Experimental investigation and numerical simulation of instabilities in a multi-parallel channel two-phase natural circulation system
ENGLISH ABSTRACT: In the present study, two-phase natural circulation flow in a multi-parallel channel system is investigated using experimental and numerical modelling. The experimental model consisted, essentially, of four 25 mm diameter and two-meter long vertically orientated transparent polycarbonate pipes connected to a common manifold at the bottom and a relatively large steam drum at the top; three one-meter long electrical resistance heating elements were inserted into the lower end of three of the vertical pipes. Tests were conducted using different combinations of input power and as-defined and so-called open, closed and heat pipe system operating modes. A water-cooled condenser was placed in the upper portion of the steam drum and an expansion tank was connected to the lower manifold. For different power inputs and operating modes twelve temperatures and three flow rates as a function of time were recorded. In this way start-up transients and dynamical oscillatory responses were captured. So-called Type I instability were observed at low power inputs and open system operating modes (system open to the atmosphere). Type II instabilities and flashing instability were observed at medium and high-power excitations for the open system mode of operation. The fluid flow became more stable and less oscillatory at all power excitations for the closed system operation mode (system not open to the atmosphere). For the heat pipe mode of operation so-called geysering, followed by flashing-induced boiling was observed. After boiling had commenced downward single phase flow was invariable noted to occur in the central of the three hearted risers, even when all three heater power inputs were the same. Also, after boing had started a further increase in power input did not necessarily result in an increase in flow rate. The experimental system was discretized into a number of control volumes. The conservation equations, mass, momentum and energy were applied to each control volume and a set of time dependent temperature-coupled finite difference equations simulating the thermal-hydraulic behaviour of the system thus derived. This set of difference equations was solved using an explicit solution method. An encouragingly-good correspondence between the experimental and theoretical simulation model of the temperature and flow rate in the system was obtained.