Heat transfer and friction characteristics in decaying swirl flow generated by different radial guide vane swirl generators

Heat transfer and friction characteristics in decaying swirlflow generated by different radial guide vane swirl generators

Mehmet Yilmaz a,*, Omer Comakli a, Sinan Yapici b, O. Nuri Sara b

a Atat€uurk €UUniversitesi, M€uuhendislik Fak€uultesi, Makina M€uuhendisli�ggi B€ool€uum€uu, 25240 Erzurum, Turkeyb Atat€uurk €UUniversitesi, M€uuhendislik Fak€uultesi, Kimya M€uuhendisli�ggi B€ool€uum€uu 25240 Erzurum, Turkey

Received 22 August 2001; accepted 21 January 2002

Abstract

In radial guide vane swirl generators, the flow direction must change from the radial direction to the axialdirection. This can be achieved either abruptly or by means of a fairing section, and each technique can beused in conjunction with an inserted centre body (deflecting element). This research was conducted to studythe effect of the geometry of the deflecting element in the radial guide vane swirl generator on the heattransfer and fluid friction characteristics in decaying swirl flow. The radial guide vane swirl generators usedin this investigation had three different configurations related to the deflecting element: the swirl generatorwith conical deflecting element, with spherical deflecting element and with no deflecting element. Theseswirl generators were compared with each other by taking into account their heat transfer and frictioncharacteristics. An augmentation up to 150% in Nusselt number relative to that of the fully developed axialflow was obtained with a constant heat flux boundary condition, depending upon the vane angles, Reynoldsnumbers and type of the swirl generators. It was observed that the swirl generator with no deflecting el-ement gave the highest Nusselt numbers and also gave the highest pressure drop in both the swirl generatorand the test pipe. Evaluating the effectiveness of the swirl generators for enhancing heat transfer, it wasfound that using the swirl generator with no deflecting element may be advantageous in terms of heattransfer enhancement and energy saving in comparison with swirl generators with a deflecting element.� 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Swirl flow; Decaying swirl flow; Radial guide vane swirl generator; Deflecting element; Heat transfer

Energy Conversion and Management 44 (2003) 283–300www.elsevier.com/locate/enconman

*Corresponding author. Tel.: +90-442-231-1441; fax: +90-442-233-6961.

E-mail address: [email protected] (M. Yilmaz).

0196-8904/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S0196-8904(02)00053-5

mail to: [email protected]

1. Introduction

Heat transfer enhancement may be achieved by numerous techniques, and these techniques canbe classified into three groups: passive, active and compound techniques [1]. The passive tech-niques, such as swirl flow devices, treated surfaces, rough surfaces, extended surfaces, displacedenhancement devices, coiled tubes, surface tension devices and additives for fluids, do not requiredirect application of external power, whereas the active techniques, such as mechanical aids,surface vibration, fluid vibration, electrostatic fields, suction or injection and jet impingement,require an external activator/power supply to bring about the enhancement. In the compoundtechniques, such as rough surface with a twisted tape swirl flow device and two or more of theactive or passive techniques may be utilised simultaneously to produce an enhancement that ismuch higher than the techniques operating separately.Swirl flow devices are designed to impart a rotational motion about an axis parallel to the flow

direction to the bulk flow and is one of the passive enhancement techniques used for increasing therate of heat transfer [2]. Swirl flows are found in nature, such as tornadoes, and are utilized in avery wide range of applications, such as cyclone separators, Ranque–Hilsch tubes, agriculturalspraying machines, heat exchangers, gasoline engines, diesel engines, gas turbines and many otherpractical heating devices [3].‘‘Swirl flow’’ is the descriptive term for a fluid flow in which the tangential component of the

main stream velocity is a significant contribution to the resultant velocity. Swirl flows may beclassified into three groups depending upon characteristic velocity profiles: curved, rotating andvortex flow [4]. These velocity profiles are different, depending upon the particular flow geometryand swirl generation methods. Curved flow is produced by a stationary boundary, causing acontinual bending of the local velocity vector, and complex secondary flows with an appreciablevelocity component normal to the instantaneous osculating plane are generated. Curved flows canbe generated by inserting coiled wires, twisted tapes and helical vanes into the pipe, by coiling thetube helically or by making helical grooves in the inner surface of the duct. Curved flow is alsocalled ‘‘continuous swirl flow’’. Rotating flow is generated by a rotating boundary, either con-fining the flow (as for a rotating tube) or locally influencing the flow field (as for a spinning bodyin a free stream). Vortex flow arises when a flow with some initial angular momentum is allowedto decay along the length of a tube [4]. Vortex flow is also called ‘‘decaying swirl flow’’. Decayingswirl flows are generated by the use of tangential entry swirl generators and guided vane swirlgenerators. Tangential entry of the fluid into a duct stream can be achieved by using a singletangential inlet duct or more than one tangential entry [5].Guided vane swirl generators may be grouped into two types: radial guide vane and axial guide

vane. Axial vane swirl generators consist of a set of vanes fixed at a certain angle to the axialdirection of the duct, which give a swirling motion to the fluid [6–11]. Generally, the vanes aremounted on a central hub, and they occupy space in an annular region. Even one single helicalvane or twisted tape can be used as a means of generating decaying swirl flow.Radial guide vane swirl generators are generally mounted between two disks, and the vanes are

so constructed as to be adjustable to obtain the desired initial degree of swirl. Radial generatorsare capable of generating much more intense swirls, and they cause more complex velocity profilesthan axial generators, since the flow direction must change from radial inward to axial down-stream, which can occur either abruptly or by means of a fairing section. An inserted centre body

284 M. Yilmaz et al. / Energy Conversion and Management 44 (2003) 283–300

(deflecting element) can be used in radial generators whose function is to deflect the flow into thepipe as smoothly as possible (Fig. 1).In the heat transfer studies on decaying swirl flow in the Thermomechanics Research Labo-

ratory of the Aerospace Research Laboratories (ARL) [12–15], radial swirl generators with fixedguide vanes were used. The guide vane apparatus utilised 18 fixed vanes oriented nearly tangentialto the vortex generator circumference and provided a simple transition section by means of apolished wall fairing of radius 2.54 cm. Lavan and Fejer [16], examined the velocity profiles for aconfined vortex with induced flow/radial generator using adjustable guide vanes and a transitionsection employing an internal wall fairing with an insert plug with the exit of vortex tube directedinto a radial diffuser section. Yajnik and Subbaiah [17] investigated the effects of swirl on internalturbulent flows by conducting experiments on turbulent flow with variable initial swirl. Thewooden contraction cone of the entry section in the swirl generator they used had a surface ofrevolution generated by a cubic curve. Clayton and Morsi [18] studied turbulent swirling flowsalong the annulus formed between two concentric stationary cylinders. They used hot wire ane-mometry to measure the time mean parameters and turbulence components of the velocitycomponents. In the swirling flow rig, they used a perspex bell-mouth whose function was totransport the vortex flow into a settling chamber with as little disturbance as possible. Algifri et al.

Fig. 1. Schematic diagram of the experimental setup.

M. Yilmaz et al. / Energy Conversion and Management 44 (2003) 283–300 285

[19] investigated the heat transfer in turbulent decaying swirl flow generated by a radial bladecascade, and they used a bell-mouth in their radial swirl generator. Kitoh [20] performed anexperimental investigation to obtain systematic data about swirling flow through a pipe and tounderstand its physics. He used a radial swirl generator and a bell-shaped cone at the centre of theswirler to deflect smoothly the radial flow into the axial direction. Also Kito [21] and Kito andKato [22] used a bell-shaped cone in their radial swirl generators. Yilmaz et al. [23] investigatedthe enhancement of heat transfer by turbulent decaying swirl flow generated by a radial guidevane swirler with conical deflecting element. As seen from the literature, the investigations inwhich radial swirl generators are used have been scant and have usually dealt with the flowcharacteristics, such as velocity, turbulence, shear stress etc., and generally, a conical insertedcentre body (deflecting element) has been used to deflect smoothly the radial flow into the axialdirection in these investigations (Table 1).In this study, in order to investigate the effects of the geometry of the deflecting element in

radial guide vane swirl generators on the heat transfer and friction characteristics in decayingswirl pipe flow, an experimental system was designed and constructed. In this system, three typesof swirl generators were used: the swirl generator with conical deflecting element, with sphericaldeflecting element and with no deflecting element.

2. Experimental setup and procedure

A general arrangement of the experimental equipment is shown in Fig. 1. Air was supplied tothe experimental equipment by a centrifugal fan (1), which was driven by an induction motor. Theinlet of the fan was equipped with a sliding vane whose function was to regulate the flow rate.

Table 1

Summary of radial guide vane swirl generator investigations

Investigator Configura-

tion

Working

fluid

Investigated character-

istics

Deflecting element

Dervage [12] Annulus Air Velocity Wall fairing

Mc Kelvey [13] Annulus Air Heat transfer Wall fairing

Steele [14] Annulus Air Velocity Wall fairing

Loosley [15] Annulus Air Heat transfer Wall fairing

Scott and Rask [24] Annulus Air Velocity Cone

Clayton and Morsi [18] Annulus Air Velocity and turbulence Bell-mouth

Lavan and Fejer [16] Pipe Air Velocity Wall fairing

Yajnik and Subbaiah [17] Pipe Air Velocity Cone

Kitoh [20] Pipe Air and

Water

Velocity Cone

Kito [21] Pipe Water Velocity Cone

Kito and Kato [22] Pipe Air Velocity Cone

Algifri et al. [19] Pipe Air Heat transfer Bell-mouth

Yilmaz et al. [23] Pipe Air Heat transfer Cone

Yilmaz et al. (Present) Pipe Air Heat transfer Cone, sphere and no

deflecting


Prior to entry into a flow stabilizer (2), an orifice meter (3), constructed in accordance withASME standards, was used to measure the flow rate. The stabilizer included a conical diffuser (4),a fluid straightener (5) and two wire mesh screens. The function of the conical diffuser was toallow the placement of the radial passage and to lead the air stream from the orifice meter into theflow stabilizer at a lower velocity. Air entered radially into the inward flow passage (6) whereguide vanes (7) were installed and acquired angular momentum in passing the cascade of vanes inthe radial flow passage. Namely, the swirl generator received the radial inflow coming from theflow stabilizer, imparted a swirling motion to it and finally directed the flow into the axial di-rection. At the center of the radial flow passage, three different designs related to the deflectingelement were designed, and thus, three different swirl generators were constructed:

(i) The swirl generator with the conical deflecting element: A conical deflecting element wasused in this type of swirl generator, and this generator was called the ‘‘first type swirl genera-tor’’. The cone base diameter and the cone height were 14 and 13 cm, respectively (Fig. 1a).(ii) The swirl generator with the spherical deflecting element: This generator was called the ‘‘sec-ond type swirl generator’’, and there was a hemispherical deflecting element whose radius was11 cm (Fig. 1b).(iii) The swirl generator with no deflecting element: In this type of swirl generator, there was nodeflecting element, and it was called the ‘‘ third type swirl generator’’ (Fig. 1c).

The swirling motion was generated by 12 flat guide vanes of chord 13 cm placed symmetricallyaround the radial flow passage, and the flow passed radially inward along these vanes. These guidevanes could be turned equally by a specially designed mechanism. Varying these vane anglescontrolled the swirl intensity at the inlet section of the test pipe. The vane angle could be fixedfrom zero to 75� with respect to the radial direction. The vane angle setting, corresponding to astraight axial flow, is designated as zero degrees, and the swirl angle was set by adjusting theangular orientation of each vane (h) to a specified value between 0 and 75� relative to the radialdirection. When all the guide vanes were directed radially 0�, the velocity distributions in the pipewere almost the same as in the usual parallel pipe flow and were axially symmetric [21]. In thisexperiment, five different swirl intensities were used, which were generated by setting the guidevane angles at 15�, 30�, 45�, 60� and 75�.A steel pipe of 73.5 mm inner diameter was used as test pipe (8). An electrically insulated

heating wire was wound continuously around the test pipe, and thus, a constant heat fluxboundary condition was obtained. The outside of the test pipe was insulated with glass woolinsulation (9) to prevent heat transfer from the test section into the environment. At the down-stream end of the test pipe there was a mixing chamber in which the outlet temperature of the airwas measured. The inlet and outlet bulk temperatures of the air and the temperature of the pipewall at various stations were measured by using copper–constantan thermocouples (10). The bulktemperature of the inlet air was measured by using a thermocouple positioned at the flow sta-bilizer. The exit bulk temperature was measured at the centerline of the mixing chamber. Acalibrated 12 channel thermometer (11) was used to measure all the temperatures. All tempera-tures were measured with a precision 0.1 �C. The pressure drop across the test section and acrossthe swirl generator were determined using pressure taps and a glass tube manometer with amanometer fluid having a relative density of 0.784 (12).


In swirling flow, an apparent mean friction factor, fa, can be defined by reference to the dif-ference in wall pressures at two stations distance L apart by the following relation [25,26], and thefollowing relation was used to evaluate the friction factor:

Dp ¼ faLd

qU 2

2ð1Þ

The local heat transfer coefficients were calculated using:

hx ¼ qx= Twð � TbÞ ð2Þwhere Tw is the local wall temperature, Tb the local bulk temperature and qx the local heat flux. Thevalues of the local wall temperature Tw were available from the experimental data. The local bulktemperature of the air at the measuring section was computed from the inlet temperature, flow rateand actual power input. A linear variation in bulk temperature from the inlet to the exit of the testtube was assumed [19,27]. The local Nusselt numbers were calculated using the following relation:

Nux ¼ hxD=k ð3Þwith k at the local bulk temperature.The Reynolds number based on the average bulk mean properties and the pipe inner diameter

ranged from 32 000 to 111 000. All measurements were taken after reaching steady state condi-tions. Because of the thermal capacity of the experimental setup, the system had to be run about2 h before steady state conditions were established.

3. Results and discussion

3.1. Heat transfer and friction characteristics

The experimental average Nusselt number results are presented in Figs. 2–4 in terms of the vaneangle and Reynolds number for the swirl generator with the conical deflecting element, with thespherical deflecting element and with no deflecting element, respectively. These curves indicatethat the Nusselt numbers in swirling flow are higher than the Nusselt numbers in axial flow for thesame Reynolds numbers for all the types of swirl generators. The results also indicate that as theReynolds number and vane angle increase, the Nusselt number increases for all the types of swirlgenerators. The effect of the vane angle on the Nusselt number is more pronounced for the highervane angles. The Nusselt numbers increased in the range of 4–99%, 12–119% and 9–148% de-pending upon the Reynolds number and the vane angle for the first, second and third type swirlgenerators relative to that in fully developed axial flow, respectively (Table 2). The maximumincrease in Nusselt number obtained was of the order of 148%, corresponding to the third type ofswirl generator and the maximum vane angle of 75�. The experimental results were correlated bythe method of least squares, using simultaneous multiple regression in the case of more than oneindependent variable. The average Nusselt number correlation equations for the first, second andthird type of swirl generators were found to be as follows, respectively:

Nu ¼ 0:133Re0:65Pr0:4 1ð þ tan hÞ0:406 ð4Þ




From the literature review, it can be seen that the use of swirl flows could provide a Nusseltnumber enhancement (Nua=Nuo) up to 4–5 at a constant Reynolds number and 1.3–1.8 on aconstant pumping power basis [4]. In this investigation, the Nusselt number enhancement variedbetween 1.04 and 1.99, 1.12 and 2.19 and 1.09 and 2.48 at a constant Reynolds number for thefirst, second and third type of swirl generator, respectively (Table 2).

Fig. 2. Nusselt number as a function of Reynolds number for different vane angles for the swirl generator with conical

deflecting element.

Fig. 3. Nusselt number as a function of Reynolds number for different vane angles for the swirl generator with spherical

deflecting element.


The friction factor ratio is defined as the ratio of the friction factor in swirling flow to that inaxial flow at the same Reynolds number. The experimental friction factors were used as the axialflow friction factors. Figs. 5–7 show the influence of vane angle and Reynolds number on thefriction factor ratio for the swirl generator with the conical deflecting element, with the sphericaldeflecting element and with no deflecting element, respectively. These curves indicate that thefriction factor ratios are very high and come much higher especially at higher vane angles. Thefriction factors increased in the range of 244–1442%, 261–1351% and 236–1699% for the first,second and third type swirl generators relative to that in the fully developed axial flow, respec-tively, depending upon Reynolds number and vane angle (Table 3).The presence of swirl introduces some important changes to the flow. These changes are:

(i) to impose a tangential velocity component, thus introducing an angular acceleration to thefluid flow;(ii) to increase the flow path length in the flow channel;(iii) to decrease the free area;

Fig. 4. Nusselt number as a function of Reynolds number for different vane angles for the swirl generator with no

deflecting element.

Table 2

Increase in average Nusselt number in comparison with axial flow

Vane angle First type swirl generator

(conical deflecting)

Second type swirl generator

(spherical deflecting)

Third type swirl generator

(no deflecting)

Minimum %

increase

Maximum

% increase

Minimum %

increase

Maximum

% increase

Minimum %

increase

Maximum

% increase

15� 4 28 12 44 9 45

30� 21 47 21 59 32 63

45� 36 46 32 63 36 65

60� 53 62 47 72 47 79

75� 79 99 90 119 106 148


(iv) to cause the occurrence of centripetal forces.

As a result of these changes, a substantial enhancement in both the Nusselt number and thefriction factor occurs in both continuous and decaying swirl flow. This enhancement in swirl flowis generally referrable to at least eight enhancement mechanisms: kinematic instability, thermalinstability, longitudinal vortex structure, intensification of velocity fluctuations, recirculation,increased roughness due to inserts, fin effects of inserts and emphasized entry length effect [4].Radial guide vane swirl generators generate a vortex type of swirl flow, namely a decaying type

of swirl flow. In this type of swirl generators, some of the above mentioned mechanisms, such asincreased roughness due to inserts and fin effects of inserts have no effect on the heat and mo-mentum transfer enhancement because there are no inserts in these swirl generators. The increase

Fig. 5. Friction factor ratios for the swirl generator with conical deflecting element.

Fig. 6. Friction factor ratios for the swirl generator with spherical deflecting element.


in the Nusselt number and friction factor in this investigation results from the remaining mech-anisms: kinematic instability, thermal instability, longitudinal vortex structure, intensification ofvelocity fluctuations, recirculation and entry length effect:

(i) Since the circulation of decaying swirling flow decreases with respect to radius, the swirl flowobtained using a radial guide vane swirl generator is unstable to a radial displacement near thetube wall regardless of the tangential velocity distribution because of the no-slip condition. As aresult, the boundary layer is always unstable for this type of flow.(ii) Since the instabilities caused by centripetal forces originating from a curvilinear flow fieldand by buoyant forces arising in a thermally stratified layer are combined, centripetal forcesfurther contribute to convection heat and momentum transfer because the heat transfer is inthe direction from larger to smaller radii in the present experiments.(iii) It is known that for turbulent flow, the radial turbulent velocity fluctuations are amplifiednear a concave wall and tend to be suppressed near a convex wall [28]. As a result, the decayingswirl flow obtained using a radial guide vane swirl generator would improve both convectionheat and momentum transfer at the outer tube wall.

Fig. 7. Friction factor ratios for the swirl generator with no deflecting element.

Table 3

Increase in friction factor in comparison with axial flow

Vane angle First type swirl generator

(conical deflecting)

Second type swirl generator

(spherical deflecting)

Third type swirl generator

(no deflecting)

Minimum %

increase

Maximum

% increase

Minimum %

increase

Maximum

% increase

Minimum %

increase

Maximum

% increase

15� 244 325 261 322 236 336

30� 262 337 265 329 248 340

45� 281 348 269 349 258 357

60� 407 496 343 476 321 523

75� 915 1442 785 1351 943 1699


(iv) In decaying swirl flow, the velocity characteristics have a large effect on the heat transferand momentum characteristics. In smooth pipes, the maximum tangential velocity componentusually occurs close to the pipe wall, especially for the entrange region. As the swirl decays dueto viscous effects at the wall, the point at which the tangential velocity is a maximum movesradially inward [29–31]. On the other hand, for swirling pipe flow, the maximum axial velocitycomponent tends to occur near the pipe axis, and with increasing swirl intensity, the location ofthe maximum axial velocity moves progressively further away from the pipe axis [10,17] etc. Asa result of this effect, there is a corresponding reduction in the axial velocity at the pipe axis,and at sufficiently high intensity of swirl, a reversed flow region is established about the pipeaxis. There is no general rule for the occurrence of the reverse flow because various factors, es-pecially the method of generating swirl, have an influence. The reverse flow is sometimes called‘‘recirculation’’. The effect of recirculation and boundary layer eruption is to enhance the con-vection heat and momentum processes. The reverse flow or recirculation region improves con-vection so that it increases the effective axial Reynolds number, since the same throughput mustbe accommodated by a reduced cross-sectional flow area, and it results in more severe meanvelocity and temperature gradients which produce higher fluxes of heat and momentum dueto the larger effective driving potential for each. On the other hand, the boundary layer erup-tion causes global mixing of the core region with the wall region, thus enhancing the convectiveprocess [4].(v) Since the decaying swirl flow obtained using a radial guide vane swirl generator is entirelyundeveloped, it exhibits higher convection coefficient because of this entry length effect.

3.2. The comparison of the swirl generators

Figs. 8 and 9 show the comparison of the swirl generators on the basis of the local Nusseltnumber. In Fig. 8, the local Nusselt number was plotted as a function of the dimensionless tube

Fig. 8. Comparison of the swirl generators by the local Nusselt number (Re ¼ 52300 and h ¼ 75�).


length for Re ¼ 52300 and h ¼ 75�. In Fig. 9, the local Nusselt number at x=d ¼ 8 was plotted asa function of the Reynolds number for the vane angle of 75�. The differences in the axial distri-butions of the local Nusselt number shown in Fig. 8 and in the value of the local Nusselt numbersshown in Fig. 9 as a function of Reynolds numbers reflect the geometry of the deflecting element,which has a determinant effect on the flow behaviour. The graphs of the axial distribution of localNusselt number show that:

(i) For axial flow, the local Nusselt number decreased approximately up to ten tube diameters,then it remained nearly constant because of the thermally developed flow.(ii) Swirling flow gave higher values of local Nusselt number than those for fully developedaxial flow for all vane angles and for all the types of swirl generators. The enhancement inthe local Nusselt numbers compared to values obtained in the fully developed axial flow washigher at higher vane angles because of the higher swirl intensity. The increase in the local Nus-selt numbers decreased with decreasing swirl intensity.(iii) For swirling flow, the local Nusselt number decayed along the test pipe in the axial direc-tion as the axial distance increased, but the effect of swirl was still considerably evident at theend of the test pipe. The decay was more pronounced at higher vane angles than that at lowervane angles. As the fluid flows along the pipe, the angular momentum and tangential velocitydecay due to friction losses at the wall. Some investigators, including Baker and Sayre [10],Kreith and Sonju [29] and Wolf et al. [32], observed that the decay process showed an approx-imately exponential behaviour. It was also observed that the decay of swirl decreases with in-creasing axial Reynolds number. Clayton and Morsi [18] found that the decay rate increasedwith increasing initial vane angle and annulus ratio. In general, it has been found that the swirlintensity decays faster for smaller Reynolds numbers and/or larger initial swirl, and that thedecay is approximately exponential [4]. An examination of Fig. 8 reveals that the swirl flow de-cays towards the end of the test pipe, and that the decay of the swirl flow for the swirl generatorwith no deflecting element is faster than that of the other two swirl generators.

Fig. 9. Comparison of the swirl generators by the local Nusselt number (x=d ¼ 8 and h ¼ 75�).


The comparison of the swirl generators by the average Nusselt number is presented in Fig. 10for a vane angle of 75�. An inspection of Fig. 10 indicates that the average Nusselt number in-creases with increasing Reynolds number for all the types of swirl generators. This means that theswirl is more enhancing with respect to heat transfer at higher Reynolds numbers than at lowerReynolds numbers. It is also evident that the swirl generator that gives the maximum heat transferenhancement is the swirl generator with no deflecting element, and that the swirl generator withthe spherical deflecting element gives higher values of Nusselt numbers than those for the swirlgenerator with the conical element. The same results can also be seen in Table 2, which shows thepercent increase in the average Nusselt numbers in comparison with axial flow. The minimum andthe maximum percent increase in the average Nusselt numbers are also presented. The differencesin the Nusselt numbers arise from the different geometry of the deflecting element used in the swirlgenerator and means that the geometry of the deflecting element has an important effect on theflow behaviour. A tabulation of the friction factor ratios is shown in Table 3. The table shows thatthe results of the friction factor ratios are parallel to those of the Nusselt numbers. It may bebriefly expressed that the friction factor ratios increase with increasing Reynolds numbers, andthat the swirl generator giving the maximum increase in the friction factor ratios is the swirlgenerator with no deflecting element. The comparison of Tables 2 and 3, however, reveals that theincrease in the friction factors is much higher in comparison with that in the Nusselt numbers.Figs. 11–13 show the comparison of the swirl generators by the pressure drop in the test pipe,

the pressure drop in the swirl generator and total pressure drop, which includes the pressure dropin the test pipe and the pressure drop in the swirl generator, respectively, for the vane angle of 75�.It is obvious that the pressure drop increases with increasing Reynolds number. The swirl gen-erator with no deflecting element gives the highest pressure drop in the test pipe and in the swirlgenerator, hence the total pressure drop is higher than that of the other two types of swirl gen-erator, which use the deflecting element. These results are parallel to those of the Nusselt numbersand friction factor ratios.A general overview of the results obtained in this study reveals that the swirl generator with no

deflecting element gives the maximum increase in Nusselt number, the pressure drop in the testpipe, the pressure drop in the swirl generator and the friction factor ratios. The reason for this

Fig. 10. Comparison of the swirl generators by the average Nusselt number.


situation arises from the types of swirl generators used. In the radial guide vane swirl generators, ifno retarding forces are present, the moment of momentum is constant as the flow spirals inwardand a free vortex type of flow is generated. The drag of the guide vanes, the presence of wakesbehind the guide vanes and the side wall plate drag complicate the actual situation. In the case ofthe radial generator with no deflecting element, the flow with a tangential component cannot enterinto the test pipe smoothly, the flow is less stable than the case with a deflecting element, andhence, the turbulent mixing is better. As a result, in addition to enhancing the heat transfer, theunsteadiness increases the pressure drop in the test pipe and the swirl generator. Therefore, theNusselt number enhancement and also the pressure drop and the friction factor increase forthe swirl generator with no deflecting element is higher than that for the other swirl generators,which use the deflecting element. This result is in agreement with other reported investigations,because several investigators who have used radial guide vane swirl generators have observedoscillatory behaviour unless an insert plug was used together with a cubic wall transition section[4]. Yajnik and Subbaiah [17] used a cone with a cubic profile to stabilise the flow and to provide a

Fig. 11. Comparison of the swirl generators by the pressure drop in the test pipe.

Fig. 12. Comparison of the swirl generators by the pressure drop in the swirl generator.


gradual reduction in cross-sectional area. They concluded that the unsteadiness was markedlydiminished after installation of the cone. In the study by Kitoh [20], it was found that the un-steadiness was markedly diminished after installation of the cone. In the studies in the Ther-momechanics Research Laboratory of the ARL, the plug was found to be necessary in order toavoid oscillations in the data, suggesting some form of inlet instability associated with the velocityvector having to turn from radial inward to axial downstream. By using the deflecting element, theswirl flow is transported into the test pipe with as little disturbance as possible, so that a pro-gressive swirling flow is developed before passing on to the test section. This situation, however, isnot preferable from the point of heat transfer enhancement. Therefore, the use of the swirlgenerator with no deflecting element may be preferred from the point of heat transfer enhance-ment. However, it is necessary to check if the increased pressure drop diminishes the benefits ofthe heat transfer enhancement.

3.3. Performance characteristics

It is important to note that the increase in the Nusselt numbers with swirling flow was less thanthe increase in friction factors at constant Reynolds numbers. The maximum increase in Nusseltnumber was 2.48 times the axial flow value with no swirl, while the corresponding friction factorwas approximately 17 times the axial flow value with no swirl (Tables 2 and 3). In all cases ofswirling flow, the heat transfer rates increased at the expense of increased pressure drop. In ad-dition, the results showed that the swirl generator with no deflecting element appears attractiveover the other two swirl generators when a comparison is made on the basis of Nusselt numberenhancement. On the other hand, the same generator gives the maximum pressure drop in theswirl generator and in the test pipe, hence maximum friction factors. Therefore, the advantage ofthe swirl generator with no deflecting element may be diminished when a comparison is made onthe basis of the pressure drop and friction factor characteristics. This situation requires a com-prehensive analysis of enhancement techniques to determine their potential benefits, and it isnecessary to check whether these techniques are advantageous in terms of energy savings ornot. Although several performance criteria have been developed for evaluating heat transfer

Fig. 13. Comparison of the swirl generators by the total pressure drop.


enhancement, there is no general procedure that would allow comparison of different enhance-ment techniques, since heat transfer enhancement can be used to accomplish several goals. Here,the swirl generators will be compared using the pumping power ratios for the same heat transferrate, and the comparison of the swirl generators by the energy and enhancement efficiencies werestudied in another paper in detail by Yılmaz et al. [33]. For the same heat transfer rates, thepumping power (P) required is given by [34]

_PPa_PPo

¼ fafo

Dh;o

Dh; a

� �4 ReaReo

� �3

ð7Þ

where _PP is the power, Dh is the hydraulic diameter, subscript a refers to the augmented case,subscript o refers to the unaugmented case.The comparison of the swirl generators by the pumping powers required for the same heat

transfer rate are given in Table 4 for the vane angle of 75�. By comparing the swirl generators onthe basis of the pumping power ratios for the same heat transfer rate, it may be concluded that:

(i) The pumping power ratios for the same heat transfer rate are lower than one if one useshigher vane angles and relatively lower Reynolds numbers. Therefore, using all types of swirlgenerators with higher vane angles and relatively lower Reynolds numbers may be advanta-geous in terms of heat transfer enhancement and energy savings. However, it must be consid-ered that even for the swirl generator with no deflecting element, the pumping power ratios arehigher than one at higher Reynolds numbers.(ii) The most preferable swirl generator must be the swirl generator with no deflecting elementbecause this generator requires less pumping power for the same heat transfer rate. The resultsobtained for other vane angles were found to be in agreement with the results mentioned above.

4. Conclusions

The present study explored the effects of the geometry of the deflecting element in radial guidevane swirl generators on the heat transfer and friction characteristics. Key findings from the studymay be summarized as follows:

(i) Nusselt numbers increased in the range of 4–99%, 12–119% and 9–148% for the first, secondand third type swirl generators, respectively, relative to that in the fully developed axial flow,

Table 4

Pumping power ratios for the same heat transfer rate (h ¼ 75�)Type of swirl generator Reynolds number

35 000 60 000 85 000 111 000

First type swirl generator 0.743 0.943 1.100 1.232

Second type swirl generator 0.527 0.789 1.025 1.243

Third type swirl generator 0.383 0.609 0.823 1.028


depending upon Reynolds number and the vane angle. Friction factors, however, increased inthe range of 244–1442%, 261–1351% and 236–1699% for the first, second and third type swirlgenerators, respectively, relative to that in the fully developed axial flow, depending upon Rey-nolds number and the vane angle.(ii) In swirling flow, increasing the Reynolds number and the vane angle increased the Nusseltnumber.(iii) The decay rates of the local Nusselt number in decaying swirl flow were more pronouncedat the higher vane angles.(iv) From the comparison of the swirl generators on the basis of heat transfer, pressure dropand friction characteristics, it was found that the swirl generator with no deflecting elementgave the highest pressure drops in both the swirl generator and the test pipe and also gavethe highest Nusselt numbers. Evaluating the effectiveness of the swirl generators for enhancingheat transfer, it was found that using the swirl generator with no deflecting element may be ad-vantageous in terms of heat transfer enhancement and energy saving in comparison with swirlgenerators which use the deflecting element.(v) To obtain lower pumping powers for the same heat transfer rate, higher vane angles andrelatively lower Reynolds numbers must be employed.

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Documents

Heat transfer and friction characteristics in decaying swirl flow generated by different radial guide vane swirl generators