ABSTRACT
R.V.A. Oliemans A key aspect of gas-liquid flow in ducts is how to calculate the pressure gradient. A characteristic feature of two-phase flow in ducts is that the liquid holdup being different from the liquid volume fraction due to slip between the phases. The slip effect is different for different flow patterns. It is large for separated flow and small for mixed flow. Figure 2.1 shows the flow patterns that can occur in vertical and horizontal pipes. For vertical upflow we see (from left to right), as the amount of gas is gradually increased, the following flow patterns evolve: bubbly flow, slug flow, churn flow, and annular flow. In the horizontal situation, gravity tends to change the distribution of the phases. In this figure (from bottom to top), we see, as the amount of gas is gradually increased, the following flow patterns evolve: dispersed bubble flow, slug (intermittent) flow, stratified flow, and annular dispersed flow. For annular flow, the liquid film at the wall no longer has a uniform thickness. It is very thin at the top of the pipe and can be thick at the bottom. In the case of dispersed bubble flow, the bubble concentration is far from homogeneous, since the bubbles tend to accumulate at the top of the pipe. Finally, stratified flow is a flow pattern typical for horizontal and near-horizontal flows. Depending on the gas velocity, stratified flow can occur with a smooth liquid-gas interface without any droplets entrained in the turbulent gas layer, or it can occur with a wavy interface with strong entrainment of droplets.
