5 Pool Boiling

From the viewpoint of thermodynamics, when a body of fluid in the purely liquid phase is heated at a constant pressure, its temperature and volume will both increase, as reflected in the line from A to B in Figure 5.1. Point B represents the maximum temperature and volume of saturated liquid at the given pressure, and between points B and C, any additional heat added to the system results in a change from the liquid phase to the vapor phase. In this region, liquid and vapor phases coexist in equilibrium. The amount of heat required to convert a unit of mass from liquid to vapor is called the latent heat of vaporization hfg (= hg - hf), where hg and hf are the enthalpy of saturated vapor and liquid, respectively. In this region, the addition of heat increases the vapor fraction and the volume of the mixture; however, the temperature of the two-phase mixture still remains at the saturation temperature. At point C, all the liquid has vaporized, and the vapor is now referred to as saturated vapor. Further heating will result in a temperature rise of the vapor (superheated vapor) along the constant-pressure line.

Boiling is the process of evaporation associated with vapor bubbles in liquid. The change of phase from liquid to vapor is caused by heat transfer or by pressure changes. The two basic types of boiling are pool boiling and flow boiling. The former is boiling on heated surfaces submerged in a pool of stationary liquid, while the latter is boiling on the surface of a flowing stream of liquid. Boiling can also be classified according to the liquid temperature, into subcooling boiling, in which the bulk fluid temperature is below the saturation temperature; and saturated boiling, in which the bulk fluid temperature is uniformly at the saturation temperature.