Association **Question Set P83 4-9
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This post was last edited by 2005180005 on 2016-8-18 10:39. What measures are effective in increasing the heat transfer coefficient for liquid boiling? 1. Increase the temperature difference between the liquid and the wall surface ； 2. Try to convert film boiling into nucleate boiling ； 3. Roughen the smooth surface ； 4. Reduce the surface tension of liquids. The answers are 2, 3, and 4. Why is 4 the correct choice? The surface tension of the liquid decreases, making it harder for bubbles to form; therefore, the heat transfer coefficient should decrease. I can’t understand this – please help explain it
As the surface tension of the liquid decreases, bubbles should form more easily.
The boiling of a liquid is a phase transition process; taking interface effects into account, surface tension contributes positively to the Gibbs free energy of the nucleation process. Even if this increases the system’s Gibbs free energy, the process cannot proceed spontaneously. Reducing surface tension helps to lower the energy barrier for the formation of a new phase, thereby facilitating bubble formation. Or, from the perspective of vapor pressure, due to surface tension, the pressure inside a bubble must be higher than that of the liquid; this means that the liquid needs to be superheated in order for small bubbles to form. As surface tension decreases, the difference between the pressure inside the bubble and that of the liquid also decreases, which means that less superheating is required to form small bubbles, thereby facilitating their formation.