Experiments with a fluid characterized by a high value of Prandtl number in a multi-interconnected single-phase natural circulation loop

Single-phase natural circulation is used in many fields of engineering because it allows heat to be transferred from a hot source to a cold sink without the use of active parts such as pumps and fans. Particular attention must be paid to the choice of heat transfer fluid, which should ensure heat transfer without triggering phase transition phenomena between liquid and vapor. The fluids used to date are in liquid phase or gaseous phase. In both cases, there are limitations due to temperature (liquid-vapor transition) and operating pressure (gas phase). It is possible to increase the operating temperature without causing a liquid-vapor transition by using a mixture of water and glycerin. In this work, a mixture of 55 % glycerin and 45 % double-distilled water (by weight) was used in a natural circulation loop circuit containing three parallel interconnected loops. This mixture, as the operating temperature varies, exhibits a Prandtl number ranging from 27 to 75 at temperatures between 20 °C and 50 °C. Fluids with a high Prandtl number are characteristic of molten salts used in new-generation nuclear power plants. The tests were conducted both as the power varied and as the angle of inclination of the interconnected circuits varied. The circuit located on the upper part was the most susceptible to inclination, showing lower thermal performance than the other circuits even when maintained at constant power. Moreover, a new Figure of Merit is proposed that serves to compare the behavior of the thermal circuit with other fluids on reduced gravity fields.