Want more heat?..... Reduce the inlet size to reduce the speed of the air passing through the muff.
Want more heat?..... Reduce the inlet size to reduce the speed of the air passing through the muff.
Yes/no. It’s not quite so straight forward. Yes, reducing the mass flow will raise the HEx air outlet temp; however , the overall heat transfer goes down. Most of the related parameters are fixed. The primary heat transfer influence is the mean temperature difference (MTD); Heat exchanger temp to air temp.
Increasing surface area of the heat exchanger relevant surface area (A) can increase heat transfer though it would have to have physical contact with the hot surface or it could act like a sink.
Q=U*A*MTD. U being a function of film coefficients and fluid specific heats for those who suffered through Heat Transfer in college.
It’s a balance.
Well, It's a good thing that I didn't read this fifteen years ago.
I installed a 1.5 inch scat to the inlet of my heat muff with no noticeable change to outlet temps or comfort in the cabin. What I do notice is way more service room to the back of the motor. I do live in mild climate.
I sure can't argue with the educated. Please disregard my response to the OP.
Bronze would probably be great; at first. It's a better conductor but once the surface started to oxidize, that would quickly diminish.
Thank you.
Except for the servicing issue, I'd pay that much _per flight_ to stay warm.
Posted this earlier… works great!
https://vansairforce.net/community/showpost.php?p=1573767&postcount=6
Put the supply air into the aft end of your muff, with heated air coming out the forward end of it. I've not measured how much difference this would be, but it is likely quite a bit.
Use caution with the bronze wool - it might only be a plated mild steel. I've found many "SS" wools to also be just a plated mild steel, which will rust fairly quickly in this environment. (Edit: it appears that the OP's bronze wool is indeed just plated - the label says "rust resistant").
Posted this earlier… works great!
https://vansairforce.net/community/showpost.php?p=1573767&postcount=6
Very nice.
Key to maintaining its effectiveness would be maintaining solid contact with the pipe. Did you do anything to tension it? Stainless strip?
Darin, your modification looks great! could you in some way "quantify" the improvement? some or a lot more heat?
Somewhat challenged to quantify, but I would say it was chilly at -10C with “normal chothing” prior to shirtsleeves now and at -20C you want to start to zip up your jacket. My wife and I were up at 17,500 on a cold day over Montana once and the OAT was reading -53C on the way to Mexico, and it did get uncomfortably cold…I think it was -30ish on the ground that day.
I once made a skydive from 25,000 msl when the OAT was -40F with a windchill factor of over -100F. That was cold! See why I want a heat muff in my airplane.
Are there any metallurgists out there that could give an opinion on making Darin’s fin assemblies out of Aluminum vs. his SS? Obviously, AL is much easier to source and work with. Thanks in advance.
i also wonder to make these things from aluminum sheet. melting temp of 2024 is around 900 - 1200 F. we know that exhaust gas temps can get higher but i have no idea how hot the exhaust pipe itself gets. stainless steel has around 2500 F melting point. so there's a significant difference. but my gut feeling tells me that we should get away with aluminum.
anyone could shed some light?
You nailed on the first strike. Significantly different alphas, expansion rates as related to temperature, would be strike two (but overcome-able). Solid contact between the tube and fin material is important. Tensioning the fin wrap is the typical answer.
Strike three would be stainless and aluminum don't play well from a galvanic standpoint. The heat they would be exposed to would greatly accelerate this electrolysis/corrosion. Once this corrosive layer separated/insulated the "feet" of the fin from the hot exhaust tube surface, the local heat conduction (tube to fin) would rapidly shift towards zero making the fins next to worthless.
Designed a lot of HEx's a while back; one of the many things I've gotten to do as a professional. Most finned tubes utilized "wrapped" fins along the tube. For some critical applications, the fins and tubes were a single element. We had a vendor in Tulsa OK that would mechanically extrude fins from very thick walled pipe. I was an impressive feat of machining, if you're a geek.
Re galvanic corrosion - it requires that liquid be present, so it won't occur in this application.
Attached are the dimensions I used. As it was 2014 I don’t recall if the shop I had do them used 1/16”, 1mm or 1.5mm, as I told them any of those would be fine with whatever they had available. I did find that they charged $4.50/each (CAD) to laser cut them.
Good luck and use at your own risk, no warranty is implied nor given, your specific results may vary, side effect may include dizziness, disorientation, vomiting, diarrhea, headaches, nausea and erectile disfunction, see your doctor if conditions persist.
Are there any metallurgists out there that could give an opinion on making Darin’s fin assemblies out of Aluminum vs. his SS? Obviously, AL is much easier to source and work with. Thanks in advance.
Somewhat challenged to quantify, but I would say it was chilly at -10C with “normal chothing” prior to shirtsleeves now and at -20C you want to start to zip up your jacket. My wife and I were up at 17,500 on a cold day over Montana once and the OAT was reading -53C on the way to Mexico, and it did get uncomfortably cold…I think it was -30ish on the ground that day.
I think this re-qualifies your wife as a keeper. A lot of wives would consider a flight in an RV at -53C OAT and monitoring that the O2 was still flowing a deal breaker.
I do like your brilliant SS heater strips and the helical airflow and heat transfer they provide.