Abstract A numerical analysis has been performed on the three-dimensional natural convecrive heat transfer characteristics of a porous medium enclosed by a vertical concentric curved annulus heated from the inner surface and cooled from the outer surface with relation to the thermal insulation layer in the high-temperature ducting system of a high-temperature gas-cooled reactor. Darcy's law and the Boussinesq approximation are assumed to be applicable. The governing equations are transformed into finite-difference equations, which are numerically solved by a successive over-relaxation procedure for a range of RaDa (a product of the Rayleigh number and the Darcy number) from 100 to 800. Two typical vertical arrangements (case A, in which a 90° bend is attached at the upper part of a vertical straight tube, and case B, in which it is attached at the lower part) were analyzed and compared with each other. The numerical results show that the flow field and the temperature profile have characteristics of those for both horizontal and vertical annuli, and the insulation performance in case B becomes worse than in case A for a whole range of RaDa. Information useful for the thermal insulation design of a high-temperature ducting system was obtained.
Flow characteristics and heating limits of downward two-phase flow in single or parallel multi-channels were investigated experimentally and analytically. The heated channel used in the experiments was made of a glass tube with a heater rod inserted, and the flow regime inside was observed. In single channel experiments under low flow rate conditions, it was found that the gas phase which flows upward against the downward liquid phase flow condenses and diminishes as it rises, being cooled by inflowing liquid. But as the heating power was increased, a portion of the gas phase reached the top and accumulated to form a liquid level, which eventually caused the flow excursion as well as the dryout. On the other hand, under high flow rate conditions, the flooding initiated at the bottom of the heated section was the cause of the dryout and the flow excursion. In parallel multi-channel experiments, reversed (upward) flow leading to the dryout was observed in some of these channels with low flow rates. The situation was the same in the single-channel case under high flow rate conditions. Analyses were carried out to predict the onset of the dryout using the drift flux model and the Wallis'(1969) flooding correlation. The analytical results agree well with the experimental results.
The dynamics of the evaporated material which is formed around the solid hydrogen pellet subject to a plasma is studied. Basic equations are transformed into ordinary differential equations by the profile method, and solved numerically. In this analytical model, it is assumed that the evaporated pellet material is a neutral molecular gas and the phase equilibrium exists at the pellet surface, and that the electron energy flux is attenuated by the elastic scattering with the neutral molecules. The electron energy flux coming into the pellet surface is calculated using the electron transport equation in a spherical shell coordinate system. This transport equation prevalently includes the effects of the scattering and the absorption of the electron by the evaporated material and also the emission of the electron from the pellet and the evaporated material.Numerical results are obtained by changing parametrically τ0 characterizing the extinction of incident electrons to the pellet surface. The time dependence of the pellet radius and the outer radius of the evaporated material is obtained. For τ0=0.01, the outer radius of the evaporated material is about six times of the initial radius and the life time of the pellet is 10-2 times compared with previous results. Also it is shown that the electron scattering in the evaporated material will be important to evaluate the electron energy flux.
Abstract An analysis on the hydrodynamic instability of two-phase flow in parallel multichannels is conducted. Occurrence of instabilities and their modes of oscillations can be evaluated by investigating into a characteristic equation, its roots and composing channel transfer functions. It is also shown that a governing matrix is reduced to a diagonal one by using its eigenvalues, the oscillation modes being divided into N (number of channels) separate fundamental modes. Characteristics of each oscillation mode are given by examining corresponding characteristic equations. The derived equations are applied for the prediction of oscillation modes of systems composed of a few slightly different channels. The analysis successfully predicts the modes which have been experimentally observed. KEYWORDS: hydrodynamic instabilitytwo-phase Howparallel channelsoscillation modetransfer functionstability
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