Today one of my instructor told me that gases cannot have heat transfer thru conduction because molecules are far apart and so it cannot transfer heat,infact the diffusion process of gases transfers the heat. my thot at this point was,even though molecules are far apart,they transfer momentum to next molecules and we can feel the pressure and temperature(Kinetic energy).But why not heat? if we compress the gas at very high pressure, when molecules are very closer to each other, it will start transfer of heat by conduction? can someone shed some light on this?
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asked Feb 7, 2014 at 15:04
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5 Answers
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In general gases consisting of single atoms, i.e., the noble gases are much poorer heat conductors than molecular gases. The reason is that if a molecule hits another molecule or a hot wall its internal states, usually vibrations, can get excited. In other words, they pick up a little energy and if they then encounter another molecule or a cold wall, this energy can then be transferred to the other molecule or the cold wall.
This mechanism does not exist for noble gases and that is why noble gases are preferably used for double glass windows. In that case energy can only be transferred by a change in velocity during collisions. Air, which consists of nitrogen and oxygen is less well behaved. In particular the water vapour in wet air can spoil the insulation properties.
In general, the more complicated the molecules, the more vibrational degrees of freedom they possess, so the more energy they can pick up. This possibility of picking up energy plays an important role of global heating.
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you can transfer heat trough gas, it's only a bad heat conductor. In gasses heat will be mainly transported by convective heat transfer. So for example for air, if you are able to eliminate these convective currents or ''lock the air into place'' then it's practically an isolator. This is actually used in double glass windows, there they squeeze a gas in a thin layer between two pieces of glass so that it would attribute to the isolation of heat.
A fist-rule: ''Good thermal conductors are usually good electrical conductors'', the most famous exception is diamond which is an excellent thermal conductor, but a poor electrocal conductor.
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$\begingroup$ Dielectrics with high Debye temps will be good thermal conductors (Moissonite). Electical conductors with low Debye temps will be poor thermal conductors (lead). Solids that conduct electron but scatter phonons (filled scutterudites) decouple electrical and thermal conductivities. Make diamond absent 1.1% natural abundance C-13 and thermal conductivity zooms (no phonon scattering). The fun is in the footnotes. $\endgroup$– Uncle AlFeb 8, 2014 at 0:09
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$\begingroup$ @UncleAl, ok at that point you might be right. I was more thinking in terms of every-day life objects, there the fist-rule might be a good approximation/assumption. But indeed there are cases where it doesn't work out. $\endgroup$– NickFeb 8, 2014 at 10:12
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$\begingroup$ @Nick When a gradient temperature exist in a gas will the gas transfer heat with both convection and conduction? Are these types of transfer occuring simultaneously? I couldn't understand what you mean "eliminate these convective currents". Do you mean that no heat is transfered to the gas by a source like fire etc? $\endgroup$ Jun 7, 2020 at 16:14
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Gas heat transfer rate primarily depends upon thermal conductivity (inversely proportional to molecular weight) and specific heat. Thus hydrogen and helium are excellent heat transfer fluids...but SF6 is substantially better than either for tis much higher specific heat (downrated for viscosity re pumping).
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$\begingroup$ Does this mean that slightly pressurized helium will transfer heat to a metal container better than air? $\endgroup$– gotjoshNov 1, 2022 at 8:12
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Sudden heating of a region of gas can produce a shock-wave which propagates temperature increase, with the leading edge of the shock-wave at the speed of sound within the gas being considered as a boundary or container, and the depth and density of the shock-wave being subject to temperature increase according to Boyle's law. (followed by a wave of decreased pressure of lesser amplitude, then another smaller wave of increased pressure etc. as the gas's elasticity allows.)
In a closed container of dimensions much smaller than the wavelength of the speed of sound in that gas medium, equilibrium can be seen to be fairly quickly reached, much more quickly than by Brownian motion or convection alone.
However, this is a more poorly understood subject than one might expect, See here.
answered Feb 8, 2014 at 11:16
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All heat transfer media absorb and shed their energy in different methods relative to their thermal mass, thermal conductivity, and specific heat. If your professor thinks that gases can not be used to transfer energy in the form of heat we had better shut down the world. In heat transfer applications there are three factors. The initial heat transfer media, the heat exchange barrier, and the useful heat transfer media. All boilers employ a chemical reaction whether nuclear or combustion is utilized to heat gas which is pushed and/or pulled through a heat exchanger, the heat exchange barrier, to heat a working fluid water such as water (steam) or other heat transfer media, molten salts, thermal fluids (oils), or thermal transfer gases. Conductivity, Convection, Radiation F=MA The force as energy in gases has small mass but their ability to accelerate is orders of magnitude higher than liquids or solids. Just compare their specific heat and thermal conductivity numbers The heat transfer from hot exhaust gas mixes through boiler tubes to pressurized water is typically pretty low 80% to at best 90%. The heat transfer from hot exhaust mixes through a heat exchanger to another gas or vapor or finely dispersed particles is particularly efficient 90 to 99% efficient.
