Magneto-fluidic materials and systems for energy applications

• Main Author: Pattanaik, Mekap Subhasish
• Other Authors: Raju V. Ramanujan
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials::Magnetic materials; Engineering::Mechanical engineering::Fluid mechanics
• Online Access: https://hdl.handle.net/10356/147837
• Institution: Nanyang Technological University

Magnetic cooling (MC) is a thermal management passive heat pump technology based on the thermomagnetic convection (TMC) of a ferrofluid (FF). Thermomagnetic convection is the phenomenon of convective heat transfer by the motion of a FF, induced by the combined effect of thermal gradient and the local variation in the fluid’s magnetization, in the presence of an external magnetic field. TMC results from the temperature dependent magnetization of a differentially heated FF, which results in a non-uniform magnetic volume force. Such heat pumps do not need any external power, are vibration free, noiseless and require minimum maintenance. Hence, they offer enhanced reliability, low maintenance cost, little or no external energy requirement, and are environmentally friendly. Such magnetic cooling (MC) technology can replace conventional liquid-based cooling technologies due to the absence of mechanical moving parts and its capability to self-regulate over a range of heat load temperatures and power values. Earlier research on MC systems have been intended for cooling microscale and small-scale devices. However, large scale applications of MC systems are still a challenge due to limitations such as device length scale, flow channel diameter, device geometry, form factor, heat load power, temperature, FF volume, and low boiling point carrier fluid. Also, there are no experimental and numerical investigations on the effect of device and FF parameters on MC. Hence, the aim of this research is to investigate the cooling performance of large-scale MC devices, capable of cooling high power heat loads and transferring waste heat over long distance. Experimental and numerical study of enhanced thermomagnetic cooling performance based on device and ferrofluid parameters are also studied. The major research findings of this thesis work are the demonstrations of the capability of novel passive magnetic cooling (MC) systems to cool high power heat loads and to transfer waste heat over longer distances. This work also reported device and ferrofluid (FF) parameters which can significantly increase the cooling performance. This work identified the optimum combination of device configuration, magnet position, and FF properties for magnetic cooling of the heat load. The research findings are further described in the subsequent paragraphs. A multi-torus MC device, capable of kilowatt level cooling, has been fabricated and its cooling performance assessed. The heat load temperature drop improved from 148˚C to 214˚C when heat load power was increased from 0.5 kW to 1 kW, respectively, demonstrating the self-regulating nature of the device. The heat load cooling performance was evaluated for various magnet positions. Simulated surface velocity vector plots revealed ferrofluid vortices near the heat load, resulting in enhanced mixing of hot and cold ferrofluid, leading to improved cooling. The present device offers lower thermal resistance per unit length and lower ferrofluid thermal resistance per unit volume of FF compared to conventional heat pipes. The effect of device characteristics and properties on the cooling performance was also investigated. The effect of thermal conductivity of the tube of the device and device geometry on the cooling profile was determined. We developed several racetrack shaped magnetic cooling devices with low device footprint and high thermal conductivity. A hybrid copper-silicone device exhibited highest temperature drop of 123°C for a heat flux value and initial heat load temperature of 3.47 kW/m2 and 197°C., respectively. The complete copper device cooled the heat load even when the magnet was placed away from the heat load region. Interestingly, the extent of cooling was higher for a hybrid device with higher silicone content. Magnetic cooling devices with simple geometry can transfer heat over long distances. An 8 m long racetrack MC device was developed for the first time, and its cooling performance was studied. The extent of heat load cooling was examined over a wide range of heat flux values. This device transported heat from devices with heat flux values of up to 8.85 kW/m2. The drop in temperature is up to 41°C for a heat load temperature of 197 °C. The local Nusselt number exhibited a maximum near the magnet, enhancing heat load cooling. The performance metrics, non-dimensional parameters, and heat load cooling were calculated to analyze the TM cooling performance of various ferrite and metallic based FF. The resultant magnetic pressure, friction factor, power transferred, and the exergy loss were derived as a function of magnetic and thermophysical parameters to predict the performance of FF based MC devices. Numerical simulations were performed to investigate the effect of magnetic properties of the nanoparticles viz., bulk saturation magnetization, Curie temperature, pyro magnetic coefficient, and initial magnetic susceptibility on the cooling performance. Ferrite ferrofluids prepared from γ-Fe2O3, Fe3O4, and CoFe2O4 exhibited superior cooling. For metallic/alloy-based ferrofluids, FeCo ferrofluid exhibited the best cooling performance followed by Fe and FeNi ferrofluids. Bulk saturation magnetization of the suspended magnetic nanoparticles in the FF and the viscosity of the FF were the parameters which significantly enhanced the convective heat transfer and the heat load cooling. These results can be used to select the optimum ferrofluid for enhanced TM cooling. The novelties of this thesis work are manifold. We developed a multi-torus MC device to cool heat loads having power and temperature as high as up to 1 kW and 580°C, respectively. Secondly, circular and race-track shaped MC devices having different form factors were developed for the first time from copper, silicone, and hybrid (copper + silicone) tubes. The hybrid and copper-based racetrack shaped MC device yielded significant heat load cooling of 121°C and 113°C, respectively, for an initial heat load temperature of 200°C. An 8 m perimeter length racetrack shaped MC device was developed, and its cooling performance was evaluated for the first time. The long-distance heat transfer and cooling performance was investigated for a wide range of heat load power density and temperature values, relevant to low-grade or medium-grade waste heat values. Finally, the device performance was modelled with respect to the magnetic properties of suspended MNP and the thermophysical properties of the FF. It was predicted that Fe3O4, CoFe2O4, and FeCo ferrofluids should exhibit the best cooling performance. These experimental and numerical investigations are useful to find the conditions for obtaining the best cooling performance. Two large-scale MC systems were developed and investigated for their use in cooling of high-power heat loads and transferring waste heat over long distances. The performance can be further improved by using a low viscosity, and highly magnetic FF. A hybrid MC device, due to the presence of larger thermal gradient regions, can improve the cooling performance significantly. These developed self-regulating, and passive magnetic cooling devices may meet specific industrial and household cooling needs, increasing their efficiency, service life and reliability.