Does advection requires heat in any way or can heat (instead of materials) be advected?
I do not agree that they means the same.
As stated by @ Filippo Maria Denaro , advection is the physical process involved with the transport of any quantity (mass, heat, momentum, etc). The word CONVECTION is the combination of the words CONduction + adVECTION = CONVECTION. This reflects the heat transfer mechanism produced between a fluid and a solid wall, as mentioned by @ Shobhan Roy.
At a solid wall, where no slip condition forces the fluid to zero relative velocity, the only mechanism of heat transfer is conduction (neglecting the radiation, of course). It is only as one moves away from the solid, where the velocity is non zero, and the flow might even be turbulent, that mass transport phenomena take place because of the fluid motion. The combination of these two mechanisms is convection.
Talking about 'convection' in an isothermal problem is not correct.
In heat transfer, convection is a process in which heat is carried from place to place by the bulk movement of a fluid. As convection is dependent on the bulk movement of a fluid, it can only occur in liquids, gases and multiphase mixtures.
Convective heat transfer is split into two categories: natural (or free) convection and forced (or advective) convection, also known as heat advection.
In general, the "advection" term addresses to the macroscopic transport of a variable due to the mass flux. Convection is more focused on heat transfer process.
Both mean the same. Historically, researchers in atmospheric science distinguish between direction of propagation, but both of these relate to transport of mass/momentum/ energy.
At least in the literature of Environmental Fluid Mechanics (EFM) and groundwater contamination those terms are used interchangeably. And both refer to the same concept: Movement of a constituents or heat in a water body because of the movement of water
I have come across the usage of "convection" more often when heat transfer takes place across dissimilar states of matter, as in convective heat transfer from a solid wall to the boundary layer fluid. It seems to bridge conduction and advection in this notion. This is only from my understanding; more accurate historic references suggested by senior researchers in previous comments may indicate otherwise. Please correct me if I am wrong.
Dear Shobhan Roy
Convection heat transfer only occured in the fluids, but conduction may be occured in the solids, liquids and gases. When the fluid is stagnant, the heat transfer is in conduction mode.
Dear Shobhan Roy ,
Look at any book using convection equation as the model. And you will note that it is about signal propagation, as I stated earlier: It is about propagation of any physical quantity (mass/momentum/energy), be it in acoustics, geophysics, electromagnetic wave propagation, fluid mechanics/ heat transfer. One can add on the list.....
I want to believe that their differences is not just in their directions (horizontal and vertical directions respectively) as some scholars previously stated. An individual even even equates advection to force convection. How true are this assertions?
I do not agree that they means the same.
As stated by @ Filippo Maria Denaro , advection is the physical process involved with the transport of any quantity (mass, heat, momentum, etc). The word CONVECTION is the combination of the words CONduction + adVECTION = CONVECTION. This reflects the heat transfer mechanism produced between a fluid and a solid wall, as mentioned by @ Shobhan Roy.
At a solid wall, where no slip condition forces the fluid to zero relative velocity, the only mechanism of heat transfer is conduction (neglecting the radiation, of course). It is only as one moves away from the solid, where the velocity is non zero, and the flow might even be turbulent, that mass transport phenomena take place because of the fluid motion. The combination of these two mechanisms is convection.
Talking about 'convection' in an isothermal problem is not correct.
Although what Professors Denaro and Sengupta is what I am familiar with (and convinced of) from my education, I must say that I come across a number of workers in the field of dispersed multi-phase flow who make a distinction between "advection" and "convection".
For example, this short excerpt from Michaelides' Heat and Mass Transfer in Particulate Suspensions about Stokesian heat transfer from spheres:
"Heat convection is comprised of two parts: conduction and advection. When Pes
I am not sure that "convection" stands for conduction+advection ... We generally adopt model equations such as the "convection-diffusion equation" explicitly denoted.
My idea is that the nomenclature is more a specific field issue. I am used in fluid dynamics to denote both advection and convection as nomenclature for the macroscopic transport term in the equation. But often I see "convective heat transfer" as a unique term in the heat transfer field. Thus, I suppose that there is no unique accepted definition.
See
https://en.wikipedia.org/wiki/Convection%E2%80%93diffusion_equation
https://en.wikipedia.org/wiki/Convective_heat_transfer
I think it is sometimes good to look at the isolated meaning of the word.
ConVection is build with 2 latin words : cum (which means "with") and vehere (which means to carry) so that the meaning when the word was applied to heat transfer was " the heat was carried by the flow" as a difference from conduction which is related to "leading" the heat in the present material without displacement.
Dear Yusuf, researchers,
ADVECTION is a movement due to deformability of flowing volumes of fluids. its a relative movement inside the volume. Two advected particles are two particules who are approaching (or moving away) isinde the volume as the fluid flows. They have their own movement inside the volume which has its average velocity.
CONVECTION is a movement due to the expasion of fluid volumes receiving heat from any external or internal source. It is the displacement of the entire volume when the heat transfer by simple conduction is no longer sufficient to evacuate the energy received from the source by the fluid, and the density of warm fluid volume is less than the density in the vicinity.
Advection exists regardless of the source of fluid motion (It also exists inside convected volumes).
Convection exists thanks to the supply of sufficient quantity of heat (There are critical conditions for convection).
Hi Haikel Ben Hamed ,
Could you provide reference text for your definitions?
The definition of "Advection" sounds very similar to that of Diffusion. But diffusion is and should be different from advection. Could you please clarify.
The definition of convection you gave here is definitely interesting.
I just wrote these definitions. I did not use special references. I'll try to clarify.
ADVECTION is a relative motion of one or more species within the flowing FLUID volumes. It is given by the term V.grad(V) in the Navier-Stockes equation:
dV/dt is the acceleration of the entire fluid volume.
V.grad(V) is the acceleration of particles due to the deformability of the fluidvolume. This term does not exist in perfect solids.
DIFFUSION is another physical phenomenon that concerns at least two species. Diffusion can exist even in solid alloys.
In the specie's equation of binary mixture, for example, we write:
dC/dt + V.grad(C) = D grad² (C)
where C is the concentration and D is the diffusion coefficient of Fick.
You can clearly distinguish the term of advection V.grad(C) (which takes more mathematical meaning) but which also means variation of the concentration inside the flowing volume because of the movement of the fluid mixture, and the term of diffusion on the right which means that the molecular species move because of concentration gradients to standardize this concentration on the entire volume.
Consider V=0 (perfect solid case) : you'll remove advection. The variation of concentration versus time is only due to diffusion.
"CONVECTION is a movement due to the expansion of fluid volumes receiving heat from any external or internal source. It is the displacement of the entire volume when the heat transfer by simple conduction is no longer sufficient to evacuate the energy received from the source by the fluid, and the density of warm fluid volume is less than the density in the vicinity."
Convection is of 2 kinds:
- free convection which is due to the ascending mouvement of the heated layers in contact with the warm wall and it is called "convection" because the heat is transported by the fluid flow as difference to conduction which is the heat propagation without mass mouvements
- forced convection which is due to an imposed (forced) fluid flow movement.
What you mention as "expansion" is valid ONLY for the free convection.
You consider convection as an extension of conduction which is totally WRONG. In fact in the contact layers heat enters the fluid by conduction and because (for the free or natural convection only) of the temperature rize according to Archimede princip the hot layer having a lower density will go up and be replaced by a cooler layer which at its turn will be heated and follow the precedent. Convection is not related to a limit of conduction it is a totally different way of heat transfer.
@ Nick Hamburger
Thank you for your response.
I don't consider convection as extension of conduction. I agree that it is a different mode of heat transfer.
In the first response I consider convection by comparison to advection (read the subject of discussion). So I consider it as SOURCE OF MOTION and I did not give its definition as heat transfer mode.
Even in forced convection there is expansion (or narrowing) of forced flowing volumes near the heat (or cold) sources, otherwise there will be no thermal boundary layer and no convective structures (rolls, cells, vortex etc...), neither convective and absolute instabilities.
Consider a fluid flowing over a plate, and both are exactly at the same temperature. There is no convection (heat transfer) and there will be no convective structures because absence of expansion.
Nick Hamburger
Just look at forced convection as free convection in a moving Galliliean reference.
What about expansion in this case ?
I am not the one to fight windmills :
You are right !
You win: only expansion is the reason for heat transfer!
Happy ?
As for the boundary layer you have to consider several other aspects:
- even if no temperature gradient is present a boundary layer does exist if the fluid in moving (in a forced flow) since it is due to fluid viscosity and its adherence of to the wall.
- near to the wall heat transfer occurs by conduction from layer to layer and because layers are moving heat is transported by the fluid i.e. carried by it = this is the basic meaning of "convection" as I explained a bit earlier.
- you have turbulence even if you do not have a temperature gradient between wall and fluid.
The consequence of heating is no doubt "expansion" but the free movement is due, in the earth gravitational field, to the consequences of it : the lower density of the hot fluid in comparison with the density of the cold one. Expansion is a volumetric effect and if density would no change no free convection would occur!
The problem with the thermal boundary layer has to be looked at through the viscosity changes due to temperature increase or decrease.
If the system is in a gravity free environment there is NO free convection, only conduction is present in absence of an imposed fluid movement, only forced convection could occur.
I totally disagree with you assumption that forced convection is "moving" free convection it is wrong for the simple reason that the shear forces in the two cases are of different origins and the turbulence (as a general meaning) is due to the relative decreasing of the shear forces in the flow between two adjacent layers ( explained in rough, primitive, form). If the shear forces are relatively high there is no turbulence. Look at the criterium for turbulence : Reynolds = w*d/nu
What is it in your opinion ?
What does it mean from the force balance point of view in the fluid flow ?
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