In case of top cooled horizontal porous wicked condenser plane surfaces, liquid may move by capillary action into a two dimensional plane along the porous body rather than dropping down in the direction of gravity. Also, there is a chance for liquid to drop rather than moving by capillary action. What properties define the rate of liquid droplet from horizontal porous surface rather than capillary driven movement?
When liquid condensers on the underpart of horizontal plate ( with gravity acting downwards), the liquid film becomes unstable and droplets form. This is known as the Rayleigh-Taylor problem. You have likely seen evidence of this instability - its the array of pendant droplets that form on the underneath part of a cooling pipe. As the droplets grow in size, a point is reach when individual drops cannot support their own weight and begin dripping. A detailed analysis of this process for the two dimensional case can be found in my publication list:
https://www.researchgate.net/publication/233795982_Rayleigh-Taylor_instability_in_thin_viscous_films?ev=prf_pub
It is a complex interplay between capillary forces, and gravity.
In your situation, since the plate is porous, liquid can migrate from the pendant drop into the porous plate and be wicked away, thereby reducing the size of the pendant drop. The strength of wicking effect depends on the Darcy-like flow in the porous structure AND the local wetting properties of the pore surfaces, viz., the contact angle the liquid makes with the pores). Of course viscosity of the fluid is also important.
This problem may have been analyzed already, but I am not sure.
Article Rayleigh-Taylor instability in thin viscous films
Thank you for the clarification and sporting documents and I will look in to the recent publications further.
Imbibition of liquid into porous media should also be accounted for.
I think this pazzle is related to the so-called capillarity, it is about the liquid transporting ability of porous structure. the condensing rate of vapor should be considered and if the porous structure can suck this amount of liquid, then the droplet would not form.
And also, the total amount of liquid is critical, if there were excess liquid that porous stucture can not absorbe, then these excess water will very likely form a droplet on the upper surface.
If the porous structure undergoes a drying process, then the excess liquid would form droplet as well.
- Badges
- Science topic
- Similar topics
- Engineering
- Thermal Engineering
- Thermofluid
- Heat Transfer