Research On Heat Transfer Characteristic Of The Pebble Bed Channels With Internal Heat Generation

Water-cooled pebble-bed reactor has many advantages such as miniaturization, long lifeand high safety. So more and more researchers have paid attention to it. Water-cooledpebble-bed reactor core is typical random packed pebble-bed structure with an internal heatsource and based on safety consideration, many issues, such us heat transfer characteristic insingle phase flow and onset of nucleate boiling become the focus of attention. So far, therelevant research announced in an open manner is very limited. Therefore, the study ofconvection heat transfer characteristics and law of onset of nucleate boiling are of greatsignificance.Based on the pebble-bed channels with internal heat source, this paper mainly researchesthe flow and heat transfer characteristics of single-phase forced convection, the law of onsetof nucleate boiling and boiling hysteresis. A three-dimensional stochastic model of packedbed was constructed, and the internal parameters of pebble-bed channels were analysised bynumerical calculation method. The main work and conclusions are summarized as follows:1. The single phase convection heat transfer in pebble-bed channels with internal heatsource was studied. According to the experimental results, mass velocity, inlet watertemperature and pebble diameter are the principal factors. When mass velocity increases, inlettemperature and pebble diameter decrease, the heat transfer coefficient increases obviously.Considering the effects of these parameters on the heat transfer, based on the analysis of alarge amount of experimental data, a dimensionless heat transfer correlation for pebble-bedchannels with internal heat source was fitted by the multiple linear regression method. Therelative deviation between the calculated value and the experimental value is within±20%.2. A three dimensional stochastic model of packed-bed was constructed to obtain thedistribution of pebble-bed internal parameters such as heat transfer coefficient, temperature,pressure and velocity by numerical calculation, and the reliability of the model is verified bythe comparisons with experimental results. Link the two adjacent balls with a line, if theintersection angle of the line and vertical direction is between0to45degree, fluid velocity islower in the gap, so the temperature is higher in this area. If the intersection angle of the lineand vertical direction is between45to90degree, the velocity of fluid increase after flowing into this area, leading to the increase of heat transfer coefficient, so the fluid temperature inthis area is lower.3. The main influencing factors of onset of nucleate boiling in pebble-bed channels withinternal heat source were studied. According to the experimental results, the subcooling ofworking fluid, mass velocity and pebble diameter are the principal factors. The heat fluxdecreases with decreasing in subcooling of working fluid; and increasing in mass velocity andpebble diameters. Considering the effects of these three parameters, a criterion formula whichcan give a good prediction of onset of nucleate boiling in pebble-bed channels was fitted andthe relative deviation between the predicted value and the experimental value is within±20%. Based on the study of onset of nucleate boiling, by visual observation, and consideringthe structure characteristics of pebble-bed channels, the bubbles inside the channel can bedivided into four types: the bubble on the normal surface; the bubble in the corner of twoadjacent balls; the bubble between several compact pebbles; and the bubble inside the largevolume of loose arrange area. The law of bubble growth and detachment were proposed byanalysing the force on bubbles.4. The various hysteresis phenomenas in pebble-bed channels were studied. Usinghysteresis temperature and hysteresis area to token the degree of nucleation hysteresis andtemperature deviation hysteresis, and analysed the causes and the variation law of the varioushysteresis phenomenas. The results indicate that the nucleation hysteresis are more severeclose to the outlet of the pebble-bed or the radial centre location. When mass velocity andsurface rough degree increase, the temperature deviation hysteresis increases. Because thereare a lot of high temperature corner areas in pebble-bed channels, which can provide stablenucleation sites, have a great deal of restraint to hysteresis phenomena.