Is Mass Transport the Same as Forced Convection in Thermal Engineering?

[svishal03]

Hi All,

I'm looking at the fundamental aspects of heat transfer - as I'm not a mechanical/thermal engineer but soon to undergo a project wherein I need to deal with some aspects of thermal engioneering.

In the text, which I've been reading (for finite element analysis), it talks of mass transport wherein the medium of interest is a flowing fluid.
In such a case, heat flows by conduction, convection, and via motion of the
media. The last effect, referred to as mass transport.

My question is 'mass transport' same as forced convection? I guess it is because in forced convection aslo the fluid is in motion and so is in mass transport.

Can anyone throw some light please??

Vishal

 

[AlephZero]

No, they are two different things.

In most thermal models, the motion of any fluids is assumed to be known, not calculated as part of the model.

So convection usually refers to heat transfer at the boundary between a solid and a fluid. This is often modeled by Newton's law of cooling, using a heat transfer coefficient that is an empirical function of the fluid flow (where the function has been validated by experiment). The formula for the HTC usually depends on several dimensionless fluid dynamics parameters including Reynolds number, and the depedence on Re accounts for the effect of the fluid velocity on the rate of heat transfer.

Mass transfer is the transport of internal energy (i.e. temperature) caused by the motion of the fluid (e.g. flow through a pipe in a heat exchanger) and does not involve any other "flow" of heat energy such as conduction or convection. Of course in a "real life" situation, conduction, convection and mass transfer often all occur simultaneously.

Modelling the actual convective flow pattern of the fluid caused by the heat transfer (e.g. the way convection currents might develop in a pan full of water if you heat it on a stove) is much more complicated, and irequires solving some form of the Navier Stokes equations, not just the usual heat transfer (or diffusion) equation. That type of model is ot usually classified as a heat transfer problem, even through heat transfer forms part of it. Because of the existence of turbulence, often there isn't reallly a unique solution to that type of problem, and more assumptions need to be made to get a useful solution that "averages out" the turbulence in some way.

 

[svishal03]

Does it mean that convection refers to a body sorrounded by a STATIONARY fluid whilst mass transfer deals with a moving fluid?

Vishal

 

[AlephZero]

Mass transfer only deals with moving fluids, by definition.

Convection can refer to fluids that are moving or at rest.

Mass transfer is only relevant when the finite element model contains some elements that represent the fluid explicitly. Often, the only thing in a thermal model that represents "convection" is the boundary conditions between a solid and a fluid (Newton's law of cooling).

 

[svishal03]

Thanks a lot, but what is the difference between convection and mass transfer conceptually?

Because, convectionn occurs wherein there is movement of the atoms and molecules in a liquid (whereas conduction in solid is due to the vibration of attoms and moelecules and movement of electrons).

So, in both convection and moving fluid we have movement of atoms and m,olecules.Then how is convection different from mass transfer in this aspect (of movement of atoms and molecules).

Vishal

 

[AlephZero]

You can split the motion of the atoms and molecules into two components.

1. Mass transfer is related to the "global" motion of the fluid. For example if fluid is being pumped through a pipe, the flow velocity and the mass flow rate along the pipe does not depend on the fluid temperature, but only on the pump.

2. Convection is related to the "local" motion of individual atoms and molecules in the fluid, where according to the kinetic theory of heat, the temperature is a measure of the internal kinetic energy of the atoms or molecules in the fluid.