Experimental investigation of single-phase natural circulation in a mini-loop driven by heating and cooling fluids

Abstract

The heat transfer characteristic of natural circulation in a rectangular mini-loop is experimentally investigated with the height 250 mm and width 325 mm. Heating and cooling sections of the mini-loop are horizontally constructed with 245 mm length. The uniform diameter of the mini-loop is 4 mm with a wall thickness 2 mm. Distilled water is chosen as the loop fluid and secondary working fluids. The effects of heating fluid temperature and mass flow rates of both secondary fluids are investigated with a fixed cooling fluid temperature. The comparison between the experimental results and CFD simulation is performed. The experimental results show that thermos-hydraulic behavior of the mini-loop always tends to be stable from an overshoot and the required time of start-up from quiescent state becomes shorter with the increase of heating fluid temperature and mass flow rates of both secondary fluids. As for the steady-state results, Reynolds number and heat transfer rate of natural circulation are between 9.7 and 43.4, 1.46 W and 6.2 W, respectively, and increase with the increase of Rayleigh number. From the thermal resistance analysis, it is found that the thermal resistance between heating fluid and wall surface is very small, thus the mass flow rates of secondary fluids have no significant effect on the thermal performance of the mini-loop. Besides, the real driving temperature difference and Nusselt number of loop fluid in heater and cooler are also given.