Experimental study of relative velocity and drag force in large diameter pipes

Joshua P Schlegel, Purdue University

Abstract

The US NRC is planning to use advanced best estimate thermal hydraulics computer codes to analyze the safety systems of next-generation nuclear power plants in order to streamline the licensing process and improve the viability of nuclear power for electric utilities. In advanced BWR designs the chimney section above the core is generally modeled as a large diameter pipe. Advanced codes utilize the two-fluid model, which treats both phases with separate equations for conservation of mass, momentum and energy. The two-fluid model has been shown to work extremely well; however it has one weak link in the constitutive models for the interfacial interactions. For this approach to work, the computer codes require accurate models for a wide variety of two-phase flow phenomena. One of the interfacial transfer terms is the interfacial drag force, which is essential for accurately calculating the void fraction in dispersed bubbly flow. Current advanced codes utilize a drift-flux based approach to this modeling. As each of these flow regimes differ significantly in their governing physical mechanisms, accurate flow regime data is also essential to any modeling effort. It has been found that insufficient data exists to accurately assess the existing models for interfacial drag in large diameter pipes at high void fractions. In light of this, data has been collected in pipes of diameter 0.152 m and 0.203 m with liquid velocity conditions of 0.05 m/s to 1.0 m/s, gas velocities of 0.01 m/s to 5.0 m/s and void fractions from 0.05 to 0.8 in order to augment the existing void fraction database, develop accurate models for the flow regime transitions, and evaluate the current models for the prediction of interfacial drag in large pipes. A new approach to modeling the drag force in the two-fluid model, a scheme for drift-flux modeling in pipes and a new transition model for the cap bubbly to churn-turbulent flow regime transition have been proposed. The use of impedance meters in pipes with diameters up to 0.203 m was validated, Pressure was found to have negligible effect on the accuracy of the recommended drift-flux models, Hydraulic diameter was found to have no effect on the two-phase flow for non-dimensional diameters greater than 55 and axial development of the flow regime was found to have little effect on the void fraction prediction. Finally, a new method of flow regime identification has been proposed and the recommended drift-flux models for pipes have been validated. In addition, though the proposed flow regime transition criterion has not been validated, what results are available from the flow regime identification seems to support the proposed transition. Additional work is required to find an accurate method of determining the flow regime in large pipes through the use of impedance meters. Also, additional work should be done collect local data so that interfacial area relations can be developed in large pipes and the proposed model for interfacial drag can be validated.

Degree

M.S.

Advisors

Ishii, Purdue University.

Subject Area

Mechanical engineering|Nuclear engineering

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