Chris Hoxie

Experimental Investigation of Inverted Annular Film Boiling in a Rod Bundle during Reflood Transient

Abstract Heat transfer results for subcooled and saturated inverted annular film boiling (IAFB) obtained from a 7×7 rod bundle during transient reflood are presented in this paper. The test section consists of heater rods of 9.5-mm diameter and 12.6-mm pitch arranged in a square array. Flooding rates considered are 0.076 and 0.152 m/s, pressure varied from 138 to 414 kPa, and inlet subcooling up to 83 K. Evaluation of the data includes estimation of the local void fraction and Nusselt number during IAFB as well as in the inverted slug film boiling (ISFB) regime, which occurs when the inverted annular liquid column disintegrates. Experimental heat transfer results are compared with several film boiling models, and a new correlation for the Nusselt number is proposed for the IAFB and ISFB regimes. Predicted Nusselt numbers using the new correlation deviate from the experimental data by an average error of 15% and root-mean-square error of ∼30%.

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.

Experimental Studies of Spacer Grid Thermal Hydraulics in the Dispersed Flow Film Boiling Regime

Abstract Spacer grids have been found to enhance downstream convective heat transfer and to strongly influence droplet size distributions through early spacer grid rewet and droplet breakup. Existing models for enhancement of heat transfer and droplet breakup, however, do not appear to accurately account for these interactions between the coolant and the spacer grid. Data from two series of rod bundle heat transfer tests, low injection rate forced reflood tests, and droplet injection tests are presented in this paper to describe the effects of the spacer grids during dispersed flow film boiling. Heat transfer downstream of the spacer grids is clearly enhanced by the presence of the droplets, while the downstream droplet size was found to depend on the condition of the spacer grid: dry or wetted. Results of this study demonstrate the need to adequately account for the separate modes of dry and wet spacer grid heat transfer enhancement in predicting the thermal-hydraulic behavior during reflood transients.

Effects of Substrate Materials and Surface Conditions on the Minimum Film-Boiling Temperature

Abstract Film boiling is an important phenomenon in the evaluation of an emergency core cooling system following a hypothetical loss of coolant accident in a nuclear reactor. This study investigates the effects of liquid subcooling, surface oxidation, and surface materials on the minimum film-boiling temperature . Quenching experiments were performed using stainless steel and zirconium (Zr) test samples. The samples were heated to a temperature well above then plunged vertically in various degrees of liquid subcooling pools. A visualization study using a high-speed camera was conducted to capture the quenching behavior. Additionally, surface characterization analyses including X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy were performed to quantify the surface conditions. Results indicate that liquid subcooling has a strong influence on . The visualization study shows a very thin vapor formation around the test sample for higher subcooling pools which explains the enhancement in the heat transfer. It is observed from the surface characterization analyses that the variations in the surface condition of the stainless steel and Zr causes the vapor bubbles to depart differently in the nucleate boiling regime. Furthermore, the effect of surface oxidation is clearly noticeable in the Zr test sample compared to the stainless steel test sample due to the oxidation kinematic of each substrate material. It is found that the substrate thermophysical properties have a significant impact on . Comparing the bare substrates shows that for the same degrees of liquid subcooling pool, the value of for the Zr sample is ∼30°C to 60°C higher compared to the stainless steel sample. Moreover, increasing the degrees of liquid subcooling contributes to a significant increase in that varies between ∼50°C and 70°C for both samples.