- Email: [email protected]
Heat Transfer and Friction Factor Studies in a Circular Tube Fitted with Twisted Tape Consisting of Wire-nails
RESEARCH NOTES Chinese Journal of Chemical Engineering, 18(6) 1038—1042 (2010)
Heat Transfer and Friction Factor Studies in a Circular Tube Fitted with Twisted Tape Consisting of Wire-nails P. Murugesan1,*, K. Mayilsamy2 and S. Suresh3 1 2 3
Department of Mechanical Engineering, K.S. Rangasamy College of Technology, Tiruchengode-637215, India Department of Mechanical Engineering, Institute of Road and Transport Technology, Erode-638316, India Department of Mechanical Engineering, National Institute of Technology, Tiruchirappalli-620015, India
Abstract This paper deals with the experimental investigation on Nusselt number, friction factor and thermal enhancement factor of a double pipe heat exchanger equipped with twisted tape consisting wire nails (WN-TT) and plain twisted tapes (P-TT) with three different twist ratios of y = 2.0, 4.4 and 6.0. Test runs are conducted using the water as the working fluid with Reynolds number range between 2000 and 12000 for WN-TT and P-TT. It is found that Nusselt number, friction factor and thermal enhancement factor in the tube equipped with WN-TT appreciably higher than those in the tube fitted with P-TT and plain tube. Over the range considered Nusselt number, friction factor and thermal enhancement factor in a tube with WN-TT are respectively, 1.08 to 1.31, 1.1 to 1.75 and 1.05 to 1.13 times of those in tube with P-TT. The better performance of WN-TT is due to combined effects of the following factors: (1) common swirling flow generated by P-TT, (2) additional turbulence offered by the wire nails. Empirical correlations for Nusselt number, friction factor and thermal enhancement factor are also formulated from the experimental results of WN-TT and P-TT. Keywords heat transfer enhancement, friction factor, plain twisted tape, turbulence, swirl flow
1
INTRODUCTION
The performance of the heat exchanger can be improved by heat transfer augmentation or intensification techniques. Heat transfer intensification techniques widely used in areas such as heat recovery process, air conditioning and refrigeration systems, chemical reactors, heating and cooling in evaporators, thermal power plants, radiators for space vehicles, automobiles and solar water heater. Heat transfer augmentation techniques have been discussed in detail by Webb and Hyun Kim [1]. The published works [2-21] on twisted tapes reported that (1) the use of twisted tapes increases convective heat transfer rate to desired level while pumping power may increase significantly and ultimately the pumping cost becomes high, and (2) enhancement effect of twisted tapes strongly depends on geometry of the tapes. Therefore, the recent research [22-29] on twisted tape is focused on modification of the geometry of plain twisted tapes (P-TT) in the form of broken, serrated, delta-winglet, peripherally-cut, square-cut ones, and twisted tapes consisting of centre wings and alternate-axes assure enhancement of both heat transfer rate and thermal enhancement factor. The present paper reports the modification on tape geometry by inserting small wire nails on the P-TT and it is believed that the small nails are promoted the turbulence in addition with the swirl flow generated by the P-TT. In the experimentation, heat transfer rate and friction factor characteristics of double pipe heat exchanger fitted with P-TT and twisted tape consisting wire nails (WN-TT) for the twist ratios of 2.0, 4.4 and 6.0 are studied with the Reynolds
number between 2000 and 12000. The experimental results are obtained for the tube fitted with WN-TT was also compared with those for the tube fitted with P-TT, S-TT (square-cut twisted tape) and the plain tube. 2
TWISTED TAPES WITH WIRE NAILS
Twisted tapes used for the present work were made from the aluminum strips of thickness 1.5 mm, width 23.5 mm and length 2000 mm. In experiment, tapes with three different pitch lengths H = 50, 110 and 150 mm were used with the twist ratios respectively y = 2.0, 4.4 and 6.0. WN-TT was obtained by punching small holes and carefully inserting wire nails on P-TT. Small wire nails were preferred on the P-TT, to avoid the damage to the inner surface of the test section and to facilitate the insertion and removal of the tape from the test section during the experiment. This type of tape differs from the configurations already available in the literature, i.e. broken, serrated, delta winglet, peripherally-cut, square-cut and wing-cut twisted tapes in the aspect without cutting the edges of the P-TT and using small wire nails to promote the turbulence along with the swirl stream generated by P-TT. Photographs of P-TT and WN-TT are shown in Figs. 1 (a)-(e). 3 EXPERIMENTAL SET UP AND DATA REDUCTION The experimental set-up, data reduction steps and procedures are similar as that described in the previous paper [29] except the new WN-TT was tested.
Received 2010-06-05, accepted 2010-10-19. * To whom correspondence should be addressed. E-mail: [email protected]
Chin. J. Chem. Eng., Vol. 18, No. 6, December 2010
1039
(a) P-TT
(b) WN-TT
(c) Details of WN-TT
(d) Details of WN-TT
(e) Wire-nail
Figure 1 Geometries twisted tapes
4
RESULTS AND DISCUSSION
Nusselt number, friction factor and thermal enhancement factor of a double pipe heat exchanger fitted P-TT and WN-TT for the twist ratios y = 2.0, 4.4 and 6.0 are presented and discussed in the following section. 4.1 Effect of twisted tapes with wire nails (WN-TT) on heat transfer Variation of Nusselt number with Reynolds number in the tube fitted with WN-TT, P-TT, and S-TT and also the plain tube was presented in Fig. 2. It was observed that for all cases, Nusselt number increases with increasing Reynolds number. From Fig. 2, it is clear that Nusselt number in the tube with twisted tapes having wire nails (WN-TT) were higher than those in the plain tube, plain twisted tapes (P-TT) and square-cut twisted tapes insert. Referring to the past investigations [22-29], the reason for better heat transfer enhancement of WN-TT attributed to the collective effect of (1) small wire nails on the tape produce the additional disturbance of turbulence
Figure 2 Nusselt number vs. Reynolds number for twisted tapes consisting of wire nails (WN-TT), plain twisted tapes (P-TT) and square-cut twisted tapes(S-TT) ◆ WN-TT 2.0; S-TT 2.0; ◇ P-TT 2.0; ▲ WN-TT 4.4; + S-TT 4.4; △ P-TT 4.4; ■ WN-TT 6.0; S-TT 6.0; □ P-TT 6.0; × plain tube
in the flow, and (2) main swirl flow generated by P-TT. The vortex generation by the attached wire nail on twisted tapes plays vital role for the higher heat transfer
1040
Chin. J. Chem. Eng., Vol. 18, No. 6, December 2010
rate of WN-TT. In the range of the present experiments, mean Nusselt numbers for tube equipped with WN-TT with twist ratios of 2.0, 4.4 and 6.0 are respectively 1.98, 1.73 and 1.47 times of that plain tube, 1.19, 1.15 and 1.11 times of that for the tube fitted with P-TT, and 1.05, 1.08 and 1.10 times that with S-TT.
Correlations (5) and (6) match with the experimental results within ±9%, ±10% respectively.
4.2 Effect of twisted tapes consisting of wire nails (WN-TT) on friction factor
4.4 Thermal enhancement factor for P-TT and WN-TT
Variation of friction factor with Reynolds number in the tube fitted with WN-TT, P-TT, and S-TT and also the plain tube are presented in Fig. 3. It shows that WN-TT yields higher pressure drop than those in the plain tube as well as the tubes fitted with P-TT and S-TT. This is because of additional disturbance increases the tangential contact between main swirl and the wall surface of the tube. Mean friction factor for the WN-TT with twist ratios of 2.0, 4.4 and 6.0 are respectively, 4.26, 3.66 and 3.22 times of that for the plain tube ,1.22, 1.25 and 1.31 times of that for the tube with P-TT, and 1.14, 1.12 and 1.11 times that with S-TT.
According to literature [24, 25, 29], a comparison of heat transfer coefficients in a plain tube (p) and the tube fitted with turbulator (t) was made at the same pumping power since it is relevant to operation cost. For constant pumping power, the same power is expressed by (V Δp ) = (V Δp ) (7)
Nu = 0.063Re0.789 Pr 0.33 y −0.257
(5)
f = 28.91Re −0.731 y −0.255
(6)
p
t
Based on Eq. (7) and the definition of friction factor, Δp f = ( L / di )( ρ u 2 / 2)
the relationship between friction factor and Reynolds number can be derived as [1] ( fRe3 ) p = ( fRe3 ) t
(8)
Using Eqs. (2), (4), (6) and (8), the Reynolds number for the plain tube (Re)p may be written as a function of Reynolds number (Re)t for the tubes with P-TT (9) and WN-TT (10): Rep = 2.436 Ret0.962 y −0.115 Rep = 3.518Ret0.922 y −0.098
Figure 3 Friction factor vs. Reynolds number for twisted tapes consisting of wire nails (WN-TT) and plain twisted tapes and square-cut twisted tapes (S-TT) ● WN-TT 2.0; S-TT 2.0; ◇ P-TT 2.0; ▲ WN-TT 4.4; + S-TT 4.4; △ P-TT 4.4; ■ WN-TT 6.0; S-TT 6.0; □ P-TT 6.0; × plain tube
4.3 Nusselt and friction factor correlations of twisted tapes consisting of wire nails Correlation and regression analysis were used to develop the Nusselt number and friction factor correlations for plain tube [Eqs. (1) and (2)], P-TT [Eqs. (3) and (4)] and WN-TT [Eqs. (5) and (6)]. Nup = 0.00595 Re0.95 Pr 0.33
(1)
f p = 0.255 Re −0.374
(2)
Nu = 0.027 Re0.862 Pr 0.33 y −0.215
(3)
f = 2.642 Re −0.474 y −0.302
(4)
(9) (10)
Thermal enhancement factor (η) under equal pumping power is defined as ratio of the convective heat transfer coefficient of the tube with turbulator to that of the plain tube which can be expressed as
η = ht / hp
pp
(11)
Employing Eqs. (1), (3), (5) and (11), thermal enhancement factor for the P-TT (12) and WN-TT (13) can be written as
ηP-TT = ht / hp
pp
= 1.95 Ret−0.052 y −0.106
(12)
η WN-TT = ht / hp
pp
= 1.919 Ret−0.032 y −0.165
(13)
Thermal enhancement factor (η) for P-TT and WN-TT at different twist ratios y = 2.0, 4.4 and 6.0 calculated from Eqs. (12) and (13) respectively for PTT and WN-TT were presented in Fig. 4. The thermal enhancement factor in the tube equipped with square-cut twisted tape (S-TT) is also plotted for comparison in Fig. 4. The thermal enhancement factors for WN-TT were found to be higher than those for the tube fitted with S-TT and P-TT with same Reynolds number. The thermal enhancement factor for all the twisted tapes
Chin. J. Chem. Eng., Vol. 18, No. 6, December 2010
2
3 4
5
6
Figure 4 Thermal enhancement factors vs Reynolds number for tubes with P-TT and WN-TT ● WN-TT 2.0; S-TT 2.0; ○ P-TT 2.0; ▲ WN-TT 4.4; + S-TT 4.4; △ P-TT 4.4; ■ WN-TT 6.0; S-TT 6.0; □ P-TT 6.0
tends to decrease with increasing Reynolds number. With the use of WN-TT offered thermal enhancement factors were in a range between 1.27 to 1.33, 1.12 to 1.16 and 1.06 to 1.11 respectively for the twist ratios y = 2.0, 4.4 and 6.0. At the higher Reynolds number, WN-TT is offered the thermal enhancement factor for the twist ratios of 2.0, 4.4 and 6.0 are respectively, 6.2%, 8.1% and 13.3% higher than P-TT and 3.5%, 4.5% and 7% higher than for the tube fitted with S-TT. This is mainly due to that the disturbance in the flow path is higher in the WN-TT than the tube fitted with S-TT and P-TT. Therefore, the WN-TT can be used in place P-TT and S-TT to reduce the size of the heat exchanger.
7
8
9
10
11
12
NOMENCLATURE dh dw f H h Lw Nu Pr Δp Re W y η
head diameter of wire nail, m diameter of wire nail, m friction factor pitch length based on 180° heat transfer coefficient, W·m−2·K−1 length of wire nail, m Nusselt number Prandtl number pressure drop Reynolds number volume flow rate, m3·s−1 width of twisted tape twist ratio thermal enhancement factor
p pp t
plain tube pumping power turbulator
V
Subscripts
13
14
15
16
17
18
REFERENCES 19 1
Webb, R.L., Hyun Kim, N., Principles of Enhanced Heat Transfer,
1041
2nd edition, Taylor Francis (2005). Dewan, A., Mahanda, P., Sumithra Raju, K., Suresh Kumar, P., “Review of passive heat transfer augmentation techniques”, J. Power Energy, 218, 509-525 (2004). Wang, L., Sunden, B., “Performance comparison of some tube inserts”, Int. Comm. Heat Mass Transfer, 29, 45-56 (2002). Eiamsa-ard, S., Thianpong, C., Promvonge, P., “Experimental investigation of heat transfer and flow friction in a circular tube fitted with regularly spaced twisted tape elements”, Int. Comm. Heat Mass Transfer, 33, 1225-1233 (2006). Naphon, P., “Heat transfer and pressure drop in the horizontal double pipes with and without twisted tape insert”, Int. Comm. Heat Mass Transfer, 33, 166-175 (2006). Promvonge, P., Eiamsa-ard, S., “Heat transfer behaviors in a tube with combined conical-ring and twisted-tape insert”, Int. Comm. Heat Mass Transfer, 34, 849-859 (2007). Sivashanmugam, P., Suresh, S., “Experimental studies on heat transfer and friction factor characteristics of laminar flow through a circular tube fitted with helical screw-tape inserts”, Appl. Therm. Eng., 26, 1990-1997 (2006). Sivashanmugam, P., Suresh, S., “Experimental studies on heat transfer and friction factor characteristics of turbulent flow through a circular tube fitted with helical screw-tape inserts”, Chem. Eng. Process, 46, 1292-1298 (2007). Sivashanmugam, P., Suresh, S., “Experimental studies on heat transfer and friction factor characteristics of laminar flow through a circular tube fitted with regularly spaced helical screw-tape inserts”, Exp. Therm. Fluid Sci., 31, 301-308 (2007). Sivashanmugam, P., Suresh, S., “Experimental studies on heat transfer and friction factor characteristics of turbulent flow through a circular tube fitted with regularly spaced helical screw-tape inserts”, Appl. Therm. Eng., 27, 1311-1319 (2007). Hasim, F., Yoshida, M., Miiyashita, H., “Compound heat transfer enhancement by a combination of a helically ribbed tube with twisted tape inserts”, J. Chem. Eng. Japan, 36, 1116-1122 (2003). Krishna, S. R., Pathipaka, G., Sivashanmugam, P., “Heat transfer and pressure drop studies in a circular tube fitted with straight full twist”, Exp. Therm. Fluid Sci., 33, 431- 438 (2009). Bharadwaj, P., Khondge, A.D., Date, A.W., “Heat transfer and pressure drop in a spirally grooved tube with twisted tape insert”, Int. J. Heat Mass Transfer, 52, 1938-1944 (2009). Eiamsa-ard, S., Thianpong, C., Eiamsa-ard, P., Promvonge, P., “Convective heat transfer heat transfer in a circular tube with short-length twisted tape insert”, Int. Comm. Heat Mass Transfer, 36, 365-371 (2009). Eiamsa-ard, S., Thianpong, C., Eiamsa-ard, P., “Turbulent heat transfer enhancement by counter/co-swirling flow in a tube fitted with twin twisted tapes”, Exp. Therm. Fluid Sci., 34, 53-62 (2010). Thianpong, C., Eiamsa-ard, P., Wongcharee, K., Eiamsa-ard, S., “Compound heat transfer enhancement of a dimpled tube with a twisted tape swirl generator”, Int. Comm. Heat Mass Transfer, 36, 698-704 (2009). Al-Fahed, S., Chamra, L.M., Chakroun, W., “Pressure drop and heat transfer comparison for both micro fin tube and twisted-tape inserts in laminar flow”, Exp. Therm. Fluid Sci., 18, 323-333 (1999). Manglik, R.M., Bergles, A.E., “Heat transfer and pressure drop correlations for twisted-tape inserts in isothermal tubes (II) Transition and turbulent flows”, Trans. ASME J. Heat Transfer, 115, 890-896 (1993). Agarwal, S.K., Raja Rao, M., “Heat transfer augmentation for the flow of a viscous liquid in circular tubes using the twisted tape inserts”,
1042
20
21
22
23
24
25
Chin. J. Chem. Eng., Vol. 18, No. 6, December 2010
Int. J. Heat Mass Transfer, 39, 3547-3557 (1996). Al-Fahed, S., Chakroun, W., “Effect of tube-tape clearance on heat transfer for fully developed turbulent flow in a horizontal isothermal tube”, Int. J. Heat Fluid Flow, 17, 173-178 (1996). Tijing, L.D., Pak, B.C., Baek, B.J., Lee, D.H., “A study on heat transfer enhancement using straight and twisted internal fin inserts”, Int. Comm. Heat Mass Transfer, 33, 719-726 (2006). Chang, S.W., Yang T.L., Liou, J.S., “Heat transfer and pressure drop in a tube with broken twisted tape insert”, Exp. Therm. Fluid Sci., 32, 489-501 (2007). Chang, S.W., Jan, Y.J., Liou, J.S., “Turbulent heat transfer and pressure drop in tube fitted with serrated twisted tape”, Int. J. Thermal Sciences, 46, 506-518 (2007). Eiamsa-ard, S., Wongcharee, K., Eiamsa-ard, P., Thianpong, C., “Heat transfer enhancement in a tube using delta-winglet twisted tape inserts”, Appl. Therm. Eng., 30, 310-318 (2010). Eiamsa-ard, S., Seemawute, P., Wongcharee, K., “Influences of pe-
26
27
28
29
ripherally-cut twisted tape insert on heat transfer and thermal performance characteristics in a laminar and turbulent tube flows”, Exp. Therm. Fluid Sci., 34, 711-719 (2010). Chang, S.W., Lees, A.W., Chang, H.T., “Influence of spiky twisted tape insert on thermal fluid performance of tubular air-water bubbly flow”, Int. J. Therm. Sci., 48, 2341-2354 (2009). Chiu, Y.W., Jang, J.Y., “3D numerical and experimental analysis for thermal-hydraulic characteristics of air flow inside a circular tube with different tube inserts”, Appl. Therm. Eng., 29, 250-258 (2009). Eiamsa-ard, S., Wongcharee, K., Eiamsa-ard, P., Thianpong, C., “Thermo hydraulic investigation of turbulent flow through a round tube equipped with twisted tape consisting of centre wings and alternate-axes”, Exp. Therm. Fluid Sci., 34, 1151-1161 (2010). Murugesan, P., Mayilsamy, K., Suresh, S., “Turbulent heat transfer and pressure drop in tube fitted with square-cut twisted tape”, Chin. J. Chem. Eng., 18, 609-617 (2010).
Heat Transfer and Friction Factor Studies in a Circular Tube Fitted with Twisted Tape Consisting of Wire-nails P. Murugesan1,*, K. Mayilsamy2 and S. Suresh3 1 2 3
Department of Mechanical Engineering, K.S. Rangasamy College of Technology, Tiruchengode-637215, India Department of Mechanical Engineering, Institute of Road and Transport Technology, Erode-638316, India Department of Mechanical Engineering, National Institute of Technology, Tiruchirappalli-620015, India
Abstract This paper deals with the experimental investigation on Nusselt number, friction factor and thermal enhancement factor of a double pipe heat exchanger equipped with twisted tape consisting wire nails (WN-TT) and plain twisted tapes (P-TT) with three different twist ratios of y = 2.0, 4.4 and 6.0. Test runs are conducted using the water as the working fluid with Reynolds number range between 2000 and 12000 for WN-TT and P-TT. It is found that Nusselt number, friction factor and thermal enhancement factor in the tube equipped with WN-TT appreciably higher than those in the tube fitted with P-TT and plain tube. Over the range considered Nusselt number, friction factor and thermal enhancement factor in a tube with WN-TT are respectively, 1.08 to 1.31, 1.1 to 1.75 and 1.05 to 1.13 times of those in tube with P-TT. The better performance of WN-TT is due to combined effects of the following factors: (1) common swirling flow generated by P-TT, (2) additional turbulence offered by the wire nails. Empirical correlations for Nusselt number, friction factor and thermal enhancement factor are also formulated from the experimental results of WN-TT and P-TT. Keywords heat transfer enhancement, friction factor, plain twisted tape, turbulence, swirl flow
1
INTRODUCTION
The performance of the heat exchanger can be improved by heat transfer augmentation or intensification techniques. Heat transfer intensification techniques widely used in areas such as heat recovery process, air conditioning and refrigeration systems, chemical reactors, heating and cooling in evaporators, thermal power plants, radiators for space vehicles, automobiles and solar water heater. Heat transfer augmentation techniques have been discussed in detail by Webb and Hyun Kim [1]. The published works [2-21] on twisted tapes reported that (1) the use of twisted tapes increases convective heat transfer rate to desired level while pumping power may increase significantly and ultimately the pumping cost becomes high, and (2) enhancement effect of twisted tapes strongly depends on geometry of the tapes. Therefore, the recent research [22-29] on twisted tape is focused on modification of the geometry of plain twisted tapes (P-TT) in the form of broken, serrated, delta-winglet, peripherally-cut, square-cut ones, and twisted tapes consisting of centre wings and alternate-axes assure enhancement of both heat transfer rate and thermal enhancement factor. The present paper reports the modification on tape geometry by inserting small wire nails on the P-TT and it is believed that the small nails are promoted the turbulence in addition with the swirl flow generated by the P-TT. In the experimentation, heat transfer rate and friction factor characteristics of double pipe heat exchanger fitted with P-TT and twisted tape consisting wire nails (WN-TT) for the twist ratios of 2.0, 4.4 and 6.0 are studied with the Reynolds
number between 2000 and 12000. The experimental results are obtained for the tube fitted with WN-TT was also compared with those for the tube fitted with P-TT, S-TT (square-cut twisted tape) and the plain tube. 2
TWISTED TAPES WITH WIRE NAILS
Twisted tapes used for the present work were made from the aluminum strips of thickness 1.5 mm, width 23.5 mm and length 2000 mm. In experiment, tapes with three different pitch lengths H = 50, 110 and 150 mm were used with the twist ratios respectively y = 2.0, 4.4 and 6.0. WN-TT was obtained by punching small holes and carefully inserting wire nails on P-TT. Small wire nails were preferred on the P-TT, to avoid the damage to the inner surface of the test section and to facilitate the insertion and removal of the tape from the test section during the experiment. This type of tape differs from the configurations already available in the literature, i.e. broken, serrated, delta winglet, peripherally-cut, square-cut and wing-cut twisted tapes in the aspect without cutting the edges of the P-TT and using small wire nails to promote the turbulence along with the swirl stream generated by P-TT. Photographs of P-TT and WN-TT are shown in Figs. 1 (a)-(e). 3 EXPERIMENTAL SET UP AND DATA REDUCTION The experimental set-up, data reduction steps and procedures are similar as that described in the previous paper [29] except the new WN-TT was tested.
Received 2010-06-05, accepted 2010-10-19. * To whom correspondence should be addressed. E-mail: [email protected]
Chin. J. Chem. Eng., Vol. 18, No. 6, December 2010
1039
(a) P-TT
(b) WN-TT
(c) Details of WN-TT
(d) Details of WN-TT
(e) Wire-nail
Figure 1 Geometries twisted tapes
4
RESULTS AND DISCUSSION
Nusselt number, friction factor and thermal enhancement factor of a double pipe heat exchanger fitted P-TT and WN-TT for the twist ratios y = 2.0, 4.4 and 6.0 are presented and discussed in the following section. 4.1 Effect of twisted tapes with wire nails (WN-TT) on heat transfer Variation of Nusselt number with Reynolds number in the tube fitted with WN-TT, P-TT, and S-TT and also the plain tube was presented in Fig. 2. It was observed that for all cases, Nusselt number increases with increasing Reynolds number. From Fig. 2, it is clear that Nusselt number in the tube with twisted tapes having wire nails (WN-TT) were higher than those in the plain tube, plain twisted tapes (P-TT) and square-cut twisted tapes insert. Referring to the past investigations [22-29], the reason for better heat transfer enhancement of WN-TT attributed to the collective effect of (1) small wire nails on the tape produce the additional disturbance of turbulence
Figure 2 Nusselt number vs. Reynolds number for twisted tapes consisting of wire nails (WN-TT), plain twisted tapes (P-TT) and square-cut twisted tapes(S-TT) ◆ WN-TT 2.0; S-TT 2.0; ◇ P-TT 2.0; ▲ WN-TT 4.4; + S-TT 4.4; △ P-TT 4.4; ■ WN-TT 6.0; S-TT 6.0; □ P-TT 6.0; × plain tube
in the flow, and (2) main swirl flow generated by P-TT. The vortex generation by the attached wire nail on twisted tapes plays vital role for the higher heat transfer
1040
Chin. J. Chem. Eng., Vol. 18, No. 6, December 2010
rate of WN-TT. In the range of the present experiments, mean Nusselt numbers for tube equipped with WN-TT with twist ratios of 2.0, 4.4 and 6.0 are respectively 1.98, 1.73 and 1.47 times of that plain tube, 1.19, 1.15 and 1.11 times of that for the tube fitted with P-TT, and 1.05, 1.08 and 1.10 times that with S-TT.
Correlations (5) and (6) match with the experimental results within ±9%, ±10% respectively.
4.2 Effect of twisted tapes consisting of wire nails (WN-TT) on friction factor
4.4 Thermal enhancement factor for P-TT and WN-TT
Variation of friction factor with Reynolds number in the tube fitted with WN-TT, P-TT, and S-TT and also the plain tube are presented in Fig. 3. It shows that WN-TT yields higher pressure drop than those in the plain tube as well as the tubes fitted with P-TT and S-TT. This is because of additional disturbance increases the tangential contact between main swirl and the wall surface of the tube. Mean friction factor for the WN-TT with twist ratios of 2.0, 4.4 and 6.0 are respectively, 4.26, 3.66 and 3.22 times of that for the plain tube ,1.22, 1.25 and 1.31 times of that for the tube with P-TT, and 1.14, 1.12 and 1.11 times that with S-TT.
According to literature [24, 25, 29], a comparison of heat transfer coefficients in a plain tube (p) and the tube fitted with turbulator (t) was made at the same pumping power since it is relevant to operation cost. For constant pumping power, the same power is expressed by (V Δp ) = (V Δp ) (7)
Nu = 0.063Re0.789 Pr 0.33 y −0.257
(5)
f = 28.91Re −0.731 y −0.255
(6)
p
t
Based on Eq. (7) and the definition of friction factor, Δp f = ( L / di )( ρ u 2 / 2)
the relationship between friction factor and Reynolds number can be derived as [1] ( fRe3 ) p = ( fRe3 ) t
(8)
Using Eqs. (2), (4), (6) and (8), the Reynolds number for the plain tube (Re)p may be written as a function of Reynolds number (Re)t for the tubes with P-TT (9) and WN-TT (10): Rep = 2.436 Ret0.962 y −0.115 Rep = 3.518Ret0.922 y −0.098
Figure 3 Friction factor vs. Reynolds number for twisted tapes consisting of wire nails (WN-TT) and plain twisted tapes and square-cut twisted tapes (S-TT) ● WN-TT 2.0; S-TT 2.0; ◇ P-TT 2.0; ▲ WN-TT 4.4; + S-TT 4.4; △ P-TT 4.4; ■ WN-TT 6.0; S-TT 6.0; □ P-TT 6.0; × plain tube
4.3 Nusselt and friction factor correlations of twisted tapes consisting of wire nails Correlation and regression analysis were used to develop the Nusselt number and friction factor correlations for plain tube [Eqs. (1) and (2)], P-TT [Eqs. (3) and (4)] and WN-TT [Eqs. (5) and (6)]. Nup = 0.00595 Re0.95 Pr 0.33
(1)
f p = 0.255 Re −0.374
(2)
Nu = 0.027 Re0.862 Pr 0.33 y −0.215
(3)
f = 2.642 Re −0.474 y −0.302
(4)
(9) (10)
Thermal enhancement factor (η) under equal pumping power is defined as ratio of the convective heat transfer coefficient of the tube with turbulator to that of the plain tube which can be expressed as
η = ht / hp
pp
(11)
Employing Eqs. (1), (3), (5) and (11), thermal enhancement factor for the P-TT (12) and WN-TT (13) can be written as
ηP-TT = ht / hp
pp
= 1.95 Ret−0.052 y −0.106
(12)
η WN-TT = ht / hp
pp
= 1.919 Ret−0.032 y −0.165
(13)
Thermal enhancement factor (η) for P-TT and WN-TT at different twist ratios y = 2.0, 4.4 and 6.0 calculated from Eqs. (12) and (13) respectively for PTT and WN-TT were presented in Fig. 4. The thermal enhancement factor in the tube equipped with square-cut twisted tape (S-TT) is also plotted for comparison in Fig. 4. The thermal enhancement factors for WN-TT were found to be higher than those for the tube fitted with S-TT and P-TT with same Reynolds number. The thermal enhancement factor for all the twisted tapes
Chin. J. Chem. Eng., Vol. 18, No. 6, December 2010
2
3 4
5
6
Figure 4 Thermal enhancement factors vs Reynolds number for tubes with P-TT and WN-TT ● WN-TT 2.0; S-TT 2.0; ○ P-TT 2.0; ▲ WN-TT 4.4; + S-TT 4.4; △ P-TT 4.4; ■ WN-TT 6.0; S-TT 6.0; □ P-TT 6.0
tends to decrease with increasing Reynolds number. With the use of WN-TT offered thermal enhancement factors were in a range between 1.27 to 1.33, 1.12 to 1.16 and 1.06 to 1.11 respectively for the twist ratios y = 2.0, 4.4 and 6.0. At the higher Reynolds number, WN-TT is offered the thermal enhancement factor for the twist ratios of 2.0, 4.4 and 6.0 are respectively, 6.2%, 8.1% and 13.3% higher than P-TT and 3.5%, 4.5% and 7% higher than for the tube fitted with S-TT. This is mainly due to that the disturbance in the flow path is higher in the WN-TT than the tube fitted with S-TT and P-TT. Therefore, the WN-TT can be used in place P-TT and S-TT to reduce the size of the heat exchanger.
7
8
9
10
11
12
NOMENCLATURE dh dw f H h Lw Nu Pr Δp Re W y η
head diameter of wire nail, m diameter of wire nail, m friction factor pitch length based on 180° heat transfer coefficient, W·m−2·K−1 length of wire nail, m Nusselt number Prandtl number pressure drop Reynolds number volume flow rate, m3·s−1 width of twisted tape twist ratio thermal enhancement factor
p pp t
plain tube pumping power turbulator
V
Subscripts
13
14
15
16
17
18
REFERENCES 19 1
Webb, R.L., Hyun Kim, N., Principles of Enhanced Heat Transfer,
1041
2nd edition, Taylor Francis (2005). Dewan, A., Mahanda, P., Sumithra Raju, K., Suresh Kumar, P., “Review of passive heat transfer augmentation techniques”, J. Power Energy, 218, 509-525 (2004). Wang, L., Sunden, B., “Performance comparison of some tube inserts”, Int. Comm. Heat Mass Transfer, 29, 45-56 (2002). Eiamsa-ard, S., Thianpong, C., Promvonge, P., “Experimental investigation of heat transfer and flow friction in a circular tube fitted with regularly spaced twisted tape elements”, Int. Comm. Heat Mass Transfer, 33, 1225-1233 (2006). Naphon, P., “Heat transfer and pressure drop in the horizontal double pipes with and without twisted tape insert”, Int. Comm. Heat Mass Transfer, 33, 166-175 (2006). Promvonge, P., Eiamsa-ard, S., “Heat transfer behaviors in a tube with combined conical-ring and twisted-tape insert”, Int. Comm. Heat Mass Transfer, 34, 849-859 (2007). Sivashanmugam, P., Suresh, S., “Experimental studies on heat transfer and friction factor characteristics of laminar flow through a circular tube fitted with helical screw-tape inserts”, Appl. Therm. Eng., 26, 1990-1997 (2006). Sivashanmugam, P., Suresh, S., “Experimental studies on heat transfer and friction factor characteristics of turbulent flow through a circular tube fitted with helical screw-tape inserts”, Chem. Eng. Process, 46, 1292-1298 (2007). Sivashanmugam, P., Suresh, S., “Experimental studies on heat transfer and friction factor characteristics of laminar flow through a circular tube fitted with regularly spaced helical screw-tape inserts”, Exp. Therm. Fluid Sci., 31, 301-308 (2007). Sivashanmugam, P., Suresh, S., “Experimental studies on heat transfer and friction factor characteristics of turbulent flow through a circular tube fitted with regularly spaced helical screw-tape inserts”, Appl. Therm. Eng., 27, 1311-1319 (2007). Hasim, F., Yoshida, M., Miiyashita, H., “Compound heat transfer enhancement by a combination of a helically ribbed tube with twisted tape inserts”, J. Chem. Eng. Japan, 36, 1116-1122 (2003). Krishna, S. R., Pathipaka, G., Sivashanmugam, P., “Heat transfer and pressure drop studies in a circular tube fitted with straight full twist”, Exp. Therm. Fluid Sci., 33, 431- 438 (2009). Bharadwaj, P., Khondge, A.D., Date, A.W., “Heat transfer and pressure drop in a spirally grooved tube with twisted tape insert”, Int. J. Heat Mass Transfer, 52, 1938-1944 (2009). Eiamsa-ard, S., Thianpong, C., Eiamsa-ard, P., Promvonge, P., “Convective heat transfer heat transfer in a circular tube with short-length twisted tape insert”, Int. Comm. Heat Mass Transfer, 36, 365-371 (2009). Eiamsa-ard, S., Thianpong, C., Eiamsa-ard, P., “Turbulent heat transfer enhancement by counter/co-swirling flow in a tube fitted with twin twisted tapes”, Exp. Therm. Fluid Sci., 34, 53-62 (2010). Thianpong, C., Eiamsa-ard, P., Wongcharee, K., Eiamsa-ard, S., “Compound heat transfer enhancement of a dimpled tube with a twisted tape swirl generator”, Int. Comm. Heat Mass Transfer, 36, 698-704 (2009). Al-Fahed, S., Chamra, L.M., Chakroun, W., “Pressure drop and heat transfer comparison for both micro fin tube and twisted-tape inserts in laminar flow”, Exp. Therm. Fluid Sci., 18, 323-333 (1999). Manglik, R.M., Bergles, A.E., “Heat transfer and pressure drop correlations for twisted-tape inserts in isothermal tubes (II) Transition and turbulent flows”, Trans. ASME J. Heat Transfer, 115, 890-896 (1993). Agarwal, S.K., Raja Rao, M., “Heat transfer augmentation for the flow of a viscous liquid in circular tubes using the twisted tape inserts”,
1042
20
21
22
23
24
25
Chin. J. Chem. Eng., Vol. 18, No. 6, December 2010
Int. J. Heat Mass Transfer, 39, 3547-3557 (1996). Al-Fahed, S., Chakroun, W., “Effect of tube-tape clearance on heat transfer for fully developed turbulent flow in a horizontal isothermal tube”, Int. J. Heat Fluid Flow, 17, 173-178 (1996). Tijing, L.D., Pak, B.C., Baek, B.J., Lee, D.H., “A study on heat transfer enhancement using straight and twisted internal fin inserts”, Int. Comm. Heat Mass Transfer, 33, 719-726 (2006). Chang, S.W., Yang T.L., Liou, J.S., “Heat transfer and pressure drop in a tube with broken twisted tape insert”, Exp. Therm. Fluid Sci., 32, 489-501 (2007). Chang, S.W., Jan, Y.J., Liou, J.S., “Turbulent heat transfer and pressure drop in tube fitted with serrated twisted tape”, Int. J. Thermal Sciences, 46, 506-518 (2007). Eiamsa-ard, S., Wongcharee, K., Eiamsa-ard, P., Thianpong, C., “Heat transfer enhancement in a tube using delta-winglet twisted tape inserts”, Appl. Therm. Eng., 30, 310-318 (2010). Eiamsa-ard, S., Seemawute, P., Wongcharee, K., “Influences of pe-
26
27
28
29
ripherally-cut twisted tape insert on heat transfer and thermal performance characteristics in a laminar and turbulent tube flows”, Exp. Therm. Fluid Sci., 34, 711-719 (2010). Chang, S.W., Lees, A.W., Chang, H.T., “Influence of spiky twisted tape insert on thermal fluid performance of tubular air-water bubbly flow”, Int. J. Therm. Sci., 48, 2341-2354 (2009). Chiu, Y.W., Jang, J.Y., “3D numerical and experimental analysis for thermal-hydraulic characteristics of air flow inside a circular tube with different tube inserts”, Appl. Therm. Eng., 29, 250-258 (2009). Eiamsa-ard, S., Wongcharee, K., Eiamsa-ard, P., Thianpong, C., “Thermo hydraulic investigation of turbulent flow through a round tube equipped with twisted tape consisting of centre wings and alternate-axes”, Exp. Therm. Fluid Sci., 34, 1151-1161 (2010). Murugesan, P., Mayilsamy, K., Suresh, S., “Turbulent heat transfer and pressure drop in tube fitted with square-cut twisted tape”, Chin. J. Chem. Eng., 18, 609-617 (2010).