forced convection boiling

1. 

2.  Known as flow boiling or two-phase flow.
 In forced convection boiling , the fluid is forced to
move by an external source such as a pump as it
undergoes a phase-change process.
 It exhibits the combined effects of convection and
pool boiling.

3.  Two-phase heat transfer describes phenomena
where a change of phase occurs during or due to
the heat transfer process.
 Two-phase heat transfer generally considers
processes that occur at a solid/fluid interface and
are therefore a sub-field of convection.
 The latent heat (hfg) of the fluid must be
considered due to the change of phase.

4. 1. External Forced
convection boiling
 If fluid is forced to
move over a heated
surface (on external
side)
2. Internal Forced
convection boiling
 If fluid is forced to
move inside a heated
surface/tube (on
internal side)

5.  External forced
convection boiling over a
plate or cylinder is similar
to pool boiling, but the
added motion increases
both the nucleate boiling
heat flux and the
maximum heat flux
considerably.
 The higher the velocity,
the higher the nucleate
boiling heat flux and the
critical heat flux.
 Internal forced
convection boiling,
commonly referred to as
two-phase flow, 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.

6.  It is complicated as there is no space for vapor
for escape, so vapor and liquid flow together,
hence two-phase flow occurs.
 Different flow regimes are present depending on
relative amounts of vapor and liquid present.

8.  Total liquid.
 In the inlet region the
liquid is subcooled
and heat transfer to
the liquid is by forced
convection (assuming
no subcooled boiling).
 The heat transfer
mechanism is single-
phase forced
convection.

9.  The temperature of the
liquid is below the
saturation temperature
.
 Bubbles formed at the
surface may
condensed in liquid.

10.  When bubbles appear
in liquid.
 Individual bubbles.
 Low mass qualities.
 The vapor bubbles in
the bubbly flow region
rise due to buoyancy
force and the external
pressure difference.

11. o Bubbles grow and
coalesce into slugs of
vapor.
o Coalesce means come
together to form one
mass or whole.
o Moderate mass
qualities.

12.  Core of the flow
consists of vapor
only(vapor core).
 Liquid adjacent to
the walls(liquid film).
 Very high heat
transfer coefficients.

13.  Complete dry spot
form on tubes
inner side.

14.  When the liquid film on
the inner surface of the
tube is completely
evaporated, the flow
pattern becomes drop or
mist flow and the wall
temperature abruptly
increases.
 Liquid drops suspended
in vapor.
 A sharp decrease in the
heat transfer coefficient.

15. o Total vapors.
o The liquid phase is
completely evaporated
and vapor is
superheated.
o the heat transfer
mechanism is single-
phase forced
convection.

16.  Heat and Mass Transfer: Fundamentals &
ApplicationsFourth Edition
Yunus A. Cengel, Afshin J. Ghajar McGraw-Hill, 2011
 https://www.thermalfluidscentral.org/encyclopedia/index.p
hp/Flow_Regimes_in_Horizontal_and_Vertical_Tubes
 http://s3.amazonaws.com/boilingheattransfer-
130123095242
 http://www3.nd.edu/~dgo/teaching/AME60634/lectures/A
ME60634_S13_ht_two-phase.pptx