1、A Comparative Study of the Airside Performance of Winglet Vortex Generator and Wavy Fin-and-Tube Heat Exchangers C.C. Wang, Ph.D. Y.J. Chang Member ASHRAE C.S. Wei ABSTRACT This study presents airside performance of the delta winglet vortexgenerator (VG) vs. a wavvfn surface in both dv and wet condi
2、tions. TheJin pitch of the test samples is 1.7 mm and the number of tube rows is 2 and 4, respectively. For the airside performance tested in dry condition, the heat transfer coeficient for the wavyJin surface is only 6% higher than that of the winglet VG at N = 2, but thepressure drop is about 15%
3、higher: Conversely, the heat transfer coeficients for the winglet surface are higher than those of the wavyJin surface by approximately 5% with comparable pressure drop at N = 4. For the airside performance tested in wet condition, the heat transferperformance of the winglet VG is superior to that o
4、f the wavy surface for both N = 2 and N = 4. Furthermore, the pressure drop for winglet VG is considerably lower (15-40%). It is likely that this phenomenon is due to better condensate drainage caused by the swirled motion of the VG surface. INTRODUCTION Extended surfaces or fins are employed in hea
5、t exchang- ers for effectively improving the overall heat transfer perfor- mance. There is extensive literature on this subject for applications in connection to compact heat exchangers. The most common enhanced surfaces are the interrupted surfaces in the form of slit and louver. A recent review ar
6、ticle by Wang (2000) clearly identified the progress of patents relevant to the enhanced surfaces. Of the 50 patents surveyed, 90% of them are related to the interrupted surfaces. Generally, the associ- ated pressure drops of the interrupted fin surface are tremen- dous, irrespective of their signif
7、icant improvement of the heat transfer performance. However, a recent design called a vortex B.C. Yang Member ASHRAE generator can improve the heat transfer performance without pronounced increase in pressure drop. The vortex generator provides the swirl motion in which additional transverse velocit
8、y components do not directly contribute to the increase of pressure drop as that of longitudinal velocity gradient. As a consequence, the heat transfer performance is improved with only a moderate increase of pressure drop (Jacobi and Shah 1995). There are various types of vortex generators used in
9、aerodynamic application (wedge, plough, ramp, scoop, dome, wheeler, wing type, and wave element ESDU 19931). For applications to compact heat exchangers, most of the previous research has been related to the delta-winglet vortex generator, such as that done by Fiebig (1998) and his Co-workers. Recen
10、tly Wang et al. (2002a, 2002b) conducted flow visual- ization experiments of the vortex generator having winglet configurations in enlarged fin-and-tube heat exchangers for both in-line and staggered arrangements. Their flow visual- ization results have shown that the use of the vortex generator in
11、a fin-and-tube heat exchanger is promising. The purpose of this study is full-scale testing of vortex generators in fin-and- tube heat exchangers to examine their feasibility during prac- tical application. EXPERIMENTAL APPARATUS The test samples consisted of two vortex generators having two and fou
12、r rows, respectively, with a tube diameter before expansion of 7.94 mm. The corresponding longitudinal and transverse tube pitch is 19.05 and 25.4 mm and the fin pitch is 1.7 111111. The fin thickness is O. 15 mm. For comparison purposes, two wavy fin-and-tube heat exchangers with iden- tical tube r
13、ow and fin pitch were made. Detailed dimensions of the fin patterns for both wavy and vortex generator are given C.C. Wang, Y.J. Chang, C.S. Wei, and B.C. Yang are with the Energy Wang et al. 2001). The test conditions approximate those encountered with typical air-conditioning applications. The ene
14、rgy balance between the airside and tube side was within 3% for both dry and wet tests. RESULTS AND DISCUSSION Figure 3 presents the test results for the winglet VG and wavy fin surface in dry condition. Results are presented as heat transfer coefficients and pressure drops vs. frontal velocities ra
15、nging from 0.7 to 2.0 ms. As expected, both heat transfer coefficients and the pressure drops increased with the frontal 54 ASHRAE Transactions: Research velocity. For the dry test condition at N = 2, the heat transfer coefficients for the wavy fin exceed those of the VG surface by approximately 3-5
16、%, but the accompanying AP is 15% higher. However, the heat transfer performance is reversed for N = 4. As shown in Figure 3, the heat transfer coefficients for VG surfaces are about 10% higher than for the wavy fin surface, and the corresponding pressure drops are marginally lower than with the wav
17、y fin surface. As is well known, the presence of round tube may induce longitudinal horseshoe 0.6 1 1.6 2 2.6 VJmM Figure 3 Airside performance for wavy and winglet VG surface in dry conditions. vortex that will spiral around the tube periphery and toward the downstream. The horseshoe vortices play
18、an important role in heat transfer augmentation by providing better mixing of the core airflow with air adjacent to the fin surface. For a smaller number of tube rows, such as N = 2, the presence of an addi- tional vortex generator (winglet VG) may not be so effective. In this case the heat transfer
19、 coefficient at the entrance of the fin-and-tube heat exchanger is already high and the horseshoe vortex generated by the tube row only improves the heat trans- fer performance thereafter. Furthermore, the heat transfer coefficient can be slightly improved by the present wavy fin geometry. The wavy
20、height of this study is 1.18 mm and the corrugation angle is only 13.9 degrees; based on previous studies by Wang et al. (1999) and Ramadhyani (1986), appre- ciable heat transfer augmentation can only be obtained at a corrugation angle larger than 20“. As a consequence, one can see the heat transfer
21、 coefficient with vortex generators is slightly lower than that of the wavy fin surface at N = 2. Never- theless, one can see the pressure drop for the VG surface is roughly 15% lower than the wavy fin surface. This is probably due to (1) removal ofthe ineffective secondary flow behind the tube row
22、with the presence of the vortex generator (see Figure 4d about the flow pattern of the winglet VG) and (2) the high pressure drop caused by the wavy fin surface. For the flow field inside a wavy and plain channel, Wang et al. (2003) conducted an experiment of flow visualization via the injected dye
23、technique. At a corrugation angle of 15“ and Re 500, they reported an unsteady swing of flow field after the third corrugation. In addition, the unsteady dye streak shows a slightly swirled motion where the axis of the rotation is some- what perpendicular to the flow direction. This may eventually l
24、ead to an increase of pressure drop of the wavy channel. Conversely, the heat transfer coefficients for the winglet VG at N= 4 are about 3-5% higher than those of the wavy fin Figure 4 (a) Horseshoe vortex at thejrst row forplainjin patternjn surface. (5) Ineffective secondaryjow behind tube row. (c
25、) Flowjeld with the presence of winglet VG. (d) Flowjeld behind the tube row with the presence of winglet VG. ASHRAE Transactions: Research 55 Figure 5 Airside performance for wavy and VG surface in wet conditions. geometry. Explanations of this phenomenon are twofold. First, the strength of the ind
26、uced horseshoe vortex by the pres- ence of tube row is reduced considerably with the increase of tube row. This phenomenon can be made clear from a flow visualization experiment in a scale-up model carried out by Wang et al. (2002b). As seen in Figures 4a and 4b, although the horseshoe vortex is cle
27、arly seen in the first row at a Reynolds number of 500, it quickly loses its strength as the swirled flow meets the subsequent tube row (N = 3). The flow may be evenly separated before hitting tube row 3. Their visual results substantiate the loss swirled strength at the downstream row. On the other
28、 hand, the winglet VG still preserves certain swirled momentum at the downstream tube (see Figure 4c). Therefore, the pressure drops of the winglet VG relative to the wavy fin are comparably increased. Moreover, the ineffective area behind the tube row can be significantly improved by the winglet VG
29、 as schematically shown in Figure 4d relative to the ineffective area behind the tube for continuous fin (see Figure 4a). Notice that the ineffective secondary flow behind the tube not only contributes to the decrease of the heat transfer coef- ficient but also to an increase in the corresponding pr
30、essure drops. As a result, the heat transfer coefficient for winglet VG is higher, but it shows a comparable (or slightly lower) pres- sure drop than that of the wavy fin. In addition to the performance conducted in dry condi- tions, tests are also performed in wet conditions in which both heat and
31、mass transfer take place simultaneously. Unlike those shown in dry conditions, the heat transfer coefficients of the winglet VG are higher than those of the wavy fin irrespective ofN=2 orN=4. ForN=2, theratios ofhvC/h,yare approx- imately 1.02-1.04 and the ratios are increased to approxi- mately 1.0
32、5-1 .O9 for N = 4. In the meantime, the reduction of pressure drop relative to wavy fin geometry ranges from 15-40% for N= 2 and from 5 - 30% for N= 4. Apparently, the airside performance of the winglet VG relative to that of a wavy fin surface in wet conditions is better than in dry condi- tions. N
33、ote that the most crucial difference between dry and wet conditions is the presence of water condensate. For fin- and-tube heat exchangers under dehumidifying conditions, the water condensate may block the fin if the fin spacing is sufficiently close. As a result, pronounced pressure drops as well a
34、s lower heat transfer coefficients may be encountered. With the winglet VG, it is likely that the airflow may spiral around the fin surface, which may help to remove the adhered condensate on the surface, thereby causing better condensate drainage. As a consequence, the pressure drops for vortex gen
35、erators are considerably lower than that of the wavy fin surface. With better condensate drainage, the heat transfer performance is also improved. CONCLUSIONS This study presents airside performance of the winglet vortex generator and wavy fin surface in both dry and wet conditions. The correspondin
36、g fin pitch of the test samples is 1.7 mm and the number of tube rows are two and four, respec- tively. Major conclusions of this study are summarized as follows: 1. For the airside performance tested in a dry condition, the heat transfer coefficient for the wavy fin surface is slightly higher than
37、that of winglet VG at N= 2 but the pressure drop is about 15% higher. Conversely, the heat transfer coeffi- cients for the winglet surface exceed those of the wavy fin surface by approximately 5% with comparable or slightly lower pressure drops at N = 4. The airside performance of the present wingle
38、t VG surface relative to the wavy fin surface increased with the number of tube rows. For the airside performance tested in wet conditions, the heat transfer coefficients for winglet surface are superior to those of the wavy fin surface. Furthermore, the pressure drop for the winglet VG is considera
39、bly lower than that of the wavy fin surface. It is likely that this phenomenon is due to the better condensate drainage by the swirled motion. 2. 3. ACKNOWLEDGMENTS The authors would like to express gratitude for the Energy R&D Foundation funding from the Energy Commission of the Ministry of Economi
40、c, Taiwan. NOMENCLATURE ho = air-side heat transfer coefficient dp = pressuredrop N = the number of tube rows 56 ASHRAE Transactions: Research Re = Reynoldsnumber V+ = frontalvelocity Subscripts VG = vortexgenerator wavy = wavyfin REFERENCES ASHRAE. 1993. 1993 ASHRAE Handbook-Fundamen- tals, SI edit
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