Bubbly, Slug, and Annular Two-Phase Flow in Tight-Lattice Subchannels

AbstractAn overview is given on the work of the Laboratory of Nuclear Energy Systems at ETH, Zurich (ETHZ) and of the Laboratory of Thermal Hydraulics at Paul Scherrer Institute (PSI), Switzerland on tight-lattice bundles. Two-phase flow in subchannels of a tight triangular lattice was studied experimentally and by computational fluid dynamics simulations. Two adiabatic facilities were used: (1) a vertical channel modeling a pair of neighboring subchannels; and (2) an arrangement of four subchannels with one subchannel in the center. The first geometry was equipped with two electrical film sensors placed on opposing rod surfaces forming the subchannel gap. They recorded 2D liquid film thickness distributions on a domain of 16×64 measuring points each, with a time resolution of 10 kHz. In the bubbly and slug flow regime, information on the bubble size, shape, and velocity and the residual liquid film thickness underneath the bubbles were obtained. The second channel was investigated using cold neutron tomography, which allowed the measurement of average liquid film profiles showing the effect of spacer grids with vanes. The results were reproduced by large eddy simulation+volume of fluid. In the outlook, a novel nonadiabatic subchannel experiment is introduced that can be driven to steady-state dryout. A refrigerant is heated by a heavy water circuit, which allows the application of cold neutron tomography

Tomography of Annular Flow in a BWR Geometry with Spacers

The annular flow is the predominant flow regime in the upper part of Boiling Water Reactor (BWR). It consists of a thin liquid coolant film on the fuel rods and a fast moving gas core between the fuel rods. The liquid phase extends to the gas core due to whisps and entrained droplets. As the liquid film removes heat from the fuel rods by evaporation cooling, it loses mass due to the phase transition. Droplets entrained in the gas core hardly contribute to the heat removal. Therefore functional spacers have been used in the recent decades to counteract the thinning of the film, mitigating the occurrence of dryout. This doctoral thesis builds around the new DoToX facility that reproduces the dryout in a lab scale manner. Within an undisturbed subchannel between four heating rods an organic model liquid is evaporated to the point of forming annular flow. A generic functional spacer induces a swirl in the subchannel. By varying the flow rate of the coolant at constant heating power the point of dryout is reached in a steady state downstream of the spacer. The test channel can be rotated such that the recordings of a common X-ray machine allow to compute a three dimensional tomographic reconstruction. With advanced post processing methods the local, time averaged Liquid Film Thickness (LFT) and the liquid holdup of entrained droplets in the gas core can be extracted from the reconstruction data. In summary this thesis validates the concept of annular flow and dryout investigation by means of X-ray tomography in fuel bundle geometry. Local distributions of the LFT are visualized and thus dryout patches with high spatial resolution. The evolution of the holdup fraction of the gas core downstream of the spacer is resolved in sections of the subchannel. The additional droplet deposition of the spacer vanes might not always and everywhere compensate for the liquid film thinning due to increased shear stress downstream of the functional spacer. The data acquired and to be acquired with the DoToX provides a valuable source for CFD validation. The well known temperature and flow boundary conditions allow for tailored simulations that, in a later stage, can transfer the findings of DoToX to real reactor conditions

Time averaged tomographic measurements before and after dryout in a simplified BWR subchannel geometry

For the validation of advanced two-fluid CFD models at boiling water reactor (BWR) conditions, it is necessary to have good experimental data challenging for the code by reflecting relevant flow phenomena. The present experimental study aims to provide detailed film thickness distributions in a BWR-like subchannel geometry, including spacer grids and heat transfer. This is achieved using chloroform as a working fluid, indirectly heated by hot water channels simulating BWR fuel rods. X-ray equipment was used to produce tomographic 3D-data of time-averaged attenuation coefficients which essentially represent the liquid holdup. The conversion to film thickness was done by integrating the attenuation coefficient along lines perpendicular to the rod wall and using an effective chloroform attenuation coefficient determined by numerical simulation of the experimental setup, including X-ray sensor behavior. The simulation method was validated by using two different sensors types. Measurements were performed over the whole cross-section of the channel and an axial length of 11 cm. The experiments cover a range of liquid flow rates in the pre- and post-dryout regime.ISSN:0029-5493ISSN:1872-759