SIMULATION OF THE UNSTABLE OSCILLATORY BEHAVIOR OF SINGLE-PHASE NATURAL CIRCULATION WITH REPETITIVE FLOW REVERSALS IN A RECTANGULAR LOOP USING THE COMPUTER CODE ATHLET

Experiments carried out in three rectangular loops differing in diamet er showed instability in the loop with the largest diameter. The obser ved oscillatory behavior involves repetitive flow reversals, the frequ ency of which is regular (periodic) at low powers and irregular (chaot ic) at high powers. The flow reversal frequency increases with heater power. Due to the flow reversals the Reynolds number vary between + 10 000 and - 10 000 causing the flow to pass through the laminar and tur bulent regime repetitively. Simulation of the oscillatory behavior usi ng the computer code ATHLET did not show instability with a coarse nod alization. The nodalization was then progressively refined and with a fine nodalization ATHLET was able to predict the instability behavior and to reproduce all the essential characteristics of the observed osc illatory behavior.