Question

What is boiling? What mechanisms are responsible for the very high heat transfer coefficients in nucleate boiling?

Short answer

Answer: The high heat transfer coefficient in nucleate boiling is due to micro-layer evaporation, bubble nucleation, growth, and departure, as well as liquid agitation and mixing caused by rising bubbles.

Step-by-step solution

Define boiling

Boiling is a phase change process in which a liquid turns into a vapor when its temperature reaches the boiling point. The boiling point is the temperature at which the vapor pressure of the liquid equals the pressure exerted on it by the surrounding environment. Boiling occurs when the heat input to the liquid provides enough energy to overcome the intermolecular forces that hold the liquid together.

Overview of boiling regimes

There are several boiling regimes, which are distinguished based on the behavior of the liquid-vapor interface and the heat transfer mechanisms involved: 1. Natural convection: Heat transfer occurs by natural convection currents in the liquid, without boiling. 2. Nucleate boiling: The liquid forms vapor bubbles at discrete nucleation sites on the heated surface, which then grow and detach from the surface. 3. Transition boiling: A mixed-mode regime with a combination of nucleate boiling and film boiling. 4. Film boiling: The heated surface is covered with a continuous vapor film that insulates it from the liquid, leading to a low heat transfer coefficient. 5. Critical heat flux: The maximum heat flux that can be sustained without a significant drop in heat transfer coefficient, which is followed by a transition to film boiling.

Focus on nucleate boiling

Nucleate boiling is characterized by the formation of vapor bubbles on the heated surface, followed by bubble growth and detachment. This boiling regime has the highest heat transfer coefficient among the different boiling regimes. The high heat transfer coefficient in nucleate boiling is primarily due to the following mechanisms: 1. Micro-layer evaporation: At the nucleation site, a thin liquid micro-layer exists between the growing bubble and the heated surface. This micro-layer evaporates rapidly, causing the bubble to grow and transfer heat effectively. 2. Bubble nucleation, growth, and departure: Vapor bubbles form on nucleation sites on the heated surface, which then grow by absorbing surrounding liquid, and detach once they reach a certain size. This process continually exposes fresh liquid to the heated surface, promoting efficient heat transfer. 3. Liquid agitation and mixing: The motion of the rising bubbles creates local turbulence and mixing in the liquid, which enhances heat transfer by reducing the thermal boundary layer thickness and increasing the convective heat transfer coefficient.

Conclusion

Boiling is a phase change process in which a liquid turns into a vapor when its temperature reaches the boiling point. Nucleate boiling is a regime characterized by the formation and growth of vapor bubbles on the heated surface, which leads to a high heat transfer coefficient. The high heat transfer coefficient in nucleate boiling is due to micro-layer evaporation, bubble nucleation, growth, and departure, as well as liquid agitation and mixing caused by rising bubbles.