Question about convective heat transfer and convective mass transfer.

[u/FateUniverse]

Is a temperature gradient necessary for convective heat transfer (natural or forced) and a concentration gradient necessary for mass convection?

[u/ogag79]

You always need temperature gradient for heat transfer. Convection is just a means of transferring heat.

For natural convection, what drives the movement of fluid is density difference, which is affected by temperature (hot air rises). But even if you do a forced convection (by having a fan or blower), no heat transfer will happen if there's no temperature gradient between the source and the sink.

[u/MoneyMammoth4718]

Is necessary for ANY Heat Transfer Process

[u/brickbatsandadiabats]

For convective heat transfer, you need a temperature gradient. No heat transfer occurs without a temperature gradient.

In 90%+ of circumstances you need a (solubility adjusted) mass gradient to do any kind of mass transfer, convective or otherwise.

But because mass transfer equalizes chemical potential and not concentration (even incorporating solubility) per se, anything else that affects the chemical potential can also cause mass transfer to occur.

So for example, applied pressure across a semipermeable ion exchange membrane can cause mass transfer by creating a pressure gradient that can only be equalized by osmotic pressure from the other side, even if no concentration gradient exists. This is the entire concept behind reverse osmosis.

An applied voltage can do the same, although because of charged particle transport phenomena it will never be purely convective. Equivalently, any electrostatic imbalance can cause a voltage gradient that will induce mass transfer of other charged species, even if it isn't the species that's imbalanced. You'll see this in electrolysis, fuel cells, and electrodialysis, among many others.

Solution effects can cause chemical potential differences that induce mass transfer. If a mixture of component AB results in a lower chemical potential than AC, the chemical potential gradient will still induce mass transfer to replace AB with AC. Bam, that's ion exchange chromatography.

Even a temperature differential with negligible thermal expansion can cause significant mass transfer of a component if the difference causes a similarly significant change in its chemical potential, like through nonlinear behavior of the heat of solvation. I know people who spent years tinkering with this kind of effect at the supercritical transition state for carbon dioxide.

(Edit: added an example or two)

[u/Peclet1]

Yes, that is how these things are sort of defined. Sometimes you have both like in the case evaporation.

[u/Stunning_Ad_2936]

Spontaneous transfer implies potential difference/gradient. Transfer of charges - electric potential, transfer of species - chemical potential, transfer of energy - temperature, migration of people - opportunities.