Effect of Entry/Exit Length on Flow Distribution in the Test Bundle

In data analysis, the geometric information within the heated section of the rod bundle is important because the CHF occurs in the heated section. To ensure a constant geometry and to prevent adverse flow effects, it is required to extend the same geometry beyond the heated section of rod bundle geometry. Regarding to evaluate the validity of inlet boundary condition of subchannel analysis code, the effect of the entry and exit length on the flow distribution is evaluated under the various inlet flow conditions which could be produced without flow distributor or strainer. To evaluate the validation of the inlet and outlet boundary conditions used in the subchannel analysis code, a study on the effect of the entry and exit length on the flow distribution is conducted. Even though the non-uniform flow is entered inside the test bundle, the flow gets more saturated by the simple supports and frictions. Through the code calculation under various flow conditions, it is concluded that the flow is to be fully developed flow over about 40∼80 inches of the entry length. If the exit length is about 30∼40 inches, the effect of the exit pressure can be negligible. The entry and exit length in this paper is calculated based on only rod bundles and simple supports. By installing the flow distributors or strainer, these lengths can get shorter and the flow difference between the subchannels become smaller. This study could be very useful in order to confirm the validation of the boundary conditions used in the subchannel analysis code