Conference Agenda

The study of boiling flow is crucial for designing safety features of chemical and nuclear plants. The boiling behavior and bubble dynamics depend on various system parameters such as pressure, subcooling, mass flux, heat flux, and flow geometry. While a previous flow boiling experiment by the authors’ laboratory has revealed valuable parametric effects in a decent operational range, its setup alone is insufficient to reveal the influence from the channel width. A new dataset of upward flow boiling is therefore collected in a narrower annular test section. Compared to the previous configuration, this new channel has a larger inner diameter of 25.4 mm and the same outer diameter of 38.10 mm, with a heated inner rod of 3-m long. Multi-sensor conductivity probes are adopted measuring void fraction, gas velocity, and interfacial area concentration following a two-group description. Traversing mechanisms are employed allowing the probes to scan across the flow area, and a dedicated sensor pattern is designed and validated to minimize near-wall blind zones for the narrow channel. In addition, high-speed visualization is conducted recording axial flow evolution, and the Onset of Nucleate Boiling and Point of Net Vapor Generation are identified and recorded. Parametric studies are also presented investigating the flow field dependence on systematic boundary conditions. This work presents valuable new experimental data on narrow-channel flow boiling.

Due to the fuel self-shielding effect, reactor irradiation, and component arrangement, plate-type fuel elements exhibit significant transverse non-uniformity in heat generation. Consequently, the internal thermal-hydraulic characteristics of coolant channels may differ from those of conventional channels. To address this, experimental studies on single-phase flow and heat transfer in narrow rectangular channels with transverse non-uniform heating were performed using PIV visualization technology. Analysis of the flow and heat transfer characteristics in narrow rectangular channels revealed the following: Time-averaged velocity field analysis demonstrated that low-velocity regions in laminar flow regimes are more prominent compared to turbulent flow regimes, while both laminar and turbulent regimes exhibit distinct stratification phenomena in transitional flow regions. In laminar regimes, non-uniform heating reduces the nominal boundary layer thickness , with no observed correlation between velocity gradient and heating power magnitude. In turbulent regimes, non-uniform heating increases the nominal boundary layer thickness, and a decreasing trend in velocity gradient is observed with increasing non-uniform heating power.

Under the conditions of a reactor accident, heat transfer at the wall can be hindered, leading to the risk of boiling crisis. The calculation of the near-wall void fraction is crucial for predicting the boiling crisis. In narrow rectangular channels, large-scale interfaces exist near the wall due to geometric constraints. This paper is based on an improved two-phase multi-scale interface model that considers the interfacial transfer of concentration, momentum, heat, and mass for bubbles of different scales. The model is embedded within the Eulerian two-fluid model in Fluent. The results from the modified model were compared with experimental data, validating the accuracy of the model.

Flow reversal in narrow coolant channels is a critical phenomenon for the safety of research reactors, especially those designed with a downward nominal flow. During a loss of forced flow event, the downward movement of the coolant may temporarily halt before transitioning to an upward natural circulation. Fuel damage can result if dryout occurs and the fuel or cladding temperatures exceed safe limits. This study provides an in-depth examination of flow reversal in narrow rectangular channels by analyzing experimental results and an analytical approach. The analytical approach utilizes the calculated steady-state system pressure drop versus flow curves for different heat flux values. A review of the relevant literature was conducted, and selected experimental data were utilized to benchmark against the flow reversal limits predicted by the analytical approach. The experimental data is selected from tests involving flow reversal in a narrow rectangular channel. The findings were compared with successful flow reversal test data and predicted dryout power under dryout conditions. Also, the study examined the effects of inlet liquid temperature, system pressure, and localized pressure drops. The analytical approach provides physical insights into the flow reversal phenomenon. The approach may be used in conjunction with thermal-hydraulic analysis software, strengthening the confidence in the software predictions.

The present study presents a numerical investigation into the fluid flow and heat transfer performance of a straight mini-channel with an additional inter-connected mixing area in a heat sink plate. The influence of the dimension of the inter-connected area on the thermal-hydraulic performance was examined. Three different sizes of the inter-connected area, defined in terms of aspect ratio (AR), were studied to understand their effect on the thermal-hydraulic performance. Water was used as the coolant, flowing in a single-phase regime under turbulent conditions at Reynolds numbers ranging from 1000 to 4000. A constant heat flux of 10 kW/m2 was applied to both surfaces of the cooling plate. The grid independence test showed a deviation of less than 2% for the friction factor and Nusselt number, while the GCI results indicated that the deviation of the friction factor and Nusselt number was less than 2% within an asymptotic range around 1. The results demonstrated that the aspect ratio of the inter-connected area has an impact on the thermal and hydraulic performance. Both the friction factor and Nusselt number decreased with an increase in the size of the inter-connected area. Furthermore, this study revealed that the interconnection zone created two stationary circulation zones, which influenced the velocity and temperature contours. Finally, a new correlation was developed to explain the relationship between the friction factor and Nusselt number in terms of the Reynolds number and the aspect ratio of the inter-connected area.