivity to

increase heat transfer rate during operation.

Most appropriate way to analyze the solar heater air duct with

different Geometry is to analyze each of the geometry on CFD

and compare the results obtained with the results of

experimental investigation done by Rajesh Maithani and

J.S.Sainiand find out the best geometry having more heat

transfer of air in solar heater air duct

Figure 1.1: Solar Air Heater

1.2.2 SOLAR ENERGY COLLECTORS

Solar collectors are the key component of active solar-heating

systems. Solar collectors gather the sun's energy, transform its

radiation into heat, then transfer that heat to water, solar fluid,

or air. The solar thermal energy can be used in solar water-

heating systems, solar pool heaters, and solar space-heating systems.

Solar energy collectors are classified as:

(1) Flat plate collectors.

(2) Concentrating collectors.



If the area of interception of solar radiation is same as the area

of absorption, the collector is known as flat plate collector.

II PERFORMANCE EQUATIONS FOR A SOLAR

COLLECTOR

The performance of solar collector is described by an energy

balance that indicate the distribution of incident solar energy

into useful energy gain (Qu) and heat losses like bottom (Qb)

and top (Qt) as shown in Fig.1.2. The details of the

performance analysis of a solar collector are discussed by

Duffie and Beckman [31] and Goswami [41].The heat transfer

in a solar collector takes place by simultaneous radiation,

convection and conduction. The heat transfer from the top takes place by convection and radiation while from the side

and bottom is by conduction. The net rate of useful energy

collected per unit area is the difference of the amount of solar

energy absorbed and the energy loss by the collector to the

surroundings.

Objective of the Work

The main objective of the current work is

The main objective of our proposed work is

validation of the solar duct models by comparing the

shows simulated outcome.

To predict nusselt no. and friction factor for different

Relative gap width on the Circular rib with different

relative gap width roughened solar duct.

To simulate the Circular rib with different relative

gap width roughened solar duct of the different

Circular rib with different relative gap width

roughened profile and constant velocity for constant heat input.

To define average heat transfer coefficient, Nusselt

number and friction factor for the Circular rib with

different relative gap width roughened solar duct of

the different relative gap width and constant velocity

and constant heat input.

Predict temperature distribution along the Duct.

Problem Formulation

The survey of different previous works we predict the

friction factor is higher as compared to present study is shown

in our base paper. The purposes of this study reduce the

friction factor and increase the Nusselt no. at various relative

gap widths. Thus Chosen Circular rib with different relative

gap width shaped roughened solar duct absorber plate of the

different relative gap width in place of circular roughened

duct.

III LITERATURE REVIEW

3.1 CATEGORIZATION OF LITERATURE

Literature is divided into following categories:

1. Concept of artificial roughness

2. Roughness geometries used in solar air heater ducts

The thermal efficiency of solar air heaters (SAHs) has been

found to be generally poor because of the inherent poor

thermal conductivity of air. In order to make the solar air

heaters economically viable, their thermal efficiency needs to

be improved by enhancing the heat transfer coefficient. There

are different factors affecting the SAH efficiency, for example

collector length, collector depth, type of absorber plate, glass cover plate, wind speed, and so forth.

The shape factor of the absorber plate is the most important

parameter in the design for any type of SAH. Increasing the

absorber plate shape area will increase the heat transfer to the

flowing air. On the other hand, it will increase the pressure

drop in the collector; this increases the required power

consumption to pump the air flow crossing the collector [1].

Sahu and Bhagoria(2005) [1]], The investigation had been

done on this paper is the heat transfer coefficient by using 90˚

broken transverse ribs on absorber plate of a solar air

heater,the roughened wall being heated while the remaining three walls are insulated. We found the entire range of

Reynolds number and Nusselt number is also increases attains

a maximum for artificial roughness pitch of 20 mm and

decreases with an increase of artificialroughness pitch.The

experimental values of the thermal efficiency of the three

roughened absorber plates tested have been compared with the

smooth plates. A plate having roughness pitch 20 mm and

plate gives the highest efficiency of 83.5%.

Anil Kumar et al.(2009)[2]–- Stated artificial roughness in

the form of repeated ribs is one of the effective way of

improving the performance of a solar air heater ducts. It has

been carried out to determine the effect of different artificial roughness geometries on heat transfer and friction

characteristics in solar air heater ducts. The objective of that

study is to review various studies, in which different artificial

roughness elements are used to enhance the heat transfer co-

efficient with little penalty of friction factor. On the basis of

correlations developed by various investigators for heat

transfer coefficient and friction factor, an attempt has been

made to compare the thermo-hydraulic performance of

roughened solar air heater ducts. It has been found that lot of

experimental and analytical studies reported in that paper.

Kumar and Saini[3] carried out CFD based analysis to fluid flow and heat transfer characteristics of a solar air heaters

having roughened duct provided with artificial roughness in

arc shaped geometry. The heat transfer and flow analysis of

the chosen roughness element were carried out using 3-D

models. Authors reported that Nusselt number has been found

to increase with increase in Reynolds number where friction

factor decreases with increase in Reynolds number for all

combinations of relative roughness height (e/D) and relative

arc angle (á/90).

Karmare and Tikekar [4]- performed a CFD simulation of

fluid flow and heat transfer in a solar air heater duct with

metal grit ribs as roughness elements employed on one broad wall of a solar air heater. The circular, triangular and square

shape rib grits with the angle of attack of 54°, 56°, 58°, 60°

and 62° were tested for the same Reynolds number. Authors

reported that the CFD results give the good agreements with

experimental results. Hence, CFD techniques can be used for

analyzing and optimizing complex type of roughness surface.

Soi et al. presented a CFD based investigation of heat transfer

and friction characteristics of artificially roughened duct of a



solar air heater. K-shaped (combination of transverse and V-

up) roughness geometry was used as an artificial roughness.

Authors observed, for a given type of artificial roughness,

Nusselt number increases with an increase in Reynolds

number for smooth as well as roughened plate. Sharma et al.

designed a solar air heater duct in order to carry out CFD based investigation. CFD based performance analysis of solar

air heater duct provided with artificial roughness in the form

of square type protrusion shape geometry was reported. 2-D

computational domain and grid were selected. Non-uniform

meshing was generated over the duct.

Sharma and Thakur [5] conducted a CFD study to

investigate the heat transfer and friction loss characteristics in

a solar air heater having attachments of V-shaped ribs

roughness at 60° relative to flow direction pointing

downstream on underside of the absorber plate. Authors

observed,Nusselt number increases with increase in Reynolds

number where friction factor decreases with increase in Reynolds number for all combinations of relative roughness

height (e/D) and relative roughness pitch. In the present work

a computational investigation of turbulent forced convection

in a two-dimensional duct of a solar air heater having

rectangular rib roughness on the absorber plate is conducted.

The upper wall is subjected to a uniform heat flux condition

while the lower wall and two other side walls, except inlet and

outlet are insulated.

Sachin Chaudhary et al. [6] - having more concern with

enhancement of heat transfer coefficient using artificial

roughened absorber plate on solar air heater. The increment in heat transfer also leads to increase in friction factor which

leads to increase in pumping power. In this study M shape

geometry has been studied which is having different

orientation. The effect of roughness parameters relative

roughness height (e/D), relative roughness (P/e) and angle of

attack (á) on Nusselt number and friction factor have been

seen. The range of Reynolds number 3000-22000, e/D, P/e

and á are 0.037-0.0776, 12.5-75 and 30-60° respectively. It

has been found out that providing the artificial roughness of M

shape increases heat transfer upto 1.7-1.8 times over the

smooth duct

V COMPUTATIONAL FLUID DYNAMICS (CFD)

5.1 INTRODUCTION: Computation Fluid Dynamics (CFD)

is the branch of fluid science which deals with a variation

occurs on fluid flow, basically computational fluid dynamics

opt an finite volume method as methodology and for base

equation it follows the eulerian equation, i.e. when gravity

forces were not consider, pressure force and viscous force are

used to simulate the desired fluid flow problem.

VI MODEL FORMULATION

6.1 PREPARATION OF THE CAD MODEL

The measurements of the computational area sun based pipe

depended on the work by Rajesh Maithani, J.S. Saini. After

this procedure the imperative are connected and along these

lines the model is accomplished in demonstrating

programming UNIGRAPHICS The accompanying section

(4.1.1) demonstrates the parameters of sun based air radiator

channel roughened misleadingly with V-ribs and Semi-

circular V-ribs.

6.2 Pre-processing

6.2.1 To prepare A CAD model and geometry

dimension

Figure 6.1: Figure 4.2 Duct with different circular gap width

absorber plate

6.2.2 Meshing

Type of meshing: - Tetrahedral No. of element: - 106660

No of face: - 163801

No of nodes: -57141 Interval Size: - 1

Figure 6.2: Meshing of under floor air distribution system

VII DATA REDUCTION 7.1 The procedure of calculation of above mentioned parameters are given below:

Velocity of air through duct

The velocity of air is calculated from the knowledge of mass

flow rate and area of flow as,

V=m/(ρ.W.H)

(ii) Equivalent hydraulic diameter

The hydraulic diameter of the rectangular section of the duct is

determined from the relationship as given below,



D = (4 x Area of flow)/ Perimeter

(iii) Reynolds number (Re)

The Reynolds number of air flow in the duct is calculated

from the following relationship,

(iv). Nusselt Number for roughened duct

Nu = h D /K

Where k is the thermal conductivity of air andD is the

hydraulic diameter.

(v) Nusselt Number for smooth duct

Nusselt Number for smooth roughened duct can be obtained

by Dittus–Boelter correlation

Nus = 0.023 Re0.8Pr0.4

(vi). Friction factor for roughened duct

f=(ΔP/L)D/(2ρV^2 )

Average pressure drop (∆P) can be obtained directly from

FLUENT.

(vii). Friction factor for smooth duct

Friction factor for smooth roughened duct can be obtained

byBlasius correlation

f_s=0.079 R_e^(-0.25)

(viii). Thermal enhancement factor

Thermal enhancement factor=(Nu⁄Nu_s )/(f⁄f_s )^(1/3)

VIII RESULT AND DISCUSSION The effects of relative gap width (g/e) and Reynolds

number(Re) on the heat transfer and friction characteristics for

flow of air in a roughened rectangular duct are presented

below.The results have been compared with experimental value of same parameter and also compare with smooth ducts

operating under similar operating conditions to discuss the

enhancement in heat transfer and friction factor on account of

artificial roughness.

Figure 8.1 Comparison of different relative gap width (2, 2.5,

3, 4) for Nusselt no

Table No. 8.1.1 Comparison of experimental, Relative gap

width (g/e)= 4 and smooth plate for Nusselt No.

Reynolds

No.

Nusselt No. of

experimental

Nusselt No

of Relative

gap width

(g/e) = 4.

Smooth

3000 41 65.184 60.481

5000 81 120.346 120.412

7000 143 208.749 200.347

9000 159 220.563 208.963

11000 168 229.761 225.852

13000 171 230.753 234.743

15000 173 239.851 241.916

Figure 8.2 Compare with experimental in g/e=4 result with

smooth

Figure 8.3 Temperature coloured contour plots of air at outlet

at Reynolds number 3000





Figure 8.5 Temperature colored contour plots of air at outlet at

Reynolds

Number7000

Figure 8.6 Temperature colored contour plots of air at outlet at Reynolds number 9000



Figure 8.8 Comparison of different r.g.w. for Friction Factor

Table no.8.1.2 Comparison of experimental, CFD, smooth

plate for Friction Factor

Reynolds No. Friction

Factor in

EXP.

Friction

Factor in

CFD

Smooth

3000 0.0333 0.0389 0.0396

5000 0.0300 0.0359 0.0357

7000 0.0280 0.0320 0.0348

9000 0.0265 0.0330 0.0342

11000 0.0260 0.0308 0.0308

13000 0.0260 0.0310 0.0310

15000 0.0260 0.0318 0.0315



Figure No.8.9 Comparison of experimental, CFD, smooth

plate for Friction Factor

IX CONCLUSION

Average deviation of result obtained

from CFD for smooth & circular geometry for Nu

number & Friction factor lies within the range,

average Nu Number is deviate 3.76% for smooth

plate and Average Friction factor is deviate 3.91% for

smooth plate as compared to experimental work of

the Author.

Average deviation of results obtained for different circular gap width from CFD in Nu Number is

deviated by 17.01 % i.e., Nu Number increases for

circular geometry at each Reynolds number taken for

analysis.

Average deviation of result obtained for different

circular gap width from CFD in Friction factor is

deviated by 20.15% i.e., Friction factor increases for

circular geometry at each Reynolds number taken for

analysis.

Thermo-Hydraulic performance increases at

Reynolds number 3000 for different circular gap width by 6.7%; and for Reynolds number 3000, 5000,

7000, 9000, 11000, 13000, 15000it increases by

18.63%, 24.12%, and 19.35%, 16. 97% respectively.

This CFD analysis clearly indicates that different

circular gap width 3roughness increases the

turbulence in the air; and at the contact, the heat

transferring area of air is increased which results in

the increase in Nu Number and Friction factor as

compared to circular geometry.

FUTURE SCOPE:-

For the future works in this area the following outlines are -

Simulation could be done for the solar duct of roughened circular rib with different relative gap width profiles by

varying the Relative gap width or no. of gap.

Simulation could be done for the solar duct of roughened circular rib with different relative gap width profiles by

varying heat input.

Simulation could be done for the solar duct of roughened

circular rib with different relative gap width profiles by

varying all three parametersi.e. relative gap width, no. of gap, roughened profile, and velocity.

Simulation could also be done by varying Angle of Attack.

Appendix

Sample calculations for circular rib q = 1000 W/m2, e= 2mm, P= 20mm, g/e=2 and Re = 3000 are given below:

1. Hydraulic Diameter (D)

= (2×W×H)/(W+H) =(2×0.3×0.025)/(0.3 + 0.025)=0.046154 m

2. Velocity of air (V)

V=Re (ν)/D=1.3 m/s

3. Heat Transfer Coefficient (h)

Average heat transfer coefficient (h) can be obtained directly from FLUENT.

h= 23W/m2K

4. Nusselt Number for roughened duct

Nu = h D /K= 45.76523

5. Nusselt Number for smooth duct

Nusselt Number for smooth roughened duct can be obtained by Dittus–Boelter correlation

Nus = 0.023 Re0.8Pr0.4 = 60.481

6. Pressure drop (ΔP)

Average pressure drop (∆P) can be obtained directly from FLUENT.

∆P= 6 Pa

7. Friction factor for roughened duct

Friction factor for smooth roughened duct can be obtained byBlasius correlation

f=(ΔP/L)D/(2ρV^2 ) =0.0492

8. Friction factor for smooth duct

f_s=0.079 R_e^(-0.25) =0.0396



9. Thermal enhancement factor

Thermal enhancement factor=(Nu⁄Nu_s )/(f⁄f_s )^(1/3) =1.712

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NUMERICAL INVESTIGATION OF HEAT TRANSFER & FLUID FLOW … · the ribs to the passing air which...

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