O ring-gasket bead - question

[RFguy]

I have a quetsion about the use of circular or rectangular cross section gasket bead.

I want to push some silicone etc gasket bead in the fins on the top  couple of mm of the fins (so not to obstruct the available fin area more than necessary).

(why- because there is little.no clearance  for flat sheet on the outside of the fins in the region- so a pushed in bead an option if I cannot fit 2 thou SS sheet 

 

Is a circular bead goes to have better retention than a rectangular cross section cord ? it would be secured with a little suitable adhesive here and there.

suggestions. comments ? - intended use (orange bead below) and the small gap I have to work with (that reduces when it gets hot !) 

-glen

 

Edited January 26, 2021 by RFguy

[skippydiesel]

Why? Is this to do with controlling cooling air flow??

[Yenn]

Could you explain what you are trying to achieve. Is this a Jab engine? Are you using the Jab Plenum or making your own?

Are you trying to seal the gap between cylinders?

For cooling efficiency you need the air to be going between the fins, from the top, high pressure side to the bottom, low pressure side and not getting through without close contact with the fins.

[RFguy]

have a read of : 

 

[skippydiesel]

Nooooo way am I going to trawl through pages & pages of intimate Jab intercourse

[RFguy]

if you go to that post I referred to above, with a bunch of photos, that specific post, not the whole topic, you will see what I mean

 

In any case, the first post I made here has the information needed to for the question.....

[walrus]

Then in the absence of further information, the answer is: “it depends”. Why would you use stainless steel instead of alloy or fibreglass? While you need to constrain the edges of a baffle for structural reasons, why try and connect it to a (vibrating) engine? It won’t stay connected for long. What’s wrong with silicon baffle seal  material like everyone else uses? Are you solving a problem that doesn’t exist?

[RFguy]

Is anyone going to actually answer the question ?

[spacesailor]

Please, RF ,

Can you refrase the question, l cannot see your purpose in what you need.

round or square is there that much difference, if vibration problem, then square has more surface area.

spacesailor

[RFguy]

OK

I am seeking to block airflow OUT of the fins. In some areas I can use a sheet of metal.

In the deeper fins I can push into between the fins silicone gasket bead. jam it it.  can apply a few spots adhensive every inch or so. 

 

Now, the question is- will round or rectangular bead lead to better retention? The idea is that I minimise the protrusion into the fin area. a wide, and minimally deep rectangular bead might provide reduced protrusion compared to a round bead. 

 

The round bead though may have a smaller contact area, and this may  enhance retention due to a higher pressure per square mm.  the bead will deform oval, and thus will prodrude a fair way into the fin channel, and also has to be recessed into the fin because I need a flush external surface.

 

-glen

 

 

[spacesailor]

Square,

 it will hold better, and if you use oversized silicone, you can Sstretch it, to fit tighter inbetween the fins ,.

Just my idea.

spacesailor

[RFguy]

OK ! thanks for the suggestion. I will endeavour to get some  square /rectangular stuff . as well as round.  

 

[facthunter]

It's tizzy and a pretty bodgy fix. There you have it. Pushing air or any fluid through confined spaces and knowing flows is a back art. I love your work but if this is the fix, just what was the question?  Nev

[spacesailor]

Cooling air flow control ! .

That,s what l think.

spacesailor

[RFguy]

the question was round or rectangular... most areas I will use 2 thou SS shim  for the cover. or some alloy if temp comp is a problem.

 

but some small bits I may want to optimize and pushing in some silicone bead  is easier than shaping bits of metal. 

 

[facthunter]

There's a lot of weird vibrations going on there. Is there any chance of clogging . Wasps spiders grass.. dry leaves. Nev

[RFguy]

we'll see. 

 

[onetrack]

I wouldn't do it. If the exercise is to improve cooling, the silicone or o-ring material is going to reduce the fin cooling surface area, and the proper transfer of heat from the fins to the surrounding air.

 

Go with some shaped metal shrouding that is a distance away from the fins. You don't want to stop airflow around fins, you're defeating the whole purpose of finning for heat transfer.

 

I've worked on quite a number of air-cooled engines, and none of them ever had anything like this in their cooling design. Close-fitting metal shrouds in some places, but always fast-moving air flow.

[RFguy]

For 10mm long aluminium fins that are only 1.6mm thick, the top 30% does not do much at all. might as well not be there. even worse, in this design makes it hard for the air to get the the bottom of the channel where the work is done.

 

"You don't want to stop airflow around fins, you're defeating the whole purpose of finning for heat transfer."

-sure but the whole problem is the airflow is not being forced through the base of the fins, that is the whole problem with this design.

 

"Go with some shaped metal shrouding that is a distance away from the fins."

NO NO NO NO NO ! the whole point of a sheet of metal IN CONTACT with the fins, OR usijng the o ring bead is to hold the air IN THE FINS and not let it out.....

 

[onetrack]

If you're holding (now-heated) air in the fins, and not letting it out, I fail to see what you're trying to achieve. The fins are there to distribute heat to the surrounding air, for cooling purposes, are they not?

[facthunter]

Most tight fitting shrouded engines  I've worked on  (Fan cooled) end up with stuff blocking the airflow. Pine needles oil and dust insects etc.  You need to be able to get at it and check it's clean. While radiation isn't a lot it's something and every  bit counts. Roughened  thin matt black surface. The heads have lots of fins where they don't do much but in other places, hardly any or any airflow. Ports and push rods  take up most of the room on the underside and the cylinders need attention to the lower parts, especially near the exhaust valves.  Nev

Edited January 28, 2021 by facthunter

[RFguy]

Yeah with constrained paths there certainly is a higher chance of blockage due to a foreign object. That is a valid point Nev. taken on board.

 

Radiation is actually bugger all at these low temperatures. 

 

"The fins are there to distribute heat to the surrounding air, for cooling purposes, are they not?"

-No,  not. They are to have forced airflow over them in order to remove lots of heat. If they simply conduct (free convection ) to the surrounding air they do very little, less than 1/50th of what they need to do...

 

 

 

[facthunter]

There's a lot in it. Fine fins are a bit of a problem and often don't work well unless everything is going for them. They should also be tapered and radiused at the root and lead from the part you want to cool most. They can be too big especially with materials that are  poor conductors of heat .It's a surface  area thing in principle.  In the very early days they were known as "cooling flanges" in  the engineering Journals of the time.. There are paints that change colour at a prescribed temperature and are very useful at finding what has (already) happened.. This was done in the twenties with the Wright J5F  probably the best engine of the time in many ways. Nev

[RFguy]

yep ! thick at the base. trapezoidal fins are a milling manufacturing pain ! (but suit sand cast molds very well ) 

 

Even with the top 10% of the fins blocked, which as I said will be bugger all on this example considering the other variables,  the $64 question will be its performance at low / no airspeeds- that is where there is not much pressure and airflow  to do the job. .,...

 

and how much pressure drop is required to work the long fin duct paths - the paths between the fins (ducts) are long in my proposed modification. This is unwanted but there isnt any other easy way to use all the fin area .  There is an upper mound on performance due to turbulance, so   2x the airflow over 50% of the fins is far poorer than 1x the airflow over 100% of the fins at high flows.

 

The thing is, with just a tiny bit of airflow over where there is currently no forced flow is enormously beneficial.

 

[facthunter]

She'll certainly cook wherever it's stagnant. Nev