Question

Discuss the different types of pool boiling phenomena.

Describe the different behavior regions associated with pool-boiling in terms of excess temperature.

Describe the differences, if any, between burn-out point and Leidenfrost point for pool boiling.

For pool boiling, which thermo-physical variable needs to be controlled to obtain a continuous curve in the heat flux vs. excess temperature plot?

Discuss at which point does nucleate boiling regime starts and ends.

Describe the reason(s) for lower heat transfer coefficients relative to natural convection when excess temperature exceeds the burn-out point in a heat vs. excess temperature plot.

Discuss the reason(s) why quality is relevant during operation of power plant low pressure turbines.

Answer 1

• Different types/ stages of pool boiling phenomena are:

Four boiling regimes are seen in the pool boiling phenomena. These are

1. Natural convection regime
2. Nucleate regime
3. Transition regime
4. Film boiling regime

Note: In the diagram, approximately 5^oC is denoted by ~5^oC. It is not to be confused with minus or -5^oC. For example, transition boiling starts at approximately 30^oC or ~30^oC and ends at roughly ~120^oC.

1. Natural convection boiling: Theoretically, boiling starts as soon as the fluid reaches its saturation temperature. However, in reality, it needs some further temperature for boiling to be actually started. For water, this excess temperature is approximately 2^oC to 6^oC. In this condition, we can observe that the bubbles are gradually beginning to form. This is natural convection boiling in which we observe the bubbles beginning to form. Bubbles are an intrinsic part of pool boiling. All the heat transfer occurring in this stage is via natural convection currents only. This stage occurs when the fluid is slightly superheated or metastable.

2. Nucleate boiling: It is the most desirable form of heat transfer in pool boiling. In this region, bubbles begin to form at various nucleation sites. These bubbles are formed and in the first region, they are highly likely to collapse when they leave the heater surface. These are isolated bubbles. There is a significant increase in the rate of bubble formation. When bubbles leave the surface, they collapse and the space vacated is filled by the surrounding liquid and thus heat transfer is accelerated. After consuming a lot of heat, the bubbles are finally large enough to be able to rise to the pool surface. These bubbles are energy movers. Thus we can see a significant increase in the heat transfer flux with the excess temperature. In this region, continuous columns of bubbles are formed. The highest/ critical/ burning heat flux is observed here. This point is also called the Burning Point or Critical Heat Flux Point (CHF).

3. Transition boiling: It is the most unstable part of boiling. Hence it is severely undesirable. In this stage, we can observe an abrupt, huge drop in the heat flux. It happens because of the vapor blanket. Liquids take too much heat and they produce massive amounts of vapor which obscure the surface of the liquid, thereby leading to the formation of a vapor blanket or a vapor cover. Owing to this vapor blanket, the surrounding liquid particles also find it tough to get inside the heater surface. Therefore, there is a significant drop in the heat flux. Here, the heat transfer rate decreases too because of the lower heat flux.

4. Film boiling: The heat transfer suddenly increases at the end of transition boiling and the beginning of film boiling. After gaining sufficient heat energy in order to attain the critical heat flux. Hence, the heater surface absorbs heat. After some time, the liquid film leaves the heat into the liquid and the heat becomes so huge that the radiation heat transfer is also used for consideration. Hence, we observe an increase in the heat flux. In the first part part of this region we see a minimum heat flux which is referred to as the Leidenfrost Point or the minimum heat flux point (often called Critical Heat Flux or CHF). In this region, the Burn-out phenomenon can occur quite frequently.

• Different behavior regions associated with pool-boiling in terms of excess temperature:

The answer is same as above.

• Differences between Burn-out Point and Leidenfrost Point for pool boiling:

1. Leidenfrost Point is associated with the liquid itself, whereas Burn-out Point is associated with the heater or boiler material.

2. Leidenfrost Point is the point at which the liquid has attained its maximum or critical heat flux and its vapor cover starts melting, whereas Burn-out Point is the point at which the heater starts to melt or "burn-out".

3. Leidenfrost Point is higher, whereas Burn-out Point is lower in most heaters. Most material would burn out before the Leidenfrost Point is attained. Only in case of very high melting point heaters can this effect be reversed.

4. Attaining the Leidenfrost Point would not be dangerous and on the contrary, it is desirable, whereas attaining the Burn-out Point would be disastrous, leading to failure and severe damage of the heater.

• The thermo-physical variables which need to be controlled to obtain a continuous curve in the heat flux vs. excess temperature plot are:

1. Temperature

2. Pressure

3. Control Volume

• The nucleate boiling regime starting and end points are:

1. The nucleate boiling regime starts at the end of natural convection boiling at roughly 5^oC.

2. The nucleate boiling regime ends when bubbles finally rise to the free surface at approximately 30^oC.

• Reason(s) for lower heat transfer coefficients relative to natural convection when excess temperature exceeds the burn-out point in a heat vs. excess temperature plot:

The answer has already been discussed in the first question itself.