Research Article EFFECT OF PITCH SPACING OF DELTA-WINGLETS ON THERMAL CHARACTERISTICS IN A HEAT EXCHANGER TUBE

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1 Tranaction of the TSME (2016) Vol. 4, No. 2, Journal of Reearch and Application in Mechanical Engineering Copyright 2016 by TSME ISSN print DOI: /jrame Reearch Article EFFECT OF PITCH SPACING OF DELTA-WINGLETS ON THERMAL CHARACTERISTICS IN A HEAT EXCHANGER TUBE N. Koolnapadol 1 S. Sripattanapipat 2 S. Skullong 3, * 1 Department of Automotive Mechanical Engineering, Faculty of Indutrial Technology Rajabhat Rajanagarindra Univerity, Chachoengao 24000, Thailand. 2 Department of Mechanical Engineering, Faculty of Engineering, Mahanakorn Univerity of Technology, Bangkok 10530, Thailand 3 Energy Sytem Reearch Group, Department of Mechanical Engineering, Faculty of Engineering at Sriracha, Kaetart Univerity Sriracha Campu, 199 M.6, Sukhumvit Rd., Sriracha, Chonburi 20230, Thailand ABSTRACT: In thi paper, the heat tranfer, preure lo and thermal performance by uing delta-winglet pair placed on double-ided traight tape inerted into a contant heat-fluxed tube are experimentally tudied. Experimental work i conducted for the tape with different value of axial winglet pitch pacing (P) or winglet pitch ratio (P/D=PR= 0.5, 1.0, 1.5, 2.0 and 2.5). Air ued a the working fluid and flow into the tube for Reynold number (Re) ranging from 4100 to 25,400. The ue of the delta-winglet tape (DWT) with the winglet attack angle of 45 i to generate two pair of longitudinal vortex flow. The experimental reult how that the DWT provide higher heat tranfer rate and preure lo than the mooth tube. The peak heat tranfer rate i at PR=0.5 and i approximately time above the mooth tube while the friction lo i around time. To acce the real benefit of the 45 DWT inert, thermal enhancement factor (TEF) i examined and found to be in the range of for PR = 1.5. Keyword: Heat exchanger, Enhancement, Inert, Thermal performance, Vortex generator 1. INTRODUCTION Growth indutrial ector make neceary to improving the thermal performance of heat exchanger and heat tranfer device. In general, the thermal performance of heat exchanger ytem can be improve by heat tranfer enhancement method. A paive method of heat tranfer enhancement ha been of interet for many reearcher becaue additional external power i not needed. The paive technique include the ue of treated urface, external urface, rough urface, extended urface, wirl generator device, vortex generator device, etc. [1,2]. Compared with other type in the paive technique, tube inert have the advantage due to low cot, rapid manufacturing, eae of intallation/maintenance. For decade, everal invetigation have been made to examine the application of vortex generator (VG) for augmenting the heat tranfer rate in heated/cooled tube fitted with different inert-type of VG. Swirl/vortex flow device uch a coiled wire [3], twited tape [4], circular/conical ring [5,6], baffle [7], winglet [8] that belong to one key group of the vortex generator by inerting them into the tube have been extenively employed in improvement of heat exchanger ytem. A comparion of the thermal and hydraulic performance of twited tape or wire coil inert wa introduced by Wang and Sunden [9] for both laminar and turbulent flow regime. They found that the coiled wire perform effectively in enhancing heat tranfer in a higher turbulent flow region wherea the twited tape yield a poorer overall efficiency. Eiama-ard et al. [10] invetigated the thermal performance of tubular heat exchanger fitted with regularly paced twited tape element. A new deign of double V-ribbed * Correponding author: S. Skullong addre: fengp@rc.ku.ac.th, ompol@eng.rc.ku.ac.th 166 /Volume 4(2), 2016 Tranaction of the TSME: JRAME

2 twited-tape in tubular heat exchanger wa introduced by Tamna et al. [11] that howed that both the V-ribbed twited-tape and the typical twited-tape provided higher heat tranfer rate and preure drop increae. The thermal enhancement factor (TEF) of the V-ribbed twited tape increaed around 22% above that of the typical twited-tape. The thermal and friction lo characteritic in a uniform heat-fluxed tube with V-nozzle inert for different pitch ratio were tudied by Eiama-ard and Promvonge [12]. Promvonge [13] examined the effect of conical ring inert with three different ring diameter ratio where the ring were placed with three different arrangement (converging conical ring, diverging conical ring and converging diverging conical ring) on heat tranfer enhancement in a heat exchanger tube. Variou tyle of VG device, apart from wire coil and twited tape, have been widely applied in tubular heat exchanger to improve their thermal performance. The effect of baffle with variou ratio of pitch to tube diameter and baffle orientation angle on forced convection heat tranfer for turbulent flow in a circular tube wa invetigated by Tandiroglu [14]. The augmentation of convective heat tranfer in a ingle-phae turbulent flow by uing twited tape coniting of centre wing and alternate-axe wa experimentally invetigated by Eiama-ard et al. [15]. Wongcharee and Eiama-ard [16] employed the modified wing-formed twited-tape with three different hape, namely, triangular, rectangular and trapezoidal wing on the tape rim for enhancing heat tranfer in a heat exchanger tube. According to the literature above, wirl/vortex flow device are frequently introduced in round tube to enhance the degree of turbulence level and the fat fluid mixing wherea the heat tranfer enhancement trongly depend on the baffle/winglet tructure. However, the delta-winglet vortex generator are found to be widely ued in a rectangular duct/channel and a fin-tube heat exchanger and are rarely applied in a round tube. In the preent work, the inertion of a delta-winglet tape obtained by extruding and protruding the traight tape to form delta winglet mounted on the double-ided tape are examined. Hence, the aim of thi article i to tudy the effect of pitch ratio (PR) of the DWT on preure drop and heat tranfer improvement in a tubular heat exchanger. Experimental et up and correlation for meauring of Nu, f and TEF are preented. The teting range of the Reynold number (Re) i between 4100 and 25, EXPERIMENTAL SET-UP The DWT ued in the preent tudy i hown in Fig. 1. In the figure, each winged tape wa made of aluminum heet (traight tape) with 0.8 mm thickne (t) and length of 1200 mm (=L). In forming the delta-winglet pair, the tape wa partially cut and extruded/protruded to become the winglet a een in Fig. 1. The 45º DWT with five ratio of wing-pitch to tube-diameter (called "pitch ratio", P R =P/D=0.5, 1.0, 1.5, 2.0 and 2.5) were inerted into a round tube by wall-attached poition. The chematic of the experimental etup i depicted in Fig. 2. The etup mainly conited of a calm ection, tet ection and exit ection. The tet ection wa made of copper in which air flowed through the heated tube. The tube wa heated by continually winding flexible electrical wire to provide a uniform heat-flux condition. The outer urface of the tet tube wa well inulated to minimize convective heat lo to the urrounding. The length of the tet ection (L) wa 1200 mm. The inner (D) and outer tube diameter were 50.2 and 54.3 mm, repectively. The 1.5 kw blower directed the air with T in = 25 ºC to the orifice flow meter. An inverter wa utilized to adjut the air flow rate by changing the motor peed of the blower to achieve deired Reynold number between 4100 and 25,400. The inlet and outlet temperature (T in and T out ) of the bulk air were meaured at certain point with a multi-channel temperature meaurement unit in conjunction with the RTD PT-100 type temperature enor while the urface temperature (T w ) were meaured by 16 T-type thermocouple located equally on each of the top and ide wall along the tet ection. The thermocouple voltage output were fed into a data acquiition ytem (Fluke 2680A) and then recorded via a laptop. A digital manometer wa ued to obtain the preure drop of air acro the tet tube. More detail on the experimental et-up, method and uncertainty analyi were imilar a reported in an earlier author paper [17]. The parameter range of the invetigation were given in Table 1. Tranaction of the TSME: JRAME 2016, Volume 4(2) / 167

3 Fig. 1. Tet tube with 45º DWT. Table 1: Range of parameter in the invetigation Fig. 2. Schematic ketch of the experimental ytem. Working fluid Air Reynold number, Re 4100 to 25,400 Attack angle of winglet, 45 Winglet pitch ratio, P/D=P R 0.5, 1.0, 1.5, 2.0 and 2.5 Tape thickne, t 0.8 mm Tape width, W 50.2 Tape length, L 1200 mm Winglet thickne, = t 0.8 mm 3. DATA PROCESSING The purpoe of the current work i to determine the heat tranfer rate (Nuelt number, Nu), preure lo (friction factor, f), and thermal enhancement factor (TEF) in a circular tube fitted with the DWT. The data proceing i a follow: 168 /Volume 4(2), 2016 Tranaction of the TSME: JRAME

4 The parameter of interet are Reynold number (Re) and winglet pitch ratio (P R ). The Re i given by U D Re (1) The f calculated from preure lo i written a f 2 / P 2 L D U in which U i mean air velocity in the tet tube. In the experiment, air flow through the tet tube under a uniform heat-flux condition. The teady tate of the heat tranfer rate i aumed to be equal to the heat lo from the tet ection which can be expreed a: Qair Q conv (3) where Q air p,air out in m C T T (4) The convection heat tranfer from the tet ection can be written by ~ Q h A T T (5) conv in which b out b T ( T T ) / in 2 (6) and ~ T T /16 (7) where T i local urface temperature located equally along the outer urface of the tet tube. The average urface ~ temperature, T i computed by uing 16 point of local wall temperature. The average heat tranfer coefficient (h) and Nu are etimated a follow: ~ h m C T T / A T T (8) p,air out in b The heat tranfer i calculated from the average Nu which can be obtained by h D Nu (9) k All of thermo-phyical propertie of air are determined at the overall bulk air temperature (T b ) from Eq. (6). To ae the practical ue, thermal performance of the enhanced tube i evaluated relatively to the mooth tube at an identical pumping power in the form of thermal enhancement factor (TEF) which can be expreed by (2) TEF Nu Nu 0 Nu Nu 0 h h pp 0 pp f f (10) where h 0 and h tand for heat tranfer coefficient of plain tube and inerted tube, repectively. 4. RESULT AND DISCUSSION 4.1. Confirmatory tet The mooth tube wa initially teted to verify the experimental etup. A comparion of the preent Nu and f reult of the mooth tube with thoe obtained from the publihed correlation [18] of Dittu-Boelter and Petukhov, a given in Eq. (11) and (12), i depicted in Fig. 3 and 4, repectively. Tranaction of the TSME: JRAME 2016, Volume 4(2) / 169

5 Dittu-Boelter correlation: Nu 0.023Re 0.8 Pr 0.4 (11) Correlation of Petukhov, 0.79lnRe f (12) Fig. 3 and 4 how the comparion between the preent work and correlation data of Dittu-Boelter (Eq. 11) and Petukhov (Eq. 12). In the figure, the preent reult agree reaonably well within ±6% for Nu of Dittu Boelter and ±7% for f of Petukhov correlation. Thi indicate that data of the experimental etup i reliable. Fig. 3. Confirmatory tet of Nu of mooth tube Effect of DWT on heat tranfer Fig. 4. Confirmatory tet of f of mooth tube. Fig. 5a, b and c preent the relationhip of heat tranfer coefficient (h), Nuelt number (Nu) and Nuelt number ratio (Nu/Nu 0 ) with Re for uing the 45º DWT with different P R value, repectively. The reult how that the h of the DWT i excellent for the tubular heat exchanger. The h increae with the rie of Re but with decreaing P R (ee Fig. 5a). The Nu of the DWT i een to be higher than that of the mooth tube around 66 76%, and the bet Nu i for P R = 0.5 (ee Fig. 5b). Thi i becaue the preence of the DWT induce tronger flow mixing reulting in detruction of the boundary layer and create vortex-flow to prolong the reidence time of flow. The ratio of augmented Nu of inerted tube to Nu of mooth tube (Nu/Nu 0 ) plotted againt the Re i depicted in Fig. 5c. In the 170 /Volume 4(2), 2016 Tranaction of the TSME: JRAME

6 figure, Nu/Nu 0 tend to decreae lightly with the rie of Re for all cae. It i noted that the P R = 0.5 provide the highet Nu/Nu 0. Thi i caued by the higher number of the winglet for P R = 0.5 that can more often interrupt the flow leading to tronger vortex flow trength leading to higher rate of heat tranfer between the working fluid and the heated wall. The Nu/Nu 0 how a lightly decreaing trend with the increae in Re. Nu/Nu 0 value for the DWT are about , , , and time for P R = 0.5, 1.0, 1.5, 2.0 and 2.5, repectively. The DWT at P R = 0.5 yield the average Nu/Nu 0 around 6%, 11.5%, 21.7% and 26.9% higher than the one at P R = 1.0, 1.5, 2.0 and 2.5, repectively. (a) (b) 4.3. Effect of DWT on friction lo (c) Fig. 5. Variation of (a) h, (b) Nu and (c) Nu/Nu0 with Re for DWT inert. Effect of preure drop (P), friction factor (f) and friction factor ratio (f/f 0 ) are exhibited in Fig. 6a, b and c, repectively. In Fig. 6a, P for all cae how the coniderable increae with imilar trend for the increae in Re and apparently, the inerted tube give higher P than the mooth one. It i clearly oberved in Fig. 6b that the DWT provide a ubtantial increae in f above the mooth tube due to higher flow blockage epecially for uing maller P R apart from the diipation of dynamic preure of the fluid due to larger urface area. The f of the DWT increae around 90 97% above that of the mooth tube alone. Fig. 6c preent the variation of f/f 0 with Re for Tranaction of the TSME: JRAME 2016, Volume 4(2) / 171

7 variou P R value. In the figure, the f/f 0 i increaing a a function of Re for all cae. The mean f/f 0 value for P R = 0.5, 1.0, 1.5, 2.0 and 2.5 are, repectively, about 31.4, 24.4, 19.7 and 15.9 time. The maximum f/f 0 i about time at mallet pitch ratio (P R = 0.5). The f/f 0 of P R = 0.5 i, repectively, about 22.4%, 37.3%, 78.4% and 59.2% higher than that of P R = 1.0, 1.5, 2.0 and 2.5. (a) (b) 4.4. Effect of DWT on thermal performance (c) Fig. 6. Variation of (a) P, (b) f and (c) f/f0 with Re for DWT inert. The aement on potential of the DWT for real application i made in the form of thermal enhancement factor (TEF) plotted againt Re a exhibited in Fig. 7. The TEF i achieved by imultaneouly evaluating the Nu and f in the inerted tube and the mooth tube under contant pumping power condition taken into account by uing Eq. (10). In the figure, TEF how the decreaing trend with increaing Re for all inert. The TEF value for the DWT at P R = 0.5, 1.0, 1.5, 2.0 and 2.5 are, repectively, around , , , and , depending on Re value. The highet TEF of 1.55 i found for P R = 1.5 at the lowet Re. Therefore, the bet choice of thi inert device i the ue of DWT at P R = 1.5 to achieve uperior thermal performance. 172 /Volume 4(2), 2016 Tranaction of the TSME: JRAME

8 Fig. 7. Variation of TEF with Re for DWT inert. 5. COMPARISON WITH OTHER INSERTED VG DEVICES The comparion of thermal performance of the current reearch with imilar publihed work (delta-wing tape of Skullong et al. [19], rectangular-winglet tape of Praopuk et al. [20] and double-ided delta wing with alternate axi of Eiama-ard and Promvonge [21]) i hown in Fig. 8. It indicate in the figure that the preent delta-winglet tape provide coniderably higher TEF than the mentioned delta-wing tape, rectangular-winglet tape and doubleided delta wing tape with alternate axi inert. The current DWT yield the TEF higher than the wing/winglet inert [19 21] at about 3 26%. Fig. 8. Comparion of TEF of the preent DWT with other imilar VG device. 6. CONCLUSION An experimental procedure ha been conducted to invetigate a ingle phae turbulent flow and heat tranfer behavior in a tubular heat exchanger equipped with delta-winglet tape vortex generator. Air ued a the working fluid enter the tet tube in a range of Reynold number between 4100 and 25,400 under a uniform wall heat-flux condition. The preent reult how that the Nu reduce with the rie of P R while increae with the increment of Re. The f decreae with increaing Re and P R. The 45 DWT at P R = 0.5 give the highet Nu/Nu 0 and f/f 0 at about 4.30 Tranaction of the TSME: JRAME 2016, Volume 4(2) / 173

9 and 36.34, repectively. The TEF of the DWT how the decreaing trend with increaing Re and ha a maximum around 1.55 at P R = 1.5. REFERENCES [1] Liu, S., Sakr, M. A comprehenive review on paive heat tranfer enhancement in pipe exchanger, Renew. Sutain. Energy Rev., Vol. 19, 2013, pp [2] Zhang, C., Wang, D., Ren, K., Han, Y., Zhu, Y., Peng, X., Deng, J., Zhang, X. A comparative review of elfrotating and tationary twited tape inert in heat exchanger, Renew. Sutain. Energy Rev., Vol. 53, 2016, pp [3] Promvonge, P. Thermal performance in circular tube fitted with coiled quare wire, Energy Conver. Manage., Vol. 49, 2008, pp [4] Singh, V., Chamoli, S., Kumar, M., Kumar, A. Heat tranfer and fluid flow characteritic of heat exchanger tube with multiple twited tape and olid ring inert, Chem. Eng. Proce. Proce Intenif., Vol. 102, 2016, pp [5] Eiama-ard, S., Kongkaitpaiboon, V., Nanan, K. Thermohydraulic of Turbulent Flow Through Heat Exchanger Tube Fitted with Circular-ring and Twited Tape, Chin. J. Chem. Eng., Vol. 21, 2013, pp [6] Yakut, K., Sahin, B. Flow-induced vibration analyi of conical ring ued of heat tranfer enhancement in heat exchanger. Appl. Energy, Vol. 78, 2004, pp [7] Tandiroglu, A. Effect of flow geometry parameter on tranient heat tranfer for turbulent flow in a circular tube with baffle inert, Int. J. Heat Ma Tranfer, Vol. 49, 2006, pp [8] Chokphoemphun, S., Pimarn, M., Thianpong, C., Promvonge, P. Heat tranfer augmentation in a circular tube with winglet vortex generator, Chin. J. Chem. Eng. Vol. 23, 2015, pp [9] Wang, L., Sunden, B. Performance comparion of ome tube inert, Int. Commun. Heat Ma Tranfer, Vol. 29, 2002, pp [10] Eiama-ard, S., Thianpong, C., Promvonge, P. Experimental invetigation of heat tranfer and flow friction in a circular tube fitted with regularly paced twited tape element, Int. Commun. Heat Ma Tranfer, Vol. 33, 2006, pp [11] Tamna, S., Kaewkohkiat, Y., Skullong, S., Promvonge, P. Heat tranfer enhancement in tubular heat exchanger with double V-ribbed twited-tape, Cae Stud. Therm. Eng. Vol. 7, 2016, pp [12] Eiama-ard, S. and Promvonge, P. Experimental invetigation of heat tranfer and friction characteritic in a circular tube fitted with V-nozzle turbulator, Int. Commun. Heat Ma Tranfer, Vol. 33, 2006, pp [13] Promvonge, P. Heat tranfer behavior in round tube with conical ring inert, Energy Conver. Manage., Vol. 49, 2008, pp [14] Tandiroglu, A. Effect of flow geometry parameter on tranient heat tranfer for turbulent flow in a circular tube with baffle inert, Int. J. Heat Ma Tranfer, Vol. 49, 2006, pp [15] Eiama-ard, S., Wongcharee, K., Eiama-ard, P., Thianpong, C. Thermohydraulic invetigation of turbulent flow through a round tube equipped with twited tape coniting of centre wing and alternate-axe, Exp. Therm. Fluid Sci. Vol. 34, 2010, pp [16] Wongcharee, K., Eiama-ard, S. Heat tranfer enhancement by twited tape with alternate-axe and triangular, rectangular and trapezoidal wing, Chem. Eng. Proce. Proce Intenif., Vol. 50, 2011, pp [17] Skullong, S., Promvonge, P., Thianpong, C., Pimarn, M. Heat tranfer and turbulent flow friction in a round tube with taggered-winglet perforated-tape, Int. J. Heat Ma Tranfer, Vol. 95, 2016, pp [18] Incropera, F.P., Witt, P.D., Bergman, T.L., Lavine, A.S. Fundamental of Heat and Ma Tranfer, John- Wiley & Son, [19] Skullong, S., Promvonge, P., Jayranaiwachira, N., Thianpong, C. Experimental and numerical heat tranfer invetigation in a tubular heat exchanger with delta-wing tape inert, Chem. Eng. Proce. Proce Intenif., Vol. 109, 2016, pp [20] Praopuk, C., Hoonpong, P., Skullong, S., Promvonge, P. Experimental invetigation of thermal performance enhancement in tubular heat exchanger fitted with rectangular-winglet-tape vortex generator, KKU ENGINEERING JOURNAL, Vol. 43(S2), 2016, pp [21] Eiama-ard, S., Promvonge, P. Influence of double-ided delta-wing tape inert with alternate-axe on flow and heat tranfer characteritic in a heat exchanger tube, Chin. J. Chem. Eng., Vol. 19(3), 2011, pp /Volume 4(2), 2016 Tranaction of the TSME: JRAME