Theoretical Study of Magneto-nanofluid Flow and Heat Transfer in Microchannels

Abstract

Microchannel liquid flow is characterized much by laminar flow regime with low Reynolds number. This laminar flow gives rise to molecular diffusion as the main means of heat trans fer, which is not sufficient to remove the required quantity of heat from the high-performance industrial equipments. Thus, the inherent laminar flow within the microchannel limits the heat transfer. To deal with the heat transfer challenge associated with the laminar nature of microchannel flow was the focus of this dissertation, and addressed by incorporating both passive and active heat transfer enhancement methods to improve the heat transfer rate of microchannel flow, where water is used as base fluid with mixed convection. Employing permeable microchannel walls with asymmetric wall temperature/ concentration (passive), loading nanoparticles to water (passive), and imposing uniform magnetic field (active) were utilized in all the problems discussed in the study for better heat transfer performance of microchannel flow for the case of single phase and two phase approaches. In addition to heat transfer enhancement, the applied magnetic field manipulates the flow fields and the heat transfer rate. Since the thermophysical properties of working fluids in microchannel rely largely on temperature, the problems also considered temperature dependent viscosity and thermal conductivity mutually or exclusively. Moreover, the physical phenomena like thermal radiation, chemical reaction, and convective wall heating were included in some of the problems. More precisely, a two-phase model approach was employed to study the effects of variable fluid properties on unsteady mixed convection of MHD nanofluid within a permeable microchannel with mutual absence and presence of thermal radiation, chemical reaction and convective heating. Moreover, a single-phase model approach has been utilized to investigate the effects of magnetic field and variable viscosity on the heat transfer analysis of unsteady mixed convection of microchannel flow using magneto-nanofluids Fe3O4 −H2O and Cu−H2O, respectively, in the absence and presence of thermal radiation and convective heating. The governing nonlinear partial differential equations (PDEs) of the problems were formulated and then transformed into a set of dimensionless nonlinear PDEs by employing dimensionless variables and parameters. The resulting nonlinear PDEs in dimensionless form were solved numerically by semidiscretization via centered finite difference scheme with Runge-Kutta Fehlberg integration technique. Results obtained from transient analysis indicated that the velocity profiles achieved steady states faster than the temperature and concentration profiles. The findings from steady state analysis pointed out that the Nusselt number of non-radiating nanofluids decreased with rise in viscosity variation parameter and magnetic field parameter at the wall y = 0, but a reverse pattern happened with these pa rameters at the wall y = 1. Moreover, the Nusselt number of radiating nanofluid indicated an increasing behavior with increasing in thermal conductivity variation parameter, Biot number, thermal radiation parameter, Eckert number and thermal Grashof number at both walls. However, the Nusselt number of a radiating Cu−H2O nanofluid with convective wall heating decreased with increasing magnetic field parameter at both channel walls. Some of the useful results presented for the velocity, temperature and concentration profiles were validated qualitatively with the previous results reported in the literatures. In this respect, a good match was obtained. At the end, general closing remarks on the outcomes of the problems and suggestions for future investigations were made.