Science Buddies: "Ask an Expert"

1. How do you measure convection currents/ is there inexpensive equipment i could use?
2. Is there a minimum distance between two surfaces that is needed for convection to occur?

Thanks, this would help a lot.

First two preliminary notes:
I. Convection is a broad and complex subject. Read the Wikipedia article at:

http://en.wikipedia.org/wiki/Convection

to get some preliminary information.

II. I am not an expert in fluid dynamics, so you should view my ideas knowing that I may well be wrong on important points. I'll see if we have a real expert on this topic available, but I guess that we do not. If you have a university near by, you might consider looking in its catalog to find professors who teach aerodynamics or fluid mechanics who might be better sources of information than I am.

Now to your specific questions.

1. How do you measure convection currents/ is there inexpensive equipment i could use?

This will depend on the whether you are planning to use a liquid, e.g. water, or a gas, e.g. air. In both cases the most feasible method would be to place some sort of tracer particles in the fluid that would allow you to see the flow. For example, in air one might consider smoke. However this method is likely to yield qualitative rather than quantitative results. Getting quantitative results will be difficult I fear. One method I have seen involved doppler laser scattering -- that sounds to me to be complex for a science fair project. Another method used to visualize flow in gases is Schlieren photography, but it too is qualitative and not so easy to perform.

2. Is there a minimum distance between two surfaces that is needed for convection to occur?

I believe it is the other way around. If the two surfaces are too close together the effects of viscosity and diffusion will suppress large scale convection (advection to be precise). In the other direction the sky's the limit, literally: convection is a major source of heat transfer through the earth's atmosphere. Cumulus clouds are a pretty example, while hurricanes are a spectacular example.

I hope this helps. We will be able to do more when you develop specific project questions.

Feel free to keep in touch, that's what we are here for.

Hello squnkus,

John's taken good care of this one, he's provided some very helpful answers. One thing I'd like to add is this: make sure that you understand that convection takes place due to fluid flow, and does not involve two surfaces. Convection is heat transfer to/from a surface from a fluid flow; it does not involve the separation between two surfaces.

To better understand convection, look up Newton's Law of Cooling:

Q= hA(Tb-Tf)

in which Q is the energy of heat transfer, h is the heat transfer coefficient (dependent on the particular situation's fluid, means of flow, and flow pattern--something you can look up), A is the area of the surface over which the fluid passes, Tb is the temperature of that surface, and Tf is the temperature of the flow as it approaches the surface. The equation assumes Tf<Tb, which is why it's called a cooling law. It calculates the heat energy transferred to the fluid, and so if Tf>Tb, Q is negative.

This equation may help you understand convection, and may prove useful in calculating convection currents.

Be sure to supplement all this information with the Wikipedia article on convection!

Good luck with your project! We are always here if you have more questions.

squnkus wrote:2. Is there a minimum distance between two surfaces that is needed for convection to occur?

Two surfaces are not required for convection to occur. There are special fluid flow cases involving sealed volume convection currents where the geometry affects the fluid dynamics. For instance windows that have two separate pieces of glass with a gas inbetween, the geometry can significantly affect the convection current in the sealed space. For example in winter with an outside temperature of 0 F and an inside temperature of 70 F, the expected convection current will be down the colder surface and up the hotter surface. The closer the two surfaces are together, the more the downward flow will interfere with the upward flow and flow rate will be reduced which will decrease the net convection heat transfer. These fluid dynamics effects aren't specific to convection fluid flows.

In fact, no physical surface is required for convection heat transfer to occur, all that is required is some boundary. Ocean currents like the gulf stream exhibit convection heat transfers to the air.

Thank you everyone for your replies. I decided due to time constraints not to go ahead with that project at the time. I am trying something else at the moment and am targeting a science fair in Feb of next year.

Good luck with your next venture!