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With increase in excess temperature, the heat flux in boiling
- a)Increases continuously
- b)Decreases and then increases
- c)Decreases, then increases and again decreases
- d)Increases, then decreases and again increases
Correct answer is option 'D'. Can you explain this answer?
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Verified Answer
With increase in excess temperature, the heat flux in boilinga)Increas...
It first increases to 50% then decreases not to zero and again increases to its maximum value.
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With increase in excess temperature, the heat flux in boilinga)Increas...
The correct answer is option 'D': Increases, then decreases and again increases.
Explanation:
Boiling is a phase change process in which a liquid converts into a vapor due to the absorption of heat. The heat flux in boiling refers to the rate at which heat is transferred from the heating surface to the liquid during the boiling process. The heat flux is influenced by the excess temperature, which is the difference between the wall temperature and the saturation temperature of the liquid.
When the excess temperature is increased, the heat flux initially increases, reaches a maximum value, and then decreases. This can be explained through the following stages:
1. Nucleate Boiling:
- At low excess temperatures, nucleate boiling occurs.
- Nucleate boiling is characterized by the formation of small bubbles (nuclei) on the heating surface.
- These bubbles grow and detach from the heating surface, carrying away heat with them.
- As the excess temperature increases, the number and size of the bubbles increase, leading to an increase in the heat flux.
2. Transition Boiling:
- As the excess temperature continues to increase, the heat flux reaches a maximum value and then starts to decrease.
- This transition from nucleate boiling to film boiling is known as the critical heat flux (CHF) point.
- At the CHF point, a continuous vapor film forms on the heating surface, hindering the direct contact between the liquid and the surface.
- The vapor film acts as an insulating layer, reducing the heat transfer rate and causing a decrease in the heat flux.
3. Film Boiling:
- At high excess temperatures, film boiling occurs.
- In film boiling, a vapor film completely covers the heating surface, preventing any direct contact between the liquid and the surface.
- The heat transfer is dominated by convection within the vapor film, which is less efficient compared to direct contact.
- As a result, the heat flux starts to increase again but at a slower rate compared to nucleate boiling.
In summary, the heat flux in boiling initially increases with increasing excess temperature due to nucleate boiling. It then reaches a maximum at the critical heat flux point and starts to decrease as film boiling takes over. However, at higher excess temperatures, the heat flux increases again but at a reduced rate due to film boiling. Hence, the correct answer is option 'D': Increases, then decreases and again increases.
Explanation:
Boiling is a phase change process in which a liquid converts into a vapor due to the absorption of heat. The heat flux in boiling refers to the rate at which heat is transferred from the heating surface to the liquid during the boiling process. The heat flux is influenced by the excess temperature, which is the difference between the wall temperature and the saturation temperature of the liquid.
When the excess temperature is increased, the heat flux initially increases, reaches a maximum value, and then decreases. This can be explained through the following stages:
1. Nucleate Boiling:
- At low excess temperatures, nucleate boiling occurs.
- Nucleate boiling is characterized by the formation of small bubbles (nuclei) on the heating surface.
- These bubbles grow and detach from the heating surface, carrying away heat with them.
- As the excess temperature increases, the number and size of the bubbles increase, leading to an increase in the heat flux.
2. Transition Boiling:
- As the excess temperature continues to increase, the heat flux reaches a maximum value and then starts to decrease.
- This transition from nucleate boiling to film boiling is known as the critical heat flux (CHF) point.
- At the CHF point, a continuous vapor film forms on the heating surface, hindering the direct contact between the liquid and the surface.
- The vapor film acts as an insulating layer, reducing the heat transfer rate and causing a decrease in the heat flux.
3. Film Boiling:
- At high excess temperatures, film boiling occurs.
- In film boiling, a vapor film completely covers the heating surface, preventing any direct contact between the liquid and the surface.
- The heat transfer is dominated by convection within the vapor film, which is less efficient compared to direct contact.
- As a result, the heat flux starts to increase again but at a slower rate compared to nucleate boiling.
In summary, the heat flux in boiling initially increases with increasing excess temperature due to nucleate boiling. It then reaches a maximum at the critical heat flux point and starts to decrease as film boiling takes over. However, at higher excess temperatures, the heat flux increases again but at a reduced rate due to film boiling. Hence, the correct answer is option 'D': Increases, then decreases and again increases.
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With increase in excess temperature, the heat flux in boilinga)Increases continuouslyb)Decreases and then increasesc)Decreases, then increases and again decreasesd)Increases, then decreases and again increasesCorrect answer is option 'D'. Can you explain this answer?
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