Large bubbles occur in slag or plug flow and will be bigger so as to form a cylinder amid tubes and at the inside surface wall tubes still attach liquid. The condition at this region can be called as annular flow. In annular flow, liquid phase inside tube will be increasingly depleted along with increasing quality of steam formed. Thus reach a point where liquid phase at the wall tubes does not exist anymore. This point is called dryout point. But at dryout point the quality of steam is not reach yet saturation steam point.
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| Figure 1: Annular & Wispy Annular Flow in Evaporation Process |
For large heat flux, the liquid in the surface wall tubes will dry out first, while the liquid in the middle of tubes still has not evaporated. Furthermore cylinder flow will be formed with liquid in the middle of tubes. This flow is called as wispy annular flow. This flow generally occurs at condition Departure Nucleate Boiling (DNB) and the quality of steam in this flow is still low.
The limitation of annular flow according the figure above can be seen the equation below:
ρf jf2 < 1000 (kg/ms2)
ρg jg2 < 168 (kg/ms2)
The limitation of wispy annular is:
ρf jf2 < 1000 (kg/ms2)
ρg jg2 < 168 (kg/ms2)
Where:
jg = superficial velocity of gas phase
jf = superficial velocity of liquid phase
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