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technophobe February 15, 2010 12:27

Coanda effect, condensation on sphere
 
Hi Guys,
I am currently looking at the condensation of gas onto a sphere. My current assumption is that the condensate film layer will remain attached to the sphere, remaining on the sphere's surface due to the coanda effect, before detatching as a stream of liquid at the lowest point on the sphere's surface. My assumption is that the condensate will remain attached over the entire underside of the sphere until reaching the stagnation point.

Could someone please comment on the validity of this assumption? I am concerned that, for large diameter spheres or thick condensate layers, the coanda effect will be insufficient to hold onto the fluid layer all the way around the sphere and that it will detach before reaching the stagnation point. Is there any straightforward way to work out the limitation of the coanda effect and whether it will be able to support the fluid layer on the underside of the sphere?

Ahmed February 16, 2010 00:10

Quote:

Originally Posted by technophobe (Post 246091)
Hi Guys,
I am currently looking at the condensation of gas onto a sphere. My current assumption is that the condensate film layer will remain attached to the sphere, remaining on the sphere's surface due to the coanda effect, before detatching as a stream of liquid at the lowest point on the sphere's surface. My assumption is that the condensate will remain attached over the entire underside of the sphere until reaching the stagnation point.

Could someone please comment on the validity of this assumption? I am concerned that, for large diameter spheres or thick condensate layers, the coanda effect will be insufficient to hold onto the fluid layer all the way around the sphere and that it will detach before reaching the stagnation point. Is there any straightforward way to work out the limitation of the coanda effect and whether it will be able to support the fluid layer on the underside of the sphere?

The right approach to answer your questions is to first agree on the definition of the Coanda effect.
The Coanda effect refers to the effect of the presence of a solid wall on the flow field developed by a jet, and why only a jet? because a jet flow field has a natural physical characteristic which is the conservation of momentum (refer to The Theory of Boundary Layers by Schlichting), down stream of the nozzle, the velocity is decreased and in order to conserve the momentum, the jet flow field entraps mass from the surroundings, because the solid wall will prevent this mass entrainment, the jet is deflected and it is called an attached jet.
Do you have this situation in your geometry problem? Not clear to me based on your description.
Centrifugal forces develop on curved surfaces which force a flow to change direction and create an omega that will lead to the formation of vortices..etc, but you need a flow field that conserves the momentum such as Jets.
Hope that will help and good luck

technophobe February 16, 2010 07:13

Thanks Ahmed,
You are absolutely correct - this isn't strictly the Coanda effect as it involves liquid water in quiescent gas. I was using the term in the sense that the 'Coanda effect' is used to explain the deflection of water around a spoon. It isn't strictly the Coanda effect but is often referred to as such.

My problem consists of a sphere with a condensate film on its surface. My heat transfer analysis assumes, thus far, that the film layer remains attached all the way around the sphere, until it reaches the stagnation point and detaches as a single stream of fluid. I am concerned that this might not be the case and that portions of the fluid may detach before reaching the stagnation point.

I would like to know if there is some limit as to how long the condensate layer will remain attached on the underside of the sphere. I imagine that if the sphere has a particularly large diameter then the flow will detach before reaching the stagnation point.

Ahmed February 16, 2010 17:27

Quote:

Originally Posted by technophobe (Post 246163)
Thanks Ahmed,
You are absolutely correct - this isn't strictly the Coanda effect as it involves liquid water in quiescent gas. I was using the term in the sense that the 'Coanda effect' is used to explain the deflection of water around a spoon. It isn't strictly the Coanda effect but is often referred to as such.

My problem consists of a sphere with a condensate film on its surface. My heat transfer analysis assumes, thus far, that the film layer remains attached all the way around the sphere, until it reaches the stagnation point and detaches as a single stream of fluid. I am concerned that this might not be the case and that portions of the fluid may detach before reaching the stagnation point.

I would like to know if there is some limit as to how long the condensate layer will remain attached on the underside of the sphere. I imagine that if the sphere has a particularly large diameter then the flow will detach before reaching the stagnation point.

1- Loose use of well defined terminology almost certain to produce confusion between individuals communicating over the internet ( even in a personal face to face conversation) so try to avoid it, Coanda effect sounds much better than flow over a curved surface, yes I agree on that, but if it leads to confusion then there is no apparent benefit, it will not attract the attention of those who can answer your question (Basic to communicatios)
2- Separation is a question of adverse pressure gradients, you need to explore the balance of forces, gravity, pressure forces and drag forces.
You are the only person with access to the magnitudes of these forces, I guess you are the only person that can answer this situation.
Wish you good luck

If you still need help, try to attache a picture of what you are investigating and other data such as Re, etc.

technophobe February 17, 2010 11:04

Whilst I am grateful for you taking the time to respond to my post, there really was no need to adopt such a condescending tone.

Adopting a condescending tone when communicating with people, either in person or on the internet, is likely to lead to an atmosphere of hostility with no apparent benefits. People are much more likely to listen to your sage advice when proffered with some degree of humility (basic to communications).


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