**3.1 Straight wall passage**

Initially, thermal characteristics in straight wall passages have been considered to analyze the heat transfer phenomenon in channel flows. Using the finite elements method, Nusselt number and friction factor were calculated for laminar regime. An investigation on laminar-turbulent transition inside a heated horizontal tube was conducted [22]. An analytical study for joule heating in a parallel plate channel with thermally developed flow has been conducted [23]. A circular tube was examined using various different conditions for viscous flow [24]. A novel method was developed for evaluating the Nusselt number for hydrodynamic flow conditions [25]. Horizontal, inclined channels and vertical plane passages were examined for mixed convective heat transfer [26, 27]. A prediction was presented for Nusselt number for the in-tube cooling of supercritical carbon dioxide [28].
