Heat transfer to a non newtonian fluid through tube

[Neer133]

I wish to know if we can characterize the parameters which influence heat transfer for non newtonian fluid such as shear thinning and thickening fluid through tube. I am working on this to understand the capacity of my heat exchanger, to know its boundaries and limitations on working with different product viscosities.

 

[moltenmetal]

Non Newtonian to me indicates quite viscous, and you're talking about a tube, so the diameter is small. Flow will very likely be laminar, regardless of whether it's shear thinning or shear thickening. In laminar flow in tubes, the Nusselt number approaches a constant value for tubes which are long enough. In tube in tube exchangers, what ends up happening is that to a fairly close approximation, assuming the shell side does not limit and the shellside temperatures are close to constant, you get the same amount of heat transfer per unit length of tube, somewhat irrespective of the diameter of the tube. Increasing the tube diameter decreases the pressure drop and increases the area available for heat transfer, but because the Nusselt number is constant you don't get any benefit from that extra area- the film coefficient drops in inverse proportion to the area such that Q remains the same.

 

[Neer133]

Thanks Moltenmetal.
I have fixed tube diameter and area, I can vary the flow rate of both Media and Product. Our product has to be kept contantly agitated to prevent settling down , in this process we incorporate air in the form of micro airbubble, I want to understand if this air plays a significant role in affecting the viscoisty and thereby affecting heat transfer coefficient. We also face burnon problem where the food particle gets stuck to the walls.

 

[moltenmetal]

When flow is laminar, the layer next to the tube ID remains pretty static- and under those conditions, burning is likely.

High velocities come with a high pressure drop penalty.

Kenics, Sulzer and others make exchangers with static mixers in the tubes. I don't know if these can be used in hygienic/food applications, but you can talk to them about it. A static mixer is a way to keep viscous material from stagnating at the hot wall.

As to the microbubbles affecting the viscosity: I'm sure they do, but have no idea if they increase it or decrease it.

 

[Latexman]

Let you reference A. H. P. Skellend's Non-Newtonian Flow and Heat Transfer.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[EdStainless]

Adding air first reduces the density. So you need the viscosity, heat capacity, and other factors for the aerated version.
In cases that I have seen they had a fluid that shear thinned, and they designed the HX to work well into the thinned region.
Otherwise you have high viscosity, thick boundary layers, and very low and non-uniform heat transfer.

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P.E. Metallurgy, Plymouth Tube