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CHAPTER 10 Internal flow boiling is much more complicated in nature because there is
no free surface for the vapor to escape, and thus both the liquid and the vapor
are forced to flow together. The two-phase flow in a tube exhibits different
flow boiling regimes, depending on the relative amounts of the liquid and the
vapor phases. This complicates the analysis even further.
The different stages encountered in flow boiling in a heated tube are illustrated in Figure 10–19 together with the variation of the heat transfer coefficient along the tube. Initially, the liquid is subcooled and heat transfer to the
liquid is by forced convection. Then bubbles start forming on the inner surfaces of the tube, and the detached bubbles are drafted into the mainstream.
This gives the fluid flow a bubbly appearance, and thus the name bubbly flow
regime. As the fluid is heated further, the bubbles grow in size and eventually
coalesce into slugs of vapor. Up to half of the volume in the tube in this slugflow regime is occupied by vapor. After a while the core of the flow consists
of vapor only, and the liquid is confined only in the annular space between the
vapor core and the tube walls. This is the annular-flow regime, and very high
heat transfer coefficients are realized in this regime. As the heating continues,
the annular liquid layer gets thinner and thinner, and eventually dry spots start
to appear on the inner surfaces of the tube. The appearance of dry spots is accompanied by a sharp decrease in the heat transfer coefficient. This transition
regime continues until the inner surface of the tube is completely dry. Any liquid at this moment is in the form of droplets suspended in the vapor core,
which resembles a mist, and we have a mist-flow regime until all the liquid
droplets are vaporized. At the end of the mist-flow regime we have saturated
vapor, which becomes superheated with any further heat transfer.
Note that the tube contains a liquid before the bubbly flow regime and a
vapor after the mist-flow regime. Heat transfer in those two cases can be
determined using the appropriate relations for single-phase convection heat
transfer. Many correlations are proposed for the determination of heat transfer
x=1 Forced convection
droplets Low Mist flow Vapor core
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- Spring '10