Ceramic Question

[pbhuter]

I'm looking into ceramics for a high-temperature (up to 2000-deg C) high resistance application (resistance = heat). Are there ceramics that I can apply a current to (not quite sure how much power I will have available, but it should be pretty high) and have it generate enough heat to get up to 2000-deg C? I see that something like alumina loses resistivity as the temperature goes up, is that common for ceramics? Lowest temperature I need will be 650-deg C, but if I can get up to 2000, that would be better. Thanks.

 

[PKerEng]

I would hope that the temperature in your system remains as low as feasible for your application. Say, you need min. 650 C, let's hope it does not exceed 1200 C. In such case you would have a lot of leeway utilising easy to fabricate and manage metallic heating element and a variety of heat insulation. Assuming 12 kW dissipated the max temp would result from heat transfer conditions. Here geometry of the contraption, materials employed and possible flow of a fluid component thru the system would define the temperature distribution. My take here is that you can do calculations/computer simulations but there is no substitute for a quick first try just making a mock-up and have a first impression what to anticipate down the road. I would make something of a meaningful configuration first, just using metallic (say Kanthal A1) heating element, crank-up the power step-by-step and see the temperature distribution. Even if you miss in some places you will be able to define your problem much, much better. With a little bit of luck you might be able to do it almost right with the second approach. This is the route I would suggest. The respondents at the forum throw-in a lot of valuable info but, as usual, the devil is in details of your particular application. I understand that what you working on may be of a classified nature and you may not be able to share vital details and all of us find ourselves in a vicious cycle.

Slawomir

http://www.pkerengineering.com

 

[pbhuter]

PKerEng - I'm looking into doing a computer model of this, but of course that means I need data on the material (such as resistivity of the element). I'm looking at a circular element, similar to a stove (I think I stated that), but bigger. I know that the total heat will also depend on the length and cross-secional area of the element, in addition to the resistivity. I'm going to start scouring the Internet to come up with some values, and try to build a computer model using Matlab. While my application isn't classified, it's sensitive, so I appreciate you understanding that. Hopefully soon I will be able to share more details. Thanks for your support.

 

[pbhuter]

I'm looking for a couple properties of MoSi2 ceramics. Does anyone know the heat transfer coefficient (h-bar) and the emmissivity (epsilon)? I'm trying to calculate how hot it will get when I apply power and I need these two values. Thanks in advance.

 

[Ceramicguy]

Emmissivity is reported by most element manufacturers and should be readily available in print or on-line literature. H bar is dependent upon a large number of variables that all boil down to the three modes of heat transfer.

Bruce

http://www.accuratus.com

 

[pbhuter]

Ceramicguy - I found an emmissivity value, thanks. As for h-bar, how do you suggest I go about finding it? I can calculate a radiative h value (using temperature and emmissivity) that I will use in my final temperature calculation, but my equation seems to use a constant h-bar that is material specific. You don't know of a constant for MoSi2, do you, or know where I can find it? Thanks again for all your help so far.

 

[pbhuter]

Ceramicguy - I'm actually looking for the natural convection h-bar...any ideas? Thanks.

 

[IRstuff]

You need to read the Wikipedia articles. Natural convection is a property of AIR.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[pbhuter]

IRstuff - I looked at the Wikipedia article on convection and linked to the article on heat transfer coefficient, but I see nothing that would explain why the book you linked be to talks about the convection heat transfer coefficient of the resistor being some value, h-bar. For reference, I am looking at page 75, example 2.8 of the book you sent me to (page 74 if you type it in at the top of Adobe Reader).

 

[IRstuff]

And did you look at Chapters 6,7, 8? Or even Chapter 1, section 1.3?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[pbhuter]

I looked throughout the book and could not find an equation to suit my needs for calculating h-bar. I assumed that since the example gave a value for h-bar that this was some kind of constant based on the material. I did see at the beginning of the book examples for calculating h-bar, but they involve temperatures and other factors. I don't see how to calculate h-bar when I don't know the temperature differential (that is what I am ultimately trying to calculate). In the example I mentioned, a radiative h was calculated using a guess for a temperature, and then the guess was refined through calculation. Would you recommend me doing the same for h-bar? So I would do:

h-bar = (power (watts)/area (m^2))/delta-T

where delta-T is calculated as some theoretical maximum minus the theoretical ambient temperature (say 1273K - 293K).

I appreciate all the help you have given me in trying to figure this problem out. Like I said, one thermo class seven years ago and I haven't used it since. These forums have proved to be a valuable resource for someone who really has no idea what they are doing.

 

[IRstuff]

While I concede that Lienhard is a bit abstruse, Chapter 8.3 gives you the basic equation for h_bar. After some rather gory calculations, example 8.1 gives you an example calculation for h_bar. But, there are a number of resources on the web that give you similar answers, as well as on-line calculators.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[pbhuter]

IRstuff: I looked at example 8.1, but it doesn't seem to apply for my situation. The calculations in that example seem to apply to fluids because they include ?, which is a function of dynamic viscosity of a fluid. I attempted what using theoretical values for delta-T, but the result was not satisfactory. Are you aware of any other calculations for h-bar in this book? I feel like I'm getting a lot closer to finding a solution to my problem, so any additional help you can provide will be much appreciated. I also have the following values:

density
thermal conductivity
specific heat capacity
emissivity

Thanks again for your assistance so far.

 

[IRstuff]

You have a resistor in air, do you not? Air is the fluid.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[pbhuter]

I'm trying to calculate the temperature of the resistor. From there I will calculate the temperature of the air using section 3 of the book. Originally I was only trying to figure out how hot the resistor (heating element) got, but with this book, I realized I could do so much more. This has proven to be an interesting problem that I would like to solve.