This paper discusses the internal mass transfer process in annular flow dryout. The emphasis is put on the order of magnitude estimation of respective hydrodynamic and thermal mechanisms and the analysis of the heat flux effect on droplet entrainment and deposition. A simple interfacial turbulence model is developed to characterize the turbulence intensity suppression due to interface tension. The heat flux effect on droplet entrainment depends on the competition between the shear force decrease due to vapor effusion and the bubble emission: in low flow condition, the bubble emission outweighs the shear force decrease, thus the net effect is to increase the droplet entrainment; in high flow condition, the situation may reverse. The heat flux impact on droplet deposition is significant only for very fine droplets (less than 1 μm) because of the coupled effect of interface turbulence damping and the radial vapor effusion due to evaporation, but for droplets of medium and large sizes the heat flux effect is negligible. The analysis is then used to develop constitutive equations for droplet entrainment and deposition rates to take into account the interaction between thermal and hydrodynamic mechanisms, which gives improved CHF prediction for limiting quality regime (LQR) CHF experimental data. 相似文献
The transient critical heat flux (CHF) experiments with forced sinusoidal inlet flow oscillation (oscillation period in 1–11 s, normalized amplitude of inlet flow oscillation in 0–3.0) were conducted in a vertical tube under low pressure condition. To analyze the triggering mechanism and aftermath of periodic dryout, the wall temperature fluctuation characteristics at the onset of periodic dryout and during post-periodic dryout were investigated. Under inlet flow oscillation condition, periodic dryout would be triggered at the wave trough of liquid film oscillation as wall heat flux far below the stable-flow CHF. The transient periodic dryout would give rise to temperature fluctuations on the tube wall, the amplitude of which increased with oscillation period and heat flux. The large wall temperature fluctuation during long-playing periodic dryout could significantly pre-trigger continuous dryout. The changing trends of the periodic dryout heat flux show a reasonable agreement with Okawa’s theoretical model, in which the liquid film oscillation was supposed be weakened by the axial mixing of liquid film. Moreover, the droplet entrainment at the oscillatory interface also has noticeable influence on the oscillation characteristics of liquid film. Based on the analysis of parameter effects on periodic dryout, a semi-empirical correlation was proposed to predict the periodic dryout heat flux under inlet flow oscillation condition. 相似文献
The critical heat flux (CHF) mechanisms for subcooled flow boiling are reviewed. Based on experimental observations reported by previous investigators, the authors have developed a new mechanistic CHF model for vertical subcooled flow at high pressure and high mass velocity. This model is based on the dryout of a thin liquid layer (sublayer) beneath an intermittent vapor blanket due to a Helmholtz instability at the sublayer-vapor interface. The parametric trends of CHF have been explored qualitatively and quantitatively with respect to variations in pressure, mass velocity, subcooling and tube diameter. Comparisons of the model predictions with experimental data for water show good agreement in the simulation of subcooled flow conditions of pressurized water reactors (PWRs). 相似文献
Early in 1988 dryout of fuel rods occurred in the Oskarshamn 2 boiling water reactor. During refuelling it was observed that one corner rod was damaged in each of four fuel assemblies. These were of the SVEA design, SVEA being the trade name of the ABB Atom water cross fuel. The damaged zone covered about 180° of the rod periphery facing the corner sub-channel, over a stretch of about 30 cm with the upper end just below the last downstream spacer.
The dominating cause of the dryout was re-use of fuel channels for ordinary 64-rod fuel, which were located in neighbouring positions to the SVEA fuel. The re-used channels showed excessive bowing because of irradiation. This bow increased the water gap between the fuel assemblies, thus increasing the neutron moderation and the local power around one corner of the SVEA fuel. This and some other factors caused the local peaking factor for the corner rod to increase from 1.04 to 1.38.
The flow and power conditions in the damaged fuel assemblies were calculated by means of the POLCA, PHOENIX, CASMO and CONDOR computer programs. The results of these calculations were used as a base for dryout predictions, which were carried out employing eight correlations, which are available in the open literature. The Barnett, the Becker and the Bezrukow correlations predicted the dryout power within 1%. Also the Condie & Bengston, the EPRI and the XN-1 correlations yielded very good results with accuracies of, respectively, −5.1, −2.3 and 7.3%. The Becker, the XN-1, the Bezrukow and the Condie & Bengston correlations predicted dryout to occur inside of the observed dryout zone of 30 cm length.
It is concluded that the dryout in the Oskarshamn 2 nuclear power plant was not caused by any faults in the design or manufacture of the SVEA fuel, and that the re-use of fuel channels should not be permitted. 相似文献
Mechanistic models for flow regime transitions and drag forces proposed in an earlier work are employed to predict two-phase flow characteristics in multi-dimensional porous layers. The numerical scheme calls for elimination of velocities in favor of pressure and void fraction. The momentum equations for vapor and liquid then can be reduced to a system of two partial differential equations (PDEs) which must be solved simultaneously for pressure and void fraction.
Solutions are obtained both in two-dimensional cartesian and in axi-symmetric coordinate systems. The porous layers in both cases are composed of regions with different permeabilities. The finite element method is employed by casting the PDEs in their equivalent variational forms. Two classes of boundary conditions (specified pressure and specified fluid fluxes) can be incorporated in the solution. Volumetric heating can be included as a source term. The numerical procedure is thus suitable for a wide variety of geometry and heating conditions. Numerical solutions are also compared with available experimental data. 相似文献