Physics of heat transfer - conduction, convection and radiation

[Bishop]

Mod note: Posts moved from XR2 MKII thread

Hi

I've an idea! :idea:

You know the way the temperature readout ('mode 3') shows the temps at different parts of the ship?

How about when the Top is warmer than the Coolant, deploying the Radiator actually accelerates the rate of Coolant heating?

I was doing a transatlantic hop, opened the Radiator at Hi Alt and noticed even though the roof was ~300C and the Coolant ~70C, the Coolant temp was falling... :hmm: That can't be right thinks I...

You wouldn't need Sally to record anything new, 'Warning Radiator Deployed' is already recorded for Dynamic Pressure...


- Bishop

 

[tblaxland]

I was doing a transatlantic hop, opened the Radiator at Hi Alt and noticed even though the roof was ~300C and the Coolant ~70C, the Coolant temp was falling... :hmm: That can't be right thinks I...

Just because the hull temp is 300°C, doesn't mean the radiators won't radiate heat into the environment. What matters is the balance between the energy leaving the radiators vs the energy entering the radiators. In the low density upper atmosphere, conductive effects are minimal and the radiation from the radiators will be greater than the radiation from the atmosphere because of the radiator material's greater emissivity. The ISS radiators don't seem to have any trouble operating in the ~500°C thermosphere. Granted, the XR2's radiators would get some radiation from the hull, but it shouldn't be too bad with the low emissivity white paint Steve has put on there . Finally, solar radiation would only be an issue when flying on the sunny side and in specific spacecraft attitudes (it varies in proportion to the cosine of the angle of incidence) that a competent pilot would avoid anyway, so I think it is safe to ignore. You are talking about a ship that runs on fairy-dust anyway...

 

[Bishop]

WTF (what the fizzix)

Hi tblaxland!

First of all... thanks for the reply! :thumbup:

Just because the hull temp is 300°C, doesn't mean the radiators won't radiate heat into the environment.

Second... excuse me, What? :huh:

If I pump fluid @ 70C into something @ 300C the fluid will get warmer, right..?***

Or is this one of those situations where reality != common sense? Physics can do that :blink:

Nuts. I thought I had something to contribute. Oh well, I'll take what you're saying on trust and leave it at that :shrug:

This bit sold me:

The ISS radiators don't seem to have any trouble operating in the ~500°C thermosphere.

tar mate,

- Bishop

*** yes, noticed I'm assuming roof and rad temps are the same...

 

[Tommy]

If I pump fluid @ 70C into something @ 300C the fluid will get warmer, right..?***

Well, that depends. Yes, as far as it goes. But it doesn't account for all the ways that heat is transfered. Heat, like water, tends to "seek it's own level", but there are a lot of factors that determine how well it can do that. If I pump a 70C liquid into a 300C heat exchanger, then the liquid will become warmer. But then again, that's what heat exchangers are for - facilitating the transfer of heat energy to another. In this case, the heat is being transfered by conduction - the fastest way to transfer heat.

There is no such thing as cold - there is only a lack of heat. Heat is form of energy, and cold is a lack of energy - not an "anti-energy". Heat Energy is transfered from one object to another in three ways.

Conduction works much like it does with electrical energy. Different materials conduct heat better or worse than others. The cooling fluid uses conduction to absorb heat from one heat exchanger (such as a heat sink on a CPU) and also to release that heat to another (such as a radiator).

Convection is easiest to understand as "using the air to conduct heat". A standard oven uses convection - it heats the interior of the oven and the food is placed into the hot environment and absorbs the ambient heat from the air. Higher end "convection" ovens used in restaurants also circulate the air in the oven, which speeds the heat transfer and allows shorter cooking times.

The last main way is radiation, such as Infra Red light.

And the whole "environment temperature" is misleading. It's based on the amount of radiant heat an object is exposed to, but seems based on an "average" level of absorption. In space, where heat can only transfer by radiation, actual heat only exists where there is matter to hold the heat. There is lots of "potential heat", or "heat energy", but there are many things that can affect whether or not that potential heat actually gets something hot.

Once above an atmosphere, convection doesn't really apply. The "environment" is at 300C, but what does that mean. Heat doesn't exist on it's own, it needs something physical to be hot, or else it's "heat radiation", or "potential heat". The way the radiator is designed has a lot to do with how hot the radiator will actually be. A surface that is reflective to IR energy, or is in the shade, will have a much lower temperature than the "environment". There are plenty of materials available that offer good conductance and low absorption of radiant energy. This means that a radiator that is designed to reflect heat energy from external radiation can effectively radiate heat energy even if it's in sunlight, if it isn't the most powerful source of radiation around. I doubt mankind will ever even begin to get a clue as to how much heat can be "sunk" into space through radiation, "Solar System Warming" from human causes is unlikely!

 

[River Crab]

If I pump fluid @ 70C into something @ 300C the fluid will get warmer, right..?***

I think an easier way to say this would be, yes, the fluid gets warmer, but the radiator also gets cooler. And so as an effect, the rest of the hull is also cooled by the radiator.
So, yes, but it's just doing its job. No need for a warning.
Unless the coolant itself is overheating, in which case, there's already a warning for that. :thumbup:

---------- Post added at 22:45 ---------- Previous post was at 22:25 ----------

How about when the Top is warmer than the Coolant, deploying the Radiator actually accelerates the rate of Coolant heating?

I was doing a transatlantic hop, opened the Radiator at Hi Alt and noticed even though the roof was ~300C and the Coolant ~70C, the Coolant temp was falling... :hmm: That can't be right thinks I...

There it is. The rate of heating increases, but the coolant temp is still falling because the radiator is not actually working that fast. It's the coolant temp and not the radiator temp.
:idk:
God, this is how 3 Mile Island happened...

 

[tblaxland]

*** yes, noticed I'm assuming roof and rad temps are the same...

A flawed assumption IMHO. Radiators are often insulated from the structure they are mounted on, either to minimise thermal effects on the structure or reduce the load on the radiators depending on the situation.

Conduction works much like it does with electrical energy. Different materials conduct heat better or worse than others. The cooling fluid uses conduction to absorb heat from one heat exchanger (such as a heat sink on a CPU) and also to release that heat to another (such as a radiator).

Yes, but conduction also occurs across material boundaries. Essentially conduction is heat transfer due to collisions between the vibrating molecules. In a tightly bound solid, conduction is quite high because it is easy for a hot molecule to vibrate an adjoining cooler one. In something softer, or gaseous, the molecules are less tightly bound to each other so the rate of heat transfer is much lower.

Convection is easiest to understand as "using the air to conduct heat". A standard oven uses convection - it heats the interior of the oven and the food is placed into the hot environment and absorbs the ambient heat from the air. Higher end "convection" ovens used in restaurants also circulate the air in the oven, which speeds the heat transfer and allows shorter cooking times.

The pie in my oven (I love cooking pie ) is heated by both conduction and radiation, but definitely not by convection. The radiation comes from the surfaces in the oven, the walls and the elements themselves. The conduction is from the collisions of hot particles of air with the cooler particles in my pie. The convection is just responsible for moving the hot air around the oven and displacing the air that has been cooled by my pie.

And the whole "environment temperature" is misleading. It's based on the amount of radiant heat an object is exposed to, but seems based on an "average" level of absorption. In space, where heat can only transfer by radiation, actual heat only exists where there is matter to hold the heat. There is lots of "potential heat", or "heat energy", but there are many things that can affect whether or not that potential heat actually gets something hot.

To clarify: the thermosphere of Earth is quite hot but it is so thin that collisions of the molecules with each other or a solid are relatively rare so conduction is low. The temperature is not measured by any average level of "absorption", but rather by the average energy of the substance's molecules.

This means that a radiator that is designed to reflect heat energy from external radiation can effectively radiate heat energy even if it's in sunlight, if it isn't the most powerful source of radiation around.

Such material is quite difficult to make because both the radiation and absorption of electromagnetic energy is governed by the material's emissivity. It is possible to make a material that has high emissivity at some frequencies and lower emissivity at others. This is the theory behind infra-red high emissivity paints that are sold for building roofs, to radiate heat at the black body temperate of the roof but reflect other frequencies. However, given that the Q factor of such paints is quite poor and the sun generates such a broad spectrum of electromagnetic energy the best strategy is to provide shade for your radiators - this is why the radiators on the ISS truss are fixed to be perpendicular to the plane of the photovoltaic panels. For buildings, the testing I have been involved in has strongly indicated that the best strategy is a low emissivity surface backed by a low conductance insulation.

 

[Bishop]

Me head is meltin'


OH YES!!

I think an easier way to say this would be, yes, the fluid gets warmer, but the radiator also gets cooler.

That is so what I'm talking about!

I don't know if you operate the XR2 (give her a spin man, only after you've given the manual the once-over mind you) but the fluid temp falls when pumped into the hot rad***

---------- Post added at 03:29 AM ---------- Previous post was at 03:17 AM ----------

A flawed assumption IMHO. Radiators are often insulated from the structure they are mounted on, either to minimise thermal effects on the structure or reduce the load on the radiators depending on the situation.

Yep, that's what I thought, then I changed my mind back again :lol: because...in my intercontinental flights, the radiator is deployed for ages so the 'outside' conditions are the same for both, so they will stabilise (eventually) at the same temperature.***

Unless... the radiators are painted black. Or white!?

I predict an experiment with black and white ice filled metal bottles on a sunny day

I'm getting to the bottom of this! :rofl:

- B


*** I'm not in space, about 60-65K max ASL

 

[trish129]

how to do this optimisation

hello..i want to do optimisation of a heat pump...how to go about..i know engineering equation solver...what and how many equations in this?..plz help me formulate the equations...

i have attached the question ....