Ted Worosz

Considerations in the Practical Application of the Multisensor Conductivity Probe for Two-Phase Flow

Abstract This study investigates two issues in the practical application of the local conductivity probe for two-phase flow measurements. First, the effects of signal “ghosting,” an electrical interference inherent to multiplexing data acquisition systems, on the measured two-phase flow parameters are examined. A revised conductivity probe circuit is proposed to remove the effects of ghosting. The characteristics of signal ghosting are investigated experimentally with a specialized conductivity probe that enables concurrent acquisition of ghosted and unghosted signals within the same flow condition. It is demonstrated that ghosting causes bubble velocity measurements that are artificially high and, consequently, artificially low interfacial area concentration measurements that depend on sampling frequency and sensor impedance. The revised circuit successfully eliminates this variability. Second, the sensitivity of measured two-phase flow parameters to increasing data acquisition sampling frequency is investigated experimentally. Measurements are acquired at incrementally increasing sampling frequencies with a four-sensor conductivity probe in 13 vertical-upward air-water two-phase flow conditions with superficial liquid and gas velocities ranging from 1.00 to 5.00 m/s and 0.17 to 2.0 m/s, respectively. It is found that the void fraction and average bubble velocity are insensitive to the sampling frequency, while the detected number of bubbles and interfacial area concentration can demonstrate a strong dependence. Considerations for selecting appropriate sampling frequencies in different flow conditions are discussed.

Experiments in Cap-Bubbly Two-Phase Flows for Two-Group IATE Development

Abstract In the two-group interfacial area transport equation (IATE) used to calculate the interfacial area concentration (ai), bubbles are categorized into two groups. Namely, group-I consists of spherical/distorted bubbles, and group-II consists of cap/slug/churn-turbulent bubbles. Robust models for the major bubble interaction mechanisms that cause the transition from purely one-group to two-group flows are essential to the dynamic closure of the two-fluid model with the two-group IATE. Therefore, the present study seeks to establish an experimental database in cap-bubbly flows that highlights this transition to support model development. A four-sensor conductivity probe is used to obtain measurements of local time-averaged two-phase flow parameters, including the void fraction and ai, in vertical-upward air-water two-phase flows in a 5.08-cm pipe. Four flow conditions are investigated at ⟨jf⟩ = 2 m/s with increasing ⟨jg⟩ to study the generation and growth of group-II bubbles. Characteristic features of the local void fraction and ai distributions are discussed. Additionally, axial development of area-averaged void fraction and ai that is indicative of exchange between the bubble groups is presented.

Experiments on the Effects of a Spacer Grid in Air-Water Two-Phase Flow

Abstract Understanding the effects of spacer grids on the coolant flow through a nuclear reactor core is required for best-estimate design and analysis of the plant. The impact of a spacer grid on two-phase flows is of particular importance because the geometric effects of the grid can alter the two-phase flow structure and, consequently, the mass, momentum, and energy transfer characteristics. Therefore, a scaled separate-effects test facility is constructed to investigate the effects of a spacer grid on the hydrodynamics of air-water two-phase flow through a rod bundle. The test facility is scaled to maintain hydrodynamic and geometric similarity to single- and two-phase flows in a conventional pressurized water reactor and to facilitate detailed local measurements of two-phase flow parameters around the simulant fuel rods with a four-sensor conductivity probe. This paper presents measurements of local time-averaged two-phase flow parameters acquired upstream and downstream of the spacer grid with the conductivity probe in a representative subchannel of a 1×3 rod bundle for eight flow conditions. Characteristic features of the development of the two-phase flow parameters along the test section are discussed.