Thermosyphon warming process

[Malcolm Parker]

Me to!  Perhaps there is a mouse living in there

[Andy Bennett]

Ah no, much simpler than that.
The Benjamin horn has a chrome rear half to the case. This reflects all wavelengths, including IR heat. Same for the bonnet catch top right.
What you are seeing is the reflection probably from my garage light.
It isn't much, but with a cold engine it shows up as the only 'warm' thing in shot.

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And to add Colin is correct, it is pure water heat in pictures 4 and 6 building up before the thermosyphen process gets going.

[Mark Dymond]

Cool pictures ? hopefully made more informative with a bit of theory to muse over.

Heat exchangers have two properties of interest, efficiency and effectiveness. In heat exchanger terminology the effectiveness is a measure of the temperature change through a stage compared to the maximum change possible, so if the water enters the top hot and exits the bottom at room temperature (like the early photos) its running at 100% effectiveness. (probably due to slow flow flow and a cold radiator). In a thermosyphon system flow will increase - increasing the efficiency and decreasing the effectiveness until a balance is reached where the heat exchanged is equal to the heat provided by the engine.
On cold days the balance will be such that the hot side does not get very hot so you have two options, fit a thermostat or reduce the efficiency of the radiator by blocking off a portion. The difference being in the former the effectiveness with the reduced flow will be high and you will have colder coolant entering the block and in the latter you wont, but you have less control of the temperature of the hot side. Not saying thermostats are bad, but in modern cars this problem is controlled by bypassing \ mixing rather than allowing cold radiator water enter the block directly.

[Andy Bennett]

'so if the water enters the top hot and exits the bottom at room temperature (like the early photos) its running at 100% effectiveness.'

But I would suggest that in the earlier pictures it is more likely to be running at closer to zero efficiency, where there is no flow, rather heat is being conducted through the static water rather than there actually being any flow at all. What I don't know is what temperature is required to actually produce the flow without a pump to assist.

[Colin Morgan]

Thinking about how such as system works… namely, by convection - the reduced density of the hot water making it buoyant so it rises. The driving force for the circulation is the average difference in density between the water on the engine side of the circuit with that on the radiator side. As there is no obstruction in the circuit, even a small driving force from a small amount of heat in the block would start the water circulating very slowly, given a bit of time whilst it got established. As soon as the water moves, resistance to this flow starts to build around the system and this resistance counteracts the buoyancy force such that the flow settles at the level where these are equal.

[When the engine is started, water in the block starts to warm, its density drops and its buoyancy makes the hot water rise as high as it can go in the system, which is into the radiator header tank, where it mixes with the water already there. As the total volume of the water in the system doesn't change much as it warms up (expansion even over a temperature range of 60 degrees C is only about 3%), this upward water movement will start to slowly drag cold water up from below the block, and this must then result in the water around the whole circuit starting to move, though only very gradually at first. As more heat goes into the water inside the block and head, the amount of hot water in the top of the radiator increases. Some of this warmer water starts to be pulled down into the radiator where it can start to lose heat. As it travels down the radiator it continues to cool, its density increasing again. Colder water from the bottom of the radiator continues to be dragged round by the movement of the water from the block and head to radiator by the buoyancy force. From the photos, it seems that it takes a bit of time for this circulation to become properly established and there is a short period where an excess of hot water gathers at the top of the radiator before the increased flow rate allows the radiator to perform more effectively.

The flow keeps increasing until an equilibrium is reached where the heat being lost through the radiator equals the (waste) heat produced by the engine (ignoring other direct heat losses), at which point the temperatures stabilise. At equilibrium, the buoyancy force that drives the water around the circuit then equals the resistance to the flow of water through the block, head, hoses and radiator.]

[Andy Bennett]

Agree Colin
I guess the way to see when convection takes over from pure conduction would be to put a few drops of vegetable colouring in the top of the radiator and have clear hoses to see the colour moving. That's one for when I have nothing else to call on my time...

The more you think about it the more complicated it is, especially when looking at all the other cooling affects aside to the radiator. Not of course adding in the differing cooling efficiency of water to water/antifreeze. Also remembering these are all static so not with the wind in its hair.

If a 4 blade fan made all the difference to tropical survivability then these things may have a bigger contribution than we imagine.

[Bob Culver]

My car has always had a tiny hole in the radiator cap so that any steam could be observed (Had the curious effect that on the rare occassion of heavy braking a fine jet of rusty water shot over the roof! And a large quantity out the overflow. can be captured)
Becaue there is no reserve water and not above the head, and the head is alloy with ohc most modern engines are wrecked by a boil up. But provided enough water remains to ensure circulation, or at least over the head, older engines are little affected. So with reasonable care can can experiment with part blanked ardiator, no fan etc. Temp gauges in top tank can be deceptive as with no water may give an OK reading.

[Barry Townsend]

Interesting photos,
On my RN I've installed a thermostat at the bottom end of the top hose as per an article on this site some time ago.
The engine now warms up quickly irrespective of ambient and remains constant so the 'stat is doing its job.
Continuing on with this theme I've since installed a small catch tank on the bottom of the rad overflow with the tube immersed. 
Since this last little mod I've never had to top the radiator up as whatever overflows whilst hot is sucked back into the radiator as it cools down. All that is needed is a good seal on the rad cap.
I'm using the blue anti freeze year round and I religiously check oil and water on every trip.

[Bob Culver]

There is aavilable here and Oz an additive tpe B which is not expensive and based on 40 years experience very effective. But not anti freeze.

[John Cornforth]

Hi

I delved into the subject of emissivity after getting one of those cheap handheld IR thermometers. They use an assumed value of 0.95, which fortunately isn't far off for a wide range of materials including painted surfaces that aren't too glossy. However, any smooth shiny metal has a much lower value, so it's hard to get an accurate reading of the true surface temperature rather than the surrounding environment it is reflecting back.