Study On Flow And Heat Transfer Features Of Fuel Rod Bundles With Spacer Grids Under Unsteady Condition

The reactor during normal working always has the problem of unsteady flow results in instability of flow field and heat transfer among rod bundles. CFD was used for simulating the state of the flow field and heat transfer among rod bundles to study flow instability. Besides,the paper analyzed the unsteady state influence on simulating result and phenomenon of the flow field.The paper concluded the relevant articles about the flow field and heat transfer among the rod bundles and unsteady flow with regular cross-section. According to the latest progress about the rod bundle channel researches, a new research direction was put forward. The single span rod bundle channels with spacer grid are set as the model which was simplified for the complicate structure in spacer grid. Using CFD was a popular way to divide structured and unstructured grid and analyze grid number sensitivity.Secondly, this paper mentioned the control equations of computational fluid dynamics and turbulence model based on the equations including the applicable scope of the model,the applicable objects and so on. According to the characteristics of the rod bundle channels,some suitable turbulent models were put forward and compared the results with experiments.After comparison, the most suitable model was chose.Finally, comparing the heat transfer between laminar flow and turbulent flow in the rod bundle channels. Many parameters had been considered such as the pressure drop, the flow resistance coefficient, local resistance coefficient and the eddy current mix factor, cross flow mix factor and turbulent intensity factor aiming at the function of spacer grid. The contour of cross-sections upstream, downstream and spacer grid were analyzed. The obvious eddy phenomenon was found downstream spacer grid. The secondary flow seemed more obvious under turbulent flow. The eddy phenomenon disappeared earlier in the turbulent flow. The simulation result showed: in the steady state, the spacer grid had a great influence on the flow characteristic, the mixing vanes would change the direction of the flow after the fluid flowed through. The eddy generated in the center of rod bundle channels, and flow resistance increased. In uniform acceleration state, with the increasing of absolute value of the acceleration, slope of flow characteristic curve the increases as well. In pulsating flow.Resistance pressure drop of one cycle characteristic curve is flat oval, as the increase of pulsating period, the pressure drop under the same Reynolds number gap decreases. When the pulsating period is the same, with the increase of relative fluctuating value, Re number and ?p changed the proportional relationship to the exponential relation. On the aspect of heat transfer under steady and unsteady state, the distribution of surface temperature and Nusselt number along the circumferential were analyzed. Heat transfer coefficient is found higher downstream spacer grid, mainly by cross flow and uniform temperature effect caused by local components and mixing vanes,the above two factors increased downstream area of the heat transfer intensity.