Nusselt number-friction characteristic for a twisted rib roughened rectangular duct using liquid crystal thermography

Liquid crystal thermography technique is applied over a wide wall of a rectangular channel having twisted rib roughness to evaluate heat transfer distribution. The temperature distribution can be easily visualized easily as color pattern obtained by a CCD camera. The captured color pattern is digitized and converted to HIS (Hue, Saturation and Intensity) from RGB (red, Green and Blue). In this study the effect of different roughness parameters such as relative roughness pitch, twist ratio and rib inclination angle on heat transfer coefficient and friction factor are investigated. The range of Reynolds number and aspect ratio of the duct is selected those are most suitable for solar air heater. It is observed that relative roughness pitch of 8 shows the maximum heat transfer while friction factor decreases with increase in relative roughness pitch. The effect of twist ratio is to create the jet formation on the downstream side of the ribs, which creates vigorous mixing to increase in heat transfer. The increase in twist ratio decreases the number of jets formed and the Nusselt number, while friction factor increases. The rib inclination to the main flow generates secondary flow, increases turbulence level as well as Nusselt number. The secondary flow increases with increase in inclination angle to create maxima in Nusselt number at an angle of 60°. The friction factor increases with increase in rib inclination angle.