Assessment of heat transfer enhancement technique in flow boiling conditions based on entropy generation analysis: twisted-tape tube

Twisted-tape tubes are commonly used in practice to passively enhance heat transfer. A clear understanding of the effect of variations in geometrical parameters as well as flow conditions is necessary, most importantly when it comes to finding the conditions in which the application of twisted-tape tubes is justifiable. The entropy generation analysis is a suitable approach towards defining such criteria. Therefore, in this paper, the effects of tube diameter, twist pitch, mass velocity, vapor quality, and saturation temperature are discussed in detail through entropy generation analysis. The Irreversibility Distribution Ratio (IDR) and Bejan number (Be), which track the variations of the components of total entropy generation, are discussed in each of these conditions. Finally, favorable conditions for using twisted-tape tubes rather than plain straight tubes are distinguished by defining entropy generation number (N-s). The results showed that twisted-tape tubes with bigger diameters have higher entropy generations. Also, increasing twist pitch and, similarly, increasing saturation temperature result in lower entropy generation. In addition, higher qualities lead to higher entropy generations and, finally, by increasing mass flux, entropy generation initially shows a drop and later a growth. The results for N-s show that employing twisted-tape tubes in lower mass velocities (roughly G <= 80 kg/m(2)s) and lower qualities (roughly x <= 0.4) and, as well as higher saturation temperatures (roughly T-sat >= - 18 degrees C) is advantageous compared to plain straight ones.