Hydraulic nonuniformities at the exit of a heat-exchange collector in a fast-neutron reactor

Abstract

Distribution collector systems in the form of reverse axisymmetric 180 ~ turns are widely used in intermediate heat exchangers of fast-neutron reactors. In these exchangers the heat carrier flows from the neutral downpipe into a distribution collector, turns in this collector, and flows into a bundle of heat-transfer pipes which is installed in a pipe panel. A great deal of experimental data is now available on the distribution of the flow rate and the velocity of the heat carrier at the exit from distribution collector systems of this type [1-6], but there is no computational method for calculating the hydraulic nonuniformities at the exit from the system. Our objective in the present work is to develop a method for calculating the distribution of the mass flow rate of the heat carrier at the exit from the distribution collector systems with different ratios of the dimensions and hydraulic resistance in the exit part, such that the flow area of the gap between the bottom and the downpipe is not more than 10 times greater than the transverse cross-sectional area of the downpipe and the pipe panel is displaced from the end of the downpipe if the flow area of the gap is less than the transverse cross-sectional area of the downpipe or it is placed in an arbitrary position for other ratios of the indicated areas. A method for calculating the hydraulic nonuniformities at the exit from the axisyrnmetric distribution collector system without any additional constructional elements on the bottom was developed on the basis of the law of conservation of mass under the assumption that the thermophysical properties of the heat carrier remain constant and the flow of the heat carrier is a jet flow. To confirm experimentally the correctness of the computational relations, a cylindrical axisymmetric model of a distribution collector system of an intermediate heat exchanger for fast-neutron reactors with inflow at the center and outflow at the sides (Fig. 1) was investigated on an aerodynamic stand. The flowthrough part of the stand simulated the entrance part of the central downpipe, the distribution collector, and the pipe panel with a bundle of heat-transfer pipes. The collector height H, the height h of the entrance into the collector, the projection a of the central downpipe from the pipe panel, the gap between the step on the casing and the pipe panel, and the inner diameter d o of the diaphragms in the exit part of the heat-transport pipes were varied in the model. The heat carrier flow scheme adopted in this method is depicted in Fig. 2. The following basic assumptions and conditions were used for deriving the working relations. 1. In the distribution collector systems in which the flow area of the gap between the bottom and the central downpipe is less than the transverse cross-sectional area of the downpipe, the flow is compressed and in other cases the flow expands according to a law characteristic for the flow of a free submerged jet. In the first case the velocity of the flow increases and in the second case it decreases. For both constructions the flow characteristically turns at the bottom and the circular jet transforms into a flat jet which is then stabilized. The angle (~b) of unilateral expansion of the free submerged jet is assumed to be 15 ~ 2. The average velocity of the heat-carrier flow in the central downpipe at the bottom of the collector after the stabilization section and the rectilinear sections at the walls of the casing is constant. 3. The flow separates when the jet reaches the pipe panel. Part of the flow enters the openings of the pipe panel, located where the pipe panel meets the jet, and the rest of the flow spreads out along the perforated surface and the flow rate changes along the flow path. 4. The profile of the velocity in the jet striking the pipe panel depends on the ratio of the thickness of the jet and the height of the collector. As the ratio of the collector height to the jet thickness increases, the maximum velocity in the jet shifts from the periphery to the center and then in the opposite direction. For some value of this ratio the profile of the velocity in the jet stabilizes, and the velocity is maximum at the periphery of the flow.