Axial Wall Conduction in Partially Heated Microtubes

Abstract

A two dimensional numerical study is carried out to study the effect of axial wall conduction in a partially heated microtube in conjugate heat transfer situations. The flow of fluid through the microtube (inner radius 0.2 mm, total length 60 mm) is laminar, and simultaneously developing in nature. 6 mm each at the inlet and the outlet end of the microtube is insulated and the remaining 48 mm is subjected to constant wall temperature boundary condition over its outer surface, and the cross-sectional solid faces are considered adiabatic. The tube wall thickness, material, and liquid flow rate is varied and simulations have been performed for a wide range of tube wall to convective fluid conductivity ratio (ksf ≈ 2.26 - 646), tube thickness to inner radius ratio (δsf ≈ 1, 10), and flow Reynolds number (Re ≈ 100, 500). The case of fully heated microtube is also considered and a comparison is presented between partially heated and fully heated microtube. The results show that wall conductivity (ksf) and wall thickness (δsf) plays a dominant role in the conjugate heat transfer process. In the fully heated microtube the average Nusselt number (Nuavg) increases with decreasing wall conductivity. Secondly, thicker walls provide higher Nuavg. Due to higher flow development length, higher flow Re increases magnitude of average Nusselt number (Nuavg) for any value of ksf and δsf. In partially heated microtube, the average Nusselt number (Nuavg) for thicker wall microtube is found to be less compared to thinner wall microtube except at very low wall conductivity (ksf) at which it is higher than thinner wall microtube. Thus, the curves for the thin and the thick wall microtube intersect each other at lower ksf.