Types of boiling

Fundamentals

In each mode of boiling the heat transfer coefficient is different. The fundamentals of different modes of boiling are determined by the difference between the surface superheat or wall temperature (Twall) and the saturated temperature of the liquid (Tsat). The difference of Twall - Tsat determines the mode of boiling and the heat transfer coefficient (h) associated with each mode. Various empirical equations exist to calculate the heat transfer coefficient for different modes of heat transfer during boiling.

Typical modes of boiling

Natural convection, nucleate boiling, transition boiling, and film boiling are different modes of boiling, each characterized by distinct heat transfer mechanisms and surface conditions. These modes can be differentiated based on the difference between surface superheat and saturation temperature of the liquid:

Nucleate Boiling: Nucleate boiling involves the formation and growth of small vapor bubbles directly at the heated surface. This mode is characterized by a moderate surface superheat, typically ranging from 10 to 30 degrees Celsius. The nucleate boiling regime is highly efficient in heat transfer due to the large surface area covered by the bubbles, which enhances convective heat transfer.

Transition Boiling: Transition boiling occurs when the surface superheat increases further, typically between 30 to 60 degrees Celsius. The heat transfer mechanism transitions from nucleate boiling to film boiling. In this mode, vapor bubbles start to coalesce and form a continuous vapor film on the surface, reducing the direct contact between the liquid and the surface. Heat transfer becomes less efficient compared to nucleate boiling, as the vapor film acts as an insulating layer.

Film Boiling: Film boiling is characterized by a high surface superheat, exceeding 60 degrees Celsius. In this mode, the liquid completely separates from the surface, and a continuous vapor film is formed. The heat transfer is primarily through radiation from the hot surface to the vapor film. Film boiling is the least efficient mode of boiling regarding heat transfer, as the vapor film significantly inhibits the direct contact and convective heat transfer between the liquid and the surface.

Therefore, the modes of boiling can be differentiated based on the difference between the surface superheat and saturation temperature of the liquid. Natural convection has the lowest surface superheat, followed by nucleate boiling, transition boiling, and finally, film boiling, which has the highest surface superheat.

• Why should one know modes of boiling?

In each mode of boiling the heat transfer coefficient is different

• Heat transfer coefficient in different modes of boiling The heat transfer coefficient (h) can vary for each type of boiling. The calculation of heat transfer coefficient differs depending on the type of boiling regime.