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Triumph TR6 - DO NOT OPEN - when hot !

Maybe not the most appropriate time to be posting this for our northern hemishere colleagues, but I thought I'd share and invite comments on an interesting piece of trivia heard at a chat by an 'expert' on water cooled machinery. (He was actually selling something, but he did say this:)

One of the reasons you should not release the pressure in the cooling system when it is hot, is that the pressure acts to increase the boiling point of the coolant - fairly common knowledge.

The more interesting reason he gave was that the sudden reduction in pressure causes the water to boil and tiny bubbles are formed where the coolant is in contact with the engine, and in particular the cylinder bores and head where the most amount of heat is generated (and where effective heat dissipation is the most critical).

These tiny bubbles sit in the pores and on the suface of the rough casting and act as an INSULATOR!.

When you resume running the engine, these areas (cylinder bores and head) have an insulating layer of bubbles around them which prevents effective heat dissipation into the coolant which can lead to superheating of the bores etc etc etc!

........so , next time you are worried about burning yourself by removing the cap when hot, consider the warning on the cap to be to safeguard the car, not so much for your personal safety!!
Roger H

Ineresting - thanks. Another reason not to scald oneself
Peter G
Peter Gooch

Hm? The bubbles presumably come from water vaporizing to steam, but once the car cools down won't these steam bubbles condense back to liquid water?
Ignatius Rigor

To all
Nope not in a sealed or somewhat sealed system with many ups and downs in internals. Air will stay. Water will not go up in an area filled with air. Extreme case is having to burp the system to get your heating system to work? Common. Air entrained water and all water is to some extent. Less efficient as a coolant.

Bill
Bill Brayford

Hi Iggy (pardon my familiarity!)

I'm not really sure - I would have thought so but Bill's comments are interesting as he is suggesting that larger pockets of air are formed, much like in an upturned boat.
One of the major design flaws of the Triumph Stag was that the thermostat was close to the highest point in the cooling system. Air in the system meant that the thermostat may not contact the coolant and could not operate, thereby getting very toasty!!

My comments were really directed at those of us who might pull over with some problem or another with the coolant, release the pressure in the system and drive on after replacing the cap and (say) topping up a bit.

My slant on it was that by doing this, there is a small 'blanket' of bubbles, a few thou thick, insulating around the bores and the head. The coolant moves extremely slowly (if at all - in theory) at the walls of the engine. (oh no - here we go again with the fluid dynamics thing!!).

The heat dissipation relies on constant circulation of the fluid. Two things happen with the 'insulation' present.

Firstly, the bubbles themselves act as an insulator as they are tiny pockets of air. This is the basis af all insulation, be it wet suits, styrene foam, fireglass blanket or clothing. Secondly, distance at which the coolant is absorbing the heat from the bores is increased. Both these factors (together with the reduced boiling point due to less pressure) are significant. It's a good job we have cast iron heads heads!!!
Roger H

I would think that the ethylene glycol/water mixture most of us run in our cooling systems affects surface tension to minimize the 'nucleate boiling' you refer to.

Rick O.
Rick Orthen

Roger - I seem to recall your mechanic had another wild theory some weeks back....Can't recall it now, though.

The cooling system is designed to remove engine heat by warming the coolant(sensible heat), not by boiling (latent heat) - we all know that. If there is any circulation at all through the radiator any steam bubbles clinging to surfaces that were generated by removing the cap will quickly collapse, especially if the cap is put back on so the system builds pressure. It's hard for me to buy into the case for "film boiling" or "steam blanketing" to the extent that it CAUSES overheating from suddenly removing the cap, though.

Yes, there is a stagnant fluid boundary layer that effects the heat transfer coefficient. Heat has to be conducted through that layer before it's convected off by moving fluid. I think products like "water wetter" are supposed to work by reducing that boundary layer (I didn't notice a difference in cooling when I tried it, but your milage may vary). Again, though, if the heat can be removed faster than it's being formed, bubbles in the boundary layer, if there are any that small, should collapse.

If there is enough heat flux at a particular location to cause localized nucleate boiling, it will likely be there with or without the cap on. My opinion is that this doesn't happen in our engines unless something's amiss.


BB
Brent B

To Brent and all:

Ya know, I've been wondering why the beer in the middle of my bottle always seemed colder than the beer around the edges of the bottle. Now I know. It's all them dadgum bubbles that keep forming when I take the cap off.

The Other "BB" (off to solve the problem of how to get beer out of the bottle without taking the cap off. Now, where did I put my nucleate flux capacitor??? Be back after the weekend!!!)

R.C. Blair

G'day Brent (et al)

Thanks for your thoughts - just to clarify, I heard this at a Jaguar club meeting a while ago where a demonstration was given by a salesman pushing non glycol based coolants (mainly for environmental reasons). He also happend to be an overqualified engineer who probably couldn't get a job anywhere else!! - no seriously, it was his company's patented product that he was pushing, so there was a degree of my natural scepticism attached to the whole thing - sounded logical to me though, and it's interesting to hear other opinions.

I'm not sure of the 'wild theory' to which you refer, but my friend who is a mechanic has several!! Most of them are borne out of 30+ years of having, on average, 3 or 4 Triumphs in his workshop - per day - over that time. He lives and breathes them. Trained in aviation mechanics, he is a lateral thinker and loves his racing. We'll see how his wild theories go when he pits his skills up against the rest of the country's best when the 'nationals' are held here later in the year. One of his favourite stories is holding a Ferrari in check with a Triumph GT6 - seemed to send the new owner of the Ferrari into a mild depression as it was his first track outing after recently buying the car!!!...

It's 36°C today and predicted for the next week also - looks like I'm going to leave the planned upgrades to the winter. R.C. - I was wondering about those 'dadgum bubbles' also.......
Roger H

Roger - Best of luck to him in the races! Experience counts, and it sounds like he has it.

Brent
Brent B

This thread was discussed between 19/01/2004 and 24/01/2004

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