Study On Circumferential Non-Uniformity Of Heat Transfer In Tight Lattice

Due to its advantages such as increasing fuel utilization, tight lattice fuel assemblies are widely used in some nuclear reactors in operation or of conceptual design. Obvious nonuniform circumferential distribution of temperature exists in tight lattice, compared to regular rod bundles with larger pitch-to-diameter ratio. Since cladding temperature is one of the key design criteria for some advanced reactor design, it is necessary to carry out study on the behavior of circumferential heat transfer in tight lattice and develop numerical tools to predict circumferential distribution of temperature in the fuel rodTo investigate the circumferential non-uniformity of heat transfer in rod bundles, the approach of this paper is divided into several steps: firstly, to establish mathematical model and make theoretical analysis, based on which influential non-dimensional numbers are identified, and then an empirical correlation describing this phenomenon is proposed based on theoretical results and CFD sensitivity analysis; finally a three dimensional fuel rod model is introduced in sub-channel code to predict the non-uniform behavior of circumferential heat transfer.In this paper the following efforts are included.(i) The semi-analytical solutions of temperature distribution the rod bundle is obtained based on some assumptions.(ii) Nondimensional numbers which affect temperature field is summarized and the empirical correlation is proposed. The constants in this correlation is determined by CFD calculation.(iii) The correlation is employed as boundary condition of the three dimensional fuel model in subchannel code, and the new model is applied in analysis of different fuel assemblies.The results of this paper indicate that(i) For fluid of constant property, the distribution local heat transfer coefficient and wall temperature is almost a cosine function, with gap temperature being higher than central region in a sub-channel;(ii) The distribution of local heat transfer is affected strongly by pitch-to-diameter ratio. Because of the flattening effect of cladding, it is also affected by thermal conductivity ratio of cladding to fluid. When the fluid is of very low Prandtl number the non-uniformity becomes stronger with the decreasing Prandtl number;(iii) The non-uniform fuel rod model developed for the sub-channel code has the ability to calculate circumferential variation of temperature in the fuel rod, and the results agree well with the existing experimental data.The main objective of this thesis is to provide an approach of developing a model to predict circumferential non-uniformity of heat transfer in sub-channel code. The results achieved so far can be applied to the thermal-hydraulic analysis and code development for advanced reactor.