Heat Transfer from Electrically Heated Microwire in Magnetic and Optical Nanofluids

Nanofluids, due to their specific features, have been widely utilized in a wide range of applications such as biomedicine, transportation, food processing and heat transfer. The main feature of nanofluids which makes them a suitable alternative for convectional heat transfer fluids is their augmented thermal conductivity due to dispersion of metallic, carbide or oxide particles in them. The other important feature is the smaller size of particles compared to microfluids which prevents the particles from aggregation/sedimentation, and therefore maintains higher stability of the suspension. In this thesis, heat transfer in two types of nanofluids with special functionalities (ferrofluid and carbon-dots in water) is investigated using an electrically-heated wire. Ferrofluids are colloidal suspensions of magnetic nano-particles while carbon dots are photoluminescent nano-particles of amorphous carbon. The transient temperature rise of the electrically-heated wire immersed in the sample fluid shows the apparent thermal conductivity of the nanofluid, the onset of convection heat transfer and boiling heat transfer phenomena. For the case of the magnetic fluid (Fe3O4 particles in water) “thermomagnetic convection” effects are observed for the first time in the absence of any magnetic field other than that due to the electrical current in the wire. The physics of thermomagnetic convection is a multi-disciplinary area coupling the fluid dynamics and heat transfer with magnetism. The phenomenon is investigated experimentally, analytically and numerically and several aspects of heat transfer including thermomagnetic convection, onset of thermomagnetic convection and boiling heat transfer are studied. The thermal conductivity of the carbon-dot nanofluid (a novel type of nanofluid which can be applied as a tracer fluid) has also been measured for the first time. Concerning the ferrofluid, the objective of the research is to examine heat transfer from an electrically heated wire with a view to establishing the feasibility of using ferrofluid for cooling applications of electrical systems rather than deionized water. A simple and low cost miniaturized set up for the transient hot-wire technique was designed, fabricated and calibrated. A ferrofluid sample with low volume fraction was used to study the thermomagnetic convection for different currents supplied to the wire at various temperatures. Concerning the analytical component of the study, a two-dimensional model has been developed using a scaling analysis to characterize the thermomagnetic convection around the current-carrying wire. Accordingly, a magnetic Grashof number for the induced flow in relation to the applied current was derived. Also the corresponding maximum Nusselt number of the induced convection was analytically correlated to the applied current and experimentally verified. It was observed that using ferrofluid can significantly enhance the heat transfer from the heated wire due to thermomagnetic convection. The critical Fourier number for the onset of thermomagnetic convection was correlated to the magnetic Rayleigh number and the constants used in the correlation were empirically determined. It was shown that magnetic convection will onset earlier than buoyancy-driven convection for large electrical currents applied. Heat transfer of ferrofluid from the hot-wire when the temperature of the wire reaches above the boiling point of water at atmospheric pressure was experimentally studied. It was observed that the boiling heat transfer from the wire deteriorated using ferrofluid as a result of deposition of particles on wire surface. When the wire reaches the boiling temperature, water molecules evaporate leaving behind the particles in the region. Attached particles to the wire form a porous layer with cavities filled with vapor with high thermal resistance which prevents effective heat removal from the wire. The rate of particle deposition on the wire was measured with respect to the current applied, time of boiling and volume fraction of the ferrofluid. In parallel to the experimental examinations, thermomagnetic convection was investigated numerically. A two-dimensional axisymmetric incompressible laminar numerical simulation model established in COMSOL Multiphysics was used to solve the coupled conservation equations. The model was validated against experimental data collected from the heated wire. Taking into account the term for magnetic body force added to the momentum equation, the model was able to show that the observed effects can be explained by thermomagnetic convection. Finally, thermal conductivity measurements for the carbon-dot nanofluid revealed that in contrast to the predictions of empirical correlations, negligible change compared to the thermal conductivity of the base fluid is observed.