LTNE Analysis of Unsteady MHD Flow and Heat Transfer in an Expanding/Contracting Porous-Medium-Filled Pipe with Wall Injection/Suction

This study investigates magnetohydrodynamic (MHD) flow and heat transfer in a semi-infinite expanding or contracting porous pipe under local thermal non-equilibrium (LTNE) conditions with wall suction and injection. The governing momentum and energy equations for the fluid and solid phases are transformed into nonlinear ordinary differential equations using similarity transformations and then solved using the optimal homotopy analysis method (OHAM). The impacts of wall expansion/contraction, permeation Reynolds number, Hartmann number, inverse Darcy number, Prandtl number, heat capacity ratio, interphase heat transfer parameter, and porosity-scaled conductivity parameter on the flow and heat transfer characteristics are examined. The results indicate that wall expansion and suction increase the normalised centreline axial velocity and modify the magnitude of the radial velocity within the pipe, while the coupled effects of porous-medium resistance and magnetic field suppress the fluid flow. The LTNE framework reveals distinct thermal responses of the fluid and solid phases, demonstrating that the local thermal equilibrium assumption may lead to inaccurate heat transfer predictions in porous media where interphase thermal interactions are finite. The hydrodynamic results show close agreement with previously reported solutions under the corresponding limiting conditions, thereby validating the accuracy of the present flow solution.