Enhancement of Heat Transfer with Twisted Tape Inserts and Rectangular hole

Abstract

Heat transfer augmentation techniques are used to increase rate of heat transfer without affecting much the overall performance of the system. These techniques are classified as active and passive techniques. The active technique required external power such as surface vibration and electric or acoustic fields, whereas the passive techniques required fluid additives, special surface geometries, or swirl/vortex flow devices, that is, twisted tape inserts. Heat transfer augmentation techniques are commonly used in areas such as heating and cooling in evaporators, air- conditioning equipment, thermal power plants, space vehicle, automobile etc. the passive techniques can play an important role in the heat transfer augmentation if a proper configuration of the insert is being selected depending on working conditions that have been reported in the literature .The experiments were carried out for twisted tape inserts at five different width ratios (W/D) of 0.35, 0.44, 0.53, 0.62 and 0.71 at constant twist ratio (H/D) of 2.5. The experimental results show that Nusselt number increases with increasing Reynolds number and increasing width ratio (W/D) of the swirl generators. Also the friction factor decreases with increasing Reynolds number and decreasing width ratio. The comparisons showed that, compared with the plain twisted tape, the twisted tape without rectangular hole has further improved convective heat transfer performance by about 1.40 and whereas lowered flow friction. The twisted tape with rectangular hole shows about 1.50 greater thermal performances than plain twisted tape.