The present day world is energy-hungry. Energy efficient techniques are required for sustainable development. Less energy consuming equipment and appliances are very much needed, in industry as well as in homes and offices, in manufacturing sector as well as in service sector. Heat transfer enhancement techniques with minimum additional fluid pumping power requirement are aid to energy savings. Different types of twisted-tape inserts in internal flow through ducts of different cross-sectional geometry are one such passive heat transfer technique, passive in the sense that it does...