Page 275 - Fluid mechanics, heat transfer, and mass transfer

CONVECTIVE HEAT TRANSFER BASICS
256
heating surface until a maximum value for the heat
transfer rate is reached.
& Beyond the critical heat flux, two possibilities exist
depending on the heating conditions. If the surface
heat flux is controlled and increased beyond critical
point, the surface temperature increases dramatically
as shown by the dashed line from point D to D in
0
Figure 9.10.
0
& The temperature at D is often higher than the max-
imum temperature that heater surface can maintain,
and thus this heat flux is referred to as the burn out
point. Surface temperature at E is beyond melting
point of most materials.
& If the surface temperature is controlled and increased
0
beyond the temperature at D , the insulating effect
continues and the heat transfer rate decreases. This
regime, called transition boiling, is characterized by
the unstable vapor blanket that covers the surface.
The vapor blanket collapses periodically and allows
the fluid to contact the surface. This periodic motion
results in large variations in surface temperature and
a highly unstable flow.
& The periodic contacts between liquid and heated
surface in the transition boiling region of the boiling
curve result in the formation of both large amounts of
vapor, which forces liquid away from the surface, and
creates an unstable vapor film or blanket. Because of
this, the surface heat flux and the surface temperature
can experience variations both with time and position
on a heater. However, the average heat transfer
Stages in pool boiling curve.
coefficient decreases as the temperature increases,
FIGURE 9.11
because the time of contact between the liquid and
transfer coefficient can be represented by the follow- the heater surface is decreased.
ing equation: & In transition boiling region, both unstable nucleate
boiling and unstable film boiling alternately exist at
m
h ¼ aDT ; ð9:37Þ any given location on a heating surface. The variation
in heat transfer rate with temperature is primarily a
where DT ¼ T w T sat .
result of a change in the fraction of time each boiling
0
& The position of the B C part of the curve is affected by
regime exists at a given location.
the solid surface characteristics, surface tension,
& Interest in transition boiling regime arises because of
pressure, dissolved gases or solids, or presence of
its potential importance during a loss of coolant
high boiling components in the liquid mixture.
accident (LOCA) in nuclear reactors.
& At large temperature difference, large part of heater
& Point D represents start of film boiling.
surface is covered by bubbles and the curve reaches
& At large temperature differences, greater than 250 C

C, which represents maximum heat flux at C. This is
called critical heat flux (CHF). for water at atmospheric pressure, the vapor film
completely blankets the heated surface. Vapor bub-
& At the critical point, the vapor forms patches and
bles are released regularly from the surface and the
columns near the heater surface. Because of the
film is considered stable.
differences in thermal properties of the two phases,
the heat transfer rate to the vapor is considerably less & At E, heat flux reaches minimum. This point is called
than that to the liquid. Therefore, the vapor effec- Leidenfrost point.
tively insulates the surface. With increasing the & After E, heat transfer coefficient increases mainly
surface temperature, the vapor covers more of the due to the effect of radiation particularly at low flows,