Stirlingengineforum.com

EMISIVITY/ABSORBANCE – effects and possible applications in stirling engine design...

OK, I’m about to step way out or my comfort zone with this topic.

Is a materials emissivity a function that’s relative to 0deg kelvin (absolute zero)..(the higher the temp above zero, the greater the emission of radiation)?

Is material thermal conductivity/exchange a function of delta t (and also the relative different material conductivity and is the lower rated conductor the limiting factor)?

Is the gas the limiting factor for heat transfer due to its lower thermal conductivity compared to the other mediums (is the lower rated conductor the limiting factor) ie. In stirling engines, From the metal (say, aluminium 250 W/(m.K) Aluminium Oxide 30 W/(m.K)) to the gas (air, atmospheric 0.024 W/(m.K) (relative to air pressure and effective exposed area) (also I assume its inportant to consider that the gas/fluid has velocity and turbulance which changes its thermal conductivity/transfer rate/function (volume-time-turbulance, etc))?

What I am proposing is that it may be possible to increase thermal transfer to/from the working gas, in addition to regular conduction, by utilising radiation (emissivity /absorbance) not only directly into the gas (this might be quite low due to relative gas dencity) but also past/through the gas to other less utilised surface areas. ie. Transferring heat via radiation from the outer cylinder wall to the hot cap which then can conduct that heat to the working gas, or inversely having the cold cap absorb heat from the working gas and emit it to the outer wall where it can be dissipated away.. Thermal Energy is transferred back and forth to surface areas via radiation in their quest for relative equilibrium (with some interference by the gas relative to its density and gap)
I have also contemplated increasing hot and cold heat exchange by circulating fluids through the pistons/caps, so as to utilise their surface area, but although possible it is very problematic for multiple reasons.

With regards to aluminium, The surface areas could be black anodised (black/grey body) increasing emissivity/absorbance, while not interfering with thermal conductivity too much (as long as its not too thick, in the end all aluminium exposed to regular air becomes oxidised/anodised on the surface.) Im assuming that the thermal conductance of regular anodised aluminium is similar to that of colour anodising. Anodised aluminium Does seem to have a significantly higher Emissivity Coefficient than regular aluminium.

Aluminium Anodised 0.77
Aluminium Heavily Oxidised 0.2 - 0.31
Aluminium Commercial sheet 0.09
Aluminium Foil 0.04
Aluminium Highly Polished 0.039 - 0.057

It may also be worth considering making the surface less polished before anodising to reduce its reflective properties (and so increasing emissivity/absorbance).

I've read somewhere that anodised aluminium surface areas can tolerate up to and above about 180 deg Celsius before the different rates of expansions can cause crazing/cracking, but the anodising still is bonded to the surface. This might limit the workable temperature range, at least on the hot side.

These are just ramblings of some thoughts and questions I have, and none, some or all can be entertained, ignored or disputed as the reader so desires.
Has anyone got any thoughts, experiences or questions on this topic?

vamoose

I've wondered about displacers and radiation that way, but only so far as black paint, which just becomes another insulation tradeoff problem. The anodizing is interesting. I've heard of the notion of using water in air, not for phase-change, but to make a mist and increase conduction. Don't know how well or even if it works, but it would have radiant significance too. Bumpkin

Bumpkin

I reckon painting may still be worth trying on certain surfaces if not just for experimenting?? I think they use black surfaces on the plates and displacers in some of those hand temperature stirling engines to increase thermal transfer and regeneration properties.

Water probably would have radiant significance. I'm not sure what it might be though. If you can figure out the numbers for me, I’ll be your number 1 fan. I don’t mind numbers per say, but my brain can go a bit Forest Gump on me if they get bigger than my 11 fingers (jk)
I think I’ve heard of water being considered for different purposes in stirling engines. I wonder if anyone has made a kind of hybrid Atmospheric/stirling type engine before? Good old water.
I would like to know a bit more about it.
Actually here's a topic relating to it on this forum “Joule-Thompson effect cooling”
http://stirlingengineforum.com/viewtopi ... mist#p2739
I will have to give it a good read one day when I’ve got a quiet moment.



It would be nice to have 'Black Gas'

Okay so here's a thought,
what about incorporating small particulates (nano particles, carbon nanotubes, buckyballs, graphite powder etc) into the working gas. The particles would need to have certain properties like non-abrasiveness, possible lubricating effect, good emissivity/absorbance, good conduction co-efficient, low levels of static attraction so as not to group and gum up, and probably a whole lot of other things too. The particles would be generally evenly dispersed due to gas turbulence and would 'conduct' heat in or out of the gas that it is interacting with, using absorbance and emissivity (radiation) to obtain equilibrium in relation to the surrounding hot/cold sides...
Hmmm.... I wonder??
(i will have to investigate some different allotropes and compounds, and their properties)

vamoose

Vamoose, the extent of my research on radiation and water is to note that it heats up in a microwave, and is kept hot in mirrored vacuum bottles. Somewhere between or around its obvious limitations it might have a use. I only mentioned it since you're leaving no stone unturned. For now, I've decided to budget my Stirling time to just finishing what I've got. Black gas is interesting. Your open mind is inspirational and now I'm wondering just how significant radiation could be - it works for the sun. I don't know if emissivity is directly proportional to absorbance, but at some point between searching and getting lost, exact numbers aren't as meaningful as just doing the best you can. I think I'll choose black the next time I buy R.T.V. for the rubber section of a diaphragm. Bumpkin