Flow boiling heat transfer characteristics of nano

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Flow boiling heat transfer characteristics of nano-refrigerants in a horizontal tube Bin Sun*, Di Yang School of Energy and Power Engineering, Northeast Dian li University, Jilin 132012, China

article info

abstract

Article history:

The flow boiling heat transfer characteristics of four nano-refrigerants with different mass

Received 24 January 2013

fractions, qualities, and mass velocities in a horizontal tube were studied. The nano-

Received in revised form

refrigerants were Cu-R141b, Al-R141b, Al2O3-R141b, and CuO-R141b. The nanoparticle

12 August 2013

mass fractions were 0.1 wt%, 0.2 wt%, and 0.3 wt%; the quality ranged within 0.3e0.8; and

Accepted 13 August 2013

the mass velocities were 120, 210, and 330 kg m2 s1. Results showed that the flow boiling

Available online 22 August 2013

heat transfer was enhanced by nanoparticle addition. The heat transfer coefficient of the nano-refrigerant increased with increased mass fraction, quality, and mass velocity. The

Keywords:

mass fraction of nanoparticles was the main factor that influenced the heat transfer co-

Nano-refrigerant

efficient. The heat transfer enhancement effects of the different nano-refrigerants differed,

Heat transfer enhancement

with the highest enhancement achieved using Cu-R141b. At G ¼ 120 kg m2 s1 and 0.3 wt%

Flow boiling

mass fraction, the maximum heat transfer coefficient of Cu-R141b increased by 49% (average increase ¼ 27%). ª 2013 Elsevier Ltd and IIR. All rights reserved.

Caracte´ristiques du transfert de chaleur d’e´coulement en e´bullition de nano-frigorige`nes dans un tube horizontal Mots cle´s : Nano-frigorige`nes ; Ame´lioration du transfert de chaleur ; e´coulement en e´bullition

1.

Introduction

According to the Maxwell theory, adding metals, metal oxides, or other solid particles into a liquid can improve the heat transfer coefficient. However, millimeter- or micro meter-sized particles easily precipitate in a liquid, leading to clogging in pipes. In 1995, Choi of the U.S. Argonne National Laboratory proposed the concept of nanofluids (Choi, 1995). He prepared nanofluids by adding nanoparticles to a liquid, and found that the nanoparticles can significantly

increase the thermal conductivity of the fluid. Recent research on nanofluids (Masuda et al., 1993; Lee et al., 1999; Putra et al., 2003; Li and Peterson, 2007; Suresh et al., 2012) has shown that adding appropriate volume fractions of nanoparticles can significantly increase the thermal conductivity of a fluid. Based on the concept of nanofluids, the concept of nanorefrigerants has been proposed. Studies on nano-refri gerants (Kedzierski and Gongb, 2009; Trisaksri and Wongwises, 2009; Kedzierski, 2011; Peng et al., 2011) have

* Corresponding author. Tel.: þ86 432 64806902; fax: þ86 432 64807281. E-mail address: [email protected] (B. Sun). 0140-7007/$ e see front matter ª 2013 Elsevier Ltd and IIR. All rights reserved. http://dx.doi.org/10.1016/j.ijrefrig.2013.08.020


Nomenclature qw twi tf tin tout G x kl two dout din L L U I

2

heat flux (W m ) inner surface temperature ( C) fluid temperature ( C) inlet temperature ( C) outlet temperature ( C) mass velocities (kg m2 s1) quality Bo boiling number thermal conductivity of liquid (W m1 C1) outer surface temperature ( C) outer diameter (mm) inner diameter (mm) thermal conductivity of copper (W m2 K1) length of test section (mm) voltage (V) current (A)

shown that adding nanoparticles to refrigerants can improve the heat transfer coefficient of the base fluid. Simultaneously, energy conservation and emission reduction are achieved. In this study, Cu, Al, Al2O3, and CuO nanoparticles were separately added to a refrigerant at 0.1 wt%, 0.2 wt%, and 0.3 wt% mass fractions. Then, boiling heat transfer tests were performed by simulating evaporation in a horizontal tube. The heat transfer performances of the nano-refrigerants were then compared. Cu, Al, Al2O3, and CuO nanoparticles (average particle diameter ¼ 40 nm, purity ¼ 99%) were supplied by Xuzhou Hongwu Nano Materials Co., Ltd. Given that the Montreal Protocol requires the use of CFC-type and HCFC-type working fluids to be eventually terminated, alternatives to CFC refrigerants need to be identified. HCFC141b had an ozone depression potential of 0.089, global warming potential of 0.15, and boiling point (1 atm) of 32.05 C. In the experiment, nanoparticles were mixed with refrigerants under normal pressure and temperature; thus, R141b was selected.

Q P pc pin Prl Fht H M

207

heating capacity (W) inlet pressure of preheating section (kPa) critical pressure of R141b (MPa) inlet pressure of test section (kPa) Prandtl number of liquid influencing factors of nanoparticles enthalpy (J) molar mass (g mol1)

Subscripts in inlet of test section out outlet of test section pre in inlet of preheating section pre out ontlet of preheating section test test section pre preheating section

2.

Experimental apparatus and process

2.1.

Experimental apparatus

Fig. 1 shows the experimental system, including the main circuit, bypass circuit, cooling water circuits, data acquisition systems, high-speed photographic camera, and nanorefrigerant filling apparatus. The main circuit includes the preheating, test, and cooling sections. The copper tube specifications were as follows: inner diameter, 10 mm; wall thickness, 1 mm; length of preheating section, 500 mm; and length of test section, 1400 mm. To eliminate entrance effects, a 300 mm-long settling chamber was placed before the preheating section. The nano-refrigerant filling apparatus was connected with the main circuit through the shut-off valve. The nanorefrigerant passed through the filling apparatus into the experimental system. A magnetic circulation pump was used to provide power so that the nano-refrigerant can pass

Fig. 1 e Schematic of the experimental system: 1 e test section, 2 e preheating section, 3 e filling apparatus, 4 e exhaust unit, 5 e high-speed camera, 6 e light, 7 e cooler, 8 e magnetic drive pump, 9 e turbine flow transmitter, 10 e valve, 11 e data acquisition system, 12 e computer. T1eT10 are temperature transmitters. P1eP3 are pressure transmitters.

208


through the mass flow transmitter into the preheating section. After entering the test section, the preheated nanorefrigerant absorbed heat and vaporized. The liquidevapor mixture that outflowed from the test section flowed into the thermostatic water bath and then condensed into a liquid nano-refrigerant. The condensed nano-refrigerant repeatedly passed through the circulation pump within the system. The mass velocity of the nano-refrigerant was controlled by a shut-off valve after passing through the magnetic circulation pump. A shutoff valve in the bypass circuit was also used to regulate the flow rate. The core of the experimental system was the electric heating system. Using a voltage regulator to heat a NieCreAl resistance wire, the heating capacity was controlled by changing the voltage. Before heating, the NieCreAl resistance wire was insulated. Leakage was prevented by insulating the NieCreAl resistance wire with porcelain beads and then uniformly winding the wire around the copper outer wall. The electric heating section was wrapped by aluminum silicate insulation materials and glass fiber tape, which functioned as an insulator and thermal insulator, respectively. Leakage was further prevented by grounding the copper tube.

2.2.

After each test, the whole test system was repeatedly washed to prevent any effect on the test results of different nano-refrigerants.

Preparation of nano-refrigerants

Nano-refrigerants were prepared by a two-step method (Xu, 2010). First, the refrigerant was directly mixed with the nanoparticles. Second, surface dispersants were added and the mixture was shaken for 30 min in an ultrasonic shaker. The division value of the electronic balance used to weigh the nanoparticles was 0.01 g. Jiang (2009) experimentally found that dispersion performance of Span-80 is superior to that of SDBS. Bi et al. (2007) found that Span-80 can be used as a dispersant for TiO2 nanoparticles in refrigerants. Accordingly, in the present work, Span-80 was used as the surface dispersant. Adding surface dispersants can also enhance heat transfer. Peng et al. (2006) found that when a nano-refrigerant contains a dispersant, the heat transfer coefficient is higher than when no dispersant is added. In the present experiment, the transmittance was used to evaluate the stability of nanorefrigerants. We measured the transmittance with a visible spectrophotometer (Model 721-100, Shanghai Quintessence Technology Instrument, Co., Ltd). The wavelength accuracy of the visible spectrophotometer was 2 nm, the transmittance accuracy was 1% T, and the stability acuities were 0.004 A h1. The transmittance values of the four nanorefrigerants are shown in Fig. 2. The transmittance decreased with increased mass fraction, and the stability of the nano-refrigerant increased. Without a dispersant, the transmittance was remarkably higher. Thus, the stability was poor without a dispersant.

2.3.

(2) The data acquisition system was checked to ensure that the data acquisition system was properly working. (3) The cooling water flow was adjusted to ensure the stability of the water tank temperature. (4) Using the filling apparatus, the test system was filled with the nano-refrigerant. (5) The magnetic circulation pump was then run and the valve opening was adjusted. When the flow was stable, the flow was recorded. (6) The voltage regulator was adjusted to control the heat flux of the preheating and test sections. (7) When the temperature and pressure of the whole system stabilized, the temperature and pressure were recorded. (8) The flow was changed, steps (5)e(7) were repeated, and new experimental data were acquired. (9) The entire process above was repeated, and the heat transfer characteristics of the nano-refrigerant at different mass fractions and nanoparticles were examined.

Experimental process

The experiment was performed as follows. (1) Before the experiment, prevention of leakage during the filling process was ensured by checking the leakproofness of the experimental system. Afterward, the tube was emptied of residual water and air.

3. Experimental data acquisition and data processing 3.1.

Data acquisition

A Pt100 Thermal Resistance instrument was used to measure the temperature, including the inlet temperature of the preheating zone, inlet and outlet temperatures of the test section, wall temperature, and cooling water temperature. A LWGB-6 Flow Transmitter was used to measure the mass velocities. A WP401A-5G24E2N pressure transmitter was used to measure the pressure, including the inlet pressure of the preheating section as well as inlet and outlet pressures of the test section. An Advantech USB-4716 data acquisition card was used to collect data. The test instrument parameters are shown in Table 1.

3.2.

Uncertainty analysis

Uncertainty analyses were performed to determine the accuracy of the laboratory equipment (Moffat, 1998). The uncertainties of the experimental parameters are shown in Table 2.

3.3.

Data processing

3.3.1.

Heat transfer coefficient of nano-refrigerants

According to the classic convection heat transfer formula, the Newton cooling formula was used to obtain the heat transfer coefficient: qw ¼ h twi tf

(1)

where qw is the heat flux (W m2), twi is the inner surface temperature ( C), and tf is the fluid temperature ( C). tf is calculated as follows:

209


Fig. 2 e Transmittance values of nano-refrigerants at different times: (a) Cu-R141b, (b) Al-R141b, (c) Al2O3-R141b, and (d) CuO-R141b.

tin þ tout tf ¼ 2

(2)

where tin is the inlet temperature ( C) and tout is the outlet temperature ( C). Thermal resistance was used to measure the outside surface temperature of the tube, but the inner surface

temperature was required to calculate the heat transfer coefficient. Therefore, according to the Fourier’s law monolayer round wall heat conduction formula, the inner surface temperature was calculated as follows:

twi ¼ two

qtest lnðdout =din Þ 2pll

(3)

Table 2 e Uncertainty of experimental parameters. Table 1 e Range and accuracy of measurement instruments.

Experimental parameters

Instrument name

Model

Measurement range

Precision

Thermal resistance Flow transmitter Pressure transmitter

Pt100 LWGB-6 WP401A

0e400 C 0.1e0.6 m3 h1 0e1.0 MPa

0.5% 0.1% 0.5%

Pt100 Pressure Mass velocity Power input in preheater Power input in test section Heat transfer coefficient

Uncertainty 0.5% 5% 2.2% 2.5% 4.5% 7.08%

210


where two is the outer surface temperature ( C), dout is the outer diameter (mm), din is the inner diameter (mm), l is the thermal conductivity of copper (W m2 K1), l is the length of the test section (mm), qtest is the heat flux of the test section (W m2) calculated from the voltage and current of the test section.

3.3.2.

Quality calculation

The inlet quality of the test section was calculated by the inlet enthalpy and inlet pressure using Nist-refprop (National Institute of Standard and Technology, 2002). The calculation method for the outlet quality was the same as that for the inlet quality. The inlet enthalpy of the test section was calculated by the inlet enthalpy and heating capacity of the preheating section. The outlet enthalpy of the test section was calculated by the inlet enthalpy of the preheating section and the heating capacity of the entire system. The formulas were as follows: hin ¼ hpre:in þ Qpre =G

(4)

hout ¼ hpre:in þ

Qpre þ Qtest G

(5)

xin ¼ Nist hin ; pin

(6)

xout ¼ Nist hout ; pout

(7)

where Qpre is the heating capacity of the preheating section (W), G is the mass velocities (kg m2 s1), hin is the inlet enthalpy of the test section (J), hout is the outlet enthalpy of the test section (J), hpre.in is the inlet enthalpy of preheating section (J), xin is the inlet quality of test section, xout is the outlet quality of the test section, pin is the inlet pressure of the test section (kPa), and pout is the outlet pressure of the test section (kPa). The enthalpy of the preheating section was calculated by the temperature and pressure using Nist-refprop. The average quality of the test section was calculated from the following equation: xtest ¼ ðxin þ xout Þ=2

(8)

Fig. 3 e Heat transfer coefficients of nano-refrigerants at G [ 120 kg mL2 sL1: (a) Cu-R141b, (b) Al-R141b, (c) Al2O3-R141b, and (d) CuO-R141b.


4.

hl ¼ 0:023Kl =d0 ½Gd0 =ml 0:8 Pr0:4 l , ðlgpr Þ0:55 M0:5 q2=3 . and hpool ¼ 55p0:12 r

Experimental results and discussion

4.1. Comparison of the heat transfer coefficient of nanorefrigerant and pure refrigerant Before the experiment, the reliability of the experimental device was tested by measuring the heat transfer coefficient of the pure refrigerant. The formula of Chen (1997) was used for correlation determination because of its high accuracy in predicting the local boiling heat transfer coefficient in the tube (Jiang and Zhang, 2001). The experimental results of pure R141b flow boiling heat transfer were compared with the results obtained using the formula of Chen. An absolute average error of 7.4% of the calculated value compared with the experimental value was found. This result proved that the accuracy of the experimental system met the requirements; thus, the experiment on nano-refrigerants was carried out. i1=2:2 h 2:2 htp ¼ ðFhl Þ þ h2:2 pool 2:2 7 where, F ¼ 1 þ 1:75=X0:3 tt þ 4:05 10 Bo ,

211

(9)

where, hl is single-phase forced convective heat transfer coefficient, hpool is pool boiling heat transfer coefficient, Xtt is LockharteMartinelli parameter, Bo is boiling number, F is two phase convective enhancement factor, M is molar mass, pr is relative pressure. Fig. 3 compares the boiling heat transfer coefficients of the four nano-refrigerants and pure refrigerant at G ¼ 120 kg m2 s1. After adding nanoparticles, the heat transfer coefficient of the nano-refrigerant became higher than the heat transfer coefficient of pure refrigerant, which indicated enhanced heat transfer. The heat transfer coefficient of 0.3 wt% nano-refrigerant Cu-R141b increased by 49% at x ¼ 0.5. Fig. 4 compares the boiling heat transfer coefficients of the four nano-refrigerants and pure refrigerant at G ¼ 210 kg m2 s1. The findings were similar to the G ¼ 120 kg m2 s1 condition, i.e., heat transfer was enhanced after adding nanoparticles. The heat transfer coefficient of 0.3 wt% nano-refrigerant Cu-R141b increased by 36% at x ¼ 0.5.


212


Fig. 5 compares the boiling heat transfer coefficients of the four nano-refrigerants and pure refrigerant at G ¼ 330 kg m2 s1. The findings were similar to the G ¼ 120 and 210 kg m2 s1 conditions, i.e., heat transfer was enhanced after adding nanoparticles. The heat transfer coefficient of 0.3 wt% nano-refrigerant Cu-R141b increased by 29% at x ¼ 0.5. As shown in Figs. 3e5, at constant mass velocities, the concentration of nanoparticles was the main factor that influenced the heat transfer coefficient of nano-refrigerant; a larger concentration resulted in a higher heat transfer coefficient. Meanwhile, with increased quality, the heat transfer coefficient also increased.

4.2. Boiling heat transfer coefficient of 0.1 wt% nanorefrigerants Cu-R141b, Al-R141b, Al2O3-R141b, and CuOR141b Fig. 6 shows the changes in the boiling heat transfer coefficient for 0.1 wt% nano-refrigerants Cu-R141b, Al-R141b, Al2O3R141b, and CuO-R141b at G ¼ 120, 210, and 330 kg m2 s1. The heat transfer coefficient of the nano-refrigerant significantly improved with increased quality and mass velocities. At G ¼ 330 kg m2 s1 and x ¼ 0.8, the heat transfer

coefficient reached the maximum value. Different nanoparticles had different effects on the heat transfer coefficient of nano-refrigerant. At the same mass velocities, the heat transfer coefficient of nano-refrigerant Cu-R141b reached the maximum value.

4.3. Factors influencing the nano-refrigerant performance at different mass fractions Using different experimental systems inevitably lead to different heat transfer coefficients. Thus, the present research results were compared with previous ones obtained by other scholars. The influence of adding nanoparticles to refrigerant on the heat transfer coefficient was also more intuitively examined by calculating the dimensionless influencing factor Fht. Fht is the ratio of the heat transfer coefficients of nano-refrigerant to refrigerant, and was calculated by the following formula: Fht ¼ hrn =hr

(10)

where hrn is the heat transfer coefficient of nano-refrigerant (W m2 K1) and hr is the heat transfer coefficient of pure refrigerant (W m2 K1).


213


1.5

0.1%Cu+R141b 0.2%Cu+R141b 0.3%Cu+R141b

1.4

Fht

1.3 1.2 1.1 1.0 0.3

0.4

0.5

0.6

0.7

0.8

0.6

0.7

0.8

0.6

0.7

0.8

x

(a) 1.40 1.35

0.1%Cu+R141b 0.2%Cu+R141b 0.3%Cu+R141b

1.30

Fht

1.25 1.20 1.15 1.10 1.05 1.00 0.3

0.4

0.5 x

(b) 0.1%Cu+R141b 0.2%Cu+R141b 0.3%Cu+R141b

1.30 1.25

Fht

1.20 1.15 1.10 1.05

Fig. 6 e Boiling heat transfer coefficients of 0.1 wt% nanorefrigerants at various mass velocities G: (a) 120, (b) 210, and (c) 330 kg mL2 sL1. Fig. 7 shows the relationship among x, Fht (1.04e1.49), and nanoparticle mass fractions for nano-refrigerant Cu-R141b under three flow conditions. At different flow rates, all maximum Fht values were found at x ¼ 0.5e0.6. Moreover,

0.3

0.4

0.5 x

(c) Fig. 7 e Factors influencing the nano-refrigerant Cu-R141b at various mass velocities G: (a) 120, (b) 210, and (c) 330 kg mL2 sL1.

214


Table 3 e Enhanced heat transfer performance of nano-refrigerants. Mass fraction

0.1%

Mass velocity 2

1

G ¼ 120 kg m s G ¼ 210 kg m2 s1 G ¼ 330 kg m2 s1

0.2%

Maximum

Average

Maximum

Average

Maximum

Average

28% 24% 17%

17% 11% 11%

42% 28% 23%

22% 16% 18%

49% 36% 29%

27% 21% 23%

with increased quality, the enhancement in heat transfer performance decreased. This finding can be attributed to the disappearance of the annular region, which led to the direct contact of the liquid refrigerant with the wall and the deterioration of heat transfer. The nano-refrigerants had enhanced heat transfer performance. As shown in Table 3, at G ¼ 120 kg m2 s1, the maximum heat transfer coefficient of 0.3 wt% Cu-R141b increased by 49%, and the average heat transfer coefficient increased by 27%. No further enhancement was observed with increased mass velocity; instead, at high mass velocities, the enhancement in heat transfer performance decreased. The main reason was that the nanoparticles and the wall were not completely heat exchanged; they directly went into the cooling section, which weakened the heat transfer effect.

5.

0.3%

Conclusion

1) The enhancement in the heat transfer performance of nano-refrigerants was experimentally studied. The nanorefrigerants were Cu-R141b, Al-R141b, Al2O3-R141b, and CuO-R141b. Results indicated that adding nanoparticles to the refrigerant enhanced the heat transfer and that the heat transfer coefficient increased with increased nanoparticle mass fraction. 2) With increased quality, the heat transfer coefficient of nano-refrigerants increased but the nanoparticle impact factor decreased. Consequently, the enhancement in heat transfer performance decreased. 3) With increased mass velocities, the heat transfer coefficient of nano-refrigerant increased but the nanoparticle impact factor decreased. Consequently, the enhancement in heat transfer performance decreased. 4) Different types of nanoparticles also influenced the heat transfer coefficient of the nano-refrigerants. Cu nanoparticles had the best effect in enhancing the heat transfer performance of the nano-refrigerants. The reason was the high thermal conductivity of Cu, which enhanced heat transfer.

Acknowledgments This study is supported by the Program for New Century Excellent Talents in University of Ministry of Education of China (Grant No. NCET-12-0727).

references

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