An experimental study on vortex-generator insert with different arrangements of delta-winglets

Energy xxx (2015) 1e11

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An experimental study on vortex-generator insert with different arrangements of delta-winglets M. Khoshvaght-Aliabadi*, O. Sartipzadeh, A. Alizadeh Department of Chemical Engineering, Shahrood Branch, Islamic Azad University, Shahrood 36199-43189, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 August 2014 Received in revised form 24 December 2014 Accepted 24 January 2015 Available online xxx

Heat transfer enhancement in a tube using the VG (vortex-generator) insert with different arrangements of the delta-winglets is investigated. Fourteen VG inserts with the longitudinal and forward arrangement of the delta-winglets are made from the aluminum sheets with a length of 350 mm, a width of 14.5 mm, and a thickness of 0.6 mm. The heat transfer and pressure drop results achieved from the use of the VG inserts inside the tube are compared with those obtained for the plain tube. It is found that at the transitional flow through the plain tube, Notter-Rouse equation predicts the current experimental Nusselt number better than Gnielinski equation. Also, the experimental results reveal that the use of the VG inserts inside the tube yields higher heat transfer coefficient and pressure drop than the plain tube, and these parameters augment with increasing the delta-winglets. The appropriate tradeoff between the enhanced heat transfer and the friction is found by using a special arrangement of the delta-winglets on the VG insert which presents the highest heat transfer coefficient as well as the maximum values of considered PEC (performance evaluation criterion). The maximum PEC of 1.41 is found for this VG insert at Re ¼ 8715. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Heat transfer enhancement Vortex-generator insert Delta-winglet Different arrangement Experimental

1. Introduction Numerous experimental and numerical studies have been conducted on HTE (heat transfer enhancement) technologies in order to save the energy and reduce the size and cost of heat exchange devices. One of the main categories of these technologies is called as passive technique. It means that there is no need of any kinds of external forces, for example: VG (vortex-generator) and turbulator device, rough and extended surfaces, and additives for liquids and gases [1]. The VGs enhance the transport phenomena by developing longitudinal, transverse, or normal swirl flows and destabilizing the flow field. It increases the fluid mixing, breakdowns the thermal boundary layer leading to an increase in the mean velocity and temperature gradient, and thereby enhances the convective heat transfer coefficient. The wing and winglet VGs with the triangular or/and rectangular shapes have been investigated for HTE in different heat exchange systems by many researchers, such as circular and non-circular ducts [2e7], plate-fin heat exchanger [8e14], tube-fin heat exchanger [15e21], heat sink [22,23],

* Corresponding author. Tel.: þ98 9151811311; fax: þ98 58147244818. E-mail address: [email protected] (M. Khoshvaght-Aliabadi).

electronic chip [24,25], and micro-and-mini channels [26]. However, there are very scarce studies which investigated the application of the VGs inside the circular tubes as inserts. Heat transfer and flow characteristics of a tube fitted with the transverse delta-winglet on the twisted-tape insert were investigated experimentally by Eiamsa-ard et al. [27]. Their results show that the mean Nusselt number and friction factor in the tube with the delta-winglet twisted tape increase with decreasing the twisted ratio and increasing the depth of the wing cut ratio. In another work [28], they described the effects of the twisted-tapes consisting of centre wings and alternate-axes on the thermohydraulic properties of the tube. It was concluded that the superior performance of this type of the inserts could be attributed to the combined effects of the following actions: (1) a common swirling flow by the twisted-tape (2) a vortex generated by the wing (3) a strong collision of the recombined streams behind each alternate point. Eiamsa-ard and Promvonge [29] studied the convective heat transfer and friction behaviors of the turbulent air flow through a straight tape with the transverse double-sided delta-wings. Their experimental results reveal that for using the delta-wings, the increases in the mean Nusselt number and friction factor are up to 165% and 14.8 times of the plain tube, and the maximum thermal performance factor is 1.19.

http://dx.doi.org/10.1016/j.energy.2015.01.072 0360-5442/© 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Khoshvaght-Aliabadi M, et al., An experimental study on vortex-generator insert with different arrangements of delta-winglets, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.01.072

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Deltaewing, rectangular-wing, delta-winglet, and rectangularwinglet are four basic shapes of the VGs [30]. Based on the literature, the VG inserts with different arrangements of the deltawinglets have been not investigated in the past. In the present investigation, experiment are carried out to determine the heat transfer enhancement and flow pressure drop of a circular and straight tube fitted with the VG inserts fabricated with different arrangements of the delta-winglets. A performance evaluation criterion is used to evaluate the potential application of the proposed VGs as passive heat transfer enhancers in the tubular heat exchangers.

1005G, MBLD) to control the cooling fluid flow rate, i.e. tap water. The main flow line which is a straight copper tube has an internal diameter of 15 mm and an external diameter of 17 mm. The heat transfer part of the tube is 350 mm, and the entry part is 1000 mm to accomplish the fully developed flow at the entrance of the inserts. As presented in Fig. 1, the entry part is well insulated in order to eliminate the heat transfer with the ambient. Also, the constant temperature bath system is carefully insulated with 20 mm thick of glass-wool.

2. Experimental setup 3. VG (Vortex-generator) inserts Fig. 1 shows the fabricated close loop setup in the present study. As described in the figure, it consists of, I. Transmission fluid state with a stainless-steel tank to store the working fluid, a centrifugal pump (PKm60, Pedrollo) to drive the working fluid, a by-pass loop with two ball valves to adjust the flow rate, and a pressure relief valve to protect the equipment. II. Measuring instruments with an ultrasonic flow meter (Flownetix®-100series™) to measure the volumetric flow rate, two PT-100 thermocouples to measure the inlet & outlet bulk temperatures, five K-type thermocouples to measure the test section surface temperature, and two pressure transmitters (PSCH00.2BCIA, Sensys) to measure the pressure drop. III. Monitoring system with three indicators (MT4W, Autonics) to monitor the flow rate and local pressures, a temperature logger (SU-105PRR, Samwon) to monitor the bulk temperatures, and a temperature logger (SU-105KRR, Samwon) to monitor the surface temperatures. IV. Constant temperature bath system with a stainless-steel two-phase chamber, two 2 kW electrical heaters to boil the two-phase chamber fluid, a protection pressure valve to maintain safety, and a tubular level meter. V. Cooling unit with a double-tube heat exchanger for primary cooling, a plate-fin heat exchanger (B3-014C-12-3.0-H, Danfuss) for supplementary cooling, and a rotameter (LZT-

The VGs may be divided into the longitudinal and transverse shapes according to their rotating axis direction [31]. In general, the longitudinal VGs have been reported to be more effective than the transverse ones on the heat transfer enhancement [4]. It has been also reported that the heat transfer rate and thermal performance of the forward arrangement of the VGs are higher than those of the backward one [29]. Therefore, fourteen VG inserts with the longitudinal and forward arrangement of the deltawinglets are made from the aluminum sheets with a length of 350 mm, a width of 14.5 mm, and a thickness of 0.6 mm. To study the effect of the delta-winglets arrangement on the VG insert at comparable conditions, all the inserts have the same lengthwidth-thickness and similar dimension for the delta-winglets, as shown in Fig. 2(a). The specific geometrical parameters of the delta-winglets are presented in Fig. 2(b). In the present study, the delta-winglets arrangement on the VG insert are defined in the four classes as follows, o o o o

Class Class Class Class

1: 2: 3: 4:

One side cut with one delta-winglet One side cut with two delta-winglets Two side cut with two delta-winglets Two side cut with four delta-winglets

Based on the above classification, 2, 2, 5, and 5 arrangements of the delta-winglets on the VG insert are considered for the Class 1 to 4, respectively, see Fig. 2(c).

Fig. 1. Fabricated experimental close loop setup.

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Fig. 2. (a) A graphical representation of produced vortex-generator inserts (b) Specific geometrical parameters of vortex-generators (c) Classification of delta-winglets arrangements.

Please cite this article in press as: Khoshvaght-Aliabadi M, et al., An experimental study on vortex-generator insert with different arrangements of delta-winglets, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.01.072

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The performance of the inserts is evaluated relatively to the plain tube in term of a PEC (performance evaluation criterion) which can be expressed as [33],

4. Experimental procedure The experimental procedure is as follows, I. Firstly, to validate the current experimental results and create a base line, the plain tube is selected as test section. II. The constant temperature bath system is prepared. III. The reservoir is filled with the water, as working fluid. IV. The pump is switched on, and the flow rate of the working fluid through the loop is set at 2.0 L/min. V. The outlet temperature of the working fluid is restored at the constant value (30  C) by using the cooling system. VI. After the steady-state condition, the required data are recorded. VII. The flow rate through the loop is increased to 2.5 L/min and the steps (V) and (VI) are repeated. VIII. The step (VII) is repeated up to 5.0 L/min. IX. An insert is fitted inside the straight tube, and all the steps are repeated. It should be noted that all the factors are measured six times, and the most centralized four of them are chosen to calculate the average values used in the data reduction section.

h¼

ðNuE =NuNE Þ

(7)

ðfE =fNE Þ1=3

where, NuE, fE, NuNE, and fNE are the Nusselt numbers and friction factors of the enhanced (tube fitted with a VG insert) and nonenhanced (plain tube) conditions, respectively. This criterion is a ‘the larger the better’ parameter, and a high value of that indicates a delta-winglets arrangement with appropriate performance. 6. Uncertainty analysis A detailed systematical uncertainty analysis is carried out to estimate the errors associated with the experiments by using the following equation [34],

2 dR ¼ 4

M X

vR dX vXj j

j¼1

!2 31=2 5

(8)

The equation of the convective heat transfer rate is used to compute the heat transfer coefficient,

where, j, M, dR, and dXj are the specific parameter counter, number of the independent variables, uncertainties associated with the dependent, R, and independent, Xj, variables. The uncertainty values for different instruments used in the experiments are given in Table 1. Also, the maximum possible error for the parameters involved in the analysis are estimated and summarized in Table 2.


 Qconv ¼ mCP Tb;out  Tb;in

7. Results and discussion

5. Data reduction

(1)

where, m, CP, Tb,out and Tb,in represent the mass flow rate, specific heat capacity, inlet and outlet bulk temperatures of the working fluid, respectively. The effective heat transfer coefficient is estimated from the ratio of the convective heat transfer rate to the total surface area and logarithmic mean temperature difference of the wall-and-bulk fluid,

h¼

Qconv AðTw  Tb ÞLMTD

ðTw  Tb ÞLMTD

DTwb;in  DTwb;out
  ¼ log DTwb;in DTwb;out

(2)

hDh k

(3)

(4)

The pressure drop is estimated from the experimental observations and theoretical formula as given below,

DP ¼ Pin  Pout

(5)

To appraise the hydraulic performance, the Fanning friction factor is estimated from the pressure drop values by using the following equation [32],

2rDh DP f ¼ LG2 where, G is the mass velocity.

Experiments are initially conducted inside the plain tube to verify the facility reliability and check the results consistency. Experimental results including Nusselt number and friction factor are compared with those of the single phase fluid correlations in open literature, (a) Equation of Gnielinski [35]

 

where, DTw-b,in and DTw-b,out denote the differences between the wall temperature and the bulk fluid temperature at the inlet and outlet of the heat transfer section. Also, the average Nusselt number is defined as,

Nu ¼

7.1. Validation test

Nu ¼

f 2

ðRe  1000ÞPr

 1=2 
2=3 1 þ 12:7 2f 1 Pr

(9)

where,

f ¼ ð1:58 ln Re  3:82Þ2

(10)

2300 < Re < 5  106 (b) Equation of Notter-Rouse [36]

Nu ¼ 5 þ 0:015Re0:856 Pr0:347

(11)

(c) Equation of Petukov [37]

f ¼ ð0:79 ln Re  1:64Þ2

(12)

3000 < Re < 5  106 (6)

Comparison of the values calculated through Eqs. (9)e(12) and their deviations with the current experimental results are presented in Fig. 3. The comparison shows a good agreement between

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Table 1 Uncertainties of experimental instruments. Name of instrument

Range of instrument

Variable measured

Least division in measuring instrument

Min. and max. values measured in experiments

Uncertainty

Flow meter Thermocouple Thermocouple Transmitter

0e20 L min1 50e200  C 73e260  C 0e20,000 Pa

Volumetric flow rate Bulk temperature Wall temperature Local pressure

0.01 L min1 0.1  C 0.1  C 10 Pa

2e5 L min1 30e48  C 48e65  C 125e1700 Pa

0.200% 0.208% 0.153% 0.580%

the present experimental results and the previous empirical data, and it lies within ±10% error. It is found that Notter-Rouse equation [36], Eq. (11), predicts the current experimental Nusselt values better than Gnielinski equation [35], Eq. (9). Also, this figure shows a small percentage error in term of the friction factor at the lower Reynolds numbers, while a good fitting is obtained at the higher Reynolds numbers. The present plain tube results for Nusselt number and friction factor are correlated as follows,

Nu ¼ 0:00599Re0:811 Pr1:104

(13)

f ¼ 1:0123Re0:371

(14)

3485 < Re < 8715 According to the above correlations, the mean deviation between the actual Nusselt number and Eq. (13) is 4.2%, and the mean deviation between the actual friction factor and Eq. (14) is 1.6%. 7.2. Heat transfer and pressure drop results In this section, the heat transfer and pressure drop characteristics of the water flow inside the tube equipped with the VG insert at different arrangements of the delta-winglet for the volumetric flow rate between 2.0 and 5.0 L/min are described. The results of the plain tube are also reported for comparisons. The effect of the delta-winglets arrangement on the heat transfer enhancement of the VG insert is shown in Fig. 4(aed) in term of the heat transfer coefficient versus the volumetric flow rate. For all classes, the heat transfer coefficient considerably enhances with increasing the flow rate. This enhancement is responsible by a decrease of the thermal boundary thickness due to the promoted turbulent intensity. Obviously under similar conditions, the heat transfer coefficient of the tube fitted with the VG inserts is higher than that of the plain tube, and its difference enhances with increasing the flow rate. In general, the delta-winglets on the insert increase the flow mixing area and turbulence intensity, interrupt the thermal boundary layer development, and generate the swirl flows or vortices which are favorable for the increase of the heat transfer coefficient. The potential of the heat transfer enhancement for the VG inserts depends on the location and strength of the generated swirl flows relative to the delta-winglets and hot walls. As presented in Fig. 4(a), the Type 1 shows higher values of the heat transfer coefficient in comparison with the Type 2. Likewise as presented in Fig. 4(b), the heat transfer coefficient curve of the Type 3 places between those of the plain and Type 4 at the flow rates

lower than 4.0 L/min, but at the higher flow rates, the heat transfer coefficient of the Type 3 is found to be higher than that of the Type 4. It can be also seen that the heat transfer coefficient values for Class 2 are slightly higher than those of Class 1. This is attributed to the additional delta-winglets in the Class 2. A comparison between Fig. 4(b) and (c) clears that the VG inserts of the one side cut with the two delta-winglets (Class 2) and the two side cut with the two delta-winglets (Class 3) have close values of the heat transfer coefficient. However, in the Class 3, the Type 7 presents the highest values of the heat transfer coefficient, and the Type 8 comes in the second. Based on Fig. 2(c), it can be clarified that the opposite arrangements of the delta-winglets (Types 7 and 8) causes higher values of the heat transfer coefficient in comparison with the unilateral arrangements (Types 6 and 9). It is interesting to note that at the studied range, the heat transfer coefficient of the Types 5, 6, and 9 is lower than that of the Types 3 and 4. It illustrates that only some arrangements of the two side cut of the delta-winglets are effective than the one side cut of the delta-winglets, when the number of the delta-winglets is the same. It is worth to state that the heat transfer coefficient of the tube equipped with the inserts in the Class 4 is considerably improved when compared with that of the plain tube and tube equipped with the other classes. This is responsible by increasing the number of the delta-winglets and thereby formation more numbers of the swirl flows. As shown in Fig. 4(d), while the Type 11 has considerable values of the heat transfer coefficient at the lower flow rates, its heat transfer coefficient is not significant in comparison with the other types at the higher flow rates. In addition, the maximum heat transfer coefficient values at the lower and higher flow rates are obtained for the Type 14 and Type 12, respectively. In fact, the delta-winglets arrangement based on the Type 12 increases more significant the

Table 2 Maximum possible error of experimental parameters. Parameter name

Uncertainty error

Convective heat transfer coefficient Nusselt number Friction factor PEC

1.31% 1.53% 2.42% 2.86%

Fig. 3. Comparison between present experimental data and previous empirical correlations.

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Fig. 4. Effect of delta-winglets arrangement on heat transfer coefficient: (a) Class 1 (b) Class 2 (c) Class 3 (d) Class 4.

turbulence intensity and interrupts more effective the thermal boundary layer at the higher flow rates. It can be concluded that in addition to Reynolds number, the variation of the delta-winglets arrangement affects the location and strength of the longitudinal, transverse, and normal swirl flows for the tube fitted with the VG inserts. Also, the VG inserts with the maximum number of the delta-winglets, i.e. Class 4, lead to a significant exchange between the core and the wall fluids. The mentioned mechanism causes higher values of the heat transfer coefficient for these types of the VG inserts. The pressure drop results are demonstrated in Fig. 5(aed). Trend found for all classes is that the pressure drop considerably increases with increasing the flow rate, and apparently, the fitted tube presents higher values compared to the plain one. The dissipation of the fluid dynamic pressure at high viscosity loss near the tube wall and the interaction of the pressure forces with the inertial forces in the boundary layer due to the swirl flows generated by the VG inserts are the main reason for the pressure drop enhancement of the fitted tube [38]. It is found that for the considered range, different arrangements of the delta-winglets in the Class 1 and Class 2 exhibit slight variations in the pressure drop. This signifies the promising characteristic of the delta-winglets arrangement with the higher enhancement in the heat transfer. However, the pressure drop increases with increasing the number of the delta-winglets on the VG insert; the pressure drop of the Class 4 is the highest, and

that of the Class 3 and Class 2 takes the second place, while that of the Class 1 is the lowest. This can be explained by the fact that for the classes with higher delta-winglets, the change of the flow pattern leads to the increase of the flow resistance and thereby greater pressure drop than that induced by the classes with lower delta-wings. In the other words, this can be caused by the increase of shear force on the walls acted by the higher numbers of swirling flows in the tube for the VG inserts with the higher number of the delta-winglets. It can be seen that the Type 8 offers the highest values of the pressure drop for the Class 3. Also, the Type 11 and Type 10 have the highest values of the pressure drop for the Class 4, respectively. This is attributed to the low distance between two opposite delta-winglets in these arrangements of the VG inserts. To better understand the effects of the flow rate and deltawinglets arrangement on the thermal-hydraulic performance of the VG inserts, the ratios of Nusselt number and friction factor of the tube fitted with different VG inserts to those of the plain tube are also plotted in Fig. 6(aeb) as function of Reynolds number. The subscripts “E” and “NE” refer to the enhanced condition and nonenhanced condition, respectively. For a comprehensive insight, the results of all classes are displayed in the same graphs. Obviously, Nusselt number and friction factor of the tube fitted with the VG inserts are higher than that the plain tube. Although Nusselt number of each type increases with increasing Reynolds number, the Nusselt numberE/Nusselt numberNE ratio is different with

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Fig. 5. Effect of delta-winglets arrangement on pressure drop: (a) Class 1 (b) Class 2 (c) Class 3 (d) Class 4.

increasing Reynolds number. It is attributed to the difference between the thermal boundary layer thickness of each fitted tube and that of the plain tube. The friction factor and Friction factorE/Friction factorNE ratio obtained from the fitted tube with different VG inserts are almost in a similar trend and tend to decrease with the rise in Reynolds number. It is in coincident with the results obtained by Eiamsa-ard et al. [39] for the alternate twisted-tapes inserts. Under a similar operating condition, the increase in friction factor for the tube fitted with the Class 4, in general, is much higher than the other classes and plain tube. This is because of higher flow blockage from the delta-wings. Finally, the mean deviations of Nusselt number and friction factor values between the fitted tube and the plain one are summarized in Table 3, according to the following definition,

Mean diviation ð%Þ ¼

1 N

  X fE  fNE     100%  f 

(15)

NE

where, N is the number of the tested flow rate or Reynolds number. As displayed in the table, Nusselt number and friction factor of the tube fitted with the VG inserts are increased, respectively, from 9.3 to 41.5% and 9.7e78.6% as compared to those in the plain tube.

7.3. Performance evaluation criterion results The defined PEC in Eq. (7) is used to evaluate the potential of the tested VG inserts for the practical applications. The evaluation is considered under the constant pumping power for each insert with respect to the case without insert, i.e. plain tube. Obviously, when the PEC is higher than unity, it indicates that the applied insert is more in the favor of the heat transfer enhancement rather than in the favor of the pressure drop increasing. The variations of the PEC with Reynolds number for different inserts are illustrated in Fig. 7(aed). Except at the minimum Reynolds number, the PEC values of the fitted tube are higher than unity. Therefore, it can be stated that for the range investigated, the benefits from the heat transfer enhancement as a positive effect over that from the increase of friction loss as a negative effect. As shown, the PEC values of all classes almost increase, as Reynolds number increases. This implies that the use of the proposed inserts is feasible in terms of the energy saving at the higher Reynolds numbers. The average and maximum enhancement values of the PEC for the VG inserts with different arrangements of the delta-winglets are presented in Table 4. From the tabulated results, the Type 12 shows a significant average and maximum enhancements in the PEC, and the Type

Please cite this article in press as: Khoshvaght-Aliabadi M, et al., An experimental study on vortex-generator insert with different arrangements of delta-winglets, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.01.072

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Fig. 6. (a) (Nusselt number)E/(Nusselt number)NE versus Reynolds number (b) (Friction factor)E/(Friction factor)NE versus Reynolds number for different classes of delta-winglets arrangements.

14 comes in the second. This suggests that the delta-winglets arrangement presented in the Type 12 and Type 14 is more appropriate for the practical use than the others in the view point of energy as well as operating cost savings. Finally, for the range considered, the maximum PEC of 1.41 is found with the use of the Type 12 as VG insert at the maximum Reynolds number.

7.4. Comparison between VG and other types of inserts The TT (twisted-tape) insert is recognized as a well-known type of the fluid turbulators, i.e. inserts. In the recent years, different shapes of the TT inserts were proposed, for instance twin [33], perforated [40], jagged [41], alternate [39], and non-uniform [42]. A

Table 3 Mean deviations of Nusselt number and friction factor between fitted tube with different VG inserts and plain one (%). Type no.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Nusselt number Friction factor

16.8 12.9

9.3 9.7

17.6 21.1

19.6 22.7

12.7 31.3

10.2 17.2

20.4 21.2

18.1 36.6

9.8 17.1

21.4 57.3

32.6 78.6

41.5 35.2

27.6 36.9

41.9 41.8

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Fig. 7. Effect of delta-winglets arrangement on performance evaluation criterion: (a) Class 1 (b) Class 2 (c) Class 3 (d) Class 4.

8. Conclusion

comparison of the PEC values between the obtained results for the VG inserts (Class 4) in the present work and those from the previous works for the TT inserts at the range of the present work is shown in Fig. 8. As shown in the figure, at the studied range, the PEC value of different TT inserts decreases with the rise in Reynolds number. It is in coincident with the present results for the Type 11. It is attributed to the high pressure drop values of this type at the higher Reynolds numbers, see Fig. 5(d). Also, the PEC values of the Type 10 are slightly higher than those of the perforated TT and lower than those of the jagged TT. However, the PEC value of the Type 12 and Type 14 gives higher and higher values, as Reynolds number increases. It can be concluded that the main advantages of the VG inserts are their simple fabrication and considerable performance, particularly at the higher Reynolds numbers, in comparison with the other types of the inserts.

The effects of different arrangements of the delta-winglets on the heat transfer and pressure drop characteristics of the tube equipped with the VG inserts are experimentally studied in a transition flow using water, as a testing fluid, under constant temperature condition. According to the obtained results, the conclusions can be drawn as follows, ➢ Under the similar conditions, the heat transfer rate, pressure drop as well as PEC in a tube fitted with the VG inserts are consistently higher than those in the plain tube. ➢ It is found that the VG inserts of two side cut with four deltawinglets perform a better heat transfer rate than the other arrangements of the delta-winglets.

Table 4 Average and maximum enhancement values of PEC for different inserts compared to plain tube (%). Type no.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Average enhancement Maximum enhancement

14.2 20.2

6.9 14.9

11.1 19.3

12.4 17.1

3.3 7.4

4.8 8.9

13.5 21.6

6.7 11.7

4.3 7.8

4.5 8.2

9.4 12.9

28.5 40.9

15.1 20.7

26.7 35.5

Please cite this article in press as: Khoshvaght-Aliabadi M, et al., An experimental study on vortex-generator insert with different arrangements of delta-winglets, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.01.072

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Fig. 8. Comparison of PEC values between obtained results for VG inserts (Class 4) in present work and those from previous works for TT inserts.

➢ For all classes, the PEC values are apparently above unity and increase with the increase of Reynolds number. This indicates the beneficial effect for the energy saving by the uses of the proposed VG inserts, especially at the high Reynolds numbers. ➢ For the range considered, the maximum thermal performance factor of 1.41 is found. Finally, the results obtained from this experimental attempt demonstrate the ability of the studied VG inserts as heat transfer enhancers. Although the VG inserts developed in this research are tested inside a single tube, they still have great potential for use in a variety of different applications of heat exchangers. Acknowledgments The authors would like to acknowledge with appreciation to Islamic Azad University (IAU), Shahrood Branch for their supports through the set-up fabrication and research implementation. Nomenclature A Dh Cp G h L M m N Qconv T P DP R DT X

total surface area in contact with working fluid, m2 hydraulic diameter of tube, m specific heat capacity, J kg1 K1 mass velocity, kg m2 s1 heat transfer coefficient W m2 K1 tube length, m number of the independent variables mass flow rate, kg s1 number of flow rate convective heat transfer rate, W temperature, K pressure, Pa pressure drop, Pa dependent variable temperature difference, K independent variables

Greek symbols r density, kg m3 m dynamic viscosity, Pa s

k h

thermal conductivity, W m1 K1 performance evaluation criterion

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Please cite this article in press as: Khoshvaght-Aliabadi M, et al., An experimental study on vortex-generator insert with different arrangements of delta-winglets, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.01.072