“…Now, it is successfully in practical use in district heating and cooling systems [40,20]. Regarding the drag reduction of cationic surfactant solutions, the rheological properties [29], the effect of molar ratio of counterion [22,21,38], the diameter effect [37,11], the heat transfer reduction [1,28,32,33], the degradation by age [24,23], the very dilute drag reduction [14], and the high-shear drag reduction [36] have been well investigated so far.…”