Pool Boiling Regimes
From Thermal-FluidsPedia
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The classical pool boiling curve is a plot of heat flux, q'', versus excess temperature, ΔT = Tw − Tsat. As the value of the excess temperature increases, the curve traverses four different regimes: (1) natural or free convection, (2) nucleate boiling, (3) transition boiling, and (4) film boiling. Different experimental methods may be used to define the pool boiling curve; constant temperature control and constant heat flux control are the two most commonly cited. A typical boiling curve for saturated pool boiling of water at atmospheric pressure for a temperature-controlled environment is shown in Fig. 1. When the excess temperature ΔT is less than 5 °C, no bubbles form. Instead, heat is transferred from the solid surface to the bulk liquid via natural convection. Heat transfer coefficients in this regime can be calculated using the semi-empirical correlations for natural convection. When the excess temperature increases beyond 5 °C, the system enters the nucleate boiling regime – point A on Fig. 1. Vapor bubbles are generated at certain preferred locations on the heater surface called nucleation sites; these are often microscopic cavities or cracks on the solid surface. Nucleation occurs repeatedly from the same sites, indicating a causal link between bubble formation and some surface feature as well as the cyclical nature of the bubble-forming process. Small cavities and surface cracks act as sites for bubble generation because: (1) The contact area between the liquid and heating surface increases relative to a perfectly-smooth surface, so liquid trapped in these areas vaporizes first; and (2) The presence of trapped gases in such cracks creates liquid-vapor interfaces, which serve as sites where transfer of energy in the form of latent heat from the liquid to the vapor phase takes place. Once a vapor bubble has been initiated at a nucleation site, under the right conditions the bubble grows to a certain required diameter, detaches from the heating surface, and rises to the liquid free surface.
If the excess temperature remains at the low end of the nucleate boiling regime, shown between points A and B of Fig. 1, each bubble generated can grow and detach from the surface independently – that is, without interaction between bubbles. As the bubble-generating process occurs at the active nucleation sites, the surface area between these sites retains the liquid-solid contact that characterizes the natural convection regime. Convection remains the primary mechanism of heat transfer in this so-called “isolated bubble” regime. As we shall see, however, the character of this convection is markedly different from that of the natural convection encountered at lower excess temperatures (ΔT < 5 °C for water).
As the excess temperature increases beyond point B in Fig. 1, additional nucleation sites become active and more bubbles are generated. The higher density of bubbles leads to their interaction with each other. Bubbles from separate sites now merge to form columns and slugs of vapor, thus decreasing the overall contact area between the heating surface and the saturated liquid. Consequently, the slope of the boiling curve begins to decrease and the heat flux eventually reaches a maximum value, q''max, referred to as the critical heat flux. The critical heat flux, which marks the upper limit of the nucleate boiling regime, reaches a value of approximately 106 W/m2 for water at an excess temperature of about ΔTc = 30 °C. The nucleate boiling regime is most desirable for many industrial applications because of its high heat flux at relatively low levels of excess temperature (5 °C ≤ΔT≤30 °C for water). However, certain circumstances are required to avoid nucleate boiling, such as wicked heat pipes (Faghri, 1995).
As the temperature increases beyond the critical heat flux point, the rate of bubble generation exceeds the rate of bubble detachment from the heater surface. Bubbles from an increasing number of sites merge to form continuous vapor films over portions of the surface, further decreasing the contact area between the heating surface and the saturated liquid. These vapor films are not stable, however: they can detach from the surface, leading to restoration of contact with the liquid and resumption of nucleate boiling. Under these unstable conditions, the surface temperature may fluctuate rapidly, so the excess temperature shown on the ΔT-axis of Fig 1 between points C and D should be regarded as an average value. Since the boiling in this regime combines unstable film with partial-nucleate boiling types, it is referred to as the region of transition boiling. When the excess temperature becomes high enough to sustain a stable vapor film, the heat flux reaches its minimum value, q''min.
This point, known as the Leidenfrost temperature, marks the upper temperature limit of the transition boiling regime. At temperatures above the Leidenfrost temperature, the bulk liquid and the heating surface are completely separated by a stable vapor film, so boiling in this regime is known as film boiling. The phase change in film boiling occurs at a liquid-vapor interface, instead of directly on the surface, as in the case of nucleate boiling. Thermal energy from the heating surface reaches the liquid-vapor interface by convection in the vapor film as well as by direct radiation to the interface. In the film boiling regime, the surface heat flux becomes a monotonically increasing function of the excess temperature, because radiation heat transfer from the solid surface to the liquid plays a significant role at high surface temperature. Pool boiling continues in this regime until the surface temperature reaches the maximum allowable temperature of the heating surface (2042 K for platinum, for example). Beyond that point, the heating surface can melt in a potentially catastrophic failure. If protective insulation is provided, however, as in the case of refractory metals, for example, it is possible for film boiling heat flux to exceed the critical heat flux, q''max.
The boiling curve presented in Fig. 1 and described above assumes that the surface temperature is independently controlled and that the heat flux is the dependent variable. However, direct control over the surface temperature is not always possible. For example, when electric heating provides thermal energy to the solid, the controllable parameter is heat flux. Surface temperature then becomes the dependent variable, and heat flux becomes the independent variable. If the experiment of gradually increasing the added thermal energy is repeated using constant heat flux instead of constant temperature control, the resulting boiling curve matches that of controlled-temperature up to the critical heat flux, q''max. When the surface heat flux is increased slightly above the critical heat flux, however, portion C-D-E of the boiling curve is bypassed and the surface temperature increases abruptly to point E of the stable film boiling regime (Nukiyama, 1934). This abrupt increase in surface temperature is undesirable because TE usually exceeds the melting point of the solid material. The critical heat flux can thus serve as the warning point above which burnout can occur; consequently, the critical point is sometimes referred to as the burnout point. If the pool boiling curve is defined by decreasing the controlled heat flux from an initial point in the film boiling regime (point E, for example), a characteristic identical to Fig 1 appears down to the point of minimum heat flux. After that point, a continued decrease of q'' yields a second hysteresis path that leads immediately to the nucleate boiling regime between points A and B. In this case, the transition boiling regime and a portion of the nucleate boiling regime are bypassed. Therefore, the transition regime that is observed when temperature control is used to define the pool boiling curve is unavailable in the controlled-heat-flux case.
References
Faghri, A., 1995, Heat Pipe Science and Technology, Taylor & Francis, Washington, DC.
Faghri, A., and Zhang, Y., 2006, Transport Phenomena in Multiphase Systems, Elsevier, Burlington, MA
Faghri, A., Zhang, Y., and Howell, J. R., 2010, Advanced Heat and Mass Transfer, Global Digital Press, Columbia, MO.
Nukiyama, S., 1934, “The Maximum and Minimum Values of Heat Q Transmitted From Metal to Boiling Water Under Atmospheric Pressure,” Journal of Japanese Society of Mechanical Engineering, Vol. 37, pp. 367-374 (1934) (translated in International Journal of Heat and Mass Transfer, Vol. 9, pp. 1419-1433 (1966)).