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is an example of pool boiling. Pool boiling of a fluid can also be achieved by
placing a heating coil in the fluid. In flow boiling, the fluid is forced to move
in a heated pipe or over a surface by external means such as a pump. Therefore, flow boiling is always accompanied by other convection effects.
Pool and flow boiling are further classified as subcooled boiling or saturated boiling, depending on the bulk liquid temperature (Fig. 10–4). Boiling
is said to be subcooled (or local) when the temperature of the main body of
the liquid is below the saturation temperature Tsat (i.e., the bulk of the liquid is
subcooled) and saturated (or bulk) when the temperature of the liquid is
equal to Tsat (i.e., the bulk of the liquid is saturated). At the early stages of boiling, the bubbles are confined to a narrow region near the hot surface. This is
because the liquid adjacent to the hot surface vaporizes as a result of being
heated above its saturation temperature. But these bubbles disappear soon after they move away from the hot surface as a result of heat transfer from the
bubbles to the cooler liquid surrounding them. This happens when the bulk of
the liquid is at a lower temperature than the saturation temperature. The bubbles serve as “energy movers” from the hot surface into the liquid body by absorbing heat from the hot surface and releasing it into the liquid as they
condense and collapse. Boiling in this case is confined to a region in the locality of the hot surface and is appropriately called local or subcooled boiling.
When the entire liquid body reaches the saturation temperature, the bubbles
start rising to the top. We can see bubbles throughout the bulk of the liquid, Heating Heating (a) Pool boiling (b) Flow boiling FIGURE 10–3
Classification of boiling on the basis
of the presence of bulk fluid motion.
P = 1 atm P = 1 atm Subcooled 80°C
107°C Saturated 100°C
Bubble Heating Heating (a) Subcooled boiling (b) Saturated boiling FIGURE 10–4
Classification of boiling...
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This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.
- Spring '10