Condensation
and boiling heat transfer are encompassed in various engineering applications
that are well-known. In the evaporator section of a household refrigerator, for
instance, heat is absorbed from the refrigerated space as the refrigerant
undergoes boiling, while in the condenser section (situated behind the
refrigerator as long coils), heat is released to the kitchen air through
condensation. Similarly, in steam power plants, the heat is transferred to the
steam in the boiler, causing water to vaporize, and the waste heat is expelled
from the steam in the condenser, where the steam undergoes condensation. Some
electronic components are cooled by immersing them in a fluid with an
appropriate boiling temperature.
Boiling,
akin to evaporation, is a process involving a transition from
liquid to vapor phase, but notable distinctions exist between the two.
Evaporation transpires at the liquid–vapor interface under conditions where the
vapor pressure is lower than the saturation pressure of the liquid at a specified
temperature. For instance, water in a lake at 20°C will evaporate into air at
the same temperature and 60 percent relative humidity, given the saturation
pressure of water at 20°C is 2.3 kPa, and the vapor pressure of air under those
conditions is 1.4 kPa. Other instances of evaporation include the drying of
clothes, fruits, and vegetables, the cooling of the human body through sweat
evaporation, and the dissipation of waste heat in wet cooling towers. It is
noteworthy that evaporation does not involve the formation or motion of
bubbles.
Conversely,
boiling occurs at the solid–liquid interface when a liquid contacts a surface
maintained at a temperature (Ts) significantly surpassing the saturation
temperature (Tsat) of the liquid. For instance, at 1 atm, liquid water in
contact with a solid surface at 110°C will undergo boiling, as the saturation
temperature of water at 1 atm is 100°C. The boiling process is characterized
by the rapid formation of vapor bubbles at the solid–liquid interface, detaching
from the surface upon reaching a certain size and attempting to ascend to the
liquid's free surface. In cooking, it is customary not to declare water is
boiling until observable bubbles rise to the top. Boiling, due to the multitude
of variables and intricate fluid motion patterns resulting from bubble
formation and growth, is a complex phenomenon.
The
boiling process involves a phase change from liquid to vapor and hinges on
surface characteristics, surface tension, latent heat of vaporization, pressure,
density, and potentially other vapor properties, so it is advisable not to
singularly attribute it to fire. The complexity arising from numerous
variables hinders the availability of general equations or correlations for
boiling heat transfer data. Nevertheless, substantial strides have been taken
in comprehending the physical aspects of the boiling mechanism. Through
high-speed photography, distinct boiling regimes have been identified,
showcasing radical differences in heat transfer mechanisms.
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