The viscosity of various nanofluids measured in the present study is used in the experimental and computational thermal analyses, also carried out in this thesis. These properties could also be of use to other investigators, especially since there is a general shortage of detailed and reliable measurements of nanofluid viscosity. The present experimental activity also includes measurements of the viscosity of nanofluids at different temperatures and nanoparticle concentrations. Such information is useful since the nanofluids are usually subjected to large temperature fluctuations in practical applications, and dynamic viscosity can vary appreciably with nanoparticle concentration and temperature.
The thermal performance (heat transfer coefficient) and flow characteristics (pressure drop) of nanofluids measured in specific components are important. Comparisons with data in the literature are carried out and also dealing with nanofluid as a single phase or two phases flow is of vital importance, since there is a controversy of nanofluid behaviour.
Another important issue that concerns the practical relevance of the present thesis is the determination of recommended nanofluid concentrations for typical data centre cooling systems. This issue is of vital importance with regard to energy conservation in data centres. The use of a nanofluid with an optimum volume fraction is a major step towards optimizing the total cost, which comprises the cost of energy consumption due to the electronic devices (servers in the data centre), cooling system and pump plus the cost of the nanofluids itself. The volume fraction calculated for typical nanofluids under the considered conditions should benefit the energy conservation in data centres.
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1.5:
Objectives
There are three main objectives in the present work. The first objective is measuring the thermophysical properties of nanofluids and proposing a correlation viscosity. The second objective is the measurement and calculation of the thermal and flow characteristics of various nanofluids. The third objective is simulating nanofluids within a data centre cooling liquid loop using different nanofluids and predicting the recommended volume fraction for these nanofluids by the application of an economic analysis. In general, all objectives can be summarised as follows:
1. Survey and collect samples of nanofluids produced by commercial companies and also try to produce nanofluids in our laboratory.
2. Measure the viscosity of the nanofluids as a function of temperature as well as nanoparticle volume fraction.
3. Suggest and propose a correlation for viscosity as a function of volume fraction and temperature. Then compare this with current theoretical and empirical correlations. 4. Build an experimental loop to evaluate heat transfer performance and pressure drop
of commonly used nanofluids.
5. Measure thermal and flow characteristics of nanofluids by applying them in the test loop. A comparison with the conventional and existing correlation will be made in terms of heat transfer coefficient and pressure drop.
6. Survey and collect information of data centre cooling techniques and collect data regarding the thermal properties of server materials.
7. By utilising a model based on the finite-element method (COMSOL program), evaluate and compare the thermal performance and flow characteristics of a cooling nanofluid within a data centre server based on various nanoparticle volume fractions. 8. Evaluate the benefit of nanofluids within the cooled plate of immersed liquid cooling
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9. Determine the optimum nanoparticle concentration and recommended nanofluids for the application under work conditions and given types of nanofluids and nanoparticles concentration. These would be subject to certain (thermal performance) thermal conditions of the data centre and specific values of a number of economic parameters.
10. Investigate the effects of some parametric study on the optimum nanoparticles concentration such as changing the configuration of server and the effect of economic factors
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CHAPTER 2:
NANOFLUIDS PREPARATION AND
CHARACTERISTICS
2.1:
Introduction
There are many types of nanofluids available; these differ with the base liquid type such as water, ethylene glycol and oil. They also differ in regards to nanoparticle material, size, shape and concentration [90]. An important issue facing scientists in creating a nanofluid is dispersing the nanoparticles homogeneously within the base fluid and the stability of these over time. Another issue is reducing the agglomeration of nanoparticles to avoid sedimentation due to gravity [91].
There are two main techniques for dispersing nanoparticles into a base fluid when preparing a nanofluid: the single step technique and the two step technique. The single step technique involves simultaneously producing and dispersing nanoparticles into a base fluid at the same time. The two step method involves first producing the nanoparticles and then dispersing them into the base fluid, this needs high shear and ultrasound to aid dispersion of the nanoparticles into the fluid. This technique is mainly used to produce nanofluids with oxide nanoparticles and is preferred by researchers [11, 90-92] since it is easy to find the oxide nanoparticles and prepare nanofluids. However, the single step method is characterised by stability and less agglomeration of nanoparticles and also it is easier to control the size and the shape of the nanoparticles, but is more expensive.
The nanoparticles of the two step technique are produced by physical processing such as mechanical grinding and ball milling or can be synthesised by chemical processes such as Chemical Vapour Deposition (CVD) or chemical precipitation. The output of these processes is a powder with different size and shape and a variety of types such as carbon nanotubes, oxide ceramics, nitride ceramics, carbide ceramics, metals or composite materials [11, 90].
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From the previous studies, the procedures of nanofluid preparation and evaluation can be summarized in the diagram shown in Figure 2.1. The abbreviations of TEM and SEM refer to Transmission Electron Microscopy and Scanning Electron Microscopy, respectively.
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2.2:
Nanofluids Preparation
In this study, aluminium oxide (Al2O3-water), titanium oxide (TiO2-water) and copper
oxide (CuO-water) nanofluids were chosen because they are the most commonly used, easiest to prepare and are commercially available as a nanofluid.
The alumina (Al2O3-water) nanofluids were prepared using the two step technique
mentioned above. The nanoparticles were supplied by Sigma-Aldrich with an average size of 50 nm and added to distilled water using magnetic stirring and ultrasonic agitation to disperse the nanoparticles in the base fluid to get a nanofluid with mass fraction of 20 wt.%. While titanium oxide (TiO2-water) and copper oxide (CuO-water) nanofluids were
purchased premixed from Alfa Aesar as a colloidal dispersion nanofluid with mass fractions of 50 wt.% and 45 nm average nanoparticle size for TiO2-water nanofluid and
35 wt.% with nanoparticle size 30 nm for the CuO-water nanofluid. These concentrations of the nanofluids were diluted into distilled water to get nanofluids with a variety of volume fractions, as explained in the next section. The dilution process was undertaken by stirring and shaking the mixture without any additional processes.