A Simple Flow Stability Model for Figure-of-Eight Loop with Outlet Header Interconnect

Abstract

A simple algebraic model is developed to explain and predict the stability of flow to out-of-phase oscillation in the RD-14 figure-of-eight experimental loop. The model accounts for the pipe interconnecting the riser pipes between the outlet headers and steam generators, The model solves the linearized one-dimensional, homogenous conservation equations. The heater thermal response to the flow oscillation is also modelled. The flow oscillations are explained in terms of the response of pressure in the two-phase region to a change in the subcooled liquid flowrate. This response consists of two contributions, one arising from mass and another from enthalpy change in the two-phase region, Flow instability is caused by the component of the pressure response which is associated with the enthalpy change. The dominant function of the interconnecting pipe is the transfer of mass between the two-phase regions. This mass transfer has a restoring effect on the loop flow, Since the flow through the pipe is not, in general, in phase with the subcooled liquid flow, the pipe may not lead to increased flow stability under some conditions unless there is a non-zero steady state flow through it. However, by an appropriate choice of the pipe geometry, a stable flow can be obtained. A simple criterion for scaling the interconnecting pipe for stable flow is derived. It is shown that the flow can in general be stabilized by increasing the ratio of the flow area-to-length of the interconnecting pipe (i.e., by increasing the flow area and/or reducing the length of the pipe). Increasing the value of this ratio for the present interconnecting pipe geometry by a factor of 2.9 is shown to stabilize the flow in RD-14, The model predicts the flow stability trend observed in RD-14, namely that, at full power, the flow is unstable at full pressure and at reduced flowrate and stable at full flow and reduced pressure.