Hi,
Is there any criterion (e.g. a non-dimension number) to find out which heat transfer mechanism is dominant in a phenomenon?
I've solved a heat transfer problem and now I want to know which one is dominant between "conduction" and "convection"?
ِDear Ali Minaeian
You can use of " Nusselt number" and " Prandtl number".
If Nu and Pr >1 expressive heat transfer of convection is more effective and if Nu and Pr
Hi Thomas,
Thanks for your reply.
Can I conclude that Prandtl number is a more general criterion to answer this question?
Beacuse Pr is convection versus conduction, and can be used for both free or forced convection.
Regards,
Ali
I understand that you are asking for Identification of dominant mode of heat transfer.
It is dynamic and various factor dependent such at start of heat transfer (@ t=0 ) Conduction only dominating. Similarly unitless number depends on the various properties of streams. Such as high velocity dominant unit less number will be diffrent than at low velocity etc. I hope HT book such as DQ Kern can help you out
Simply, look at your Prandtl number value
Mainly, in fluids, convection is the heat transfer mechanism that involves two effects; heat diffusion from particles to their adjacent or neighboring other particles (conduction) in addition to the advection which is the heat transferred by the fluid flow (free or forced motion). These two parameters are concluded in Prandtl number as a ratio between the momentum diffusivity and thermal diffusivity. So, for Pr
Convection = conduction + advection
Nusselt number is the ratio between CONVECTION to CONDUCTION.
If the Nu = 1; it mean that heat transfer is by conduction ONLY and no advection.
If Nusselt = 2 the advection term is equal to conduction term.
if Nu >2 the heat by advection is higher than the heat by conduction.
I think you should use Nusselt number as a criteria for that issue.
Hi ali,
The Pr number is just the ration of viscosity over thermal diffusivity of your fluid, you can not conclude which mechanism is dominant without taking into consideration your thermal and geometrical configuration (via Gr or Ra). The Nusselt number is he right choice for a heat transfer between a fluid and a solid wall. Peclet number is for the heat transfer within a fluid in motion.
Hi Idir,
Yes, but Pr gives a qualitative criterion for the comparison of these two heat transfer mechanism.
Regards,
Ali
yes but only if you keep the same thermal configuration, or your working with a non dimensional model and you keep Gr or Ra constant. where are you looking to characterize the mechanism of transfer?
Hi all,
I agree with Mohamed H. Shedid and Idir Hamzaoui .
The Pr-Number is a material property, which is e.g. for air at ambient conditions constant (Pr = 0.71). But that doesn't mean that air is always conduction dominated. For heat transfer by natural convection over a small slit, conduction can be dominant, while in a large enclosure, natural convection will probably become dominant. For heat transfer in a tube at low flow velocity, heat conduction can be dominant, while at higher velocities, forced convection will at some point become dominant. In general, the heat transfer mechanism depends on material properties, boundary conditions and dimensions.
As it was already said, convective transport inside a fluid is judged with the Peclet Number, which can be determined from the Reynolds and Prandtl numbers: Pe = Re*Pr.
The ratio of convective to conductive heat transfer at a wall is determined by the Nusselt number.
Finally, you can judge natural convection a priori with the Grashof or Rayleigh number (Ra = Gr*Pr) and a threshold value given by experiments.
Best regards,
Julian
Dear Ali Minaeian .. I think that Dr. Mohamed H. Shedid, Dr. Idir Hamzaoui and Dr. Julian Vogel are right about Nu as an indication for which is more dominant, convection or conduction, rather than Pr which only gives information about the fluid type and the ratio between the thickness of both the thermal and hydrodynamic boundary layers .. Good to see such valuable comments .. Thank you all.
ِDear Ali Minaeian
You can use of " Nusselt number" and " Prandtl number".
If Nu and Pr >1 expressive heat transfer of convection is more effective and if Nu and Pr
Hello Ali Minaeian and all,
I am currently working on jet impingement cooling of surfaces, which involves both modes of heat transfer (Conduction & Convection).
I have found that a based on a single dimensionless number it will be difficult to explain which mode of heat transfer will be dominant.
I am agree with Dr. Mohamed H. Shedid , Dr. Idir Hamzaoui and Dr. Julian Vogel . Coming to a conclusion based on a single number is not advisable.
We may need to calculate Nusselt number (Nu), Prandtl number (Pr), Peclet number (Pe), Biot number (Bi), Grashof number or Rayleigh number (Ra) to understand the heat transfer behavior of our problem.
We need to apply boundary conditions.
Dear Kourosh,
Yes, I agree with you.
However, I think these are qualitative criteria and not exact.
If you solved it numerically then you can track the stream line velocity. It will give you an indication.
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