Thermal processing of viscous non-Newtonian fluids in annular ducts: effects of power-law rheology, duct eccentricity, and thermal boundary conditions

Abstract

Forced convection heat transfer in fully developed laminar flows of power-law fluids in eccentric annular ducts is computationally analyzed. With an insulated outer surface, the heating or cooling on the inner surface is modeled by two fundamental boundary conditions – uniform axial heat flux ( H1 ) and constant wall temperature ( T ) – commonly encountered in thermal processing applications. Numerical solutions for the velocity and temperature distributions, isothermal frictions factors, and Nusselt numbers for annular ducts of varying aspect ratios (0.2⩽r ∗⩽0.8) and inner core eccentricity (0⩽ε ∗⩽0.6) are presented for both shear-thinning (0.2⩽ n<1) and shear-thickening (1< n⩽1.8) fluids. Due to the geometric asymmetry of the eccentric annular cross-section, the flow tends to stagnate in the narrow section and have higher peak velocities in the wide section. This induces greater non-uniformity in the temperature field, and degradation in the average heat transfer coefficient. The nonlinear shear behavior of the fluid further aggravates the flow and temperature maldistribution, which produces a significantly anomalous thermal-hydraulic performance.