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Chapter 10: Problem 6

How does film boiling differ from nucleate boiling? Is the boiling heat flux necessarily higher in the stable film boiling regime than it is in the nucleate boiling regime?

Short Answer

Expert verified
Answer: The boiling heat flux differs between film boiling and nucleate boiling in terms of their heat transfer mechanisms and heat transfer rates. In nucleate boiling, the heat flux is generally higher due to its heat transfer mechanism through convection and direct contact between the liquid and the heating surface. In film boiling, the heat flux is relatively lower due to the insulating effect of the stable vapor film, which reduces direct contact between the liquid and the surface.

Step by step solution

01

Define Film Boiling and Nucleate Boiling

Film boiling is a type of boiling where a continuous and stable vapor film is formed between the heating surface and the liquid. Due to the low thermal conductivity of vapor, heat transfer in this scenario is mainly radiation and convection. This results in a relatively lower heat transfer rate compared to nucleate boiling. Nucleate boiling is a boiling process where bubbles form at nucleation sites on a hot surface due to the liquid absorbing enough heat and then rise through the liquid, with heat transfer mainly occurring through convection. In this process, the heat transfer rate is generally higher compared to film boiling.
02

Compare Film Boiling and Nucleate Boiling

Some of the main differences between the boiling types include: 1. Heat transfer mechanism: In film boiling, the heat transfer is mainly through radiation and convection, while in nucleate boiling, it is mostly convection. 2. Heat transfer rate: The heat transfer rate in nucleate boiling is significantly higher compared to film boiling due to the direct contact between the liquid and the heating surface. 3. Occurrence: Film boiling typically occurs at higher temperatures than nucleate boiling when the heating surface temperature exceeds the liquid's critical temperature.
03

Analyze the Boiling Heat Flux in Each Regime

The boiling heat flux represents the rate of heat transfer per unit area during the boiling process. It is crucial to determine the efficiency and stability of various boiling methods. For nucleate boiling, the heat flux is generally higher due to its heat transfer mechanism through convection and direct contact between the liquid and the heating surface. However, in the film boiling regime, a stable vapor film acts as an insulator between the liquid and the heating surface. As a result, the heat flux is relatively lower than in the nucleate boiling regime, due to the decreased contact between the liquid and the surface.
04

Conclusion

In conclusion, film boiling and nucleate boiling differ in terms of their heat transfer mechanisms, heat transfer rates, and the temperature at which they occur. The boiling heat flux in the stable film boiling regime is not necessarily higher than in the nucleate boiling regime, so it is typically lower due to the insulating effect of the vapor film in film boiling.

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Most popular questions from this chapter

Water is boiled at atmospheric pressure by a horizontal polished copper heating element of diameter \(D=0.5\) in and emissivity \(\varepsilon=0.05\) immersed in water. If the surface temperature of the heating element is \(788^{\circ} \mathrm{F}\), determine the rate of heat transfer to the water per unit length of the heating element.

Water is boiled at \(90^{\circ} \mathrm{C}\) by a horizontal brass heating element of diameter \(7 \mathrm{~mm}\). Determine the maximum heat flux that can be attained in the nucleate boiling regime.

10-59 The Reynolds number for condensate flow is defined as \(\operatorname{Re}=4 \dot{m} / p \mu_{l}\), where \(p\) is the wetted perimeter. Obtain simplified relations for the Reynolds number by expressing \(p\) and \(\dot{m}\) by their equivalence for the following geometries: \((a)\) a vertical plate of height \(L\) and width \(w,(b)\) a tilted plate of height \(L\) and width \(W\) inclined at an angle \(u\) from the vertical, \((c)\) a vertical cylinder of length \(L\) and diameter \(D,(d)\) a horizontal cylinder of length \(L\) and diameter \(D\), and (e) a sphere of diameter \(D\).

Saturated steam at 1 atm condenses on a 3-m-high and 8 - \(\mathrm{m}\)-wide vertical plate that is maintained at \(90^{\circ} \mathrm{C}\) by circulating cooling water through the other side. Determine \((a)\) the rate of heat transfer by condensation to the plate, and ( \(b\) ) the rate at which the condensate drips off the plate at the bottom. Assume wavy-laminar flow. Is this a good assumption?

In condensate flow, how is the wetted perimeter defined? How does wetted perimeter differ from ordinary perimeter?
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