چكيده لاتين
In the present study, the performance of a suspension comprising magnetic nanocapsules containing n-octadecane in natural/thermomagnetic convective heat transfer within a cubic cavity was experimentally investigated under various heat fluxes and magnetic flux densities. The magnetic nanoencapsulated phase change material (M-NEnPCM) was synthesized using the miniemulsion ultrasonic-assistant polymerization technique. The magnetic property was introduced to the nanocapsules by adding surface-modified Fe3O4 nanoparticles into the core. The dynamic light scattering (DLS) results showed an average nanocapsule size of 78 nm. The scanning electron microscope (SEM) along with transmission electron microscope (TEM) images confirmed the spherical shape with a core/shell structure of the M-NEnPCM. According to the differential scanning calorimetry (DSC), M-NEnPCM had melting and crystallization latent heats of 116.47 and 124.71 J/g, respectively. Thermogravimetric analysis (TGA) indicated improved thermal stability of the nanocapsules compared to pure octadecane and an encapsulation efficiency of 85%. Furthermore, vibrating sample magnetometry (VSM) confirmed the superparamagnetic properties of the synthesized nanocapsules. Steady state heat transfer results showed that the use of M-NEnPCM led to an increase in the convective heat transfer coefficient up to twice that of pure water under similar operating conditions. Optimal concentration of M-NEnPCM and magnetic flux density were found, at which the convective heat transfer coefficient was maximized. Additionally, in the unsteady state thermal analysis, the effect of M-NEnPCM concentration and magnetic flux density was examined on the onset of convective mechanisms dominance (th) and the onset of PCM influence (tp). Results indicated that the presence of M-NEnPCM reduced th, indicating improved convective heat transfer in the initial stages of the experiment. Conversely, further increasing the concentration of nanocapsules increased th due to the significant rise in the viscosity. Increasing the magnetic flux density up to an optimal value decreased th due to the occurrence of thermomagnetic convection, but further increases intensified magnetoviscouse effect. Futhurmore, minimum values for tp relative to M-NEnPCM concentration and magnetic flux density were observed in unsteady state analyses. The comparative analysis revealed that the superiority of the M-NEnPCM suspension over pure water in heat transfer was influenced by nanofluidity, magnetism, and the presence of PCM, contributing 64%, 19.8%, and 5%, respectively. However, studies on the effect of different configurations of temperature gradient, magnetic field, and gravity revealed that inappropriate configurations could make M-NEnPCM suspension less effective than pure water
