Nanofluids: A novel promising flow stabilizer in natural circulation systems

A natural circulation system operates on the basis of natural laws like gravity and buoyancy. Although natural circulation is a benign gift of nature for applications to several heat removal systems due to the simplicity in design, elimination of hazards related to pumps, better flow distribution, cost reduction, etc. However, the potential threat of flow instabilities still eludes for its wide applications. Although addition of local losses (orificing) may suppress instabilities, however, it is accompanied by significant flow reduction which is detrimental to the natural circulation heat removal capability. In this note, we have demonstrated experimentally, with nanofluids, not only the flow instabilities are suppressed but also the natural circulation flow rate is enhanced. The purpose of this research note is concerned with the use of metal oxide nanofluids to suppress the instabilities and enhance the flow rate in a natural circulation loop induced by a heating-cooling system. These findings were demonstrated experimentally by comparing the steady state flow rates and instability of natural circulation between water and that with three different nanofluids (Al2O3, CuO, and TiO2) having different particle sizes and concentrations. The main focus of this research was to develop a technology to eliminate the flow instabilities generally associated with the natural circulation loop without degrading the natural circulation flow rate. To substantiate the facts, we conducted experiments in a natural circulation loop with geometry as shown in Figure 1. The test facility resembles rectangular in geometry with circular flow cross-section area. The geometry is relevant to that of solar water heaters and nuclear reactors. The pipes are made of glass with inner diameter of around 26 mm. Important dimensions of the loop are shown in Figure 1. The loop was heated with electric wire which was wrapped around uniformly on the outer surface of the glass tube in the bottom horizontal leg. It was cooled at the top through a tube-in-tube type heat exchanger with tap water flowing through the annulus. An expansion tank was provided at the topmost elevation to accommodate the volumetric expansion of the fluid. It also ensures that the loop remains full of water. Thermocouples were installed at different positions in the loop to measure the instantaneous local temperature. The flow rate was measured using a differential pressure transducer installed in the horizontal leg of the loop. The instruments were connected to a data acquisition system which could scan all the channels in less than one second. The secondary side cooling water flow rate was measured with the help of a rotameter. The loop was insulated to minimize the heat losses to the ambient. Experiments were conducted for different power transients which are typical to that of a power raising and setback phenomena in any power generating system.