Plenum inlet question ?

I have read archives for two days about plenums. Airflow to the head fins is logically the objective. It is generally accepted that static pressure above cylinders creates the flow through fins and large air inlets from the cowl probably stagnate in front of the cylinder head.
Assume that the heads and steel barrels will be wrapped to contain flow to the bottom.
So my question is:
1- What portion of the heat leaves the head vs steel cylinder.
2- Can the incoming air be primarily delivered to the head area and assume the steel cylinders will be cooled adequately by static pressure developed in the plenum ?
The reason for the questions relates to simplifying the baffle/plenum/flexible cowl connection...Thanks in advance.

I cannot quantity how much. Titan cyls are manufactured with tapering fins, less fin area at the base, and cool fine.

Considering the pressure is highest when the Piston is around 15 to 18 degrees after TDC and that the leftover heat is wasted through the head out the exhaust, most of the cooling is done through the cyl head.

Larry DeCamp said:

I have read archives for two days about plenums. Airflow to the head fins is logically the objective. It is generally accepted that static pressure above cylinders creates the flow through fins and large air inlets from the cowl probably stagnate in front of the cylinder head.

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Static pressure + dynamic pressure = total pressure.

There is a good argument that high velocity air impinging on a finned body can result in good heat transfer, just because highly turbulent flow generally results in higher heat transfer from the surface to the air. However, in the context of a flat engine cowling, high velocity jets of cooling air might be very hard to control or direct across a range of airspeed and AOA.

So, as a practical matter, we generally don't worry about directing flow to any particular location. Better to shoot for an even distribution of pressure.

You mentioned large inlets. Generally, a large inlet would be paired with a small outlet, and the result is a low velocity ratio, i.e. the velocity of the air passing through the inlet is a small percentage of the freestream velocity. Most of the stagnation takes place well out in front of the inlet, not at the front cylinders. It is possible to use a high velocity ratio inlet and slow the flow internally, with a divergent diffuser. Or one could design for no slowing of the flow prior to impact with the finned surface. All would result in some degree of stagnation and static pressure increase. God is in the details.

What portion of the heat leaves the head vs steel cylinder.

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I once asked a Lycoming rep that question. The answer was "no published data".

Can the incoming air be primarily delivered to the head area and assume the steel cylinders will be cooled adequately by static pressure developed in the plenum ?

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The upper fins (3 to 9 o'clock for example) on both the heads and the cylinders receive turbulent flow due to whatever velocity may be present. It is very roughly akin to blowing on your soup. The lower fins on both the heads and cylinders are cooled by static pressure difference forcing flow between the fins with enough velocity to be turbulent, i.e. with a high Reynolds number.

Not sure what your objective is but . . .

How much total heat rejection comes to the barrels I can not quantify, but it partly depends on what goes to the piston. Cooling jets reduce piston temps and, therefore, the fin heat flow. Yes much less is needed approaching the base flange. Lycoming has a flange temperature spec. and I am nowhere near it. I wrapped my barrels and unwrapped and tested, I could not measure ANY effect on the CHT. I have not put a thermocouple down in the fins yet, but feel you can cook the piston with no outward indication on CHT. If you wrap only under the vans head metal, and leave a 4-5" gap at the top of the barrels tapering to 3 at the base then you can collect quite high pressure in the upper chamber. I have come to the conclusion that the flow through heads and barrels is controlled by the exit gap on the bottom. I have one head that likes to run 20F hotter. Filed the gaps under the plug, and some other things, but measured the gap and it is a tiny bit smaller than the others. I'll get to it some day.

If you want to go down this road, I suggest a good multi channel thermocouple data logger, and installing thermocouples in the top barrel fin, and base flange for a baseline.

BTW I do have cooling flow to bypass the shallow fins on #2 and #3.

I am wrestling now with getting exit velocity to correlate with speed increase, but that is another beer. V-exit is up, speed is not.

Bill said: "I have come to the conclusion that the flow through heads and barrels is controlled by the exit gap on the bottom."

I believe there is a NACA report that represents the exit gap is responsible for 50% of the total cooling drag.

If you're going to "wrap" the cylinders (I assume tightly) by another NACA report, you quickly run into diminishing returns as the air quickly heats in the "cooling tube" formed by the baffle and fins. The question is how best to manage?

I have an idea but would appreciate Mr. Horton's position on the point!

FWIW

Exit gap ?

I appreciate the responses. Based on Dan?s input I will not overthink the flex connector to plenum GEOMETRY and focus on effective, easy to access connection design.
I look forward the definition of exit gap. It has been posted the nominal expansion ratio of air IN vs. air OUT. Pictures show wraps on both sides of the cylinder and nominally terminated at 3 and 9 o'clock.
If I were looking down at a cylinder with a light under it, the effective area for flow is the light between fins? Therefore, the ? unwrapped rectangular ? area required for the bottom exit is the (area of the light above) X (heat expansion ratio) + (the area of fin metal in the bottom opening) ? This might require an iterative calculation since open area and fin area are dependent variables

PM/Email sent Larry. Return it and receive my barrel calculations.

Yes, flow area is through the fins only. The bottom exit area is a little more than the fin flow area if the gap is 1.25", but the head on flow from each side and 90 deg turn on the exit means we need some more exit area than just the fin area.

Bill

BillL said:

PM/Email sent Larry. Return it and receive my barrel calculations.

Yes, flow area is through the fins only. The bottom exit area is a little more than the fin flow area if the gap is 1.25", but the head on flow from each side and 90 deg turn on the exit means we need some more exit area than just the fin area.

Bill

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I'm not so sure it is really good to wrap the fins so as to only allow cooling air in the passages between the fins. That flow will warm up pretty quickly, and so there will be more cooling in the upper portion (say, 2:00 to 3:00) than the lower portion (3:00 to 4:00). By the time the flow comes out through the bottom gap, it is probably close to the fin temperature, so little or no more cooling. I can imagine that the cylinder barrel may actually go out of round because of the non-uniform radial temperature distribution caused by this. Hopefully there is enough circumferential conduction in the barrel itself to minimize this.

On the other hand, if there is some small air passage just outside the fins, in addition to the flow between the fins, then there can be some mixing of fresh air that will help cool the flow down between the fins and maintain some temperature differential between the air and the fins. The stock Lycoming intercylinder baffles are a bit crude, but I believe they allow this extra flow for this purpose. They've been in service for a lot of years on a lot of engines that make TBO or beyond.

I also think that if you are going to wrap the fins, it ought to be wrapped with metal, so there is good conduction from the fin tips into the wrap, and the wrap itself starts acting as additional fin surface.

Finally, if you control the total flow through the system by throttling the cowl exit, then allowing the extra air to go around the cylinders just outside the fins can not hurt anything, except to lower the flow velocity through the fins a little bit. No worries, you can not get it slow enough to stop being turbulent. If anything, it may reduce cooling drag by reducing the pressure loss through the cylinders, allowing higher pressure in the lower cowl, which gives higher cowl exit velocity.

scsmith said:

I'm not so sure it is really good to wrap the fins so as to only allow cooling air in the passages between the fins. That flow will warm up pretty quickly, and so there will be more cooling in the upper portion (say, 2:00 to 3:00) than the lower portion (3:00 to 4:00)...(snip)

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That would be NACA #620:

https://www.danhorton.net/VAF/Plenum Inlet Question/naca-tn-620.pdf

Take a close look at at Fig 2 and Table 1. The researchers tried all kinds of configurations, but one of the best overall was #4, a very conventional close fitting wrap with small exit widths. Ignore the 180 degree measurement location; nothing does well there due to the two flows running headlong into each other. The spread for the other locations is only 8F, and the absolute values are among the lowest in the test series.

(Note to readers; be sure to read the latter two paragraphs on page 6 so you'll understand the high temperature values at the 0 deg points for D through G.)

The single big change I'll make to the next set of baffles will be the addition of diverging exit ducts. Recall the HTR 212 epoxy with the sketchy physical properties? One of the claims was a rather high Tg. Well, I've made some molded cylinder wraps as test parts. They are in service on a VW right now. If molded glass cylinder wraps hold up as well as aluminum wraps, shapes become possible.

The stock Lycoming intercylinder baffles are a bit crude, but I believe they allow this extra flow for this purpose. They've been in service for a lot of years on a lot of engines that make TBO or beyond.

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Agree with "crude".

I also think that if you are going to wrap the fins, it ought to be wrapped with metal, so there is good conduction from the fin tips into the wrap, and the wrap itself starts acting as additional fin surface.

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Opinion, no data, but heat transfer from fin tips to lower plenum air via conduction through the baffle wrap would be minuscule. The points of contact are tiny, and full of fretted oxidation material...assuming the fins have not simply worn a hole in the wrap.

Finally, if you control the total flow through the system by throttling the cowl exit, then allowing the extra air to go around the cylinders just outside the fins can not hurt anything, except to lower the flow velocity through the fins a little bit. No worries, you can not get it slow enough to stop being turbulent. If anything, it may reduce cooling drag by reducing the pressure loss through the cylinders, allowing higher pressure in the lower cowl, which gives higher cowl exit velocity.

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Same mass (you throttled the exit), but less heat transfer (you're not ensuring all mass comes into contact with a hot surface). The result is less cooling.

Instead, I would argue that every molecule entering the cowl must carry away its equal share of heat. Let those diverging ducts take care of pressure loss.

DanH said:

That would be NACA #620:

https://www.danhorton.net/VAF/Plenum Inlet Question/naca-tn-620.pdf

Take a close look at at Fig 2 and Table 1. The researchers tried all kinds of configurations, but one of the best overall was #4, a very conventional close fitting wrap with small exit widths. Ignore the 180 degree measurement location; nothing does well there due to the two flows running headlong into each other. The spread for the other locations is only 8F, and the absolute values are among the lowest in the test series.

(Note to readers; be sure to read the latter two paragraphs on page 6 so you'll understand the high temperature values at the 0 deg points for D through G.)

The single big change I'll make to the next set of baffles will be the addition of diverging exit ducts. Recall the HTR 212 epoxy with the sketchy physical properties? One of the claims was a rather high Tg. Well, I've made some molded cylinder wraps as test parts. They are in service on a VW right now. If molded glass cylinder wraps hold up as well as aluminum wraps, shapes become possible.

Same mass (you throttled the exit), but less heat transfer (you're not ensuring all mass comes into contact with a hot surface). The result is less cooling.

Instead, I would argue that every molecule entering the cowl must carry away its equal share of heat. Let those diverging ducts take care of pressure loss.

Click to expand...

Thanks Dan.
Configuration (h) #13 is the one that seems interesting to me. If I interpret the table data correctly, it is better than #4. It allows a small amount of flow just outside the fin tips. This is the one I was advocating for. Although you might think it is wasted flow because it wouldn't contact the fins, I believe that the turbulent mixing brings it down into the fin channels in exchange for flow that was in the fin channel and has absorbed some heat. I'm not sure what the "injector" is at the wrap exit transition to the diffuser?

Convective heating of the wrap on that surface can be conducted away to other parts of the wrap system, even just off the outer face. I agree that conduction through contact at the fin tips is probabably negligible.

DanH said:

That would be NACA #620:

https://www.danhorton.net/VAF/Plenum Inlet Question/naca-tn-620.pdf

Take a close look at at Fig 2 and Table 1. The researchers tried all kinds of configurations, but one of the best overall was #4, a very conventional close fitting wrap with small exit widths. Ignore the 180 degree measurement location; nothing does well there due to the two flows running headlong into each other. The spread for the other locations is only 8F, and the absolute values are among the lowest in the test series.

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Not sure where you see the 8F spread. Looking at 90 degrees to 165 degrees, the spread for #4 is 27.5F Config (h) #13 has just 18F spread over the same stations.

Dan question ?

If the temps were plotted from 0 position to 180 position, is it correct to conclude that the steepest slope curve would be optimum? I know the area under the curve is net gain. If so, I will punch them into a spreadsheet and share.

Larry DeCamp said:

If the temps were plotted from 0 position to 180 position, is it correct to conclude that the steepest slope curve would be optimum? I know the area under the curve is net gain. If so, I will punch them into a spreadsheet and share.

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Larry, the report mentions that the temps from 0 -- 90 degrees don't mean much as far as the baffling goes, which starts at 90 degrees. Also, the 180 degree data probably isn't very helpful.

The temperatures are not the airflow temperatures, they are the temperature of the metal down at the bottom of the fin passage at the various positions around the circumference. ( I thought it was pretty clever of them to make 1-wire thermocouples by using the cylinder itself as the other wire)

What you are looking for is two things: 1) a fairly uniform temperature around the cylinder and 2) lowest temperature at, say, the 165 degree position.

If you plot the curves, look for the one with the least slope (most uniform temp distribution around cylinder) and the lowest overall value (coolest cylinder).

It looks to me like config's #7 and #8 are quite good. #13 is good as I observed before but is rather complicated, I see now, with a small inner baffle and exit diffuser inside the main outer baffle. But I think #7 and #8 support my hypothesis that mixing in some fresh air as you go around the cylinder is better than a tight wrap that has only the fin passage airflow to put heat into.

scsmith said:

Configuration (h) #13 is the one that seems interesting to me. If I interpret the table data correctly, it is better than #4. It allows a small amount of flow just outside the fin tips. This is the one I was advocating for. Although you might think it is wasted flow because it wouldn't contact the fins, I believe that the turbulent mixing brings it down into the fin channels in exchange for flow that was in the fin channel and has absorbed some heat. I'm not sure what the "injector" is at the wrap exit transition to the diffuser?

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It's a fair point; the flow mixing must be there, based on the results. And yes, temperatures are a little more uniform around the cylinder.

On the flip side, there is the authors's note regarding small benefit vs difficulty of construction. It's not just the mystery injector. To build #13, or #7, or #8, spacing the wrap away from the fins would require a box baffle, something with sidewalls to contain the flow. And we can't space the wrap away from the fins between the heads on flat engines anyway. Like it or not, all flow is going between fins; we can't change the engine architecture.

Let's back up a moment. Set aside head wraps, and consider only cylinder roundness. Can we quantify how out-of-round we can expect a cylinder to become given the relatively narrow temperature spreads for the best designs in Table 1?

Seems like a sheet steel intercylinder baffle could be made with end plates such that the assembly would pull up between the cylinders from the underside. The front and back wraps might be a little more tricky to form with end closures. I guess I am thinking more of the barrels. I see your point that the head fins essentially butt up against the next set of head fins.

Out of roundness? It would seem like all the 'good' designs in the report have narrow enough temp spread that out-of-round would be negligible (unless maybe for a sleeve-valve engine?) On the other hand, the test temperatures are fairly low. Delta-T from ambient of about 125F, so about 180F actual temp? How about when it is all at 360F? If the variation scales in proportion, the delta-T's are starting to get fairly large.

The baseline configuration that they cite (#1) is just so much worse than most of the others they tested, and apparently roundness not a big problem for that one?

Where is an FEM when I need one?

BTW, I just did a home-brew T_g test on a specimen of fiberglass with PTM&W 2080 resin. Post-cure 175F for 3 hrs. T_g by my home-brew method about 250F. The TDS says 250--260F. It is a little pricey.

Now it has to be done.

scsmith said:

How about when it is all at 360F? If the variation scales in proportion, the delta-T's are starting to get fairly large. .

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OK . . . now I absolutely have to measure some temperatures. From a piston stand point the barrel temps had better not be anywhere close to 360F. Piston crown should never be 400F, even in the center where it is the hottest. So the temps down in the bore where the barrel fins begin should not be near that hot.

Ring groove and temps will coke and rings will stick anywhere near 400F. This is why turbocharged engines have iron bands, and keystone ring groves. Piston pin bore will freeze too. And scuff the wall when starting cold. Temps this high internally will cause all sorts of problem with the existing piston design. And mine does not even have cooling jets!

No way I can surface weld a single wire TC, but I can stick a 1mm bead down in the fin root. It won't be as accurate but a ball park should be good for our purposes. And epoxy it in there. Ahhh, the epoxy will have to be good, maybe RTV but just a touch. The instrumentation guys have this tiny little spot welder to do such things and lay metal straps over the wires too. But I digress . . . tool envy . .

To the OP,
I have on a few occasions seen where the cyl heads were in a plenum but the steel cyls were not. We have no idea if it caused cyl problems or not but apparently doable.


In the air conditioning heat exchange world i.e. coil design, I have always been told that the majority of the heat exchange takes place in the edge of the fin and not the surface of the fin. (I have no documented proof).
My contention is that #13 may be the best only because it allows better heat exchange at the edge of the fin. Food for thought.

scsmith said:

The baseline configuration that they cite (#1) is just so much worse than most of the others they tested, and apparently roundness not a big problem for that one?

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Number 2 appears to be the closest to a "standard" baffle, close fit, small exit radius, no duct. It definitely has a wide temperature spread. My guess is that if spread creates an out of round condition, we're all currently flying with some.

#5 is the same spec, but with the addition of an diverging exit duct. Performance improves. Interesting correlation with your oil cooler experiment, yes?

BTW, I just did a home-brew T_g test on a specimen of fiberglass with PTM&W 2080 resin. Post-cure 175F for 3 hrs. T_g by my home-brew method about 250F. The TDS says 250--260F. It is a little pricey.

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Very useful. Thank you sir.

Mark W said: "In the air conditioning heat exchange world i.e. coil design, I have always been told that the majority of the heat exchange takes place in the edge of the fin and not the surface of the fin."

I understand that little heat is lost by radiation. Most of the heat is carried away by conduction (i.e., transfer from contact)

FWIW

Test conditions?

Now that we are discussing #620, I am not clear on the pressure delta across the test barrel. I did a SS of all the data and based on peak temp and averages, #4 and #13 gave the lowest average and a peak of 136F - 130F respectively.


The mass flow requirement between the two would be enormous if the inlet pressure were held constant. Much higher mass flow would require inlets and exit changes to balance.

DanH said:

Number 2 appears to be the closest to a "standard" baffle, close fit, small exit radius, no duct. It definitely has a wide temperature spread. My guess is that if spread creates an out of round condition, we're all currently flying with some.

#5 is the same spec, but with the addition of an diverging exit duct. Performance improves. Interesting correlation with your oil cooler experiment, yes?

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Indeed.
When I can get some good weather and a backseat volunteer, I'll report pressure measurements on the other thread.

Thanks much to Dan !

The data is truly informative. Steve is also obviously a valued contributor. Thanks for the info, I will carry on with wraps and some curved exit structure