Slag or Plug Flow in Evaporation Process

There are some kinds of flow in evaporation. One of them is slag flow. Along with the increases quality of steam in the water wall tubes, bubbles steam that arises will be greater, thus forming plug or slag inside tubes. The flow in this region is called as plug or slag flow. However, if in the slag flow there are many small bubbles or bubbly, the flow in this region is often called as random or churn flow.
Figure 1: Plug or Slag Flow and Churn Flow in Evaporation Process
Churn flow is formed from the breakage of large bubbles steam in the slag flow. This flow is sometimes referred to as semi-annular flow or slag-annular flow.  From the figure above can be known the limitations of slag flow are:

ρf  jf2 < 5124 (kg/ms2)
ρg  jg2 < 168 (kg/ms2)
jg          = superficial velocity of gas phase
jf          = superficial velocity of liquid phase

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