Hi all I hope it is the good place to ask this question :
One often says that heat transfer beetween two bodies is related to the difference of temperature beetween both bodies. Then one often says that a mass stove auto-regulates -self regulates- its emission in relation with its environment. But in the same time, one says that radiation of a body is strictly related to its temperature (at the power 4). Another think is that a body also absorbs energy from the other bodies around it.
Then : does heat transfer change in relation with its surounding? that is, does a mass stove self regulates its power in relation with its environment? or isn't it only the difference beetween emision and absorption of both bodies that changes?
PS : if not at its place, is it possible to put this message somewhere else?
Last Edit: Jul 25, 2015 3:30:25 GMT -8 by pyrophile
In my view, heat emission of a masonry body will stop when there's an equilibrium with its environment.
That's true, and the rate at which the exchange of temperature happens slows down in relation with the temperature difference. The biggest the temperature difference, the fastest the heat exchange. The smallest the temperature difference, the slowest the temperature exchange. Pretty much what Peter explains bellow.
In other words: a masonry heater in a cold room will cool much quicker as compared to a warm room. The more the surroundings are warming up, the less heat will leave the heater.
My two cents.
And regarding this Benoit,
Then : does heat transfer change in relation with its suurounding? that is, does a mass stove self regulates its power in relation with its environment? or isn't it only the difference beetween emision and absorption of both bodies that changes?
One thing to know, heat moves always from the hoter part, warming the cooler part. It can't happen any other way!
That is the point I would like to clear as,on one hand, as Newton's law of cooling says, the speed of cooling of a body is function of its environment's temperature. In the other hand, the emission of radiation of a body is function of its temperature not of its environment. And a stove replaces its losses. If we have two bread toasters facing, 1000 watts each, do they limit their raiation to nothing? They are never cooling but they still radiate. In a room, a mass stove radiates even if the weather gets very hot and heats the room. But the stove doesn't cool, as its environment heats it too. Then, can we say, as many people do (and I used to also) that it selfregulates in function of room's temperature? Or can we say that during summer, a mass stove radiates at max power, like (edit) in winter but it cools quicker in winter as it gets lower energy from its surroundings (edit)?
There is something I am missing!
Last Edit: Jul 25, 2015 2:31:26 GMT -8 by pyrophile
a mass stove radiates at max power, as in winter but it cools quicker in winter?
That's it pretty much, delta of temperature!
In a cold room, there's a great temperature difference, nature wanting to reach an equilibrium, cold air and the suroundings suck the heat at a faster rate. In a hotter room, if you have only 2 or 3 degrees difference between the stove's outside skin and the suroundings, the process will be slower.
But that happens also between the inner layer of the mass and it's outside layer too, and also between the insides of the bell and it's inner skin. The mass itself, slows down the process. So the peak heat inside the mass won't be at the top of the burn. May be not even during the burn.
I can't remember, but there's also something about the change from conduction to radiation. Tho, that might be beyond my reach.
Well, Peter wrote : "In my view, heat emission of a masonry body will stop when there's an equilibrium with its environment".
Let's suppose that the room is a sauna at 60°C and the heater at 60°C too. I would think that the heater, in equilibrium with the room, still emits at the same rate- which is related only to its temperature (stefan-Boltzmann's law). Of course, in the same time, it absorbs the emissions of its environment, which prevents it to cool. Then, I have the impression that the stove emits but doesn't cool! It will radiate for a longer time. But it didn't decrease its emission. More heat was emitted as both bodies (the stove and the room) did emit in function of their temperature but no one cooled.
The difficulty is, for me, to articulate two principles 1) a body exchanges heat with its environment in relation with the difference of energy beetween it and its surrounding (law of cooling of Newton) and 2) a body radiates without relation with its environnment but with its temperature (Stefan-Boltzmann's law).
Isn't it clearer with two electric bread roasters of 1000 watts, facing? Both emit but don't cool a lot because 1) they recreate heat at each second and 2) each one absorbs energy from the other one! 2000 watts arebemitted, not 0!
It is not easy for me but it is interesting! In France (and everywhere I suppose), we (stove builders) usually say (and I did) that a stove self-regulates its emission of heat. Is it true?
Last Edit: Jul 25, 2015 3:31:15 GMT -8 by pyrophile
You seem to know more than me. There's something i seem to remember, the stove and sauna will act as the same body somewhat, if at the same temps.
It doesn't self regulates imho. The mass of the stove being "insulated" by the suroundings being at the same temperature as the stove skin, and not being able to take more heat. The heat in the mass will tend to equalize, so the stove body will become hotter, and heat the suroundings accordingly. Then when the point of equilibrium between all parts will be reached; this means the stove mass, gases inside it, the suroundings, would it be air or mass, will be at the same temp. Then, from that point cooling will occur, heating the outsides. Depending on the heat exchange speed of different medias, some part will cool faster than others, then will be heated by the parts cooling slower, since they've becomed hotter than the rest again, they'll cool donw in turn again. Untill the equilibrium is reached with the great outsides, and then space the universeand so on! Untill everything everywhere in the universe is at about 5K° or something like this!
Now that i am beginning to reassess my understanding of conduction… I wonder if we could say: When the temperature of the stove's outer skin is the same as the temperature of the surrounding environment, radiation from the stove is happening, but conduction through its mass is at a stand still - other than whatever equilibration is occurring within the stove's mass/body. Could this be what is meant by 'self-regulating'?
Heat exchange might happen in 3 different ways: convection, conduction and radiation. Radiation is always there. All matter with a temperature greater than absolute zero emits thermal radiation. You can eliminate or significantly cut down convection and conduction but you never can avoid radiation.
I got most of my questions in the topic answered by the book "heat transfer" J.P Holman (tenth edition) but I believe that Lumped-Heat-Capacity System heat transfer can be explained here but numerical methods for real life unsteady multidimensional systems may be a bit too much..
and lots of real life examples of heat transfer calculation can be found in a book HEAT TRANSFER A Practical Approach by YUNUS A. CENGEL. Thats a real brick 1300 pages. But so much worth of reading.
Construction and gardening season has blowed out of the sudden in here so.. it's time to apply the newly gained theoretical knowledge into practice.
Radiation would be lower in a masonry bell compared to metal. Radiation moves in waves and heats objects directly - infrared (sunlight). Our sun heats the earth through radiation. Radiation does not heat air molecules. Conduction is the transfer of heat directly like when you sit on the warm bench. Convection is the heating of air molecules - this is generally what happens with a masonry bell.
Lets not forget that during colder climatic times your "room" will be loosing heat to its environment constantly and as such your heat source will be loosing heat to the "room" constantly. Variables exist such as the power that the heat source can produce the thermal retention capabilities of the "room" and the temp of the environment outside said room.
Radiation would be lower in a masonry bell compared to metal. Radiation moves in waves and heats objects directly - infrared (sunlight). Our sun heats the earth through radiation. Radiation does not heat air molecules. Convection is the heating of air molecules - this is generally what happens with a masonry bell.
doesnt the radiation depend on emissivity, where the surface of the material is more important than the material itself?
and I thought that convection is simply radiation hitting the air and dust molecules.
kkp: Mercedes: Benches don't get real hot. In fact, they are rather cool compared to other areas. You shouldn't need a mix like you described
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