I came across liquid and gas conduction in heat transfer when I was reading about conduction. Conduction normally refers to transfer of heat taking place in one or more solids that are in contact. Then what is liquid and gas conduction? They have also given some formulas regarding calculation of thermal conductivity (k) for liquids and gases that includes Boltzmann Constant in its equation. Can anyone clarify it for me? I thought convection is the one that involves transfer of heat between solid and liquid. Even liquid- liquid heat transfer is convection. Then from where fluid conduction comes into picture?
The above answers were good but they did not touch on one aspect. Right next to a solid surface (or a gas-liquid interface), you can imagine that there will be one very thin layer of fluid that has zero velocity (with respect to the surface/interface) due to viscous friction. If the velocity is zero, there can be no convection and all transfer of heat is due to conduction in the fluid layer. That is why heat flux in convection is expressed as thermal conductivity of the fluid multiplied by the temperature gradient at the wall. If this answer is not easy to understand, discuss the answer with a teacher or a friend.
If the heating element is at the bottom, some of the heat is obviously transfered to the gas/liquid through conduction, but afterward, convection currents form to transfer heat more effectively.
If the heating element is at the top on the other hand, convection currents wont form quite as much and you must rely much more heavily on conduction to transfer the heat. A similar situation occurs if you have the heating element on the side, in the middle of the gas/liquid.
Alternatively, you can conduct your experiment in a zero gravity environment. That way buoyant driven natural convection is impossible, or at least negligible.
Conduction is heat transfer through a material (solid or otherwise), caused by a temperature gradient within the material that results in a heat flux. Convection is heat transfer via the movement and mixing of a fluid.
Imagine a fluid that is (somehow, magically) stationary. If there was a temperature gradient across that fluid, I'm sure you can imagine that there would be heat transfer occurring in the fluid, just as there would be in a solid. That's conduction. Just because a real fluid would be moving doesn't mean that it wouldn't also experience conductive heat transfer. (Though it may be negligible compared to convective heat transfer.)
The above answers were good but they did not touch on one aspect. Right next to a solid surface (or a gas-liquid interface), you can imagine that there will be one very thin layer of fluid that has zero velocity (with respect to the surface/interface) due to viscous friction. If the velocity is zero, there can be no convection and all transfer of heat is due to conduction in the fluid layer. That is why heat flux in convection is expressed as thermal conductivity of the fluid multiplied by the temperature gradient at the wall. If this answer is not easy to understand, discuss the answer with a teacher or a friend.
That's a great point! (That convection actually requires conduction because of the "no-slip" condition - I'd never thought of that before.)
Think about glass wool, for example, its thermal conductivity is greater than, but of the order of, that of air. In this case, and in case of many insulating material with very high porosity, it the solid matrix that keeps the gas at rest. So movements are avoided and convection does not occur. It is a simple way to have common insulating materials.
Ram, After reading the good answer by Federico, I happened to read your question again. I realized that my answer of almost a month ago was not complete.
How the Boltzmann constant enters the picture is this way: If the radiating medium were to be replaced by another medium that only conducts, (the area, the thickness or path length of heat travel and the temperature difference being unchanged), then the medium can be said to have an effective conductivity, which will naturally depend not only the heat transfer coefficient but also the difference of temperatures. This is purely an equivalent concept. Imagine radiation taking place in vacuum!
Ram, convection is a type of conduction heat transfer. It occurs when the motion of the particle is appeared in the system. If there is a system in which heat is transfer to the liquid through the solid, assume that initially the liquid is in stationary condition. Due to heat transfer from solid to liquid, density of liquid changes and hot liquid start to move in the upward direction due to low density. This type of motion of liquid is known as convection Currents. This is the starting of heat transfer due to convection. If in the system, convection currents are not appeared, then heat transfer takes place due to conduction.
Oops, there was an inadvertent error in my post of yesterday. Instead of " then the medium can be said to have an effective conductivity, which will naturally depend not only on the heat transfer coefficient but also the difference of temperatures.", what was intended was " then the medium can be said to have an effective conductivity, which will naturally depend not only on the Boltzmann constant but also the emissivity of the surface, the radiation shape factor and the difference of temperatures". My apologies for any confusion that might have been created.
Just equate the Fourier law to the Stefan-Boltzmann law and you can derive the expression for effective thermal conductivity.
Vijay Raghavan Sir. The sentence "No local jump in temperature" , I am unable to understand this. If the solid surface temperature and the temperature of fluid layer that sticks to solid surface is same, then how the conduction takes place?
Reference: Heat transfer A practical approach by Yunus A Cengel. chapter6
I hope you will give a helpful answer.
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