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Migration properties of TiO2 nanoparticles during the pool boiling of nanorefrigerants I.M. Mahbubul, A. Kamyar, R. Saidur, and M.A. Amalina Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie302006n • Publication Date (Web): 04 Apr 2013 Downloaded from http://pubs.acs.org on April 10, 2013
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Industrial & Engineering Chemistry Research
Migration Properties of TiO2 Nanoparticles during the Pool Boiling of Nanorefrigerants I.M. Mahbubul,*,† A. Kamyar,† R. Saidur,†,‡ M.A. Amalina† a
Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
b
UM Power Energy Dedicated Advanced Centre (UMPEDAC), Level 4, Wisma R&D,
University of Malaya, 59990 Kuala Lumpur, Malaysia KEYWORDS: Nanorefrigerant; Pool boiling; Mass fraction; Heat flux; Lubricating oil. ABSTRACT: Nanofluid is a promising fluid due to its enhanced heat transfer properties. Nanorefrigerant is a kind of nanofluid that uses a refrigerant as the base fluid. It can enhance the energy and cooling performance of refrigeration and air-conditioning systems. The objective of this paper is to study the migration properties of nanoparticles during the pool boiling of the nanorefrigerant. The effects of different parameters including heat flux, liquid level height, vessel size, insulation, and lubricating oil on the migration of nano-sized particles have been investigated. TiO2 particles with average diameters of 40 nm were used with R141b refrigerant. The experimental results show that the migrated mass of nanoparticles increases with augmenting the initial mass of nanoparticles as well the applied heat flux. Besides, particle
*
Corresponding author. Tel: +603 7967 7611; Email:
[email protected] (I.M. Mahbubul).
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departure from the liquid to vapor augments by increasing the lubricating oil concentration and adding insulation to the container. However, the migration of nanoparticles decreases with the increase of initial liquid level height and boiling vessel size. Therefore, migration properties of nanoparticles during the pool boiling of nanorefrigerants have a distinct relationship with the distribution of nano-sized particles. 1. INTRODUCTION Optimization of engineering devices, that include heat transfer mechanisms, has been the major focus of many researchers since it can substantially affect the efficiency and the performance. There are various ways to achieve this goal. The most useful methods are: augmentation of heat transfer area and/or increasing the heat transfer coefficient of the fluid. The former is usually tried to be avoided due to the fact that it could cause the bulkiness of the device. The latter way, however, has been implemented by changing different parameters. A recently introduced means of increasing the heat transfer rate includes the utilization of nanofluids. These new heat transfer fluids consist of a base fluid (such as water or ethylene glycol) and nano-sized particles (1-100 nm) that can be of metallic type (Au, Cu), metallic oxides (TiO2, Al2O3) or other materials (CNT, TNT). Extensive studies have done to clarify the performance of these colloidal suspensions unanimously state that the suspension of particles leads to enhancement in the thermal conductivity and diffusivity. Conspicuous enhancements in thermal conductivity at even low volume concentrations for metallic particles1 and increase in the thermal conductivity when increasing the volume concentration2
are among these
observations. It also has been seen that nanoparticles with same concentration but large specific surface area intensify the fluids’ thermal conductivity.3
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Industrial & Engineering Chemistry Research
A particular type of nanofluids consisting of refrigerants (R134a, R123, R141b etc.) as the base fluid is called “nanorefrigerant”.4 Like other nanofluids, this type of refrigerants has shown enhanced thermal conductivity5 and heat transfer performance.6 Since the energy efficiency of air conditioning systems and refrigerators can be influenced by adding nanoparticles into the refrigerants, some research has been done in this area recently.7-8 Heat transfer analysis using refrigerants is generally deals with the phase change due to their low boiling temperature. Therefore, when dealing with nanorefrigerants, their activity under boiling and condensation is to be considered and the value for Critical Heat Flux (CHF) is to be determined. This has been the center of focus for many researchers. One of the first studies about CHF belongs to You et al.9 where an increase of 200 % was outlined for the CHF of a water/Al2O3 nanofluid for different concentrations of particles (0-0.5 g/l). Liu et al.10 also conducted an experimental study on the nucleate pool boiling behavior of water/CuO nanofluid in a flat heat pipe. Authors reported 25% and 50% increase in the heat transfer and CHF, respectively. They also mentioned, the operating pressure as well as the mass concentration are as important parameters in the boiling heat transfer, as an optimum mass concentration of 1.0 wt.% was stated at all pressures for maximum heat transfer. Zeinali Heris11 performed an experimental study on the boiling behavior of ethylene glycol-water/CuO nanofluid with low concentrations. It was announced that an increase of 55% in the heat transfer coefficient could be achieved at a concentration of 0.5 wt.%. Kwark et al.12 also performed experiments with a flat heater at the atmospheric pressure to investigate the different parameters affecting the pool boiling mechanism. With focusing on the coating created on the heater surface, the authors concluded that as the coating layer gets thicker, the wettability changes and as a result changes occur in the amount of Critical Heat Transfer (CHT) and Boiling Heat Transfer (BHT).
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Moreover, Bolukbasi and Ciloglu13 have studied the pool boiling mechanism of SiO2/water nanofluid through a vertical cylinder and observed that with increasing the particle concentration (0.05-0.1 vol.%) the pool film regions were replaced by nucleate pool boiling. They also reported no significant change in BHT for the nanofluid, yet an increase in the CHT due to the increased wettability. Jung et al.14 studied the effect of polyvinyl alcohol addition to the colloid on the pool boiling of alumina/water nanofluid12. They reported a lower boiling heat transfer coefficient for the nanofluid compared to that of pure base fluid due to the blockage in the nucleation cavities after particles being deposited. In addition, CHF was stated to be directly proportional to the heater surface area. Gerardi et al.15 investigated the pool boiling mechanism of diamond and silica nanoparticles with water for very low concentration (
