use of artificial roughness to enhance heat transfer in ... · detailed review on heat transfer...

AbstractImprovement in the thermo hydraulic performanceof a solar air heater can be done by enhancing theheat transfer. In general, heat transfer enhancementtechniques are divided into two groups: active andpassive techniques. Providing an artificial roughnesson a heat transferring surface is an effective passiveheat transfer technique to enhance the rate of heattransfer to fluid flow. In this paper, reviews of vari-ous artificial roughness elements used as passiveheat transfer techniques, in order to improve ther-mo hydraulic performance of a solar air heater, isdone. The objective of this paper is to review vari-ous studies, in which different artificial roughnesselements are used to enhance the heat transfer ratewith little penalty of friction. Correlations developedby various researchers with the help of experimen-tal results for heat transfer and friction factor forsolar air heater ducts by taking different roughenedsurfaces geometries are given in tabular form.These correlations are used to predict the thermohydraulic performance of solar air heaters havingroughened ducts. The objective is to provide adetailed review on heat transfer enhancement byusing an artificial roughness technique. This paperwill be very helpful for the researchers who areresearching new artificial roughness for solar airheater ducts to enhance the heat transfer rate andcomparing with artificial roughness already studiedby various researchers.

Keywords: solar air heater, artificial roughness,active & passive technique, heat transfer, frictionfactor

1. IntroductionThe rapid depletion of fossil fuel resources hasnecessitated an urgent search for alternative sourcesof energy. Of the many alternatives, solar energystands out as the brightest long range promisetowards meeting the continually increasing demandfor energy. Solar energy is available freely,omnipresent and an indigenous source of energyprovides a clean and pollution free atmosphere.The simplest and the most efficient way to utilizesolar energy is to convert it into thermal energy forheating applications by using solar collectors. Solarair heaters, because of their inherent simplicity arecheap and most widely used collector devices. Solarair heaters are being used for many applications atlow and moderate temperatures. Some of these arecrop drying, timber seasoning, space heating, chick-en brooding and curing / drying of concrete / claybuilding components. The thermal efficiency ofsolar air heaters is low due to two reasons: low ther-

Journal of Energy in Southern Africa • Vol 21 No 1 • February 2010 35

Use of artificial roughness to enhance heat transfer insolar air heaters – a review

Thakur Sanjay Kumar Department of Mechanical Engineering, BRCM College of Engineering & Technology, Bahal, Bhiwani (Haryana), India

N S ThakurDepartment of Mechanical Engineering, National Institute of Technology, Hamirpur, Himachal Pradesh, India

Anoop KumarDepartment of Mechanical Engineering, National Institute of Technology, Hamirpur, Himachal Pradesh, India

Vijay MittalDepartment of Mechanical Engineering, BRCM College of Engineering & Technology, Bahal, Bhiwani (Haryana), India

mal capacity of air and a low heat transfer co-effi-cient between the absorber plate and air flowthrough duct.

In order to make the solar air heater economi-cally more viable, their thermal efficiency needs tobe improved. This can be done by enhancing theheat transfer co-efficient between the absorber plateand air flow through a duct. In general, heat trans-fer co-efficient enhancement techniques can bedivided into two groups; namely active and passive.

The active techniques require external forces,e.g. electric field, acoustic and surface vibration.Over the past 70 years, the heat transfer enhance-ment by using a strong electric field has been con-tinuously studied. The electro hydrodynamic (EHT)enhancement of heat transfer refers to the couplingof an electric field with the fluid field in a dielectricfluid medium – EHD induced secondary flow orionic wind. This type of flow can be used not onlyfor the pressure drop control in a flow channel butalso for the enhancement of heat transfer.

The passive techniques require special surfacegeometries, such as rough and external surface,fluid additives and swirl flow devices i.e. twisted tapinserts to create a swirling flow. Passive techniqueshave being used by researchers for 140 years forincreasing the heat transfer rate in a heat exchang-er (Laohalertdecha, 2007).

This paper is divided in to five sections whichare further divided into various subsections. Section1 gives the introduction of solar air heaters and dif-ferent heat transfer enhancement techniques.Section 2 deals with the concept of artificial rough-ness and a brief discussion of various roughnessparameters. Section 3 deals with thermo hydrauliccharacteristics for a solar air heater. Section 4 dealswith the comparison among various geometries byplotting the Colburn factor (j factor) and friction fac-tor (f factor) against Reynolds number with the helpof correlations developed by various researchersand finally the paper is concluded in Section 5.2. Concept of artificial roughnessArtificial roughness is basically a passive heat trans-fer enhancement technique by which thermohydraulic performance of a solar air heater can beimproved. The artificial roughness has been usedextensively for the enhancement of forced convec-tive heat transfer, which further requires flow at theheat-transferring surface to be turbulent. However,energy for creating such turbulence has to comefrom the fan or blower and the excessive power isrequired to flow air through the duct. Therefore, it isdesirable that the turbulence must be created onlyin the region very close to the heat transferring sur-face, so that the power requirement may be less-ened.

This can be done by keeping the height of theroughness elements to be small in comparison with

the duct dimensions. The key dimensionless geo-metrical parameters that are used to characterizeroughness are:1. Relative roughness pitch (p/e): Relative rough-

ness pitch (p/e) is defined as the ratio of distancebetween two consecutive ribs and height of therib.

2. Relative roughness height (e/D): Relative rough-ness height (e/D) is the ratio of rib height toequivalent diameter of the air passage.

3. Angle of attack (α): Angle of attack is inclinationof rib with direction of air flow in duct.

4. Shape of roughness element: The roughnesselements can be two-dimensional ribs or three-dimensional discrete elements, transverse orinclined ribs or V-shaped continuous or brokenribs with or without gap. The roughness ele-ments can also be arc-shaped wire or dimple orcavity or compound rib-grooved. The commonshape of ribs is square but different shapes likecircular, semi-circular and chamfered have alsobeen considered to investigate thermo hydraulicperformance.

5. Aspect ratio: It is ratio of duct width to ductheight. This factor also plays a very crucial rolein investigating thermo-hydraulic performance.

3. Thermo-hydraulic characteristics forsolar air heaterNikuradse (1950) carried out experiment in order todevelop velocity and temperature distribution forroughened surfaces. Webb (1971) revealed that thevelocity profile in the turbulent flow region is strong-ly dependent upon the roughness height along withflow the Reynolds number. A correlation based onthe law of wall similarity for sand grain roughness inpipes with different grades of closely packed sandroughness (0.004< e/d < 0.0679) has been devel-oped as:

(2.1)and

(2.2)

Where e+ is called roughness Reynolds number andR (e+) is known as momentum transfer roughnessfunction.

Dipprey and Sabersky (1963) have developed arelation for heat transfer in terms of a heat transferfunction G (e+), which is expressed as:

(2.3)

Gee and Webb (1980) reported experimental infor-mation for a single phase forced convection in a cir-

36 Journal of Energy in Southern Africa • Vol 21 No 1 • February 2010

cular tube containing a two dimensional rib rough-ness with a helix angle, and developed a relation-ship for friction roughness parameter and heattransfer roughness parameter as:

(2.4)

(2.5)

Where J = 0.37 for αa < 50˚J = -0.16 for αa > 50˚

Experimental data collected by variousresearchers on various geometries of rib-roughenedsurfaces, have been utilized to develop correlationin the following form:R (e+) = R [e+, p/e, α, duct shape, rib shape] (2.6)G (e+) = G [e+, p/e, α, duct shape, rib shape] (2.7) It was realized that the statistical correlations maybe more suitable for design and also easy to formu-late. Such relationship can be expressed as: Nu = Nu [Re, e/D, p/e, α, Rib shape, Duct shape]

(2.8)f = f [Re, e/D, p/e,α, Rib shape, Duct shape] (2.9)

Several experimental studies of heat transferand friction characteristics have been carried out forsolar air heater applications. In solar air heaters,one broad wall of the rectangular section air flowpassage is subjected to uniform heat flux, while theremaining three walls are insulated. Therefore, thesolar air heaters are modelled as a rectangular

channel having one rough wall and three smoothwalls. This makes the fluid flow and heat transfercharacteristics distinctly different from those foundin the case of a channel with two opposite rough-ened walls, roughened annular and circular tubes.

Various researchers have investigated the effectof different roughness parameters of artificial rough-ness elements in thermo-hydraulic characteristicsfor solar air heater. The most important effect pro-duced by the presence of a rib on the thermo-hydraulic characteristics i.e. flow pattern, is the for-mation of a vortex between ribs filling approxi-mately two thirds of cavity, and energy interchangewith the main flow (Karwa et al., 1999). The vor-tices so generated are responsible for the turbu-lence, which result in the desirable increase in heattransfer and the undesirable drop in pressure. Theeffect of different roughness parameters on thermo-hydraulic characteristics as investigated by variousresearchers is given below.3.1 Relative roughness pitch (p/e) Various researchers have shown the effect of a rela-tive roughness pitch (p/e) on the flow pattern i.e.heat transfer coefficient and friction factor. Table 1shows the value of relative roughness pitch (p/e) fora maximum value of a heat transfer coefficient fordifferent types of artificial roughness. Figure 1(Prasad & Saini, 1988) depicts the flow patternsdownstream from a rib as a function of a relativeroughness pitch. Due to separation at the rib, reat-tachment of the free shear layer does not occur fora relative roughness pitch (p/e) – less than about 8to 10. The maximum heat transfer coefficientoccurs in the vicinity of the reattachment point. Forrelative roughness pitch (p/e) (less than 8 to 10),reattachment will not occur, which results in thedecrease of the heat enhancement rate. The rate ofincrease in the friction factor will increase with thedecrease of pitch. However, an increase in the rela-tive roughness pitch (p/e) beyond 10 resulted in thedecrease of heat transfer enhancement.


Table 1: Values of relative roughness pitch (P/e) at which maximum value of heat transfercoefficient for different roughness geometries used in solar air heater duct

Investigators Roughness geometry Value of relative roughness pitch at which maximum value of heat transfer coefficient (P/e)

Prasad and Saini (1988) Wire 10Karwa et al. (2001) Chamfered rib 7.09Bhagoria et al. (2002) Transverse wedge 7.57Sahu and Bhagoria (2005) 90˚broken transverse 13.33Jaurker et al. (2006) Transverse rib-grooved 6Karmare and Tikekar (2007) Metal grit rib roughness 17.5Layek et al. (2007) Transverse chamfered rib-grooved 6Varun et al. (2008) Combination of inclined and transverse ribs 8Saini and Verma (2008) Dimple-shape roughness 10

Figure 1: Flow patterns downstream of ribswith the roughness as a function of relative

roughness pitch Source: Prasad & Saini, 1988

3.2 Relative roughness height (e/D)Figure 2 (Prasad & Saini, 1988) and Figure 3 (Pra-sad & Saini, 1991) depict the flow pattern down-stream of a rib and effect on the laminar sub-layeras the rib height is changed respectively. Breakageof the viscous sub layer due to repeated ribs,increases the rate of heat transfer by creating localwall turbulence. If the ribs protrude beyond the vis-cous sub-layer, they would increase the heat trans-fer rate, but also cause much higher friction losses.Optimal thermo hydraulic performance conditionsare obtained when the roughness height is slightlyhigher than the transition sub-layer thickness(Prasad & Saini, 1991).

Table 2 shows the values of the relative rough-ness height (e/D) for a maximum value of heattransfer coefficient.3.3 Angle of attackVarious researchers have investigated experimental-ly, the effect of angle of attack on the flow pattern.Besides a relative roughness pitch and relativeroughness height, the parameter that has beenfound to be most influential to flow pattern is theangle of inclination of the rib i.e. angle of attack offlow with respect to the rib position. The inclined ribgives a higher heat transfer rate than the transverserib because of the secondary flow induced by therib, in addition to breaking the viscous sub-layerand producing local wall turbulence. It is pointedout that the two fluid vortices immediately upstreamand downstream of a transverse rib are essentiallystagnant relative to main stream flow which raisesthe local fluid temperature in the vortices and walltemperature near the rib resulting in low heat trans-fer.

The vortices in the case of inclined ribs movealong the rib so subsequently to join the mainstream causing the fluid to enter near the leadingend of rib and coming out near the trailing end.These moving vortices therefore bring in cooler


Table 2: Values of relative roughness height (e/D) at which maximum value of heat transfercoefficient for different roughness geometries used in solar air heater duct

Investigators Roughness geometry Value of relative roughness height at which maximum value of heat transfer coefficient (e/D)

Prasad and Saini (1988) Wire 0.033Karwa et al. (2001) Chamfered rib 0.0441Momin et al. (2002) V-shaped rib 0.034Bhagoria et al. (2002) Transverse wedge 0.033Jaurker et al. (2006) Transverse rib-grooved 0.036Karmare and Tikekar (2007) Metal grit rib roughness 0.044Layek et al. (2007) Transverse chamferedrib-grooved 0.04Saini and Verma (2008) Dimple-shape roughness 0.0379Saini and Saini (2008) Arc shaped wire 0.0422

channel fluid in contact with leading end, raising theheat transfer rate while the trailing end heat transferis relatively lower. This phenomenon thereforeresults in strong span wise variation of heat transfer.3.4 Shape of roughness elementsBesides a relative roughness pitch, relative rough-ness height and angle of attack, shapes of variousroughness elements also influence the heat transfercoefficient and friction factor. Different shapes ofroughness elements are discussed.3.4.1 V- shaped rib Muluwork et al. (1998) have investigated the effectof a staggered discrete V- apex up and down on thethermal performance as depicted in Figure 4. TheStanton number for V-down discrete ribs was high-er than the corresponding V-up and transverse dis-crete roughened surfaces. The Stanton numberratio enhancement was found to be 1.32 to 2.47 inthe range of parameters covered in the investiga-tion. Further for the Stanton number, it was seenthat the ribbed surface friction factor for V-downdiscrete ribs was highest among the three configu-rations investigated.

Momin et al. (2002) has investigated the effectof relative roughness height and angle of attack fora fixed relative roughness pitch of 10 with theReynolds number range of 2500 to 18000 for a V-shaped rib as depicted in Figure 5. It was found thatthe rate of increase of the Nusselt number with anincrease in Reynolds number is lower than the rateof increase of the friction factor. It was found that forthe relative roughness height of 0.034, the V-shaped ribs enhanced the values of Nusselt numberby 1.14 and 2.30 times over inclined ribs andsmooth plate.

Karwa (2003) has investigated and revealed theeffect of transverse, inclined, V-continuous and V-discrete patterns on heat transfer and the frictionfactor in a rectangular duct. The ribs in the V-pat-


Figure 2: Flow patterns downstream of wireswith the roughness as a function of relative

roughness height Source: Prasad & Saini, 1988

Figure 3: Roughness height with respect tolaminar sub layer

Source: Prasad & Saini (1991)

tern were tested for both pointing upstream (V-up)and down stream (V-down) to the flow. The angleof inclination of the ribs in inclined and V-patternwas 60˚. The enhancement in the Stanton numberover the smooth duct was up to 137%, 147%,134% and 142% for the V-up continuous, V-downcontinuous, V-up discrete and V-down discrete ribarrangement respectively. The friction factor ratiofor these arrangements was up to 3.92, 3.65, 2.47and 2.58, respectively. Based on the equal pumpingpower, V-down discrete roughness gave the bestheat transfer performance. 3.4.2 Broken or discrete ribBroken V-shaped or broken parallel ribs can createmore secondary flow cells and produce more localturbulence in the opposite wall region in compari-son to the continuous V-shaped or continuous par-allel ribs. The average heat transfer coefficient forthe ribbed surfaces turned out to be higher thanthose for the unribbed surface by a factor of up to 2when the transverse ribs were continuous, and by afactor of up to 3 when they were broken.

Karwa (2003) found that the best heat transferoccurs for the equal pumping power for V-downdiscrete ribs. Sahu and Bhagoria (2005) investigat-ed experimentally the effect of pitch varying from10 to 30 by taking the height of the rib to be1.5 mmand duct aspect ratio 8 on the heat transfer coeffi-cient and friction factor for 90˚ broken transverseribs. It was found that the separation occurred notonly at the top edge of the rib but also at the edgesat the end of the ribs. This secondary flow inter-rupted the growth of the boundary layer down-stream of the nearby attachment zone in case of90˚ broken ribs. It was found out that the maximumNusselt number attained for a pitch of 20 mm anddecreased with an increase in roughness pitch. Themaximum thermal efficiency of 83.5 % has been

found for a 20 mm pitch. Based on experiments, itwas found that the maximum thermal efficiency ofa roughened solar air heater was of the order of 51– 83.5 % depending upon the flow conditions.

Aharwal et al. (2006) and Aharwal et al. (2008)investigated the effect of gap to width ratio (g/e) andgap to position ratio (d/W) in an inclined split ribarrangement in a rectangular duct of a solar airheater as depicted in Figure 6. A gap in the inclinedrib arrangement enhanced the heat transfer andfriction factor. The increase in the Nusselt numberand friction factor, were in the range of 1.48 to 2.59times and 2.26 to 2.9 times of the smooth duct,respectively. The maximum values of the Nusseltnumber, friction factor and thermo-hydraulic per-formance were observed for a gap in the inclinedrepeated ribs with a relative gap position of 0.25and relative gap width of 1.0.3.4.3 Compound roughness Eiansa-ard and Promvonge (2009) investigatedexperimentally the combined effect of rib-groovedturbulators on the turbulent forced convection heattransfer and friction characteristics in a rectangularduct. There are three types of rib-groove arrange-ments: rectangular-rib with triangular-groove, trian-gular-rib and rectangular-groove and triangular-ribwith triangular-groove, which were examined. Allrib-groove arrangements significantly enhance theheat transfer rate in comparison with the smoothduct. The thermal enhancement index for the trian-gular-rib and triangular-groove was achieved andbetter than that for the rectangular-rib and triangu-lar-groove and triangular-rib and rectangular-groove were around 7% and 4% respectively.

Jaurker et al. (2006) investigated the effect ofrelative roughness pitch, relative roughness heightand relative groove position on a heat transfer coef-ficient and friction factor of rib-grooved artificial


Figure 4: Type and orientation of roughness element investigated by Muluwork et al. (1998)

Figure 5: Type and orientation of roughness element investigated by Momin et al. (2002)

roughness as depicted in Figure 7. The maximumheat transfer was obtained for a relative roughnesspitch of about 6, and it was decreased either side ofthe relative roughness pitch. The optimum condi-tion for heat transfer was found at a groove positionto pitch ratio of 0.4 as compared to the smoothduct. As compared to smooth surface, the presenceof rib grooved artificial roughness increased theNusselt number up to 2.7 times, while the frictionfactor raised up to 3.6 times in the range of param-eters investigated.

Layek et al. (2007) investigated experimentallythe effect of a relative roughness pitch, chamferangle, relative groove position and relative rough-ness height on the heat transfer and friction factorfor the chamfered rib-groove roughness as depictedin Figure 8. As compared to smooth surface, thechamfer rib-groove roughness resulted in to theincrease in the Nusselt number by 3.24 fold andfriction factor by 3.78 fold. The maximum heattransfer enhancement occurred for the relativegroove pitch of 6 and relative groove position of

0.4. The highest Nusselt number occurred forchamfer angle of 18* but the friction factorincreased monotonously with an increase in cham-fer angle.

Varun et al. (2008) and Varun et al. (2009)investigated experimentally the thermal perform-ance of a solar air heater having roughness ele-ments as a combination of inclined and transverseribs on the absorber plate as depicted in Figure 9. Itwas observed that the best thermal performanceoccurs for a relative roughness pitch (P/e) of 8.3.4.4 Small diameter protrusion wiresPrasad and Saini (1988) investigated experimental-ly the effect of relative roughness pitch (p/e) and rel-ative roughness height (e/D) on the heat transfercoefficient and friction factor of a fully developedturbulent flow in a solar air heater duct with smalldiameter protrusion wires on the absorber plate.The type and orientation of the geometry is depict-ed in Figure 10 (Prasad and, Saini 1988). Itobserved that the average Nusselt number and


Figure 6: Type and orientation of roughness element investigated by Aharwal et al. (2008)

Figure 7: Type and orientation of roughness element investigated by Jaurker et al. (2006)

average friction factor in the roughened duct wereas 2.10, 2.24, 2.38 and 3.08, 3.67, 4.25 times thatof the smooth duct for relative roughness height of0.020, 0.027, and 0.033, respectively. It has alsobeen found that increase in the average Nusseltnumber and average friction factor in the rough-ened duct were about 2.38, 2.14, 2.01 and 4.25,3.39, 2.93 times of that of the smooth duct for a rel-ative roughness pitch of 10, 15, and 20, respective-ly. The maximum enhancement in the heat transfercoefficient and friction factor were as 2.38 and 4.25times than that of smooth duct respectively.

Gupta et al. (1993) investigated the effect of aduct aspect ratio and relative roughness height at arelative roughness pitch of 10, with the Reynoldsnumber range of 3 000 to 18 000, and developedthe correlations for heat transfer and friction factorfor transverse rib roughness on the absorber plate.It has been found that the behaviour of the Stanton

number in a transitionally rough flow region wasdifferent from its behaviour in a fully rough flowregion. Correlations for transitionally rough flowregions have been developed for the range of inves-tigation. These correlations showed good agree-ment between the predicted and experimental val-ues of the heat transfer coefficient and friction fac-tor.

Verma and Prasad (2000) have investigated theeffect of similar geometrical parameters of circularwire ribs on heat transfer and friction factor. It wasobserved that the Nusselt number varied from 1.25to 2.08 times that of a smooth duct within the rangeof parameters investigated. 3.4.5 Expanded wire meshSaini and Saini (1997) investigated experimentallythe effect of expanded metal mesh geometry asdepicted in Figure 11 for fully developed turbulent


Figure 8: Type and orientation of roughness element investigated by Layek et al. (2007)

Figure 9: Type and orientation of roughness element investigated by Varun et al. (2008)

Figure 10: Type and orientation of roughness element investigated by Prasad & Saini (1988)

flow in a rectangular duct with a large aspect ratio(11:1). The effect of expanded metal mesh geome-try [i.e. relative long way length of mesh (L/e), rela-tive short way length of mesh (S/e) and relativeroughness height of mesh (e/D)] on the heat trans-fer coefficient and friction factor was investigated.The maximum Nusselt number and friction factorwere found corresponding to relative long waylength of mesh of 46.87 and 71.87 respectively forall the values of relative short way length of meshinvestigated. The maximum Nusselt number andfriction factor occurred for relative short way lengthof mesh of 25 and 15 respectively. The maximumenhancement in Nusselt number and friction factorvalues were reported of the order of 4 and 5 timesto the smooth absorber plate respectively.

Paswan and Sharma (2009) studied experimen-tally, the thermal performance of wire-screen mesh(metal) as artificial roughened on the underside ofits absorber plate. Thermal performance of a solarair heater enhanced considerably by roughing theabsorber plate with wire screen metal mesh, andthis enhancement was a strong function of geomet-rical parameters (such as diameter of wire andpitch) and operating parameters (mass flow rate,insulation and inlet temperature). Decreasing valuesof wire-pitch, leads to increasing value of thermalperformance of solar air heater.3.4.6 Chamfered ribs Karwa et al. (1999) and Karwa et al. (2001) inves-tigated experimentally the effect of repeated cham-fered rib-roughness on one broad wall and the ductaspect ratio on the heat transfer coefficient and fluidfriction as depicted in Figure 12. It has beenobserved that the presence of chamfered ribs at onebroad wall of the duct yields up to about two-foldand three-fold increase in the Stanton number andthe friction factor, respectively, as compared to thesmooth duct. The highest heat transfer and also the

highest friction factor occur for 15˚ chamfered ribs.The minima of the heat transfer function occur at aroughness Reynolds number of about 20. The heattransfer function increases with the increase in theaspect ratio from 4.65 to 9.66, and the roughnessfunction decreases with the increase in the aspectratio from 4.65 to 7.75. Thereafter, both the func-tions attain nearly a constant value.

There was an appreciable increase in the ther-mal efficiency (10 to 40 %) of the solar air heaterswith chamfered-rib roughened absorber plate dueto the enhancement in the Nusselt number of the


Figure 11: Type and orientation of roughnesselement investigated by Saini & Saini (1997)

Figure 12: Type and orientation of roughness element investigated by Karwa et al. (1999)

order of 50 to 120% over the smooth absorberplate. The enhancement in the Nusselt number, fric-tion factor and thermal efficiency were found to bestrong functions of the relative roughness height.The greatest enhancement was observed for the airheater with the highest relative roughness height. 3.4.7 Wedge shaped ribs Bhagoria et al. (2002) has performed experimentsin order to find out the effect of relative roughnessheight, relative roughness pitch and wedge angle onthe heat transfer and friction factor in a solar airheater roughened duct having wedge shaped ribroughness as depicted in Figure 13. It has beenobserved that the maximum heat transfer occurredfor a relative roughness pitch of about 7.57, whilethe friction factor decreased as the relative rough-ness pitch increased. A maximum enhancement ofheat transfer occurred at a wedge angle of about10˚. As compared to the smooth duct, the presenceof ribs yielded a Nusselt number up to 2.4 timeswhile the friction factor raised up to 5.3 times for therange of parameters investigated. 3.4.8 Arc shaped ribs

Saini and Saini (2008) investigated experimen-tally the effect of relative roughness height (e/d) andrelative angle of attack (α/90) of arc-shape parallelwire on the heat transfer coefficient and friction fac-

tor as depicted in Figure 14 and Figure 15. Themaximum enhancement in the Nusselt number wasobtained as 3.80 times corresponding to the relativearc angle (α/90) of 0.3333 at relative roughnessheight of 0.0422. However, the increment in thefriction factor corresponding to these parameterswas found to be only 1.75 times.

Kumar and Saini (2009) used ComputationalFluid Dynamics for analyzing the performance of asolar air heater duct provided with artificial rough-ness in the form of thin circular wire in arc shapedgeometry. Overall enhancement ratio with a maxi-mum value of 1.7 has been found for the roughnessgeometry corresponding to the relative arc angle(�/90) of 0.3333 at relative roughness height of0.0426 for relative roughness pitch of 10. The over-all enhancement ratio (OER) given by Wang andSunden (2002) is as below.

(3.1)


Figure 13: Type and orientation of roughness element investigated by Bhagoria et al. (2002)

Figure 14: Type and orientation of roughnesselement investigated by Saini and Saini (2008)

Figure 15: Type and orientation of roughness element investigated by Saini and Saini (2008)

3.4.9 DimpleSaini and Verma (2008) investigated the effect ofrelative roughness height (e/D) and relative rough-ness pitch (P/e) of dimple-shape roughness geome-try on heat transfer and friction factor as depicted inFigure 16. It was found that heat transfer could beenhanced considerably as a result of providing dim-ple-shape roughness geometry on the absorberplate of a solar air heater duct.

The maximum value of the Nusselt number wasfound to correspond to relative roughness height of0.0379 and relative roughness pitch of 10, whileminimum value of the friction factor was found cor-responding to relative roughness height of 0.0289and relative pitch of 10. It was concluded that theroughness parameters of the geometry can beselected by considering the net heat gain and corre-sponding power required to propel air through theduct.

Figure 16: Type and orientation of roughnesselement investigated by Saini and Verma (2008)

3.4.10 Metal grit ribsKarmare and Tikekar (2007) investigated experi-mentally the effect of metal ribs of the circular cross

section in a staggered manner to form a definedgrid as depicted in Figure 17. The effect of the rela-tive roughness height of grit (e/D), relative rough-ness pitch of grit (P/e), relative length of grit (l/s) onthe heat transfer and friction factor were investigat-ed. As compared to a smooth surface, the presenceof metal grit ribs on the collector surface of the ductyielded up to two-fold enhancement in the Nusseltnumber and three-fold enhancement in the frictionfactor. The highest heat transfer was found for l/s =1.72, e/D = 0.044 and P/e = 17.5, where the high-est friction factor was found for l/s = 1.72, e/D =0.044 and P/e = 12.5.

Optimum performance was observed for l/s =1.72, e/D = 0.044 and P/e = 17.5 for the range ofparameters studied. Enhancement in the Nusseltnumber was found to be 187% and the friction fac-tor by 213 % as compared with a smooth surface. 3.4.11 Inverted u-shaped turbulatorsBopche and Tandale (2009) investigated experi-mentally the effect of the inverted U-shaped turbu-lators on the absorber surface of an air heater ducton the heat transfer coefficient and friction factor.The inverted U-shaped turbulator showed appre-ciable heat transfer enhancement even at a lowReynolds number (Re < 5000) where ribs wereinefficient. At the Reynolds number, Re = 3800, themaximum enhancement in the Nusselt number andfriction factor were of the order of 2.388 and 2.50,respectively. The maximum enhancement in theNusselt number and friction factor values comparedto the smooth duct were of the order of 2.82 and3.72, respectively. The turbulence generated only inthe viscous sub-layer region of the boundary layerresulted in better thermo-hydraulic performance i.e.maximum heat transfer enhancement at an afford-able friction penalty.3.5 Aspect ratio Karwa (1999) has also presented the detailed ex-perimental evidence of this effect. He compared theheat transfer and friction factor characteristics forthe duct of the aspect ratio 4.8 and 7.75. He foundthat the Stanton number ratio, Str / Sts, decreased


Figure 17: Type and orientation of roughness element investigated by Karmare and Tikekar (2007)

by about 5% with an increase in the aspect ratiofrom 4.8 to 7.75. The friction factor ratio fr / fs,increased by about 20% for the corresponding flowconditions.

Karwa (2003) also compared the results for theaspect ratio 5 and 7.5 for asymmetrically heatedrectangular ducts with ribs on the heated wall in thetransverse, inclined, V-continuous and V-discretepattern. The average increase in the friction factor

ratio fr / fs is about 10.48%, with an increase ofaspect ratio from 5 to 7.5. The Stanton numberratio Str / Sts decreased by 5.12 % with an increasein aspect ratio from 5 to 7.5. Based on the experi-mental investigation carried out by various investi-gators, the developed correlations for the Nusseltnumber and friction factor are also presented inTable 3 within the range of operating parameters.


Table 3: Correlations developed for heat transfer and friction factor for different roughnessgeometries used in solar air heater duct

4. Colburn factor (j factor) and frictionfactor (f factor)The enhanced surface on one hand, provides agreater heat transfer coefficient, but also leads toincreased fluid flow friction and pressure drop.Sometimes, the benefits gained from heat transferenhancement are not great enough to offset theincreased friction losses. Clearly, then the perform-ance goal is to gain maximum enhancement of heattransfer with minimum penalty on pumping power.The criterion of selecting good geometries done onthe basis of comparing the heat transfer coefficientsor dimensionless heat transfer parameters (i.e.Nusselt number, Stanton number etc) is not good

enough. So, performance data for an enhanced sur-face are shown as curves of the Colburn factor (J)and the friction factor (f) plotted versus Reynoldsnumber (Re). The curves for J and f plotted vs. Retend to vary over a wide range, in magnitude aswell as slope. Various steps involved in the evalua-tion have been explained below:

(4.1)

(4.2)


Table 4: Coefficient for roughness and heat transfer functions for V-continuous and V-discrete ribs(Karwa, 2003)

Roughness a b a1 b1 C1Inclined 3.7135 0.12770 12.765 -0.05095 0.000506V-up continuous 3.5080 0.12195 12.382 -0.04547 0.000408V-down continuous 3.4590 0.13048 12.502 -0.11609 0.001239V-up discrete 4.0917 0.16083 11.249 -0.13120 0.001479V-down discrete 3.5341 0.19102 11.070 -0.14900 0.001757

(4.3)

The value of the heat transfer coefficient andfriction factor for roughened solar air heaters hasbeen determined from the correlations developedfor heat transfer and friction factors by severalinvestigators as given in Table. 3. The set of systemoperating parameters (shown in bracket of Table.3,column 3) for particular roughness geometry atwhich thermo hydraulic behaviour has been report-ed best, is selected for the analysis.

Figure 18 depicts the variation of the Colburnfactor (J) with the Reynolds number, for variousconsidered geometries (rough and smooth). TheColburn factors for all the geometries are higherthan that of the smooth surface for the entire range

of Re, except Gupta et al. (1993) and Saini andVerma (2008) up to Re around 4000.

It has been observed that the maximum value ofthe Colburn factor has been achieved for Bopcheand Tandale (2009) at Re 3000, but at Re 24000the maximum value of the Colburn factor has beenobserved for Saini and Saini (2008). The Colburnfactor decreases with an increase in Re in case ofsmooth surface, V-continuous, V-discrete, rib-grooved, chamfered rib-grooved, small diameterprotrusion wire and Inverted U-shaped turbulators.But the Colburn factor increases with increase in Rein case of dimple, transverse wire roughness, com-bination of inclined and transverse ribs, metal gritrib roughness, arc shaped wire roughness, expand-ed wire mesh, wedge shaped rib.

Figure 19 depicts the variation of friction factors


Figure 18: Colburn factor (j factor) versus Reynolds Number

Figure 19: Friction factor (f factor) versus Reynolds Number

with the Reynolds number. It is seen that the frictionfactor decreases with increase in Re for all types ofroughness geometries, and friction factor of anyconsidered rough surface is always higher than thesmooth surface. The friction factor, among the con-sidered geometries, in general increases in the fol-lowing sequence: smooth surface, chamfered rib-groove, chamfered rib, arc shaped wire, wedgeshaped rib, metal grit rib, transverse wire rough-ness, small diameter protrusion wire, V-continuousand V-discrete, Inverted U-shaped, rib-groove,expanded wire mesh, inclined and transverse ribsand dimple. Finally, one can conclude that there isnot a single roughness geometry which gives thebest result for the whole range of Re.

5. ConclusionsThis paper reviews the investigation carried out byvarious researchers in order to enhance the heattransfer and friction factor by the use of artificialroughness of different shapes, sizes and orienta-tions. It can be concluded that there is a consider-able enhancement in heat transfer with little penal-ty of friction.

Correlations developed for heat transfer andfriction factor for solar air heater ducts having artifi-cial roughness of different geometries for differentinvestigators are also shown in tabular form. Thesecorrelations can be used to predict the thermal effi-ciency, effective efficiency and then hydraulic per-formance of artificial roughened solar air heaterducts.

The Colburn factors are improved by usingroughened geometries in the duct of the solar airheater but friction factors are also increased. A solarair heater having a dimple has roughness elements,and is found to have a maximum value of frictionfactor in all the range of Reynolds number.

This paper is very helpful for researchers in car-rying out the experimental and numerical investiga-tions to find out and optimize the new elementgeometries for the maximum enhancement of heattransfer.

NomenclatureD equivalent diameter of the air passage (m)d/w relative gap positione height of roughness element (m)e+ roughness Reynolds numbere/D relative roughness heightfr friction factor of roughened ductfs friction factor of smooth ductg heat transfer function (g-function)g/e relative gap widthg/p groove position to pitch ratioG momentum heat transfer functionG (e+) heat transfer roughness function

H height of duct (m)l/s relative length of metal gritL/e relative long way lengthS/e relative short way lengthNu Average Nusselt numberNur Nusselt number for roughened ductNus Nusselt number for smooth ductpr Prandtl numberp oughness pitch (m)p/e relative roughness pitchR roughness functionRe Reynolds numberR (e+) Reynolds roughness numberSt average Stanton numberStr Stanton number for roughened ductSts Stanton number for smooth ductU+ dimensionless velocityY+ dimensionless distanceGreek symbolsα rib angle of attackα/90 relative angle of attackφ chamfer angleΨ wedge angle

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Received 5 March 2009; revised 2 December 2009


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