Open access under CC BY-NC-ND license.Case Studies in Thermal Engineering 2 (2014) 3641

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Case Studies in Thermal Engineering

journal homepage: www.elsevier.com/locate/csite

Enhancement of fin efficiency of a solid wire fin by oscillating heat pipe under forced convection

Tawat Samana a, Tanongkiat Kiatsiriroat a,n, Atipoang Nuntaphan b

a Department of Mechanical Engineering, Chiang Mai University, Chiang Mai 50200, Thailand

b Thermal Technology Research Laboratory, Mae Moh Training Center, Electricity Generating Authority of Thailand, Mae Moh, Lampang 52220, Thailand

a r t i c l e i n f o

Article history:

Received 22 October 2013 Accepted 25 October 2013 Available online 31 October 2013

Keywords:

Wire-on-tube

Oscillating heat pipe

Heat exchanger

Forced convection

a b s t r a c t

Enhancement of fin efficiency of solid wire fin in a wire-on-tube heat exchanger under forced convection was examined. The solid wire fin was replaced with an oscillating heat pipe filled with R123. The unit was tested in a wind tunnel by exchanging heat between hot water flowing inside the tube and the air stream flowing across the external surface. The results showed that the fin efficiency for the case of oscillating heat pipe fin was higher than that of the conventional fin around 5% depended on the mass flow rate of air stream and the geometrical parameters of heat exchanger surface. Moreover, the model of fin efficiency was developed and the results agreed well with the experimental data.

& 2013 The Authors. Published by Elsevier Ltd.

Introduction

Wire-on-tube heat exchangers have been used in refrigeration and air-conditioning systems for many years. Hot working fluid exchanges heat with the ambient air by flowing through a serpentine tube panel attached with a small diameter solid wire.

Many research works reported on the thermal performances of wire-on-tube heat exchangers. For examples, Witzell and Fontaine [1,2] showed the thermal characteristics of a wire-on-tube heat exchanger. Tagliafico and Tanda [3] investigated the radiation and the natural convection heat transfers from a wire-on-tube heat exchanger in refrigeration appliances. Lee et al. [4] proposed a correlation for evaluating the air-side heat transfer coefficient of a wire-on-tube type heat exchanger for the Reynolds number between 50 and 900. Quadir et al. [5] used the finite element method to analyze the free convection heat transfer of a wire-on-tube heat exchanger affected by the ambient temperature and the inside refrigerant mass flow rate.

However, all of the previous works used solid metal wire as an extended surface. The heat transfer from the tube surface to the fin was only by the heat conduction. Therefore, there was a limitation due to the thermal conductivity of wire material. To overcome this problem, Nuntaphan et al. [6] proposed a new design of wire-on-tube heat exchanger. The solid wire was replaced with a closed-loop oscillating heat pipe, which was the capillary tube filled with a working fluid. The total heat transfer from the main tube surface to the fin body was the combination of the heat conduction via the capillary tube material and the heat convection via the working fluid flowing inside the capillary tube. The effectiveness of this heat exchanger increased approximately 10% compared to that of a conventional wire-on-tube heat exchanger. In case of free convection, Samana et al. [7] also found the enhancement of heat transfer when the oscillating heat pipe was used as the extended surface.

n Corresponding author. Tel.: 66 53 944146; fax: 66 54 944145.

E-mail addresses: kiatsiriroat_t@yahoo.co.th, tanongkiat_k@yahoo.com (T. Kiatsiriroat).

2214-157X & 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license. http://dx.doi.org/10.1016/j.csite.2013.10.003

T. Samana et al. / Case Studies in Thermal Engineering 2 (2014) 364137

In this study, thermal performance of a wire-on-tube heat exchanger of which the wire was replaced with an oscillating heat pipe as shown in Fig. 1 under air forced convection was investigated. The effects of some geometrical parameters on the thermal performance were included in this work.

Experimental set-up

In this work, a wire-on-tube heat exchanger was installed in a wind tunnel shown in Fig. 2. It exchanged heat between the ambient air flowing perpendicular to the tube panel and the hot water flowing inside the tube. The dimensions of heat exchangers were shown in Table 1. The volume flow rate of hot water was at 1 L/min and measured by a rotameter having 70.01 L/min accuracy. The air flow across the heat exchanger was varied between 0.02 and 1.5 kg/s and measured by a hot wire anemometer having 70.1 m/s accuracy.

The inlet temperature of hot water was varied between 40 and 80 1C while the ambient air temperature was kept constant at 25 1C. The inlet and the outlet temperatures of water and ambient air and the surface temperatures of fin and tube were measured by the set of K-type thermocouples with 70.1 1C accuracy. Note that, 50% of the total volume inside the capillary tube was filled with R123 and acted as an oscillating heat pipe.

Data reduction

For forced convection, the tube-side and the air-side heat transfer rates of the tested wire-on-tube heat exchanger could be evaluated from

Q w mwCpwTwi _Two;

Q a maCpaTao _ Tai

Length

Fluid in

Fluid out

1

2

CondenserEvaporator

Capillary tubeTube diameter

Adiabatic ZoneTube pitch

Capillary tubeCapillary tube pitch

Tube

Fig. 1. Details of wire-on-tube heat exchanger with oscillating heat pipe fin.

Air Blower &Mixing DeviceTest Section

Frequency Inverter

TTT

TT

Hot Water TankMeter

Intake DuctData

Heater &Flow

LoggerComputer

Temperature

Controller

Pump

Fig. 2. A wind tunnel for heat exchanger testing.

38T. Samana et al. / Case Studies in Thermal Engineering 2 (2014) 3641

Table 1

Dimensions of wire-on-tube heat exchanger.

Tube diameterTube pitchNumber of tubes inTube lengthCapillary tube dia.Capillary tube pitchCap. Tube lengthTube &Cap.

(mm)(mm)row(mm)(mm)(mm)(m)material

9.5330105003.452012.90Copper

9.5340105003.452017.15

6.3540105003.452015.93

12.7040105003.452016.48

9.5340105002.652017.08

where Q is the heat transfer rate, m is the mass flow rate, Cp is the specific heat, T is the temperature (subscripts a; w; i; o are air, water, inlet and outlet respectively). The heat transfer rate used in the calculation was the mathematical average of the air-side and the tube-side heat transfer rates as

Q 0:5Q a Q w3

The relation between heat transfer rate and overall heat transfer coefficient of the heat exchanger is

Q UA TLMTD4

where UA is the overall heat transfer coefficient and area of heat exchanger. TLMTD is the log mean temperature difference which could be calculated from

TLMTD Twi _ Tao _ Two _ Tai5

Twi _ Tao_

ln _Two _ Tai

The heat transfer coefficients can be obtained from the following overall resistance equation as

11ln do=di16

UA ohoAo 2kt Lt hiAi

where hi and ho are the tube-side and the air-side heat transfer coefficients, Ai and Ao are the inner and the outer surface areas of each tube, di and do are the inner and the outer tube diameters, o is the surface efficiency, kt is the thermal conductivity of tube material and Lt is the total tube length.

The tube-side heat transfer coefficient could be evaluated from Gnielinski correlation [8] as

hkwReDw1000 Prwf i=27

i ___;

di12 7f =2 Pr2=31

1i

: pi_

f i 1:58 ln ReDw _ 3:28& _ 28

where ReDw is the tube-side Reynolds number, Prw is the Prandtl number of water flowing inside tube and kw is the thermal conductivity of water.The relation of the surface efficiency in Eq. (6) and the fin efficiency () is

o 1_Af1_ ;9

Ao

Ao Af Ab10

where Ao is the total air-side surface area of heat exchanger, Af is the surface area of fin and Ab is the surface area of bare tube.

For wire-on-tube heat exchanger, the fin efficiency could be evaluated from [3]

Tf _ Ta;11

Tt _ Ta

where Tf is the fin temperature, Tt is the tube surface temperature and Ta is the average air temperature across the heat exchanger.

Results and discussion

Fig. 3 shows the air-side heat transfer coefficients of the heat exchanger for the conventional wire fin and the oscillating heat pipe fin. In these cases, the wire diameter (dw), the tube pitch (st ) and the wire pitch (sw) were kept constant, while the tube diameter (dt ) was varied. It was found that the air-side heat transfer coefficients of the heat exchanger for the case of oscillating heat pipe fin closes to that of the conventional fin. Normally, the air-side heat transfer coefficient strongly depends on the velocity of air stream and the heat transfer surface of heat exchanger. For both cases, the oscillating heat pipe and the conventional fins had the same external surface condition thus same value of the convective heat transfer

T. Samana et al. / Case Studies in Thermal Engineering 2 (2014) 364139

K)

2

(W/m

o

h

OSP,dt = 6.35 mm

Conv.,dt = 6.35 mm

dw =3.45 mm,st = 40 mm,sw = 20 mmOSP,dt = 9.53 mm

Conv.,dt = 9.53 mm

.COSP,dt = 12.70 mm

Temp = 60Conv.,dt = 12.70 mm

Fig. 3. Air-side heat transfer coefficient of heat exchanger for conventional wire fin (Conv.) and oscillating heat pipe fin (OSP).

OSP,Temp 40 C

Conv.,Temp = 40 C

OSP,Temp 60 C

Conv.,Temp = 60 C

OSP,Temp 80 C

Conv.,Temp = 80 C

dt = 12.70 mm,dw =3.45 mm,st = 40 mm,sw = 20 mm

Fig. 4. Fin efficiencies of heat exchanger for conventional type (Conv.) and those of oscillating heat pipe fin (OSP) in case of tube diameters12.70 mm.

coefficient were simimilar. Moreover, it should be noted that the larger tube diameter had the lower heat transfer coefficient. This result came from the promotion of air recirculation for the case of larger tube diameter.

Figs. 46 show the fin efficiency for various conditions and the decreasing of fin efficiency with the increasing of mass flow rate was observed. Normally, the higher air mass flow rate brought to get the lower temperature of the fin body and from Eq. (11), the fin efficiency was reduced.

Actually, in case of oscillating heat pipe fin, the heat transfer between tube and fin is the combination of heat conduction and heat convection, therefore, the fin efficiency of this case should be higher than that of conventional case. The evidences of this assumption could be found in Fig. 4. The results showed that the fin efficiency for the oscillating heat pipe fin was higher than that of conventional fin around 5%.

The effect of tube diameter on the fin efficiency was shown in Fig. 5. For the case of bigger tube size, the heat transfer from tube to fin is higher than that of the smaller one. Therefore, the fin temperature in case of 12.70 mm tube diameter is higher than that of 9.53 mm. It means, the fin efficiency in case of 70 mm is higher than the other one. Similarly result was found for the wire size on the fin efficiency as shown in Fig. 6. The result showed that bigger size of wire diameter gave better fin efficiency.

The effect of tube pitch on the fin efficiency was shown in Fig. 7. Since, the narrow tube pitch gives the increasing of contact point between tube and fin which means the increasing of heat transfer rate and fin efficiency.

40T. Samana et al. / Case Studies in Thermal Engineering 2 (2014) 3641

1.00

OSP,Temp 40 C

0.98OSP,Temp 60 C

OSP,Temp 80 C

OSP,Temp 40 C

0.96OSP,Temp 60 C

OSP,Temp 80 C

0.94

0.92dt = 9.53 mm

dt = 12.70 mm

0.90d w = 3.45 mm, st = 40 mm, sw = 20 mm

0.88

00.050.10.150.20.250.3

ma (kg/s)

Fig. 5. Oscillating heat pipe fin (OSP) efficiencies at various tube diameters.

0.92

OSP,Temp 40 C

OSP,Temp 60 C

0.9OSP,Temp 80 C

OSP,Temp 40 C

OSP,Temp 60 C

0.88OSP,Temp 80 C

0.86

0.84

dw = 2.65 mm

0.82dw = 3.45 mm

dt= 9.53 mm, st = 40 mm, sw = 20 mm

0.8

00.050.10.150.20.250.3

ma (kg/s)

Fig. 6. Oscillating heat pipe fin (OSP) efficiencies at various wire diameters.

The model of fin efficiency in case of oscillating heat pipe fin was also developed as

37 098Re_ 0:0277Pr9:3026st _ dt_ 0:01619sw _ dw0:101712

D; max___

dtdw

:_

It should be noted that the model could be predicted all of the experimental data within 710% variation and the standard deviation is 0.267.

Conclusions

The oscillating heat pipe fin could promote higher fin efficiency of the wire-on-tube heat exchanger. This result came from the combination of heat transfer from the conduction through the fin body and the evaporation and the condensation of fluid slug inside the capillary tube. In case of forced convection, the parameters affecting the air-side performance were the air mass flow rate and the dimensions of heat exchanger such as the tube diameter and the tube pitch, and the wire diameter and the wire pitch.

T. Samana et al. / Case Studies in Thermal Engineering 2 (2014) 364141

0.96

OSP,Temp 40 C

0.94OSP,Temp 60 C

OSP,Temp 80 C

OSP,Temp 40 C

0.92OSP,Temp 60 C

OSP,Temp 80 C

0.9

0.88

0.86st = 30 mm

st = 40 mm

dt = 9.53 mm, dw = 3.45 mm, sw = 20 mm

0.84

00.050.10.150.20.250.30.350.4

ma (kg/s)

Fig. 7. Oscillating heat pipe fin (OSP) efficiencies at various tube pitches.

References

[1] Witzell OW, Fontaine WE. What are the heat transfer characteristics of wire and tube condensers. Refrig Eng 1975;65:337.

[2] Witzell OW, Fontaine WE. Design of wire and tube condensers. Refrig Eng 1975;65:414.

[3] Tagliafico L, Tanda G. Radiation and natural convection heat transfer from wire-and-tube heat exchangers in refrigeration appliances. Int J Refrig 1997;20:4619.

[4] Lee TH, Yun JY, Lee JS, Park JJ, Lee KS. Determination of airside heat transfer coefficient on wire-on-tube type heat exchanger. Int J Heat Mass Transfer 2001;44:176776.

[5] Quadir GA, Krishnan GM, Seetharamu KN. Modeling of wire-on-tube heat exchangers using finite element method. Finite Elem Anal Des 2002;38: 41734.

[6] Nuntaphan A, Vithayasai S, Nut Vorayos Natanee, Vorayos Kiatsiriroat T. Use of oscillating heat pipe technique as extended surface in wire-on-tube heat exchanger for heat transfer enhancement. Int. Commun. Heat Mass Transfer 2010;37:28792. [7] Samana T, Kiatsiriroat T, Nuntaphan A. Air-side performance analysis of wire-on-tube heat exchanger having oscillating heat pipe as extended surface under natural convection condition. Heat Transfer Eng 2012;33(12):10339. [8] Gnielinski V. New equation for heat and mass transfer in turbulent pipe and channel flow. Int Chem Eng 1976;16:35968.