Question

P. #3: An engineer conducted pool boiling experiments on a commonly used industrial copper substrate and obtained the following "Pool boiling Curve". He identified four different boiling regimes: (1) natural or free convection, (2) nucleate boiling, (3) transition boiling, and (4) film boiling. You, as an R&D researcher, are asked to explain the results and improve the efficiency of the phase-change process. Note: Describe your answer concisely with minimal jargon. 10 Free Convection Nucleate Transition Film Inflection point Surface Heat Flux, W/m² Onset of boiling Isolated Jets and bubbles columns 5 10 30 120 Excess Temperature, °C 1000 P. #3-3: Superhydrophilic coating: You are asked to consider a superhydrophilic coating to improve the boiling process. The coated surface can be achieved by a simple chemical treatment process. (i) Explain whether temperature associated with point A (Onset of Nucleate Boiling, ONB) increases or decreases on the superhydrophilic coated surface. Why? (ii) What happens to slop of AB curve? What does this slope represent? (iii) Also, explain whether heat flux at point C increases or decreases? Why?

Answer 1

P. #3:

Pool boiling curve:

In the pool boiling curve there are 4 phases with transition being excess temperature. They are,

1. Natural/ Free convection boiling
2. Nucleate boiling - (Unstable & Stable)
3. Transition boiling
4. Film boiling

1. Natural/ Free convection boiling:

• In this case the liquid is being slightly overheated.
• In this form of boiling the fluid motion is governed by natural currents of convection.
• At around 7oC (at point 1 to A) excess temperature.

2. Nucleate boiling:

• At an increasing number of nucleation sites, the bubbles shape at an rising pace as we pass along the boiling curve towards point C.
• Unstable boiling is at beginning part of the nucleate boiling.
• Then another component is available higher at steady boiling time.
• The heat transfer coefficient varies from around 3000 to 30000 W/m2 K for the entire boiling range of the nucleates.
• Region A-B: Isolated bubbles
• Region B-C: Numerous continuous columns of vapour in the liquid.

3. Transition boiling:

• The heat flux decreases as over temperature increases beyond point C.
• That is because a vapor film covers a significant fraction of the heater surface, which serves as an insulation.
• Nucleate and film boiling partly occur in the transition boiling regime.
• In action activity in the boiling transition regime, also called the chaotic boiling film regime, is avoided.
• The transition boiling occurs for copper substrate over the excess temperature from about 30oC to about 120oC.

4. Film boiling:

• Beyond point D a continuous stable vapor film fully covers the heater flux sheet.
• Point D, where the flux of heat reaches a peak, is called Leidenfrost.
• The presence of vapor film between the surface of the heater and the liquid is responsible for the low levels of heat transfer in the boiling region.
• The heat transfer rate increases due to radiation to the liquid, with an increase in excess temperature.