0

Convection is a combination of advection and diffusion, where the former is a macroscopic movement of bulk fluid and the latter is a microscopic movement of fluid particles. But often in literature, I see sentences like "The second and third terms of the left-hand side are the energy convected (advection) and the energy transferred (diffusion) by the pyrolysis gases, respectively."

I am confused by this. It seems to suggest that diffusion is separate from convection, when conventionally diffusion is just one form of convection and convection=advection+diffusion. Can someone explain why, often, we see convection and diffusion differentiated in literature?

user5965026
  • 101
  • I've seen this in literature also. Convection and diffusion may have different meanings in different fields. You may want to specify what field you are in. – Charles Oct 19 '16 at 02:19
  • My field on the broad scope is fluid dynamics. In particular, I deal with high-temperature reacting gas flows for hypersonic atmospheric entry phenomena. – user5965026 Oct 19 '16 at 02:22

1 Answers1

0

Convection and advection both refer to macroscopic motion of fluid. Convection, as it is usually understood, is motion of fluid induced by density difference within the fluid in the presence of gravity. Advection refers to transport of something (pollutant, dye etc.) by macroscopic motion of fluid. However sometimes the two terms are used interchangeably, so all you have remember is that both terms make reference to macroscopic motion of fluid and any transport that may result thereby.

Diffusion on the other hand refers of spreading of something (dye, heat, momentum etc.) by means of molecular motion. This mechanism is operative even when there is no macroscopic motion of fluid (i.e. when the fluid is static).

Deep
  • 6,504
  • I agree with macroscopic vs microscopic viewpoints, however, I disagree with your statement about "induced by density difference in the presence of gravity". Gravity is not required for convection to occur, and density differences are also not required for convection to occur. I simulate flows with no differences in density and there is certainly still diffusion and advection. If you accept the definition convection = diffusion + advection, then there is also convection. – Charles Oct 19 '16 at 16:21
  • @Charlie My point is convection$\neq$diffusion+advection as it is usually defined. But if you are going to define it that way and use that definition consistently, then it's fine. But while reading papers of others, applying your definition may not make sense, so you ought to be careful. – Deep Oct 20 '16 at 05:23
  • Hello @Deep, can you suggest me some source on which I can find the distinction between microscopic and macroscopic viewpoints please? – Gennaro Arguzzi Apr 29 '18 at 18:39
  • 1
    @GennaroArguzzi I am not sure what exactly you are looking for, but see this post, article1, and article2. Here is some specialized stuff. – Deep Apr 30 '18 at 04:13
  • Thank you so much @Deep Are there also sources which say that convection has not meaning in the microscopic scale? – Gennaro Arguzzi Apr 30 '18 at 11:45
  • 1
    @GennaroArguzzi Convection is a macroscopic concept as it is defined. At the microscopic scale you have molecular velocity. Suitable averaging must lead one from the microscopic molecular velocity to macroscopic convection, although the technical details of how it is done I have not studied. – Deep May 01 '18 at 03:35