Simulation of the formation of slugs in a vertical-to-horizontal obstructed flow

During a postulated Loss of Coolant Accidents, the emergency cooling water injected into the inlet and outlet headers enters to the fuel channels through the feeder pipes. Steam produced in the feeders and in the fuel channels may flow in the direction opposite to that of the water, thereby creating vertical-to-horizontal Counter-Current Flow (CCF). The rate at which the cooling water enters the fuel channel may be limited by the flooding phenomena that entrains the water in the same direction as the steam flow. Thus, the steam flowing in the direction opposite to the cooling water can bring about the formation of slug flow. Long slugs of liquid moving at relatively high speed are transported back towards the headers by the steam. This phenomenon substantially reduces the amount of cooling water that can reach the reactor core. CCF experiments using a vertical-to horizontal test section connected by 90o elbows, with an orifice installed in a horizontal leg, were carried out. During the experiments the water and air flow rates, pressures and void fraction distributions were measured. The slug propagation velocity was obtained from the void fraction signals. Moreover, it have been observed that 'soliton-type' waves generated close to the elbow propagate in the horizontal leg towards the orifice, where a partial reflection takes place. The reflected waves move in the opposite direction to that of the incident wave. Since the soliton waves are periodically generated, the incident and reflected waves interfere at some place in the horizontal leg. If the amplitude of the interference wave is high enough, the bridging of the tube occurs, which generates the slugs. Even though, for co-current flows, the formation of slugs has been explained in terms of the Kelvin-Helmoltz instability criterion, it has been not observe that the slugging phenomena were triggered by this type of instability. In this paper a model based on the Boussinesq nonlinear system of equations, that explain the formation of slugs in a CCF is presented. The predictions of the model were then compared with the slug propagation velocity data; in general, a good agreement between the predictions and the data was found. (author)