Question

What is the difference between pool boiling and flow boiling?

Short answer

Answer: Pool boiling refers to the heat transfer process in which a heated surface is submerged in a stationary liquid pool, and the heat transfer occurs at the surface-liquid interface. In contrast, flow boiling involves a liquid flowing over a heated surface and undergoing phase change. Pool boiling applications include cooling electronic devices, nuclear reactors, and boiling water for cooking, while flow boiling is applied in air conditioning systems, industrial heat exchangers, and power plants. The heat transfer process in pool boiling involves natural convection, nucleate boiling, and film boiling, whereas in flow boiling, it combines single-phase convective heat transfer with phase-change heat transfer, facilitating a more efficient process due to the continuous replenishment of the liquid-vapor interface.

Step-by-step solution

Definition of Pool Boiling

Pool boiling is the process of heat transfer that occurs when a liquid (usually referred to as a coolant) changes phase from liquid to vapor at a solid surface, while being stationary or having minimal flow. In this process, a heated surface is submerged in a pool of liquid, and the heat transfer occurs at the surface-liquid interface.

Definition of Flow Boiling

Flow boiling refers to the heat transfer process occurring when a liquid flows over a heated surface and undergoes phase change from liquid to vapor. In this case, the liquid flow helps enhance the heat transfer rate by continuously replacing the evaporating liquid layer with fresh coolant.

Applications of Pool Boiling

Pool boiling is commonly used in applications where a stationary liquid pool is present. Some typical applications include cooling of electronic devices, nuclear reactors with passive cooling systems, and boiling water for cooking purposes.

Applications of Flow Boiling

Flow boiling finds its application in systems where fluid transport is more conducive to efficient heat removal. Examples of such applications can be observed in air conditioning systems, heat exchangers used in industrial processes, and power plants with active coolant loops.

Heat Transfer Process in Pool Boiling

In pool boiling, the heat transfer process is primarily governed by three mechanisms: natural convection, nucleate boiling, and film boiling. Natural convection occurs when the heated liquid rises due to buoyancy forces, while cooler liquid moves towards the heated surface. Nucleate boiling occurs when the temperature of the heated surface becomes high enough to cause bubble formation and vapor generation. Film boiling involves vaporizing the entire liquid film adjacent to the heated surface, creating an insulating vapor layer, usually at very high surface temperatures.

Heat Transfer Process in Flow Boiling

Flow boiling heat transfer combines the mechanisms of single-phase heat transfer (convective heat transfer) and phase-change heat transfer (boiling). As the liquid flows over the heated surface, the heat transfer process begins with single-phase convective heat transfer, followed by the formation of bubbles and nucleate boiling, and ultimately film boiling, similar to the process observed in pool boiling. The key difference lies in the assisted liquid flow, which results in a more efficient heat transfer process by continuously replenishing the liquid-vapor interface.