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Computation of heat per unit volume, BUBBLY FLOW, HEAT TRANSFER

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I have a problem with phase change.

I have modelled it using bf (turbulent bubbly flow) and ht (heat transfer). Under bubbly flow is the exciting part: Mass Transfer. Since this is in a vessel which is only being heated, therefore there is only mass transfer from liquid to gas, I could put something in the "mgl" box to represent the mass transfer.

Over in the heat transfer in solids section, there is a temperature field available (T). I can compare this to the boiling point, and use the difference from boiling point over the specific heat and the latent heat of evaporation to represent the mass transfer:

if(T>tBoil,-(T-tBoil)*cpWater/hfgWater*mat2.def.rho,0)

But this is in units of kg/m3, and I need kg/m3/s. Obviously the /s part comes from the rate of heat flux. So this approach doesn't work.

I check over in the ht section and see that there is a variable available, ht.ntfluxInt which has units of W. So now if I look at what I can produce,

if (T>tBoil,-ht.ntfluxInt/hfgWater,0)

Still, no good. In this case, ht.ntfluxInt has been normalized against a unit volume, but hfgWater has not: I get units of kg/s when I need units fo kg/m3/s. And it doesn't make any sense to multiply this by the density, either.

(To lower the temperature, I was going to put the corresponding inverse of this equation in the HeatSource node under ht.)

Any help out there? What's the right approach in COMSOLand to get the units necessary? Is there a specific volume or a specific unit of time to use? Or a completely different approach? I did also look at reacf... interestingly reacf(ht.ntfluxInt) gives me exactly the units I want but I can't say that it's actually the right physical quantity.

Thanks.

4 Replies Last Post 29 juil. 2016, 16:59 UTC−4

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Posted: 9 years ago 10 sept. 2015, 19:49 UTC−4
I should mention that this was done using the time dependent solver.
I should mention that this was done using the time dependent solver.

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Posted: 9 years ago 11 sept. 2015, 10:42 UTC−4
Strangely, Equation View shows that bf.res_rhogeff, with units of Hz, includes the term bf.mgl, which comes directly from Mass Transfer, and has units there of kg/m3/s.
Strangely, Equation View shows that bf.res_rhogeff, with units of Hz, includes the term bf.mgl, which comes directly from Mass Transfer, and has units there of kg/m3/s.

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Posted: 9 years ago 15 sept. 2015, 16:03 UTC−4
I will try using the bdf operator...

if(T>tBoil,-bdf(T-tBoil,1)*cpWater/hfgWater*mat2.def.rho,0)
I will try using the bdf operator... if(T>tBoil,-bdf(T-tBoil,1)*cpWater/hfgWater*mat2.def.rho,0)

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Posted: 8 years ago 29 juil. 2016, 16:59 UTC−4
Hi,

Did you succeed in formulating with phase change (liquid to gas) in bubbly flow model? I would appreciate if you can share your insight.

Regards,
Susant
Hi, Did you succeed in formulating with phase change (liquid to gas) in bubbly flow model? I would appreciate if you can share your insight. Regards, Susant

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