Two-phase flow interfacial structures in a rod bundle geometry

Sidharth S Paranjape, Purdue University

Abstract

Interfacial structure of air-water two-phase flow in a scaled nuclear reactor rod bundle geometry was studied in this research. Global and local flow regimes were obtained for the rod bundle geometry. Local two-phase flow parameters were measured at various axial locations in order to understand the transport of interfacial structures. A one-dimensional two-group interfacial area transport model was evaluated using the local parameter database. Air-water two-phase flow experiments were performed in an 8 X 8 rod bundle test section to obtain flow regime maps at various axial locations. Area averaged void fraction was measured using parallel plate type impedance void meters. The cumulative probability distribution functions of the signals from the impedance void meters were used along with a self organizing neural network to identify flow regimes. Local flow regime maps revealed the cross-sectional distribution of flow regimes in the bundle. Local parameters that characterize interfacial structure, that is, void fraction α, interfacial area concentration, ai, bubble Sauter mean diameter, DSm and bubble velocity, vg were measured using four sensor conductivity probe technique. The local data revealed the distribution of the interfacial structure in the radial direction, as well as its development in the axial direction. In addition to this, the effect of spacer grid on the flow structure at different gas and liquid velocities was revealed by local parameter measurements across the spacer grids. A two-group interfacial area transport equation (IATE) specific to rod bundle geometry was derived. The derivation of two-group IATE required certain assumption on the bubble shapes in the subchannels and the bubbles spanning more than a subchannel. It was found that the geometrical relationship between the volume and the area of a cap bubble distorted by rods was similar to the one derived for a confined channel under a specific geometrical transformation. The one dimensional form of the interfacial area transport equation was evaluated against the one dimensional area averaged data obtained in the rod bundle. The model was evaluated for the overall development of the flow structure. The effect of spacer grid was not considered in the model evaluation scheme, in view of the observation from flow regime identification, that the flow structure developed within a short distance downstream of the spacer grid. The evaluation results showed that one-group interfacial area transport model agreed well with the experimental data. However, the two-group model prediction deviate from the experimental data. The model predicted experimental data within 30% for flow conditions with lower liquid flow rates. The model needs further developments for it to be applicable to a wide range of flow conditions.

Degree

Ph.D.

Advisors

Ishii, Purdue University.

Subject Area

Nuclear engineering

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