Heat transfer intensification of turbulent forced convection by inserting discontinuous twisted tapes in a wavy tube; hydrothermal and thermodynamics analysis

Highlights

• Effect of simultaneous use of wavy tube and twisted tape on heat transfer intensification is numerically investigated.

• The best value of twisted tape contribution on enhanced heat transfer is about 79.55%, observed at Re = 4500, IA = 15${}^{\circ }$ and TA = 180${}^{\circ }$.

• The proposed configurations can carry thermal energy up to 24.6% more than a plain tube under identical energy consumption condition.

• The second law efficiency values improve as inclination and twist angles increase.

Abstract

The current study presents a numerical investigation on turbulent hydrothermal characteristics in a wavy passage equipped with discontinuous twisted tapes. The working fluid is water and passage walls are kept under constant wall temperature. The effect of Reynolds number (Re = 2500, 4500, 6500, 8500 and 10500), twist angle (TA = 90${}^{\circ }$ and 180${}^{\circ }$) and inclination angle (IA = 15${}^{\circ }$ and 30${}^{\circ }$) on velocity fields and streamlines, temperature contours, Nusselt number, friction factor, turbulent kinetic energy (TKE), thermal performance factor (TPF), energy efficiency factor (EEF) and entropy generation rate are evaluated. More uniform temperature and velocity fields can be found as both wavy passage and twisted tape are simultaneously applied. Both heat transfer rate and pumping power rise with increasing Reynolds number, twist and inclination angles. Results reveal that the twisted tape play more remarkable role on Nusselt number and friction factor at lower inclination angle. The best value of EEF is detected 24.6% occurring for passage with IA = 15${}^{\circ }$ and TA = 180${}^{\circ }$ at Re = 2500. Among all considered cases, those of empty wavy passage with IA = 30${}^{\circ }$ at Re = 8500 and 10500 provides negative values of EEF, referring to inefficient cases from energy viewpoint. The contribution of wavy passage and twisted tape on heat transfer enhancement is individually assessed and it is found that the best value of twisted tape involvement on enhanced heat transfer is 79.6% taking place for configuration with IA = 15${}^{\circ }$ and TA = 180${}^{\circ }$ at Re = 4500. Finally, based on CFD results, passages are examined from thermodynamics viewpoint and it is shown that the second law efficiency growths with increasing twist and inclination angles.

Introduction

Today, passive heat transfer enhancement methods have been attracted a great of attention because of not only the energy consumption reduction, but also for downsizing of cooling passages and heat exchangers. Swirling flows are considered as one of the passive methods which can improve heat transfer rate at expense of not remarkable pumping power increment. The most prevalent device for creating swirling flow is twisted tapes which have been investigated by many researchers both numerically and experimentally [1]. The effect of classical twisted tape on heat transfer enhancement has been reported in boiling processes [2], [3], [4], heat recovery systems [5,6], energy storage equipment [7], impinging jet cooling techniques [8], solar collectors [9] and mitigating fouling in tubular membranes [10].

Combined use of a twisted tape and several conical rings in a plain tube was proposed by Promvonge and Eiamsa-ard [11]. The simultaneous presence of twisted tapes and conical rings provided heat transfer rate of 4 to 10% higher than that with the conical rings alone. Mengna et al. [12] used a classical twisted tube in a converging-diverging tube to experimentally investigate the combined effect of these two passive heat transfer enhancement techniques on Nusselt number and friction factor. The results showed that thermal efficiency improves as Reynolds number increases.

Zimparov [13,14] proposed a simple mathematical model to predict heat transfer coefficient and friction factor for fully developed turbulent flow in a corrugated tube fitted with twisted tape. In this regard, several comparisons between calculated results and experimental data carried out and a good agreement between them was observed.

Effect of combination of dimpled tube and twisted tube on hydrothermal characteristics of turbulent flow was experimentally evaluated by Thianpong et al. [15]. It was also reported that thermal performance increases as pitch ratio of dimpled surfaces and twisted tape decrease.

Eiamsa-ard and Promvonge [16] carried out several experiments with configurations consisted of a serrated twisted tape with different depth ratios. Under similar conditions, it was found that typical twisted tape increases heat transfer rate up to 27% relative to plain tube while serrated twisted tape leads to a 72% heat transfer enhancement as compared with simple tube. Peripherally-cut twisted tape with alternate axis (PT-A), was applied in a uniform heat flux circular tube to promote the heat transfer rate [17]. Water was chosen as testing fluid and the Reynolds number varied in the range of 5000 to 20000. To make some comparisons, peripherally-cut twisted tape (PT) and typical twisted tape (TP) were also evaluated. The heat transfer rate of tube equipped with PT-A, PT and TP was respectively enhanced up to 184%, 102% and 54% than that of a simple tube.

Murugesan et al. [18] carried out several experiments to investigate thermal performance enhancement of a double heat exchanger equipped with twisted tape involving wire nails. Over the range considered, the presence of wire nails on twisted tape can increase heat transfer rate up to 30.

Guo et al. [19] numerically evaluated the effect of center-cleared and short-width twisted tape on hydrothermal characteristics. The short-width twisted tape shows a week thermal behavior as a result of cutting off the tape edge but an outstanding heat transfer can be observed for the center-cleared twisted tape with an appropriate central clearance ratio. More recently, Saysroy et al. [20] numerically introduced the rectangular-cut twisted tapes with alternate axes and reported that Nusselt number and friction factor decrease as the length and width of cut ratio increase.

Yongsiri et al. [21] presented an evaluation on thermal performance enhancement in a circular tube by using helically twisted tape with alternate axis. The simulations showed that the twisted tape with alternate axis presents higher heat transfer rate and thermal performance than those with typical helical twisted tape approximately 14% and 2%, respectively. The use of multiple twisted tape vortex generators with special arrangements may provide a more uniform temperature field as well as heat transfer enhancement [22], [23], [24], [25], [26].

A combination of helical ribs and double twisted tape was considered by Promvong et al. [27]. Based on experimental data, this configuration performs much better than the ribbed/smooth tube alone under an identical condition. Nanan el al. [28] reported that the use of perforated helical twisted tapes leads to the reduction of pressure drop as compared to that of typical helical twisted tape. Pal and Saha [29] shows that that twisted tapes in combination with integral spiral corrugation roughness present a better thermal performance than the individual enhancement technique. A clockwise-counterclockwise twisted tape with semicircle cuts was applied by Man et al. [30] to experimentally investigate its effect on heat transfer and friction factor characteristics in a tube. Several lengths of twisted tape were chosen and it was reported that the best Nusselt number caused by clockwise-counterclockwise twisted tape is 1.9 time of that plain tube.

Bhuiya et al. [31] experimentally studied the effects of perforated double counter twisted tape on heat transfer and fluid pressure drop characteristics in a heat exchanger tube. The results indicated that the heat transfer rate and pressure drop were obtained to be approximately 80 to 290% and 111 to 335% higher than those of the simple tube values.

Hassanpour et al. [32] used artificial neural networks on experimental data to optimizing the hydrothermal characteristics of a corrugated tube heat exchanger equipped with twisted tapes. The classical, perforated, V-cut and U-cut twisted tapes were applied in a turbulent region. The best heat transfer enhancement was related to the corrugated tube with V-cut twisted tape, while the maximum pressure drop occurred for perforated twisted tape.

Hong et al. [33] experimentally investigated turbulent flow and heat transfer characteristics in a simple tube equipped by overlapped multiple twisted tapes with counter large/small dimensions. The results discovered that Nusselt number and friction factor rise with increasing tape number and decreasing overlapped twisted ratio.

Saylroy and Eiamsa-ard [34] numerically studied the heat transfer augmentation by square cut twisted-tapes for a turbulent periodic flow in a tube. The main findings showed that the heat transfer and pressure drop increase with decreasing perforated width to tape width ratio and perforated length to tape width ratio.

Effect of square wings in multiple square perforated twisted tape on Nusselt number and friction factor was experimentally evaluated by Suri et al. [35]. It is observed that the maximum increment in Nusselt number and friction factor are 6.96 and 8.34 times of those of the plain circular tube, respectively. Recently, Liu et al. [36] proposed a new type of twisted tape, called coaxial cross twisted tape, to improve thermal performance. The numerical results, obtained in Reynolds number Range of 40-1050, showed that the Nusselt number of the tube fitted by coaxial cross twisted tape is 168-426% higher than that of an empty tube. This amazing enhancement can be attributed to this fact that vortexes, generated by the coaxial cross twisted, moves high-velocity region from flow center to the vicinity of tube wall. Zhang et al. [37] experimentally investigated thermal characteristics of a double-pipe heat exchanger fitted with perforated self-rotating twisted tapes with various perforation ratios. These authors showed that perforated self-rotating twisted tapes provide a greater thermal performance than perforated stationary twisted tapes. In addition, the increase in perforation ratios raised the Nusselt number and pressure drop.

Nakhchi and Esfahani [38] numerically evaluated performance of turbulent flow through a heat exchanger tube using double V-cut twisted tape inserts and concluded that the proposed configuration can improve heat transfer rate up to 117.4%. The benefits of different compound techniques including twisted tape inserts are experimentally investigated by Emani et al. [39] in non-circular passages for laminar flow regime and an increase in heat duty of 55–79% at constant pumping power and a 23–39% reduction in pumping power at constant heat duty were observed. The experimental study on thermal performance of a twisted tube equipped with twisted tape showed that show that at a given Re, the Nusselt number and friction factor of a twisted tube mounted with a twisted tape are consistently higher than those of the twisted tube alone and a smooth circular tube [40].

More recently, Yan et al. [41] designed an open spiral tape to obtain good heat transfer performance in a furnace tube. Compared to ordinary twisted tape, the open spiral tape can greatly reduce flow loss and improve thermal performance factor. The idea of inserting of twisted tape in helical tube was suggested by Liaw et al. [42]. Based on numerical results, it was found that adding twisted tape in helical tube improves the generation of secondary flow and thus thermal energy transport. Chang et al. [43] proposed an innovative twisted-tape pin-fin array as swirling generator for increasing heat transfer rate in rectangular channel. They reported that the twisted-tape pin-fin channel provided a higher degree of heat transfer at the same pressure drop penalty and pumping power, as compared with a simple pin-fin channel.

Kumar et al. [44] experimentally studied the effect of double V-cuts in a perforated twisted tape insert and reported that the maximum increment in Nusselt number and friction factor were found to be 3.5 and 9.63. Singh et al. [45] showed that thermal performance of an evacuated tube collector solar air can be increased up to 70.9% using helical coil inserts. Bahuguna et al. [46] carried out several experiments to evaluate the thermal performance of heat exchanger tube equipped by the multiple helical tape geometry. They found that there is 3.95 and 1.22 times enhancement in the heat transfer and thermal performance, respectively. The lanced ring insert was proposed by Kumar et al. [47] to enhancement heat transfer rate via local swirling flows. The experimental data showed that the enhancements in the Nusselt number and friction factor vary in the range of 1.69–4.1 and 16–72, respectively. Effect of triple blade vortex generator inserts on tube heat exchanger was investigated from Entropy generation view point by Bahuguna et al [48]. The results showed that the presence of vortex generator inserts is responsible of less entropy generation due to local swirling flows. Srivastava et al . [49] used diverging perforated cones to enhance the thermo-hydraulic performance of a heat exchanger tube . Based on experimental data, it is discovered that the maximum thermal performance can be reached to 1.38. The application of alternate perforated V-Notch (APVN) twisted tape in a double heat exchanger was experimentally evaluated by Dehankar et al. [50], and it was reported that Nusselt number, friction factor and thermal performance factor are, respectively, 1.833–2.276, 2.155–4.549 and 1.84–1.778 times more than the values of empty tube. Altun et al [51] conducted several experiments to investigate the twisted trapezoidal tape elements effect on heat transfer enhancement in tubes and showed that the best thermal performance of 1.164 is obtained with the twisted trapezoidal tape element with the maximum thread height.

Kalateh et al. [52] not only investigated the effect of various twisted tapes on hydrothermal performance but also optimized their geometrical parameters via response surface methodology. According the results, amidst the different inserts, the clockwise-counterclockwise and alternate axis twisted tapes ranked first from the highest thermal performance factor .

Hydrothermal characteristics of a U-tube equipped with twisted-tape inserts were experimentally investigated by Feizabadi et al. [53]. It was found that for the twisted U-tube fitted with the twisted-tape, the highest augmentations of 122.4% and 78.4% are recorded for Nusselt number and friction factor as compared to the smooth U-tube.

According to the best knowledge of authors, no study has been carried out to evaluate the effect of simultaneous use of wavy passage and twisted tape on turbulent forced convection. Moreover, knowing the fact that the presence of wavy passage can dramatically promote heat transfer rate in both tube and shell side of a double pipe heat exchanger, the present work is motivated to investigate the flow and heat transfer in a wavy passage equipped with twisted tape. In this regard, the flow pattern, temperature field, heat transfer rate, pumping power, thermal performance and energy efficiency factors are numerically evaluated for various Reynolds numbers, inclination and twist angles. In addition, the contribution of each technique, i.e. wavy passage and twisted tape, on heat transfer and pumping power is individually addressed. Finally, based on CFD results, some investigations are carried out from thermodynamics viewpoints using well-known terms such as entropy generation rate, exergy destruction and second law efficiency.