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Solar Energy 85 (2011) 1109–1118

Nusselt number and friction factor correlations for solar air heaterduct having artificially roughened absorber plate

Brij Bhushan ⇑, Ranjit Singh

Department of Mechanical Engineering, Beant College of Engineering and Technology, Gurdaspur, Punjab 143 521, India

Received 12 July 2010; received in revised form 9 January 2011; accepted 4 March 2011Available online 6 April 2011

Communicated by: Associate Editor I. Farkas

Abstract

An experimental investigation has been carried out for a range of system and operating parameters in order to analyse effect of arti-ficial roughness on heat transfer and friction in solar air heater duct having protrusions as roughness geometry. An increase in heat trans-fer and friction loss has been observed for duct having roughened absorber plate. Experimental data have been used to develop Nusseltnumber and friction factor correlations as function of system and operating parameters for predicting performance of the system havinginvestigated type of roughness geometry.� 2011 Elsevier Ltd. All rights reserved.

Keywords: Solar air heater; Artificial roughness; Nusselt number; Friction factor

1. Introduction

In view of world’s depleting fossil fuel reserves and envi-ronmental threat, development of renewable energysources has received an impetus. From many alternatives,solar energy stands out as the brightest long range resourcefor meeting continuously increasing demand of energy.Flat plate collectors like solar water heaters and solar airheaters are commonly used to tap solar energy for variousthermal applications. Solar air heaters are considered to becompact and less complicated as compared to solar waterheaters. Solar air heaters can be fabricated using cheaperas well as less amount of material as compare to solarwater heaters. Thermal efficiency of a solar air heater isgenerally considered poor because of low rate of heat trans-fer capability between absorber plate and air flowing in theduct. In order to make a solar air heater more effectivesolar energy utilization system, thermal performance needsto be improved by enhancing the heat transfer rate from

0038-092X/$ - see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.solener.2011.03.007

⇑ Corresponding author. Tel.: +91 9855566294; fax: +91 1874 221463.E-mail address: brijbsaini@gmail.com (B. Bhushan).

absorber plate to air flowing in the duct of solar air heater.One of the methods for enhancement of convective heattransfer is by creating turbulence at heat transfer surfacewith the help of artificial roughness on absorber plate. Ribsprovided by artificial roughness break laminar sub-layerand create local wall turbulence due to flow separationand reattachment between the consecutive ribs. As a resultthermal resistance reduces and heat transfer rate getsgreatly enhanced. However, simultaneous increase in fric-tion loss also takes place in the duct with application ofartificial roughness. In order to reduce friction loss withapplication of artificial roughness, turbulence should becreated in the region very close to the heat transferringsurface i.e. in laminar sub-layer only. Therefore, height ofthe roughness element should be kept small in comparisonwith duct dimensions.

Many investigations on forced convective heat transferin smooth and roughened ducts have been reported in liter-ature. Artificial roughness on the surface of absorber platecan be provided by fixing small diameter wires, ribs formedby machining process, wire mesh or expanded metal meshand by forming dimple/protrusion shape geometry as has

Nomenclature

A surface area of absorber plate (m2)Ac cross sectional area of air duct (m2)At throat area of orifice plate (m2)Cp specific heat of air (J/kg K)d print diameter of protrusion (m)D equivalent diameter of duct (m)e protrusion height (m)g acceleration due to gravity (m/s2)h heat transfer coefficient (W/m2 K)Dh1 height of U-tube manometer fluid column (m)Dh2 height of micro-manometer fluid column (m)H depth of duct (m)k thermal conductivity of air (W/m K)L length of test section or longway length between

protrusions (m)_m mass flow rate (kg/s)DP1 pressure drop across orifice plate (Pa)DP2 pressure drop across test section (Pa)Qu useful heat gain (W)S shortway length between protrusions (m)Ti air inlet temperature (K)To air outlet temperature (K)Tpm mean temperature of absorber plate (K)

Tam mean temperature of air (K)W width of duct (m)

Dimensionless numbers/parameters

Cd co-efficient of discharged/D relative print diametere/D relative roughness heightf friction factor for roughened ductL/e relative longway length between protrusionsNu Nusselt number for roughened ductPr Prandtl numberRe Reynolds numberS/e relative shortway length between protrusions

Greek symbols

l dynamic viscosity (Ns/m2)q density of air (kg/m3)q1 density of fluid used in U-tube

manometer (kg/m3)q2 density of fluid used in

micro-manometer (kg/m3)b ratio of orifice diameter to pipe diameter

(dimensionless)

1110 B. Bhushan, R. Singh / Solar Energy 85 (2011) 1109–1118

been reported by Bhushan and Singh (2010), Hans et al.(2009) and Varun et al. (2007).

Experimental investigations on heat transfer and frictionin artificially roughened solar air heater duct have beenreported by Gupta et al. (1993), Jaurker et al. (2006),Karwa (2003), Karmare and Tikekar (2007), Mominet al. (2002), Prasad and Saini (1988) and Saini and Saini(1997). Nusselt number and friction factor correlationshave been developed by these investigators by using exper-imental data. Investigations have also been reported inliterature by Chyu et al. (1997), Mahmood and Ligrani(2002), Moon et al. (2000), Mohammad et al. (2008) andSang et al. (2008) in order to implement concept of artificialroughness for effective cooling in an area of gas turbine air-foil cooling system, gas cooled nuclear reactors and designof compact heat exchangers.

Considerable amount of experimentation has beenreported in literature to study effect of different type ofroughness geometries on heat transfer and friction in ductof solar air heaters. However, it has been observed that cre-ating artificial roughness on absorber plate is a tedious taskand may not be economically feasible for large scale pro-duction of solar air heaters. Therefore, a suitable geometryof roughness element needs to be selected, which should beeasy to fabricate on the surface of absorber plate. Forma-tion of dimples/protrusions on absorber plate can be con-sidered an innovative technique as dimples/protrusionsare easy to fabricate and do not add extra weight to theabsorbing plate. Saini and Verma (2008) reported an

experimental investigation for fully developed turbulentflow in rectangular duct having dimpled absorber plate.It has been revealed from literature that an experimentalinvestigation is required to be carried out on heat transferand flow characteristics of solar air heater duct havingabsorber plate roughened by formation of protrusions,because no investigation has been reported in literatureon such type of roughness geometry. In the present paper,an experimental investigation has been reported for analys-ing heat transfer and flow characteristics of duct havingprotruded absorber plate. In order to predict performanceof the system having such type of roughened absorberplate, Nusselt number and friction factor correlations asa function of system and operating parameters have beendeveloped by using experimental data.

2. Experimental program

2.1. Experimental apparatus

An experimental set-up was designed and fabricated inorder to carry out experimental investigation on heat trans-fer and flow characteristics of smooth as well as artificiallyroughened duct used in solar air heaters. Schematic andphotographic view of experimental set-up is shown inFig. 1 and Fig. 2 respectively. Rectangular duct havingaspect ratio of 10, internal size of 300 � 30 � 2400 mmand entry section, test section and exit section of length900, 1000 and 500 mm respectively was fabricated in

Air in

Air out

V

A

1 2 3

8 6

7

5 9

10

11

MM

8

12

13

(a)

4

I - Insulation AP - Absorber Plate H - Duct Height W - Duct Width EH - Electric Heater A - Ammeter V - Voltmeter MM- Micro Manometer

1. Entry Section 2. Test Section 3. Exit Section 4. Transition Section 5. GI Pipe 6. Orifice Plate 7. U-Tube Manometer 8. Control Valves 9. Centrifugal Blower 10. Electric Motor 11. Selector Switch 12. Variac 13. Temperature Indicator

(b)

H

W

AP

EH

I

I

Fig. 1. (a). Schematic of experimental set-up. (b). Sectional view of duct from entry side.

Fig. 2. Photographic view of experimental set-up.

B. Bhushan, R. Singh / Solar Energy 85 (2011) 1109–1118 1111

accordance with recommendation of ASHRAE standard(ASHRAE standard, 1977).

Galvanized iron sheet of 20 SWG having size of2400 � 335 mm was used as absorber plate and consideredas top broad wall of duct, whereas bottom and side walls ofduct were made from 12 mm thick plywood. In order tominimize heat loss, duct was well insulated by packingglass wool at the outer surface. In order to carry out indoorexperimentation, an electric heater having size of1500 � 300 mm was provided for supplying heat flux toabsorber plate. It was fabricated by combining series andparallel loops of nichrome wire on an asbestos sheet andplaced on top side of absorber plate at a gap of 75 mm.In order to control heat flux, variac was connected to theheater. Centrifugal blower was provided at exit side of

the pipe line to make forced flow of air through the duct.Control valves were provided at inlet and outlet of centrif-ugal blower for adjusting flow rate of air.

In order to measure flow rate of air, calibrated orificemeter connected with U-tube manometer was installed inthe pipe line having 80 mm diameter. An Orifice plate hav-ing 40 mm internal diameter was fabricated from 3 mmthickness brass plate. Based on standard calibration proce-dure, coefficient of discharge for orifice plate was obtainedequal to as 0.62.

In order to measure pressure drop across test section ofthe duct, micro-manometer having least count of 0.01 wasinstalled as shown in Fig. 1a. Calibrated copper–constan-tan thermocouples were used for measuring plate and airtemperature. Twelve thermocouples were fixed on test sec-tion of absorber plate for measuring plate temperature andtwelve thermocouples were installed in the duct to measureair temperature in each set of experiment as shown inFig. 3. Photographic view of absorber plate being artifi-cially roughened by formation of protrusions is shown inFig. 4. An arrangement of artificial roughness geometryused in present experimental investigation is shown inFig. 5. An arrangement for formation of protrusions onabsorber plate surface is shown in Fig. 6.

2.2. Experimental procedure

It was decided to collect heat transfer and friction datain each set of experiment under nine values of mass flow

(a)

Air

Thermocouples

900 500 333.3 333.3 333.3

All dimensions in mm

(b) Pressure tap Pressure tap

(c)

Thermocouples

Fig. 3. (a) Top view of absorber plate showing fixing of thermocouples for measuring plate temperature in test section. (b) Front view of duct showing taps formeasuring pressure drop across test section of duct. (c) Top view of duct showing installation of thermocouples for measuring air temperature in test section.

Fig. 4. Photographic view of absorber plate roughened by formation ofprotrusions.

L

S

Air d e

Fig. 5. Arrangement of protruded roughness geometry fabricated onabsorber plate.

1112 B. Bhushan, R. Singh / Solar Energy 85 (2011) 1109–1118

rate of air in order to cover range of Reynolds number.Continuity of thermocouples was checked properly beforestarting each set of experiment. In order to collect heattransfer and friction data, electric heater and centrifugalblower were switched ON for starting each set of experi-ment. Air was sucked through entry section as shown inFig. 1a and desired flow rate of air was set with the helpof control valves. In order to ensure arrival of quasi-steady state condition, temperature values indicated byall thermocouples were observed at a regular interval of15 min. On attaining quasi-steady state condition undereach value of mass flow rate of air, experimental datawere collected for U-tube manometer fluid column(Dh1), micro-manometer fluid column (Dh2), temperatureof absorber plate and air at various locations in test sec-tion of the duct and voltage and current supplied to elec-tric heater.

3. Range of parameters

Range/value of system and operating parameters usedin present experimental investigation are listed inTable 1.

Absorber Plate

2d

de

2d

Indenter

Fig. 6. Arrangement for formation of protrusions on absorber platesurface.

Table 1Range/value of system and operating parameters.

S. No. Parameter Range/value

1 Relative short way length (S/e) 18.75–37.502 Relative long way length (L/e) 25.00–37.503 Relative print diameter (d/D) 0.147–0.3674 Relative roughness height (e/D) 0.035 Duct aspect ratio (W/H) 10.006 Reynolds number (Re) 4000–20,000

Fig. 7. Comparison of experimental and predicted data of Nusselt numberfor smooth plate.

Fig. 8. Comparison of experimental and predicted data of friction factorfor smooth plate.

B. Bhushan, R. Singh / Solar Energy 85 (2011) 1109–1118 1113

4. Data reduction

Numerical values of mass flow rate ‘ _m’, useful energygain ‘Qu’, heat transfer coefficient ‘h’, Nusselt number‘Nu’, Reynolds number ‘Re’ and friction factor ‘f’ wereobtained by using experimental observations as describedin the following steps.

_m ¼ CdAt2qDP 1

1� b4

� �1=2

ð1Þ

where DP 1 ¼ q1gðDh1Þ ð2ÞQu ¼ _mCpðT o � T iÞ ð3Þ

also

Qu ¼ hAðT Pm � T amÞ ð4Þ

From Eqs. (3) and (4)

h ¼ _mcpðT o � T iÞAðT pm � T amÞ

ð5Þ

Mean value of plate temperature ‘Tpm’ was determinedfrom the detailed temperature profile of the absorber plateindicated by twelve thermocouples at various locations andmean value of air temperature ‘Tam’ was determined fromthe detailed temperature profile of air in the duct indicatedby twelve thermocouples at various locations in test sectionof duct.

Nusselt number ‘Nu’ and friction factor ‘f’ were calcu-lated by using the following relationships.

Nu ¼ hD=k ð6Þ

f ¼ 2DP 2D

4qLV 2ð7Þ

where D ¼ 4WH2ðW þ HÞ and ð8Þ

DP 2 ¼ q2gðDh2Þ ð9Þ

Based on the procedure outlined by Holman (2004), erroranalysis was carried out to obtain uncertainties in the cal-culated values of experimental data. Accordingly, uncer-tainty values for non-dimensional numbers i.e. Nusseltnumber, friction factor and Reynolds number were ob-tained as 4%, 8% and 4% respectively.

5. Validation of experimental data collected from

experimental set-up

A thorough check of experimental set-up was carriedout by conducting experimentation on smooth duct. Accu-racy of Nusselt number and friction factor data collectedfor smooth duct was verified by comparing it with the dataobtained from following Nusselt number and friction fac-tor correlations reported by Hans et al. (2010) for rectan-gular smooth duct.

Nu ¼ 0:023Re0:8Pr0:4 ðDittus Boelter CorrelationÞ ð10Þ

f ¼ 0:085Re�0:25 ðModified Blasius CorrelationÞ ð11Þ

Comparison of experimental and predicted data of Nusseltnumber and friction factor is shown in Figs. 7 and 8 respec-tively. A reasonably good agreement between experimentaland predicted data ensures accuracy of the data being col-lected with the help of experimental set-up.

6. Results and discussion

Experimental data of heat transfer and friction in theroughened duct as a function of system and operatingparameters have been reported and discussed in the presentsection. Figs. 10–12 represent effect of system and operat-ing parameters on Nusselt number. It has been observed

Fig. 10. Variation of Nusselt number with Reynolds number for range ofrelative short way length (S/e).

Fig. 11. Variation of Nusselt number with Reynolds number for range ofrelative long way length (L/e).

Fig. 12. Variation of Nusselt number with Reynolds number for range ofrelative print diameter (d/D).

Fig. 9. Mean flow structure in turbulent flow over protruded surface(Mohammad et al., 2008).

1114 B. Bhushan, R. Singh / Solar Energy 85 (2011) 1109–1118

that under the given operating conditions, Nusselt numberis a strong function of relative short way length (S/e), rel-ative long way length (L/e) and relative print diameter(d/D). During these set of experiments, relative roughnessheight (e/D) was fixed as 0.03. It can be observed from

Figs. 10–12 that for the given value of roughness parame-ters, Nusselt number increases monotonously with anincrease of Reynolds number. However, values of Nusseltnumber for protruded absorber plate are considerablyhigher as compared to those obtained for smooth absorberplate due to the fact that protrusions on absorber plateresult enhancement in heat transfer coefficient. Enhancedheat transfer coefficient may be due to main flow impinge-ment, vortex generation on both sides of the protrusionand flow separation as shown in Fig. 9. Because, decelerat-ing motion is accompanied by adverse pressure gradientwhich promotes separation, instability, eddy formationand large energy dissipation as has been reported by Ven-nard and Street (1982) and Landau and Lifshitz (2000).Main flow impinges is on the front side of protrusion andvortices are generated due to hindrance created by protru-sions. In addition, vortex originated at the former protru-sion affects the downstream protrusions located indiagonal direction and augments heat transfer coefficientsas has been reported by Sang et al. (2008).

Fig. 10 shows that Nusselt number is maximum corre-sponding to relative shortway length (S/e) of 31.25. For rel-ative shortway length (S/e) below 31.25, small distancebetween diagonal protrusions may create hindrance in for-mation of vortices, hence heat transfer coefficient corre-sponding to these values of short way length is less ascompared to that obtained for relative shortway length of31.25. Similarly for relative shortway length (S/e) above31.25, distance between diagonal protrusions may not beable to break laminar sub-layer and vortices formationmay not takes place. Hence heat transfer coefficient corre-sponding to these values of relative shortway length is alsoless as compared to that obtained for relative shortwaylength of 31.25.

Fig. 11 shows that Nusselt number is maximum corre-sponding to relative longway length (L/e) of 31.25. Itmay be due to separation of flow at the protrusion, reat-tachment of free shear layer in between two protrusionsalong longway direction. It is reported in literature thatmaximum heat transfer coefficient occurs in the vicinityof the reattachment region. For relative longway length(L/e) below 31.25 reattachment may not occur hence heattransfer coefficient correspond to these values of relative

Fig. 15. Variation of friction factor with Reynolds number for range ofrelative print diameter (d/D).

B. Bhushan, R. Singh / Solar Energy 85 (2011) 1109–1118 1115

long way length (L/e) are found to be minimum. Also withan increase in relative longway length beyond 31.25, heattransfer enhancement gets reduced as has been reportedby Prasad and Saini (1988).

Fig. 12 shows that Nusselt number is maximum corre-sponding to relative print diameter, (d/D) of 0.294. For rel-ative print diameter (d/D) below 0.294, effective short wayand long way distance between the protrusions increasesand it may not be able to break laminar sub-layer alongthe passage of air flow. Hence, heat transfer coefficient cor-responding to these values of relative print diameter is lessas compared to that obtained for relative print diameter of0.294. With increase in relative print diameter beyond0.294, vortices generated near to diagonal protrusions areaffected, which may be responsible for reduction in heattransfer coefficient and hence Nusselt number. From thewhole data, maximum enhancement in Nusselt numberhas been observed of the order of 3.8 times over the smoothduct.

Figs. 13–15 represent effect of system and operatingparameters on friction factor. It can be observed that fric-tion factor decreases monotonously with increase in Rey-nolds number as expected for fully developed turbulentflow through a duct. However, values of friction factorare higher for protruded absorber plate as compared tothose obtained for smooth absorber plate. Friction factorhave been plotted as a function of relative short way length(S/e) for different values of Reynolds number as shown inFig. 13. Friction factor is minimum corresponding to rela-

Fig. 13. Variation of friction factor with Reynolds number for range ofrelative short way length (S/e).

Fig. 14. Variation of friction factor with Reynolds number for range ofrelative long way length (L/e).

tive short way length (S/e) of 37.50 and it increases with thedecrease in relative short way length. Friction factor havebeen plotted as a function of relative longway length (L/e) for different values of Reynolds number as shown inFig. 14. Friction factor is minimum corresponding to rela-tive longway length (L/e) of 37.50 and it increases with thedecrease in relative long way length. Friction factor havebeen plotted as a function of relative print diameter (d/D)for different values of Reynolds number as shown inFig. 15. Friction factor is minimum corresponding to rela-tive print diameter (d/D) of 0.147 and it increases with theincrease in relative print diameter (d/D).

Decrease in relative short way length or decrease in rel-ative long way length or increase in relative print diameterdecreases the effective relative pitch of protruded geometrywhich increases the friction and hence the friction factor ashas been reported by Prasad and Saini (1988). From thewhole data, maximum enhancement in friction factor hasbeen observed of the order of 2.2 times over the smoothduct.

7. Development of Nusselt number and friction factor

correlations

It is revealed from experimental data that Nusselt num-ber and friction factor are strong function of system andoperating parameter. Functional relationship for Nusseltnumber and friction factor can be written as:

Nu ¼ f1ðRe; S=e; L=e; d=DÞ ð12Þf ¼ f2ðRe; S=e;L=e; d=DÞ ð13Þ

In order to predict performance of an artificially roughenedduct of a solar air heater having protrusions as roughnessgeometry, Nusselt number and friction factor correlationswere developed for range of system and operating parame-ters by following the procedure reported by Singh et al.(2006). Sigma Plot software has been used to carry outthe regression analysis.

7.1. Nusselt number correlation

Experimental data of Nusselt number was plotted onlog–log scale as shown in Fig. 16. It shows that Nusselt

Fig. 16. Plot of Nusselt number vs. Reynolds number.

1116 B. Bhushan, R. Singh / Solar Energy 85 (2011) 1109–1118

number and Reynolds number have almost a linear rela-tionship. From first-order regression of the data, it hasbeen found that an average slope of all lines is 1.452.

Variation of function NuRe1:452 with relative short way length

(S/e) was plotted on log–log scale as shown in Fig. 17 andfrom the second order regression, following correlation hasbeen obtained;

Nu

Re1:452¼ 6:46� 10�14ðS=eÞ12:94 exp½ð�10:4ÞflogðS=eÞg2�

ð14Þ

Similarly, variation of function NuRe1:452ðS=eÞ12:94 exp½ð�10:4ÞflogðS=eÞg2�

with relative long way length (L/e) was plotted on log–logscale as shown in Fig. 18 and yielded following correlation;

Fig. 17. Plot of Nu/Re1.452 vs. S/e.

Nu

Re1:452ðS=eÞ12:94 exp½ð�10:4ÞflogðS=eÞg2�¼ 5:51 � 10�74ðL=eÞ99:2 exp½ð�77:2ÞflogðL=eÞg2� ð15Þ

and variation of functionNu=Re1:452

ðS=eÞ12:94 exp ð�10:4ÞflogðS=eÞg2½ �ðL=eÞ99:2 exp½ð�77:2ÞflogðL=eÞg2�with relative

print diameter (d/D) yielded following correlation;

Nu

Re1:452ðS=eÞ12:94 exp½ð�10:4ÞflogðS=eÞg2�ðL=eÞ99:2 exp½ð�77:2ÞflogðL=eÞg2�¼0:059�ðd=DÞ�3:9 exp ð�7:83Þflogðd=DÞg2

h i

ð16Þ

or

Nu ¼ 2:1� 10�88Re1:452ðS=eÞ12:94ðL=eÞ99:2ðd=DÞ�3:9

� exp ð�10:4ÞflogðS=eÞg2h i

exp ð�77:2ÞflogðL=eÞg2h i

� exp ð�7:83Þflogðd=DÞg2h i

ð17Þ

which is the required correlation for Nusselt number

7.2. Correlation for friction factor

A similar procedure has been employed to develop fric-tion factor correlation as shown in Figs. 19–22 and follow-ing friction factor correlation has been obtained:

f ¼ 2:32Re�0:201ðS=eÞ�0:383ðL=eÞ�0:484ðd=DÞ0:133 ð18Þ

Validity of Nusselt number and friction factor correlationswas checked by comparing experimental data of Nusseltnumber and friction factor and that predicted from thesecorrelations. A good agreement was observed as has beenshown in Figs. 23 and 24. Maximum deviation of ±15%and ±10% has been observed for Nusselt number and fric-tion factor respectively.

Fig. 18. Plot of Nu/Re1.452(S/e)12.94 exp[(�10.4){log(S/e)}2] vs. L/e.

Fig. 20. Plot of f/Re�0.201 vs. S/e.

Fig. 21. Plot of f/Re�0.201(S/e)�0.383 vs. L/e.

Fig. 23. Plot of predicted data vs. experimental data of Nusselt number.

Fig. 19. Plot of friction factor vs. Reynolds number.Fig. 22. Plot of f/Re�0.201(S/e)�0.383(L/e)�0.484 vs. d/D.

Fig. 24. Plot of predicted data vs. experimental data of friction factor.

B. Bhushan, R. Singh / Solar Energy 85 (2011) 1109–1118 1117

1118 B. Bhushan, R. Singh / Solar Energy 85 (2011) 1109–1118

8. Conclusions

In the present paper, an experimental investigation hasbeen reported to analyse effect of protruded roughnessgeometry on heat transfer and flow characteristics ofduct used in solar air heaters. It has been observed thatprotruded absorber plate results in higher heat transfercoefficient as compared to smooth plate at an added fric-tion penalty. Maximum enhancement of Nusselt numberand friction factor has been found 3.8 and 2.2 timesrespectively in comparison to smooth duct for the inves-tigated range of parameters. Maximum enhancement inheat transfer coefficient has been found to occur for rel-ative shortway length (S/e) of 31.25, relative long waylength (L/e) of 31.25 and relative print diameter (d/D)of 0.294. In order to predict performance of the systemhaving investigated type of roughness geometry, Nusseltnumber and friction factor correlations based on experi-mental data have been developed and reported in thepresent paper for range of system and operatingparameters.

9. Future scope of work

(i) In order to bring into account effect of friction pen-alty induced with application of artificial roughness,thermohydraulic performance of investigated typeof artificially roughened duct may be evaluated.

(ii) In order to predict performance of solar air heaterunder actual climatic conditions, mathematicalmodel may be developed by using Nusselt numberand friction factor correlations reported in the pres-ent paper.

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