EXPERIMENTAL STUDY ON LOW PRESSURE FLOW INSTABILITY

The experiment was performed on the test loop (HRTL-5), which simulates the geometry and system design of the 5 MW reactor. The flow behavior for a wide range of inlet subcoolings, in which the flow undergoes from single phase to two phase, is described in a natural circulation system at low pressure (p = 0. 1, 0. 24 MPa). Several kinds of flow instability, e. g. subcooled boiling instability, subcooled boiling induced flashing instability, pure flashing instability as well as flashing coupled density wave instability and high frequency flow oscillation, are investigated. The mechanism of flashing and flashing concerned flow instability, which ha$ never been studied well in this field, is especially interpreted. The experimental results show that, firstly, for a low pressure natural circulation system the two phase flow is unstable in most of inlet subcooling conditions, the two phase stable flow can only be reached at very low inlet subcoolings; secondary, at high inlet subcoolings the flow instability is dominated by subcooled boiling in the heated section, and at middle inlet subcoolings is dominated by void flashing in the adiabatic long riser; third, in two phase stable flow region the condition for boiling out of the core, namely, single phase flow in the heated section, two phase flow in the riser due to vapor flashing, can be realized. The experimental results are very important for the design and accident analysis of the vessel and swimming pool type natural circulation nuclear heating reactor.