The research aimed at developing an experimental apparatus that was employed to estimate the convective heat transfer rate of nanofluids during cooling. The project started with the design and fabrication of the previously mentioned apparatus and continued with the evaluation of its effectiveness to test several different variables such as nanoparticle concentration and Reynolds Number. In all the experiments conducted, the cooling was achieved by natural convection with the ambient air. Along with the apparatus, a spreadsheet based data analysis algorithm was developed to analyze the data acquisition system. The research also attempted to model the convective heat transfer coefficient of nanofluids based as a function of the Reynolds Number and the nanofluid concentration. Even though some conclusions could be made, there were issues with the quality of data obtained from the experiment. Due to the low temperature difference between the ambient air and the nanofluid and the short length of the tube, the temperature difference was small relative to the error associated with each sensor. An interesting observation during this investigation is a lack of dependency of the Nusselt Number on the Reynolds Number which is contrary to much of the reported literature. The data did, however, show a functional relationship between the Nusselt Number and the volume fraction of nanoparticles where Nu=aφ0.25. Many methods of reducing the errors were determined such as the addition of an outer controlled environment that would increase the temperature difference between the nanofluid and the outer wall of the tubes in order to enhance the rates of heat transfer and in turn increase the temperature difference between the sensor locations along the fluid flow. With the employment of these methods on the current apparatus, it could be a very successful method of quickly and easily determining convective heat transfer coefficients especially with the use of the algorithm imbedded in the spreadsheet to determine important factors.

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abstract: As miniature and high-heat-dissipation equipment became major manufacture and operation trends, heat-rejecting and heat-transport solutions faced increasing challenges. In the 1970s, researchers showed that particle suspensions can enhance the heat transfer efficiency of their base fluids. However, their work was hindered by the sedimentation and erosion issues caused by the relatively large particle sizes in their suspensions. More recently, nanofluids--suspensions of nanoparticles in liquids-were proposed to be applied as heat transfer fluids, because of the enhanced thermal conductivity that has generally been observed. However, in practical applications, a heat conduction mechanism may not be sufficient for cooling high-heat-dissipation devices such as microelectronics or powerful optical equipment. Thus, the thermal performance under convective, i.e., flowing heat transfer conditions becomes of primary interest. In addition, with the presence of nanoparticles, the viscosity of a nanofluid is greater than its base fluid and deviates from Einstein's classical prediction. Through the use of a test rig designed and assembled as part of this dissertation, the viscosity and heat transfer coefficient of nanofluids can be simultaneously determined by pressure drop and temperature difference measurements under laminar flow conditions. An extensive characterization of the nanofluid samples, including pH, electrical conductivity, particle sizing and zeta potential, is also documented. Results indicate that with constant wall heat flux, the relative viscosities of nanofluid decrease with increasing volume flow rate. The results also show, based on Brenner's model, that the nanofluid viscosity can be explained in part by the aspect ratio of the aggregates. The measured heat transfer coefficient values for nanofluids are generally higher than those for base fluids. In the developing region, this can be at least partially explained by Prandtl number effects. The Nusselt number ( Nu ) results for nanofluid show that Nu increases with increasing nanofluid volume fraction and volume flow rate. However, only DI-H2O (deionized water) and 5/95 PG/H2O (PG = propylene glycol) based nanofluids with 1 vol% nanoparticle loading have Nu greater than the theoretical prediction, 4.364. It is suggested that the nanofluid has potential to be applied within the thermally developing region when utilizing the nanofluid as a heat transfer liquid in a circular tube. The suggested Reynold's number is greater than 100. / Dissertation/Thesis / Ph.D. Mechanical Engineering 2010

In this experimental investigation the evaporative behaviour of liquid droplets of both pure fluids and fluids containing nanoparticles was studied. Initial tests were conducted on drops of pure volatile liquids using IR thermography, and the effect of substrate material, drop composition, and substrate temperature was investigated. The effect of the addition of nanoparticles to the liquid drops was then investigated using a contact angle analyser which could record the drop profile in time. The effects of liquid composition, nano-particle composition, nanoparticles concentration, substrate hydrophobicity, and substrate temperature were all studied. Results obtained from IR thermography showed that there exists interfacial temperature instabilities in evaporating volatile drops, the appearance of these fluctuations was found to be dependent on the liquid and substrate in question and are self generated temperature gradients resulting from non-uniform evaporation. A stability analysis was conducted and the results give a good agreement with experimental results. The addition of nanoparticles to a liquid drop was found to alter the evaporative behaviour by enhancing pinning of the drop contact line and preventing the drop radius from shrinking. By manipulating the concentration of the particles suspended in a drop, a stick-slip evaporative process was achieved, leading to rings of particulate material formed upon total evaporation. By varying parameters such as substrate hydrophobicity, nanoparticle concentration, liquid composition, and substrate temperature, many distinct nanoparticle deposit patterns were observed upon total evaporation. It was shown that by varying these parameters, many different patterns could be achieved, and that inside these deposit patterns regular formations such as particulate rings, radial lines, and cellular structures were present.

AN ABSTRACT OF THE THESIS OF Daniel Grohmann, for the Master of Science degree in Mechanical Engineering on May 5, 2014, at Southern Illinois University Carbondale DESIGN AND CHARACTERIZATION OF A COMPACT HEAT EXCHANGER FOR USE WITH NANOFLUIDS Major Professor: Dr. Kanchan Mondal This research is aimed to design and characterize an experimental plain channel, plate heat exchanger and further compare the performance of water based nanofluids as with that of water. The thesis discusses the designing and fabrication of the heat exchanger such that several parameters such as temperature, flow rate, nanoparticle concentration, and length of channels can be varied. Three sizes of heat exchangers were fabricated. Experiments were conducted to remove heat from air at different temperatures by the various heat exchanger fluids, namely water, 0.5, 0.75 and 1 vol.% alumina in water. Flow rates of the cold fluid were varied in order to change the Reynolds Number while maintaining a laminar regime. In the experiments with water as the heat exchanger fluid, it was found that the heat exchange did not follow pure counter current flow conditions presumably due to end effects. A correction factor, F, for the log mean temperature difference value was calculated for each case to estimate the mean temperature difference. The effectiveness values were found to be greater than 0.75 for most cases. It was also found that the thermal entrance length was larger than the length of the channels for the shortest heat exchanger. In addition, it was observed that the laminar regime Nusselt number values were similar to values reported in literature for flow through mini and micro channels. Convective heat transfer coefficients were calculated and were found to be of the order of those reported for water. It was also discovered that Prandtl number was the most influential property for this study. As opposed to expectations, the use of nanofluids was found that it did not significantly improve heat transfer than that of the water alone. The only case that the nanofluids had a significant enhancement in the performance was that of the 6in plate at 1 vol.% of 11% increase. The rest of the study showed that it had no increase or negative effects.

Natural convection as a heat transfer mechanism plays a major role in the functioning of many heat transfer devices, such as heat exchangers, energy storage, thermal management and solar collectors. All of these have a large impact on the generation of solar power. Considering how common these devices are not only in power generation cycles, but in a majority of other thermal uses it is clear that increased performance for natural convection heat transfer will have consequences of a high impact. As such, the purpose of this study is to experimentally study the natural convection heat transfer behaviour of a relatively new class of fluids where nano-sized particles are mixed into a base fluid, also known as a nanofluids. Nanofluids have attracted widespread interest as a new heat transfer fluid due to the fact that the addition of nanoparticles considerably increases the thermophysical properties of the nanofluids when compared to those of the base fluid. Furthermore, if these nanoparticles show magnetic behaviour, huge increases in the thermal conductivity and viscosity of the nanofluid can be obtained if the fluid is exposed to a proper magnetic field. With this in mind, the study aimed to experimentally show the behaviour of these so-called magnetic nanofluids in natural convection heat transfer applications. In this study, the natural convection heat transfer of a magnetic nanofluid in a differentially heated cavity is investigated with and without an applied external magnetic field. The effects of volume concentration and magnetic field configuration are investigated. Spherical nanoparticles with a diameter of 20 nm are used with a volume concentration ranging between 0.05% and 0.3%, tested for the case with no magnetic field, while only a volume concentration of 0.1% was used in the magnetic cases. The experiments were conducted for a range of Rayleigh numbers in . The viscosity of the nanofluid was determined experimentally, while an empirical model from the literature was used to predict the thermal conductivity of the nanofluids. An empirical correlation for the viscosity was determined, and the stability of various nanofluids was investigated. Using heat transfer data obtained from the cavity, the average heat transfer coefficient, as well as the average Nusselt number for the nanofluids, is determined. It was found that a volume concentration of 0.05% showed an increase of 3.75% in heat transfer performance. For the magnetic field study, it was found that the best-performing magnetic field enhanced the heat transfer performance by 1.58% compared to the 0.1% volume concentration of the nanofluid with no magnetic field. / Dissertation (MEng)--University of Pretoria, 2017. / Mechanical and Aeronautical Engineering / MEng / Unrestricted

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yes / The thermal behavior of aluminium oxide-water nanofluid in a double pipe carbon steel heat exchanger was investigated in the present study. The overall heat transfer coefficient, Nusselt, and heat transfer coefficient of nanofluid were compared with the base fluid. The volume fraction of the nanoparticles was 1%. By adding nanoparticles to the fluid, the thermal properties of the base fluid improved significantly. The hot and cold fluid flow was considered counter-current, and the nanofluid was pumped into the inner tube and once into the outer tube, and the flow rate of each fluid was 0.05 kg/s. The convective heat transfer and the overall heat transfer coefficient enhanced 94% and 253% for the hot fluid flow in the outer tube and 308 % and 144% for the hot fluid flow in the inner tube, respectively. The pressure drop calculations also showed that the pressure drop would not change significantly when using nanofluid.

In spite of recent attention to renewable sources of energy, liquid hydrocarbon fuels are still the main source of energy for industrial and transportation systems. Manufactures and consumers are consistently looking for ways to optimize the efficiency of fuel combustion in terms of cost, emissions and consumer safety. In this regard, increasing burning rate of liquid fuels has been of special interest in both industrial and transportation systems. Recent studies have shown that adding combustible nano-particles could have promising effects on improving combustion performance of liquid fuels. Combustible nano-particles could enhance radiative and conductive heat transfer and also mixing within the droplet. Polymeric additive have also shown promising effect on improving fire safety by suppressing spreading behavior and splatter formation in case of crash scenario. Polymers are also known to have higher burning rate than regular hydrocarbon fuels. Therefore adding polymeric additive could have the potential to increase the burning rate. In this work, combustion dynamics of liquid fuel droplets with both polymeric and nanoparticle additives is studied in normal gravity. High speed photography is employed and the effect of additive concentration on droplet burning rate, burning time, extinction and soot morphology is investigated. Polymer added fuel was found to have a volatility controlled combustion with four distinct regimes. The first three zones are associated with combustion of base fuel while the polymer burns last and after a heating zone because of its higher boiling point. Polymer addition reduces the burning rate of the base fuel in the first zone by means of increasing viscosity and results in nucleate boiling and increased burning rates in the second and third stages. Overall, polymer addition resulted in a higher burning rate and shorter burning time in most of the scenarios. Colloidal suspensions of carbon-based nanomaterials in liquid fuels were also tested at different particle loadings. It was found that dispersing nanoparticles results in higher burning rate by means of enhanced radiative heat absorption and thermal conductivity. An optimum particle loading was found for each particle type at which the maximum burning rate was achieved. It was observed that the burning rate again starts to reduce after this optimum point most likely due to the formation of large aggregates that reduce thermal conductivity and suppress the diffusion of species.