Ever imagined how boiling takes place? How is boiling different from evaporation? What is pool boiling and what are the different stages of pool boiling?
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The processes of boiling and evaporation both involve liquid to vapour phase change, but then how do they differ? Evaporation takes place at the interface of liquid vapour at a specific condition when the vapour pressure is less than the liquid saturation pressure. In evaporation, there is no bubble formation. While boiling takes place at the interface of solid liquid when the liquid makes contact with a solid surface whose temperature is higher than the saturation temperature of the liquid. Boiling involves bubble formation at the solid-liquid interface. Boiling is characterized as a convection mode of heat transfer. However, at supercritical pressures and temperatures, boiling can occur without any bubble formation.
Types of boiling based on bulk motion
Boiling can be categorized into two divisions - pool boiling and flow boiling.
Pool boiling - When the boiling occurs in absence of bulk motion of fluid, it is called pool boiling. The fluid remains stationary and the motion occurs due to natural convection. The bubble motion is majorly influenced due to the presence of buoyancy. Example - Boiling of water in a vessel.
Flow boiling - Also, termed forced convection boiling, it takes place in presence of bulk motion of the fluid. When a pump pushed the fluid to move on a heated surface or a heated pipe, flow boiling takes place.
Types of boiling based on bulk liquid temperature
Subcooled boiling - When the liquid temperature is less than the saturation temperature, it's termed subcooled boiling or local boiling.
Saturated boiling - When the temperatures of liquid equal saturation temperature, it's termed saturated boiling
Let's understand pool boiling with a physical example. We take water at room temperature and place it over a hot pan/vessel to allow it to boil. With a rise in temperature, there occurs motion in the water due to natural convection. Following this, we see the formation of vapour bubbles at the surface of the pan. The bubbles detach from the surface, rise in the liquid and further collapse in the zone where water is comparatively cooler. We term this stage as subcooled boiling as saturation temperature is not yet reached by the bulk of the liquid. With the further rise in temperatures, the bubble formation increase and the bubbles rise to the top of the water surface, thereby a saturated boiling is observed.
S. Nukiyama through his experiments identified different zones of boiling depending on the value of excess temperature. Below, the boiling curve for water has illustrated which is essentially the plot between boiling heat flux and the excess temperature. It should be noted, that the typical shape of the boiling curve will remain more or less the same for any liquid.
Regimes of flow boiling
Thus, we observe the following four regimes.
Natural Convection Boiling - In this mode, the fluid motion by natural convection takes place. The heated surface transfers heat to the fluid by the method of natural convection.
Nucleate boiling - At this stage, the first bubbles start forming and thereby keep forming at an increasing rate. Nucleate boiling can be further identified into two distinct regions. The bubble formed in the regions A-B gets dissipated in the liquid when they detach from the surface. Thus the bubbles are formed at discrete nucleation sites. Whereas, in regions B-C, the bubble formation is at a greater rate. We observe continuous vapour columns in liquid. These bubbles rise to the surface where they break up. With large values of temperature excess, the surface is covered with numerous bubbles and the liquid fails to reach the surface to wet these bubbles. Thus, at this stage, the rate of the flux increased at a slower value even though the temperature excess is large. The heat flux at this stage is called critical heat flux. In the figure, point C represents this point.
Transition boiling - Beyond the point C, on further increase in excess temperature, the heat flux decreases. This happens due to the formation of vapour film due to excessive bubble formation which now acts as an insulator as the thermal conductivity of vapour is lower than that of liquid.
Film boiling - In this stage, the heated surface is completely covered by the formation of vapour film. The heat flux reached a minimum value called the Leidenfrost point. The heat transfer rate is low in film boiling due to the presence of the vapour film. On the further increase of excess temperature, the heat transfer rate increases due to the radiation mode of heat transfer from the vapour to the liquid.
Enhancement of heat transfer in pool boiling
Following ate the methods by which we can increase the rate of heat transfer
By proving irregularities to the heating surface. The heat flux can be increased by a factor of 10 by providing roughness to the surface, This roughness provides a nest for bubble formation, thereby increasing the heat transfer rates. However, this effect decays with time.
A special coating on the heated surface is done. This coating is either of a porous material or forms cavities on the surface which allows for continuous vapour formation.
Use of finned heated surfaces
Mechanical agitation and surface vibration are other methods but these are limited in physical use due to the large complexities involved.
Pool boiling occurs in absence of bulk motion of the liquid.
Pool boiling occurs due to natural convection and buoyancy.
The various stages of flow boiling are - natural convection boiling, nucleate boiling, transition boiling and film boiling