Page 277 - Fluid mechanics, heat transfer, and mass transfer

CONVECTIVE HEAT TRANSFER BASICS
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& In flow boiling, fluid is forced to move in a heated & If one of the components has very high boiling point,
pipe or surface by external means, for example, by a essentially making it nonvolatile, the effect of
pump. Flow boiling is, simply stated, boiling taking increasing its concentration is to shift the curve B C
0
place while the liquid is flowing. to the right reducing heat transfer rates. Accumula-
& Examples of flow boiling include boiling in thermo- tion of this heavy component in the nuclei cavities
siphon and pump-through reboilers, which operate can make them inactive for boiling. For this reason,
with a net liquid flow past the heat transfer surface. sometimes, special surfaces are used to provide
Under these conditions, shear stress on the layer of washing effect of the circulating fluid on the heavy
liquid immediately adjacent to the heat transfer components from the cavities.
surface will influence the boiling process. . What are the additional factors involved in nucleate and
& In flow boiling, nucleation processes are suppressed film boiling phenomena inside tubes?
and liquid gets superheated and transported from the & The vapor and liquid inside a tube must travel
tube wall by turbulent eddies to the vapor–liquid together. The pattern of the resulting two-phase flow
interface, where vaporization takes place. affects both heat transfer and pressure drop, because
& Heat transfer coefficients are much higher in flow of the changing vaporization and relative vapor–
boiling processes compared to nucleate boiling liquid loads.
processes. & In film boiling, while the liquid does not contact the
& Film boiling is also possible under forced convection tube surface, it will be in one of the following forms:
vaporization if the tube wall temperatures are high. ➢ A dispersed spray of droplets, normally encoun-
In these cases, sometimes mist flow is encountered in tered at void fractions in excess of 80% (liquid-
which vaporization takes place from suspended dro- deficient or dispersed flow film boiling regime).
plets in the superheated vapor. ➢ A continuous liquid core (surrounded by a vapor
& In such cases, heat transfer coefficients become annulus that maycontain entrained droplets) usually
very low and design of thermosiphon and pump- encountered at void fractions below 40% (inverted
through reboilers avoid conditions of high tube wall annular film boiling or IAFB regime).
temperatures. ➢ A transition between the above two cases, which
. What is Leidenfrost phenomenon? can be in the form of an inverted slug flow for low
& Leidenfrost noted that when liquids were spilled/ to medium flow.
placed on very hot surfaces, drops were formed that & Changes in pressure drop along the tubes cause
did not contact the surface but floated above it and changes in boiling point of the liquid.
slowly vaporized. When surface temperature was . What is inverted annular film boiling (IAFB)? Describe
reduced below a certain value, the drops contacted heat transfer processes during inverted annular film
the surface and rapidly vaporized. This phenomenon boiling.
is called Leidenfrost phenomenon and point E on the & IAFB refers to the film boiling type characterized by
boilingcurve(Figure9.10)iscalledLeidenfrostpoint.
a vapor layer separating the continuous liquid core
& Temperature at point E is known by different terms
from the heated surface.
that include Leidenfrost temperature, minimum film & In the inverted annular flow regime, few entrained
boiling temperature (T MFB ), rewetting temperature,
droplets are present while the bulk of the liquid is in
quench temperature, and film boiling collapse
the form of a continuous liquid core that may contain
temperature.
entrained bubbles.
. How does nucleate boiling phenomenon affect the
& At dry out, the continuous liquid core becomes
boiling of mixtures?
separated from the wall by a low-viscosity vapor
& In binary mixtures where both components are vol-
layer.
atile, the boiling curves for the mixtures are usually & The heat transfer process in IAFB can be considered
lying between those for pure components. by the following heat flux components:
& With mixtures, effects of mass diffusion, local con- ➢ Convective heat transfer from the wall to vapor.
centration gradients caused by the greater vaporiza-
➢ Radiation heat transfer from the wall to liquid.
tion rates of the more volatile component and the
➢ Heat transfer from vapor to the vapor–liquid
resultant effects on the physical properties of the
interface.
mixture as well as changes in interface saturation
➢ Heat transfer from thevapor–liquid interface to the
temperatures during bubble growth influence the
liquid core.
boiling curves.