Experimental study on flow instability in parallel channels with supercritical water

Due to the urgent need for validation data in supercritical flow instability analysis, an experimental study has been performed in Nuclear Power Institute of China (NPIC) on flow instability in two parallel channels with supercritical water. The heated pipes have a length of 3000 mm and inner and outer diameters of 6 mm and 11 mm. Experimental conditions included pressures of 23–25 MPa, mass fluxes of 600–800 kg/m2 s, and inlet temperatures of 180–260 °C. In the experiments heat flux was increased gradually to obtain possible parallel instability boundaries while other parameters were adjusted in advance to desired values and kept constant. The evolution of flow rates during the heat flux increase has been analysed and divided into four stages. Discussions indicate that the asymmetry of flow rate between the parallel channels would be enlarged with relatively higher fluid temperature or total mass flow rate, subsequently making the occurrence of parallel flow instability more difficult in the experiments. The way of defining the onset of parallel instabilities has been proposed for supercritical water based on the experimental phenomena. Short-life transients are disregarded and only the sustained out-of-phase oscillations accompanied by evident amplitude enlargement are considered. Parametric studies show that the flow becomes more stable with increasing pressure or decreasing inlet temperature in the range of present experiments, and the mechanisms have been discussed compared to that for two-phase flow. Finally, the stability boundaries are illustrated in a two-dimensional plane using two dimensionless parameters proposed for supercritical flow.

► Flow instability experiments have been carried out with supercritical water.
► Validation data was obtained for numerical analysis.
► The evolution of flow rates in stability experiments was shown and discussed.
► The way of defining the onset of flow instability was proposed.
► The flow is more stable with increasing pressure or decreasing inlet temperature.