Combustion with and without Viscous Heating

Vallum123 · December 3, 2018, 13:15

Hello!

I am simulating CH4 combustion in a experimental rocket combustion chamber.

I am evaluating the effect of the Viscous Heating Option in the Viscous Model Dialog Box in ANSYS Fluent.

I simulated 2 cases, where the only difference is, that Viscous Heating has been switched on (Case 1) or off (Case 2).

I plotted two contours of the difference (Case 2 - Case 1) between both cases. One for the Eddy Viscosity and one for the temperature. (see attached pictures).

For example in the middle of the chamber the temperature without Viscous Heating is colder. The Eddy Viscosity in this area is higher.

My question is: Why is that?

What does the Viscous Heeaing option change here? What does it do physically?
And follow up: I would have expected that higher eddy viscosity brings higher temperature. But exactly the opposite is the case.

Please can somebody explain this to me.

Kind regards and thank you!

acalado · December 3, 2018, 15:49

Viscous heating is the effect of increase in temperature due to the viscous effect between fluid particles or fluid-wall interface. It should mostly be used for compressible flows or where viscous forces are of importance (lubrication, polymer processing)

By default for incompressible low Mach number flows this is turned off by default.
See below for more info

https://www.sharcnet.ca/Software/Flu...ug/node568.htm

LuckyTran · December 3, 2018, 16:50

I'm guessing if you plot the velocity and mach number, they'll also be higher in the middle.

Vallum123 · December 4, 2018, 10:22

First things first: Thank you for your replies!
I am familiar with the theory on this topic. What I don't understand is:
When do i get more viscous dissipation? When the velocity is low (so near the walls), because the eddys are getting smaller and are dissipated? But this would mean that the temperature near the walls should be getting higher, since, as you said, the dissipation increases the temperature.
And why is the eddy viscosity lower in regions of high velocity? That doesn't make sense to me, because i thought high velocity means high turbulence which means high eddy viscosity?

Where is my mistake in my thoughts? Feel free to ask, if I wasn't clear enough.

LuckyTran · December 4, 2018, 11:47

It's hard to say because you are looking at non-linearities and coupling between equations.

(Eddy) viscosity by itself doesn't generate heat (you need also the velocity gradient or the strain rate tensor). You can imagine the viscosity goes up but the strain goes down and vice-versa.

If you want to see the impact of only the viscous heating term what you can do is run the case without viscous heating. Then turn on the viscous heating option but solve only the energy equation (turn off / freeze the flow and turbulence equations).

December 3, 2018, 15:49		#2
acalado Senior Member André Join Date: Mar 2016 Posts: 133 Rep Power: 10	Viscous heating is the effect of increase in temperature due to the viscous effect between fluid particles or fluid-wall interface. It should mostly be used for compressible flows or where viscous forces are of importance (lubrication, polymer processing) By default for incompressible low Mach number flows this is turned off by default. See below for more info https://www.sharcnet.ca/Software/Flu...ug/node568.htm __________________ Sapere aude!

December 3, 2018, 16:50		#3
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,762 Rep Power: 66	I'm guessing if you plot the velocity and mach number, they'll also be higher in the middle.

December 4, 2018, 10:22		#4
Vallum123 New Member Johnny Banana Join Date: May 2018 Posts: 4 Rep Power: 8	First things first: Thank you for your replies! I am familiar with the theory on this topic. What I don't understand is: When do i get more viscous dissipation? When the velocity is low (so near the walls), because the eddys are getting smaller and are dissipated? But this would mean that the temperature near the walls should be getting higher, since, as you said, the dissipation increases the temperature. And why is the eddy viscosity lower in regions of high velocity? That doesn't make sense to me, because i thought high velocity means high turbulence which means high eddy viscosity? Where is my mistake in my thoughts? Feel free to ask, if I wasn't clear enough.

December 4, 2018, 11:47		#5
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,762 Rep Power: 66	It's hard to say because you are looking at non-linearities and coupling between equations. (Eddy) viscosity by itself doesn't generate heat (you need also the velocity gradient or the strain rate tensor). You can imagine the viscosity goes up but the strain goes down and vice-versa. If you want to see the impact of only the viscous heating term what you can do is run the case without viscous heating. Then turn on the viscous heating option but solve only the energy equation (turn off / freeze the flow and turbulence equations).