CFD Modeling of Wall Steam Condensation: Two-Phase Flow Approach versus Homogeneous Flow Approach
The present work is focused on the condensation heat transfer that plays a dominant role in many accident scenarios postulated to occur in the containment of nuclear reactors. The study compares a general multiphase approach implemented in NEPTUNE_CFD with a homogeneous model, of widespread use for engineering studies, implemented in Code_Saturne. The model implemented in NEPTUNE_CFD assumes that liquid droplets form along the wall within nucleation sites. Vapor condensation on droplets makes them grow. Once the droplet diameter reaches a critical value, gravitational forces compensate surface tension force and then droplets slide over the wall and form a liquid film. This approach allows taking into account simultaneously the mechanical drift between the droplet and the gas, the heat and mass transfer on droplets in the core of the flow and the condensation/evaporation phenomena on the walls. As concern the homogeneous approach, the motion of the liquid film due to the gravitational forces is neglected, as well as the volume occupied by the liquid. Both condensation models and compressible procedures are validated and compared to experimental data provided by the TOSQAN ISP47 experiment (IRSN Saclay). Computational results compare favorably with experimental data, particularly for the Helium and steam volume fractions.
Frequency analysis of chugging condensation in pressure suppression pool system with pattern recognition
Abstract Direct contact condensation (DCC) phenomena in boiling water reactor (BWR) pressure suppression pool systems need to be understood to properly assess the performance of the pool as a heat sink and as a safety critical structure. Condensation oscillations in the form of chugging are challenging to predict by computational fluid dynamics (CFD) methods but safety relevant because of associated high dynamic loads on in-pool structures and the pool itself. Recently, new measurement methods for CFD validation purposes have become available. One of these techniques is visual observation using the high-speed cameras and suitable data processing method. Pattern recognition is a well suited technique for the determination of large oscillating bubble dynamics in a pressure suppression pool. In this work, the formation and collapse of the steam bubbles in chugging condensation mode are evaluated by using the pattern recognition algorithm. The pattern recognition algorithm is based on video material recorded during the direct contact condensation experiment DCC-05 of the PPOOLEX test facility. The formation speed, the shape and size of the steam bubbles and the acceleration of collapsing bubbles are estimated with the algorithm. Fast Fourier transform (FFT) is used for frequency analysis of the pattern recognized data. The frequencies found are compared to the frequency data of the pressure transducers collected during the experiment and to the previous results of the NEPTUNE_CFD simulations of the same experiment. The frequency analysis shows that the chugging frequencies of the steam bubbles range from 1 to 3 Hz, as predicted. Also the natural frequencies of the bubbles are visible around 53 Hz. Another frequency spike was observed close to the 125 Hz. This frequency is close to the mechanical resonance frequencies of the suppression pool and the blowdown pipe. Because of neither the pressure suppression pool nor the blowdown pipe are visible to the pattern recognition, the spike of the higher frequencies is most likely from the interfacial area of the bubble which resonates with the suppression pool system, affecting rapid condensation at a certain point.
A New Vacuum Pressure Infiltration CFRP Method and Preparation Experimental Study of Composite
In order to prepare a carbon-fiber-reinforced polymer composite (CFRP) with ideal microstructure and properties, a new vacuum pressure infiltration CFRP method is proposed based on an analysis of existing CFRP preparation process methods. Research on composite material preparation systems was carried out by using this new method principle. The system mainly includes a fiber pre-forming module, a vacuum heating infiltration module, a hot-press curing molding module, and a data acquisition control module. Under the conditions of natural curing at 0 MPa + 6 h + 25 °C, vacuum heating curing at –0.05 MPa + 30 min + 80 °C, and hot-press curing at 0.7 MPa + 5 min + 50 °C, a two-dimensional (2D) CFRP with excellent microstructure and properties was successfully prepared. Observing the microstructure of the prepared composite material, it can be found that the inside of the composite material was sufficiently and uniformly infiltrated, and common preparation defects such as holes and delamination were effectively controlled. Through the performance test, the bending strength of the material reached 790 MPa.