Study on the transient and stability behaviour of a boiling two-phase natural circulation loop with Al2O3 nanofluids

The transient and stability behaviour of a boiling two-phase natural circulation loop with water and water based Al2O3 nanofluid have been investigated. For this, experiments were conducted in a natural circulation loop at different operating pressures with small incremental step power raising process with water and Al2O3 nanofluid having concentration of 1% by wt. separately. The experimental facility consisted of a vertical tubular heater directly heated by electric current up to a maximum power of 50 kW and pressure varying from 1 to 20 bar. The inner diameter of heated section is 52.5 mm and that of other sections in the loop is 49.25 mm. The height of the loop is about 3.35 m and width is about 3.4 m. Sub-cooled water enters the heater at the bottom and gets heated as it rises through the test section due to buoyancy resulting circulation. The test results revealed that while the natural circulation flow behaviour with rise in power is similar with water and Al2O3 nanofluid in single-phase condition, however, the buoyancy induced flow rates are found to be relatively higher with nanofluid than with water alone for corresponding operating conditions. In addition, with nanofluid, the boiling induced Type I flow instabilities are found to be significantly suppressed. The operating pressure is found to have negligible effect on single-phase natural circulation, both for water and nanofluid. However under boiling conditions, the flow instabilities are strongly reduced with increase in pressure. Besides, it is observed that boiling is induced at lower power with nanofluid than with water for the corresponding operating condition.