Numerical study of axial wall conduction in partially heated microtubes

Abstract

A numerical analysis has been carried out for internal convective flows in a microtube subjected to conjugate heat transfer situation. This analysis has been carried out to understand the effect of axial wall conduction for simultaneously developing laminar flow and heat transfer in a circular microtube subjected to constant wall temperature boundary condition imposed on its outer surface.In this background, we have considered for four most practical cases of partial heating of microtubes: (i) the outer surface of the microtube is subjected to constant wall temperature over its full length (ii) 10% of total length at both the inlet and the outlet end are insulated while the remaining length is subjected to constant wall temperature boundary condition (iii) 10% of total length at inlet is insulated while the remaining length is subjected to constant wall temperature boundary condition (iv) 10% of total length at outlet is insulated while the remaining length is subjected to constant wall temperature boundary condition. Simulations have been carried out for a wide range of pipe wall to fluid conductivity ratio (ksf: 2 - 646), pipe wall thickness to inner radius ratio (äsf:1, 10), and flow Re (100 - 500). For partial heated microtube it is found that the average Nusselt number (Nuavg) is increasing with decreasing ksf. At very low ksf it is found that for smaller äsf, (Nuavg) again starts decreasing, but at higher ksf it continues to increase. This is due to absence of axial wall conduction and lower total heating length. But at higher ksf the effective heating length increases due to axial wall conduction. Overall, at higher äsf, the trend is same as full heating case because of axial wall conduction at both the inlet and the outlet ends due to the thick solid wall.