Flow visualization and analysis of self-rewetting fluids in a model heat pipe

Experiments are performed to study the fundamental physics and the two-phase flow heat transfer in diluted water/butanol solutions with unusual dependence of the surface tension with temperature (self-rewetting fluids). The heat transfer and the dry-out behaviour are investigated in a grooved heat pipe model, increasing the power supplied at the evaporator section with an electric cartridge heater and cooling the condenser section with a water loop. The experimental configuration includes a top transparent window and a lighting system with diffused light, enabling visualization of the liquid in the groove with a CCD camera. Self-rewetting fluid behaviour is compared with ordinary heat transfer liquids, including an environmentally sustainable engineered liquid, Novec™ 7100 (Methoxy-nonafluorobutane), and bi-distilled water. Experiments with ethanol, butanol and a water/ethanol mixture have been also carried out to compare self-rewetting fluids with ordinary liquids and binary solution having no minimum in surface tension. It is found that the heat pipe filled with Novec™ exhibits good heat transfer performances at relatively low input power and temperatures, before the evaporator begins to dry. Similar behaviour is observed when ethanol is considered. For the heat pipe filled with water the evaporator becomes completely dry at larger power (40 W). At the same input power, in the case of the self-rewetting fluid, the liquid is strongly flowing from the cold to the hot region due to the inverse Marangoni effect and this beneficially, rewets the evaporator. This phenomenon is never observed when dry out is established with other working fluids. Numerical simulations are carried out to explain some experimental results in case of pure liquid. A possible explanation of rewetting in the case of water/butanol mixture is given.