Heat Transfer of Boiling R134a and R142b on a Twisted Tube with Machine Processed Porous Surface*

Chinese Journal of Chemical Engineering, 16(3) 492ü496 (2008)

Heat Transfer of Boiling R134a and R142b on a Twisted Tube with Machine Processed Porous Surface* GAO Xuenong (‫غ‬༰૊)1,**, YIN Huibin (࿣݉μ)1, HUANG Yuyou (ܻံည)2, LING Shuangmei (঑ഀਜ)1, ZHANG Zhengguo (჆ჾ‫)ڳ‬1 and FANG Yutang (ֺံ൶)1 1

Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, South China University of Technology, Guangzhou 510640, China Zhuhai Gree Corporation, Zhuhai 519070, China

2

Abstract The objective of this work was to investigate nucleate pool boiling heat transfer performance and mechanism of R134a and R142b on a twisted tube with machine processed porous surface (T-MPPS tube) as well as to determine its potential application to flooded refrigerant evaporators. In the experimental range, the boiling heat transfer coefficients of R134a on a T-MPPS tube were 1.82.0 times larger than those of R134a on a plain tube. In addition, the developed experimental correlations verified that the predictions of the heat transfer coefficients of boiling R134a and R142b on a T-MPPS tube at the experimental conditions were considerably accurate. Keywords nucleate pool boiling, heat transfer enhancement, twisted tube with machine processed porous surface

1

INTRODUCTION

In recent years, considerable efforts have been spent in finding new refrigerants with no/less influence on atmospheric environment and studying their heat transfer performance. Heat transfer enhancement techniques could be adopted consequently to improve the heat transfer performance of those refrigerants [13]. It is very difficult to solve the problems related to boiling heat and mass transfer of the refrigerants on enhanced surface only with theoretical calculation due to the complex hydrodynamic characteristics and heat transfer mechanism. To improve physical models and study mechanism of boiling heat transfer on an enhanced surface, it has been recognized that experimental research is very important. Several experimental or semi-experimental correlations have been developed and applied in industrial practice. The Cooper correlation [4] and the Gorenflo correlation [5] are typical empirical correlations to predict the boiling heat transfer coefficients. As to each kind of enhanced tubes, many experiments on boiling alternative refrigerants have been conducted in recent years. Memory et al. [6, 7] developed a nucleate pool boiling heat transfer experiment on an enhanced tube with R124 at saturation temperature of 2.2°C. Li [8] studied boiling heat transfer of R134a and R22 on horizontal single mechanically fabricated enhanced tubes at saturation temperatures of 10°C, 14.5°C and 19.6°C. Webb and Pais [9, 10] studied nucleate pool boiling on five different horizontal tube geometries using six refrigerants. Gorenflo et al. [11] studied nucleate pool boiling on a plain tube, a low-fin tube and a Gewa-TX tube with R134a and R22. As a result, each kind of enhanced surface geometries could obviously improved the heat transfer performance of boiling alternative refrigerants. Otherwise, some of those experiment results showed that there was certain compatibility between refrigerants and tube geometries.

In this article, the nucleate pool boiling heat transfer performance and mechanism of R134a and R142b on a twisted tube with machine processed porous surface (T-MPPS tube) are presented, and its potential application to flooded refrigerant evaporators is also explored. 2

EXPERIMENTAL

The experimental apparatus consists of a boiling pool (evaporator), a condenser and other accessories. The tested evaporator was a stainless steel tube with an inner diameter of 159 mm and a length of 600 mm. Two glass windows were designed for visual observation as shown in Fig. 1. The test tube with outer diameter of 16 mm and length of 600 mm was installed along the axis of the evaporator. It was a twisted tube with machine processed porous surface (T-MPPS tube) which the twisted pitch ratio (twisted pitch/tube outer diameter) is about 3.8 and the ellipticity of its transverse section is 1.92, the pore density on the tube exˉ ternal surface is 250 cm 2 and the diameters of these pores are 0.30.5 mm. The sketching structure of the tube is shown in Fig. 2. O-type gaskets and impacted rings were used together to ensure that the system was well air-tight. To maintain the same surface conditions, the plain tube and the T-MPPS tube were careful1y polished and passivated by chemical processing. The schematic diagram of the experimental system is shown in Fig. 3. Refrigerant (R134a or R142b) in the evaporator 6 was heated to its saturated temperature by the hot water flowing through the tube. The generated refrigerant steam flew easily to the coil double-pipe condenser 7 and was cooled by the cooling water. Then, the refrigerant condensate flowed back to the evaporator. During the experiment, the heat flux was adjusted by changing the temperature of the hot water at a constant flow rate, and the flow rate of the cooling water had to be adjusted in order to

Received 2007-09-25, accepted 2008-04-10. * Supported by the Guangdong Provincial Scientific and Technological Development Program (2004B10201008). ** To whom correspondence should be addressed. E-mail: [email protected]

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Figure 1 The schematic diagram of evaporator 1ütest tube; 2üO-type gasket; 3üpressurize ring; 4üimpact flange cover; 5üremovable flange; 6üoil-proof gasket; 7üflange; 8ülighting glass; 9üobserving glass; 10ürod for supporting thermal couples; Aüvapor outlet; Büpressure measurement site; Cüthermal couples inlet; Düdrain out or return

matically monitored by a data acquisition/switch unit (Agilent 34970A). 3 3.1 (a) Cross-sectional view

RESULTS AND DISCUSSION Boiling heat transfer on a plain tube

Previous researchers [915] have accumulated a great deal of experimental data for boiling heat transfer on a plain tube with refrigerant R134a and R142b. Most of the results showed that the nucleate boiling heat transfer performance of R134a on the plain tube had good agreement to the predicted results of the Cooper correlation. The Cooper correlation can be expressed as 0.55

0.12  0.21lg Rp

(b) Planform Figure 2

The surface structure of experimental tube

D b 90qb0.67 M 0.5 Pr (1)   lg Pr Ignoring the fwM term, the Gorenflo correlation can be expressed as

D b / D b* where f q  qb , Pr

f p  Pr ˜ f q  qb , Pr ˜ f wR  Rp

(2)

f p  Pr 1.2 Pr0.27  2.5 Pr  Pr / 1  Pr ,

 qb / qb*

0.9  0.3 Pr0.3

, f wR  Rp

R

p

/ Rp*



2 /15

,

and D b* is the boiling heat transfer coefficient at the reference state of qb* Rp*

20 kWǜm

0.4 ȝm. As to R134a,

D b*

ˉ2

, Pr*

0.1 and

4500 Wǜm 2ǜK 1; ˉ

ˉ

and for R142b, D b* 3200 Wǜm 2ǜK 1. For the plain tube, Pr 0.2 , Rp 2 ȝm. Eqs. (1) and (2) could be simplified to ˉ

Db Figure 3 Scheme of experimental set-up 1üheated water circle pump; 2üU-shaped electrical heating rods; 3ü heated water container; 4ü heated water pump; 5 ü temperature controller; 6 ü boiling pool (evaporator); 7ücondenser; 8üheated water Rota meter; 9ücooling water Rota meter; 10ücooling water pump; 11ücooling water container; 12üworking fluids container

keep the saturated boiling pressure of the experimental refrigerant. The temperature variations were auto-

D b / D b*

ˉ

99.54qb0.67 M 0.5 0.00141qb0.71489

(3) (4)

As seen in Fig. 4, the predicted heat transfer coefficients of boiling R134a on plain tube by Cooper correlation are lower than the experimental values, and the maximum deviation is 13.9%. But the values predicted by the Gorenflo correlation are in good agreement to the experimental values, and the maximum deviation is only 4.1%.

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Figure 4 The predicted and experimental data of boiling R134a on a plain tube (Pr˙0.2) Ƶ exp. data;ƻ Gorenflo correlation;ƸCooper correlation

It can be found from Fig. 5 that the predicted heat transfer coefficients of boiling R142b on plain tube by Cooper correlation are higher than the experimental values. The maximum deviation can reach up to 36.3%. However, the deviations between the values predicted by Gorenflo correlation and the experimental values are small. The maximum deviation is less than 11.3%.

Figure 7 The wall superheat curves of R142b (Pr˙0.2) plain tube;ƷT-MPPS tube

ƻ

whereas the wall superheat for the T-MPPS tube is only 0.921.04°C. The boiling heat transfer coefficients of R134a and R142b on horizontal single tube were shown in Figs. 8 and 9 respectively. It could be found that the boiling heat transfer coefficients on the T-MPPS tube were evidently higher than those on the plain tube. At the same heat flux, the heat transfer coefficients of boiling R134a on the T-MPPS tube are 1.82.0 times higher than those on the plain tube. And the heat transfer coefficients of boiling R142b on the T-MPPS tube are 3.14.0 times higher than those on the plain tube.

Figure 5 The predicted and experimental data of boiling R142b on a plain tube (Pr˙0.2) Ƶ exp. data;ƻGorenflo correlation;ƸCooper correlation

3.2 Performance of boiling heat transfer on the T-MPPS tube

Figure 8 The boiling heat transfer coefficients of R134a on a horizontal tube (Pr˙0.2) ƻ plain tube;Ʒ T-MPPS tube

The T-MPPS tube has good performance for boiling heat transfer. The wall superheat curves of boiling R134a and R142b on the plain tube and the T-MPPS tube have been shown in Figs. 6 and 7 respectively. From Fig. 6, the wall superheats ǻTsat of boiling R134a can be observed. At the contrast pressure (Pr˙ P/Pc) of 0.2, the wall superheat for plain tube is 1.892.93°C. However, the wall superheat for the T-MPPS tube can be reduced 1.01.3°C at the same heat fluxes. The wall superheat ǻTsat of boiling R142b was shown in Fig. 7. At the contrast pressure of 0.2, the wall superheat for plain tube is 2.234.38°C,

Figure 9 The boiling heat transfer coefficients of R142b on a horizontal tube (Pr˙0.2) ƻ plain tube;Ƶ T-MPPS tube

Figure 6 The wall superheat curves of R134a (Pr˙0.2) ƻ plain tube;Ʒ T-MPPS tube

There are many mutual connective capillary tunnels and triangular pores in the surface of a T-MPPS tube, which constitute a great deal of concave cavities and have an excellent gas-collecting capacity. These cavities can also become active nucleation sites at the conditions of high hydraulic pressure and high supercooling, so the required wall superheat at the incipient boiling is relatively low, which can be identified from Figs.5 and 6. In other words, there are more nucleation sites on the T-MPPS tube than those on the plain tube at the same heat flux. After the beginning of incipient

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boiling, the number of nucleation sites on the porous surface of the T-MPPS tube increases gradually with the increase of heat flux and the boiling heat transfer coefficients increase rapidly. This stage is a flooding evaporation process in which the natural convection heat transfer is the dominant process. 3.3 Effects of system pressure on boiling heat transfer

Many previous experiments of boiling heat transfer have shown that the boiling heat transfer could be enhanced with the increase of boiling pressure. According to the Clausius-Clapeyron equation:

 wP / wT sat

'hfg / >T  v"  v' @ ,

(5)

it can be concluded that (wP / wT )sat is always greater than 0. It means that the saturation temperature of the boiling liquid increases with the increase of boiling pressure, the bubble radius will increase accordingly and more concave cavities can become active nucleation sites. To study the effects of boiling pressure on boiling heat transfer coefficients of the T-MPPS tube, the single-tube boiling heat transfer experiments at the contrast pressure of 0.1 and 0.2 were carried out. The effects of boiling saturation pressure on nucleate boiling heat transfer coefficients of the plain tube and the T-MPPS tube were shown in Figs. 10 and 11, respectively. When the boiling saturation pressure is small, the heat flux during the incipient boiling heat transfer is also small. Because the radii of the micro pores on a T-MPPS tube are larger than the initial radii forming the nucleation sites, the separation radius of the bubbles is relatively small. The volume of gas reduces accordingly after the bubbles leave the wall, which

has fewer roles on the bubble nucleate process. Therefore, the boiling pressure has strong impact on plain tube and the T-MPPS tube. At high boiling pressure, the initial heat flux of the incipient boiling is relatively high and the boiling pressure has stronger effect on the T-MPPS tube than plain tube. For the T-MPPS tube, some cavities on the surface with small aperture begin to become the active nucleate sites as the boiling pressure increases. 3.4

Experimental correlation

According to the least square method, the experimental heat transfer coefficients of boiling R134a and R142b can be correlated as

D b mqbn (6) where the coefficients of m and n are related to the factors impacting the boiling heat transfer such as saturation pressure, heat flux and heating surface, etc. The coefficient m is the function of the critical state parameters of the working fluid, the molecular weight and the boiling pressure. The contrasts between experimental data and calculated results of boiling R134a and R142b on a single tube were shown in Figs. 12 and 13. The corresponding correlations were shown in Table 1. The maximum deviation of the developed correlations is

Figure 12 Experimental data and calculated results of R134a on single tube (Pr˙0.2) Ʒ exp. data; üü calculated values

Figure 10 Boiling heat transfer coefficients of R142b on the plain tube at different pressures Pr:Ƶ0.2;Ƹ0.1 Figure 13 Experimental data and calculated results of R142b on single tube (Pr˙0.1 and 0.2) Ƶ Pr˙0.2 (exp.); ƷPr˙0.1 (exp.); üücalculated values Table 1

Figure 11 Boiling heat transfer coefficients of R142b on the T-MPPS tube at different pressures Pr:Ƶ0.2;Ƹ0.1

Experimental correlation of R134a and R142b boiling on single tube

Experimental condition

Correlation

Pr˙0.2/R134a

D b 12.6qb0.72

Pr˙0.2/R142b

Db

0.69 b

Pr˙0.1/R142b

Db

Heat flux/kW Max deviation

17.4154.48

3.50%

37.3q

87.26131.80

0.45%

96.2qb0.58

25.5882.92

1.93%

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less than 3.50% within the experimental range. 4

CONCLUSIONS

The heat transfer performances of boiling R134a and R142b on the T-MPPS tube have been studied. The results show that the boiling heat transfer performances of R134a and R142b on the T-MPPS tube are superior to that on plain tube. At the same heat flux and contrast pressure, the boiling heat transfer coefficients of R134a on the T-MPPS tube are 1.82.0 times larger than those of R134a on plain tube. At the same contrast pressure of 0.2, the boiling heat transfer performance of R142b on the T-MPPS tube is better than that of R134a on the T-MPPS tube. It shows that there is obvious compatibility between the refrigerants and tube geometries. By correlating the experimental data, some experiential correlations about the heat transfer coefficients of boiling R134a and R142b on the T-MPPS tube have been developed with a maximum deviation less than 3.50%. NOMENCLATURE fp fq fwM fwR ǻhfg M P Pc Pr q Rp T ǻT vƍ vƎ Į

influencing factor of saturation pressure influencing factor of heat flux influencing factor of material influencing factor of microstructure ˉ latent heat of vaporization, J·kg 1 molecular weight pressure, Pa critical pressure, Pa reduced pressure ˉ heat flux, W·m 2 surface roughness temperature, °C temperature difference, °C ˉ specific volume of saturated liquid, m3·kg 1 ˉ specific volume of saturated vapor, m3·kg 1 ˉ ˉ heat transfer coefficient, W·m 2·K 1

*

reference state

Superscripts

Subscripts b sat

boiling saturation

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7

8 9 10 11

12

13 14 15

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