Question about joule heating (multi terminal)

[driga]

Hello all,

I would like to heat a surface using Joule heating principle. The picture is shown in attachment. The top surface of the glass body is coated with ITO and silver beads to connect it with electric source. With my arrangement is it possible to independently control the heat flux at each section between the two opposite poles.

Thank you.

 

[IRstuff]

Why wouldn't it be? You have resistor (ITO) between each terminal.

The main question is what kind of uniformity is required, because your architecture will lead to circumferential nonuniformity in temperature.

TTFN

FAQ731-376

 

[driga]

What do you mean by circumferential non uniformity in temperature?

The idea is since on top of surface there is a convective heat transfer, I might need less heat flux at the downstream. So that I can have uniform temperature along the flat surface (longitudinal).

 

[IRstuff]

OK, maybe your picture is being misinterpreted. Is the image supposed to be a plan view of a flat surface with ITO running along the top edge? If so, then you will get a vertical nonuniformity going down your image.

I really don't know if I'm looking a box, a sheet, or a tube.

TTFN

FAQ731-376

 

[driga]

It is not a plan view, it is a front view. I have added top view, hopefully it is more understandable now. Do you think there still be non uniformity? The grey area is ITO coating. The stripes are silver beads.

 

[IRstuff]

Is that the only area you want to be heated?

Even if yes, there will be a nonuniformity along the sides and ends, because of the asymmetry of current and heat flow along the open edges. What uniformity are you trying to achieve?

Your ITO surface is the only part that's getting direct heating. That means that everywhere else in your system will be cooler, and the farther away from that surface, the cooler it will get.

TTFN

FAQ731-376

 

[driga]

Yes, the area needs to be heated is the top surface only. The one coated with ITO and silver beads. The glass window will be placed on a channel. I do not need the temperature uniformity within the glass part as well, just the surface of the ITO and silver beads.

 

[IRstuff]

Even so, as I mentioned before, the edges of your last image will be at a lower temperature than the centers of the areas between the electrodes. If you want really uniform temperatures, then the middle electrodes need to move outward, and the ITO will also need to be split, with separate electrodes, parallel to the long dimension in the drawing. This will allow you to compensate for the heat losses along the edges by running more heat into those areas.

TTFN

FAQ731-376

 

[driga]

I am sorry but I am not quite understand by moving the middle electrode outward. Lets say, from the first picture the electrode at the left most is number 1 followed by number 2, 3 and number 4 will at the right side. Which one needs to be move outward?

The other thing is what do you mean by splitting the ITO along the long dimension?

A sketch would be helpful.

Thanks.

 

[IRstuff]

Something more like:

http://files.engineering.com/getfil...8-d7ae-4d28-aea5-138edc7a4128&file=heater.gif

where there are 5 heaters, 4 for the edges and 1 for the middle.

TTFN

FAQ731-376

 

[driga]

I see..

Is there any simple calculation to prove that the arrangement will ensure a uniform temperature?? So basically based on your sketch, there will be two positive and two negative poles, right?

What do you think with this top arrangement (see attachment)? Will it gives the same effect as your sketch?

 

[IRstuff]

Hypothetically yes, but much harder to optimize the power, since there is guaranteed interaction amongst all the heated areas.

In my drawing there are 5 separate heaters, and the ones that are symmetrically placed might be paralleled, but that's about it. If you want uniformity, don't skimp on the control circuitry.

TTFN

FAQ731-376

 

[driga]

Thanks IRstuff for such an insight discussion. I have one difficulty to understand your sketch is where the silver beads need to be positioned? Since I do not want to to take much of the space with the cost of losing optical access. That is the reason I would like to use ITO to have a transparent heater.

 

[IRstuff]

They should run along the short dimension of each of the 5 heater sections.

TTFN

FAQ731-376

 

[driga]

Would be like this? (see attachment). If it is correct then it would be quite difficult in accessing the middle heater.

And what do you like think would be the best for the control circuit? Where should I start?

 

[IRstuff]

That's true; you'll have to make a compromise somewhere, leaving a couple of gaps where you have the least requirement for uniformity. But, it seems to me that you can run the center strip's electrodes to hookup wire immediately in place and then run the wire out to sides above the electrodes of the side strips with minimal loss of transmission.

TTFN

FAQ731-376

 

[driga]

I want to make things clearer so I made new modification on the previous figure.

So i have divided the plate into 5 zones, each zone has two electrodes (positive and negatives). Each zone has independent source of electricity, right?

When you said center strip, do you mean the part shown by arrow with A? I do not quite understand about your explanation. I would like to have a smooth surface since on top of it there is a fluid flow.

 

[Skogsgurra]

IR the hard-working EE
Going to send a bill for your work soon?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[IRstuff]

If you're running fluid, then why do you need to uniformity?


S, no just slumming ;-) since my BSEE is probably lapsed from disuse.


TTFN

FAQ731-376

 

[driga]

Well that is because what I would try to achieve. So far for constant temperature condition, I use a copper plate, and it is heated using water flow from thermostat. What I would like to try is having a constant temperature condition instead of constant heat flux for electric heater.