Experimental investigations on flow characteristics of two parallel channels in a forced circulation loop with supercritical water

The instability of supercritical water flow inside two parallel channels is investigated experimentally in this paper. Two types of instabilities with various oscillation periods occur at different power region during the heating process, indicating the existence of two types of dynamic instabilities. The type I instability occurs when the outlet temperature of the test section surpasses the pseudo-critical temperature. Sudden increase of the pressure drop cause a big disturbance to the system which results in a periodic feedback between the pressure drop and the mass flow rate. Large oscillations were found in the entire system at low heating power during which the two parallel channels oscillates in phase with the whole system with a long oscillation period (20-300 s). The type II instability occurs only in the heated section at high powers at which the two parallel channels oscillate 180° out of phase while the rest of the system is stable. The period of type II instability is relatively small, i.e. 2-5 s. With Ambrosini's non-dimensional parameters, a stability map was generated using the obtained instability data. Analysis of the instability map showed the expected result that no instability occurs when the exit fluid temperature is below the pseudo-critical temperature regardless of the system pressure, inlet temperature, heating power, and local loss coefficient. The influences of inlet mass flow rate, inlet fluid temperature and system pressure on instabilities were also studied.