Study On The Characteristics Of Flow And Heat Transfer In Manifolds Of Heat Exchanger

During severe accidents the integrity of double-layer concrete containments which are final barriers to prevent outward leakage of radioactive material is of importance. Therefore,new generation nuclear plants adopt not only active spray systems, but also passive containment cooling systems (PCCS) to exhaust heat from the containments. Key devices of the PCCS are often the type of manifold heat exchangers, whose flow distribution and flow loss characteristics are crucial for the safe and steady operation of the PCCS. Thereby it is necessary to study on the flow and heat transfer characteristics in the manifolds of heat exchanger.By using the ANSYS FLUENT CFD software the pressure distribution in the headers,the flow distribution, the flow loss and the heat transfer performance in the manifolds of heat exchanger are studied in this paper. The pressure distribution in the headers determines the flow distribution in manifolds of heat exchanger. For the momentum regaining manifolds of heat exchanger, when the numbers of tubes are less than 30 to 40, U-type arrangement is the best choice, because the uniformity of flow distribution is better than Z-type arrangement and the flow loss is less than central-type arrangement. When the numbers of tubes further increase, the central-type arrangement is the best choice, because the flow distribution uniformity of central-type is better than U-type and Z-type arragements. The research on the header matching of the U-typed manifolds of heat exchanger shows when the area ratio of the dividing header to the combining header is about 0.6 the flow distribution is very unifom. In addition the research on the entrance effect of reducer union shows that it may enhance the uniformity for U-typed manifolds of heat exchanger, and easily leads to backflow phenomenon for Z-typed manifolds of heat exchanger.The entrance effect caused by T-junction in the dividing header affects the flow characteristics of the central-type manifolds of heat exchanger. In the visualized experiments and the numerical simulations the first vortex and the second vortex are observed. The numerical simulations show that the first vortex is the main factor affecting the flow distribution in central-type manifolds of heat exchanger. Adjusting the distance between two tubes near the T-junction (i.e. central pitch) can weaks the effect of the first vortex on the flow distribution, and when the central pitch is ten times the diameter of the dividing header the entrance effect of the T-junction almost disappears. Because the entrance effect of the T-junction can enhance uniformity of the flow distribution six times the diameter of header is better choice. The analysis on the header sizes of central-type manifolds of heat exchanger shows that increasing the header diameters reduces the flow loss and enhance the uniformity of flow distribution. When the area ratio of the flow cross-section of the all tubes to the header is less than 4.0, SEi (the factor used to measure the integral flow uniformity of the manifold exchanger) is less than 5.0, the impact of the header diameter on the flow distribution is relatively small. In addition, the diameter of dividing header should be greater than combining header.The influence of the flow maldistribution on the performance of manifolds of heat exchanger is analysed from two aspects of flow resistance and heat transfer. In the aspect of flow resistance, the calculation method of flow loss in manifolds of heat exchanger was proposed,and the flow loss increases with the flow distribution uniformity decreasing. The influence of the flow maldistribution on heat transfer is small. The relative deviation of the flow loss between the heat exchangers with the SEi of 7.6 and 20.0 is 46%, but the total heat transfer coefficient of the manifolds of heat exchanger with SEi of 10.0 is only lower 2.85%than the manifolds of heat exchanger which flow distribution is uniform.