Experimental and numerical investigations on the reverse flow phenomena in UTSGs

•The reverse flow in UTSG is investigated experimentally and numerically.•A new method is developed to model the flow and heat transfer in UTSGs.•The transient process from normal flow to reverse flow is analyzed.•The U-tube length ratio has a great effect on the critical condition of reverse flow.

Aiming at the reverse flow phenomena in the inverted U-tube steam generators (UTSGs), the experimental and numerical simulations are performed. A new method is developed to model the flow and heat transfer in the steam generator based on the system analysis code RELAP5/MOD3.3. The reverse flow phenomenon observed experimentally is simulated well by the new method. The experimental and numerical results show that the reverse flow occurs in the adjacent shorter U-tubes. For single U-tube, the mass flow rate of reverse flow is generally greater than that of normal flow U-tube. When the reverse flow occurs, the negative pressure drop between the inlet and outlet plenums and the heat transfer of the UTSG reduce significantly. The numerical simulations also show that the reverse flow occurs more easily in UTSGs with the bigger tube length ratio.