The extended Graetz problem with piecewise constant wall temperature for laminar and turbulent flows through a concentric annulus

►We investigate axial heat conduction in flows through a heated concentric annulus analytically. ► Two different boundary conditions for the inner wall are taken into account. ► We see large differences between laminar and turbulent flows. ► Axial heat conduction is not neglectable for small Peclet numbers and short heating zones.