Local flow regime and bubble size distribution in the slender particle-containing scrubbing-cooling chamber of an entrained-flow gasifier

Abstract

A dual-tip conductivity probe was used to measure local radial gas holdups and bubble chord lengths to study the local flow regime and bubble size distribution in a scrubbing-cooling chamber cold model apparatus containing slender particles. The results showed that the cross-section averaging gas holdups with different fiber volume fractions and aspect ratios could be estimated within errors of ±10% using the modified Kataoka-Ishii bubbly flow semi-empirical correlation at superficial gas velocities ranging from 0.074 m/s to 0.37 m/s. The local flow regime map showed that an obvious bubbly flow formed near the inner wall of the liquid bath due to the effect of wall shear stress, while a cap-bubbly flow formed in other annulus regions due to the emergence of cap bubbles. Chord length distributions were transformed into bubble size distributions by decomposing the measured chord length distributions and estimating the bubble shape factors. Bubble size was categorized into two types—small spherical and large non-spherical bubbles—depending on if the equivalent diameter of the bubbles was smaller or larger than 2 mm. Liquid turbulence, shear stress between the fluid and the wall and liquid backmixing were enhanced by increasing the superficial gas velocity, which affected the size distributions of rising and descending bubbles as well as that of bubble populations. The extent of turbulence suppression and “fiber separation walls” varied with the fiber volume fractions, which affected the bubble size distribution in different regions. The fiber number density characterizing the variation of the fiber aspect ratio with different fiber diameter and length was used to study the bubble size distribution. The bubble size distribution at different aspect ratios was affected by interactions among fibers, bubbles, the fluid and the wall.