The migration characteristics of nanoparticles in the pool boiling process of nanorefrigerant and nanorefrigerant–oil mixtureCaractéristiques de la migration des nanoparticules lors de l'ébullition libre d'un nanofrigorigène et d'un mélange nanofrigorigène/huile
Introduction
A nanofluid is a mixture of host fluid and nanoparticles, and it has much higher thermal conductivity than the host fluid (Choi, 1995, Lee et al., 1999, Wang et al., 1999, Xuan and Li, 2000, Liu et al., 2005). When the host fluid is refrigerant, the nanofluid is called nanorefrigerant (Wang et al., 2005, Jiang et al., 2007). Just like other nanofluids, nanorefrigerant has higher thermal conductivity than the host refrigerant (Jiang et al., 2007), and researches on improving the energy efficient of refrigeration systems by using nanorefrigerants have been carried out (Wang et al., 2005, Wang et al., 2007, Wang et al., 2003, Bi et al., in press). As the boiling heat transfer coefficient of refrigerant inside evaporator has important influence on the energy efficiency of a refrigeration system, the boiling heat transfer process of nanorefrigerant should be studied in order to improve the performance of a refrigeration system using nanorefrigerant. The concentration of nanoparticles in nanorefrigerant has influence on the boiling heat transfer coefficient because the thermo-physical properties influence the boiling heat transfer coefficient, such as thermal conductivity and viscosity, which change with nanoparticles concentration (Choi, 1995, Lee et al., 1999, Wang et al., 1999, Xuan and Li, 2000, Das et al., 2003a, Das et al., 2003b, Liu et al., 2005, Jiang et al., 2007). The migration of nanoparticles in the boiling process of nanorefrigerant results in the change of nanoparticles concentration, so the migration characteristics of nanoparticles in the boiling process of nanorefrigerant should be known. Meanwhile, the working fluids in most vapor compression refrigeration systems contain lubricant oil. Therefore the influence of oil on the migration characteristics of nanoparticles should also be considered.
The existing researches have not drawn the same conclusions on the boiling heat transfer characteristics of nanofluids (Das et al., 2003a, Das et al., 2003b, Tu et al., 2004, Zhou, 2004, Wen and Ding, 2005) as well as those of nanorefrigerants (Park and Jung, 2007, Wu et al., 2008). Experiments on pool boiling of CNTs (Carbon nanotubes)-R123 and CNTs-R134a nanorefrigerants by Park and Jung (2007) showed that CNTs enhanced the pool boiling heat transfer coefficients of refrigerants in all cases, and the enhancement could reach 36.6% in low heat flux. Experiments on pool boiling heat transfer characteristics of TiO2-R11 nanorefrigerant by Wu et al. (2008) showed that the boiling heat transfer was enhanced under the condition of low nanoparticles concentration but deteriorated under the condition of high nanoparticles concentration. However, until now, there is no study on the migration characteristics of nanoparticles in the boiling process of nanorefrigerant and nanorefrigerant–oil mixture.
Pool boiling is the basic type of boiling heat transfer, and this paper will focus on the migration characteristics of nanoparticles in the pool boiling process of nanorefrigerant and nanorefrigerant–oil mixture. On the basis of experimental study, the mechanism of migration characteristics of nanoparticles will be analyzed and a numerical model will be established to predict the migrated mass of nanoparticles in the pool boiling process of nanorefrigerant and nanorefrigerant–oil mixture.
Section snippets
Material and apparatus
R113 was used as the host refrigerant because it is in liquid phase at ambient temperature and pressure (its boiling point is 47.6 °C at atmospheric pressure), and the preparation of nanorefrigerant can be easily performed at ambient pressure. CuO nanoparticles were used because this kind of nanoparticles is commonly used (Karthikeyan et al., 2008). The average diameter of CuO nanoparticles is 40 nm and the TEM photograph of CuO nanoparticles is shown in Fig. 1. Lubricant oil RB68EP was used
Influence of the original mass of nanoparticles on the migrated mass of nanoparticles
Totally eight experimental cases for investigating the influence of the original mass of nanoparticles on the migrated of nanoparticles, as listed in Table 1, were carried out. The relationship between the migrated mass of nanoparticles and the original mass of nanoparticles is shown in Fig. 3, where Δm represents the migrated mass of nanoparticles and mn represents the original mass of nanoparticles. It shows that the migrated mass of nanoparticles increases with the increase of the original
Mechanism of migration of nanoparticles
The above experimental results show that nanoparticles can escape from liquid phase refrigerant to gas phase in the pool boiling process. The authors consider that this phenomenon is caused by the following two ways.
Conclusions
- (1)
The migrated mass of nanoparticles in the pool boiling process of nanorefrigerant and nanorefrigerant–oil mixture increase with the increase of the original mass of nanoparticles and the mass of refrigerant. The migration ratio of nanoparticles decreases with the increase of the volume fraction of nanoparticles. The migration ratio changes from 5.84% to 1.14% when the volume fraction of nanoparticles in the nanorefrigerant changes from 0.0912 vol% to 1.536 vol%, and changes from 5.25% to 0.93%
Acknowledgements
The authors gratefully acknowledge the supports from the Program for New Century Excellent Talents in University of Ministry of Education of China (Grant No. NCET-05-0403), Shanghai Municipal Government (Grant No. 05QMH1410), Research Foundation of Shanghai Jiaotong University and International Copper Association.
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