Visualization Study Of Boiling Process In Packed Beds With Internal Heat Source

The water-cooled packed bed reactor is an innovative conceptual nuclear reactor. It utilizes the light water reactor technology but uses the coated spherical fuel elements. It has the characteristics of being simple in design, modular, long-life time, high inherent safety and proliferation resistance. The reactor core,composed of particle fuel elements,is a packed bed channel with internal heat source. To perform a study on vapor-liquid flow patterns and the flow pattern evolutions is helpful to deeply understand the flow boiling heat transfer mechanisms in packed bed channels and it can provide reliable experimental data and technical supports for the research and development of the water-cooled packed bed reactor.The test section was a quartz glass tube with an inner diameter of 75 mm and it was packed with oxidized carbon steel balls of 5.09, 8.02 and 12.04 mm diameters. The packed bed channel was volumetrically heated by electromagnetic induction. A high speed camera was employed for the visualization study of the flow boiling process.Flow visualization images were analyzed and the flow boiling patterns in the packed bed channel were categorized into bubble flow, slug flow and annular flow, and the bubble flow was further divided into two parts, namely the bubble flow with bubble diameters smaller than the pore size and the bubble flow with bubble diameters larger than the pore size.Effects of the heat flux, mass flux and particle diameter on flow patterns were obtained.With the increase of the heat flux or decrease of the mass flux, bubble flow is replaced by slug flow and slug flow gives way to annular flow. For a given mass flux, to get the same flow pattern, a higher heat flux is needed for a larger particle diameter.Based on the experimental data, two types of flow pattern maps during flow boiling in a packed bed channel were developed, one of them was plotted in terms of mass flux G versus vapor quality x and the other flow pattern map was constructed by using the combinations of dimensionless numbers Bo/Re and Re-Co as the abscissa and the ordinate, respectively.Bubble size, shape and velocity were studied at the pore level under the bubble flow regime. Results indicate that the bubble size number density distribution in terms of probability density agrees with lognormal distribution and the distinct peak of the bubble size distribution located at the characteristic length of the packed bed corresponds to the average-sized bubbles. With the increase of vapor quality or particle diameter, the average size of bubbles increases and the number density of average-sized bubbles decreases. The proportion of bubbles with roundness over 0.75 exceeds 90 % in the packed bed channel and the number density of rounder bubbles is higher. As the vapor quality or the particle diameter increases, the number of spherical bubbles decreases. Individual bubble velocities vary a lot depending on where bubbles are located in the pore, whereas the mean bubble velocity is almost uniform for the given vapor and liquid superficial velocity. When the liquid superficial velocity is constant, mean velocity of bubbles increases linearly with the vapor superficial velocity.Bubble interactions in the packed bed were researched and two dominant bubble coalescence mechanisms are identified. The first coalescence mechanism,which is termed as parallel coalescence, occurs when two bubbles are trying to pass through a narrow channel at the same time and the second coalescence mechanism termed overtaking coalescence is due to a sudden decrease in the velocity of a leading bubble. Two types of breakup mechanisms termed as shearing breakup and acceleration breakup are identified and the shearing breakup mechanism is divided into equal shearing breakup and unequal shearing breakup according to whether the two daughter bubbles are equal in size.Forces acting on the vapor and liquid phases were analyzed and the relationship between void fraction and superficial fluid velocities was obtained. The upper and lower limits of void fraction in slug flow and the correlations for length and diameter of slugs were proposed on the basis of analysis of visualization images. Based on these above, a mathematical model is developed to predict the transition boundaries for various two-phase flow patterns during flow boiling in the packed bed channel.