Thermal diffusion coefficient（about Soret effect）

sasenbb · November 23, 2011, 01:23

Hi,

Recently I'm doing my project about Rayleigh-Benard convection in binary mixtures.
In my work, the Soret effect(thermal diffusion) is considered. In the equation of Soret coefficient ,St=Dt/D, where St is Soret coefficient (1/k), Dt is thermal diffusion coefficient(m*m/s*k) and D is mass diffusivity(m*m/s) . Here comes the problem: when I try to input the thermal diffusion coefficient, I found the unit of thermal diffusion coefficient in FLUENT is (kg/m*s), not the same as (m*m/s*k). For example, the thermal diffusion coefficient Dt I want to input is -1.143e-11(m*m/s*k), so how much should I input in the unit of (kg/m*s)?

Any help would be appreciated!

Thanks, Sasenbb.

Amir · November 23, 2011, 05:05

Dear 李炳威,

Any diffusion coefficient dimension is (m2/s); like momentum (nu), thermal (alpha) and mass (D) coefficients. In FLUENT when the dimension required is (kg/m.s), you have to use (rho* diffusion coefficient) which rho is base fluid density in which the property would be diffused. But as I know, thermal diffusion coefficient is alpha and its dimension is (m2/s) not (m2/s.k)!!!

Bests,

sasenbb · November 23, 2011, 09:32

Thanks Amir for the response.But the thermal (alpha) diffusion coefficient you mentioned which dimension is (m2/s) is not the same parameter as I mentioned. Usually we call the thermal (alpha) diffusion coefficient (m2/s) thermal diffusivity, and call the Dt in Soret effect thermal diffusion coefficient(m2/s.k). And this can be found in many paper about Soret effect(thermal diffusion). And according to the Flunet 6.3 user's guide, thermal diffusion coefficient should be (m2/s.k), but when comes to the input interface, it change into (kg/m.s).

Amir · November 23, 2011, 11:02

Quote:

Originally Posted by sasenbb

Thanks Amir for the response.But the thermal (alpha) diffusion coefficient you mentioned which dimension is (m2/s) is not the same parameter as I mentioned. Usually we call the thermal (alpha) diffusion coefficient (m2/s) thermal diffusivity, and call the Dt in Soret effect thermal diffusion coefficient(m2/s.k). And this can be found in many paper about Soret effect(thermal diffusion). And according to the Flunet 6.3 user's guide, thermal diffusion coefficient should be (m2/s.k), but when comes to the input interface, it change into (kg/m.s).

Are you sure?! I just find one definition in manual; Eq. 8.9-1, and as I said before, its dimension is: kg/m3 * m2/s = kg/m.s
According to this equation, maybe you're considering D/T as a new coefficient! is it the case?!

Bests,

sasenbb · November 23, 2011, 22:14

Thank you again Amir, I think I get it. According to <The soret effect: A review of recent experimental results>Eq.(1) ,and Eq. 8.9-1 in the Fluent 6.3 user's guide, DT,i in the Eq. 8.9-1 is not the parameter in the St=DT/D. And DT,i = rho*DT*C0(1-C0)*T, C0 is the mass fraction , DT,i(kg/m.s) DT(m2/s.k)

November 23, 2011, 01:23	Thermal diffusion coefficient（about Soret effect）	#1
sasenbb New Member 李炳威 Join Date: Nov 2011 Posts: 3 Rep Power: 3	Hi, Recently I'm doing my project about Rayleigh-Benard convection in binary mixtures. In my work, the Soret effect(thermal diffusion) is considered. In the equation of Soret coefficient ,St=Dt/D, where St is Soret coefficient (1/k), Dt is thermal diffusion coefficient(mm/sk) and D is mass diffusivity(mm/s) . Here comes the problem: when I try to input the thermal diffusion coefficient, I found the unit of thermal diffusion coefficient in FLUENT is (kg/ms), not the same as (mm/sk). For example, the thermal diffusion coefficient Dt I want to input is -1.143e-11(mm/sk), so how much should I input in the unit of (kg/m*s)? Any help would be appreciated! Thanks, Sasenbb.

November 23, 2011, 05:05		#2
Amir Senior Member Amir Join Date: May 2009 Location: Shiraz, Iran Posts: 719 Blog Entries: 1 Rep Power: 12	Dear 李炳威, Any diffusion coefficient dimension is (m2/s); like momentum (nu), thermal (alpha) and mass (D) coefficients. In FLUENT when the dimension required is (kg/m.s), you have to use (rho* diffusion coefficient) which rho is base fluid density in which the property would be diffused. But as I know, thermal diffusion coefficient is alpha and its dimension is (m2/s) not (m2/s.k)!!! Bests, __________________ Amir

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November 23, 2011, 09:32		#3
sasenbb New Member 李炳威 Join Date: Nov 2011 Posts: 3 Rep Power: 3	Thanks Amir for the response.But the thermal (alpha) diffusion coefficient you mentioned which dimension is (m2/s) is not the same parameter as I mentioned. Usually we call the thermal (alpha) diffusion coefficient (m2/s) thermal diffusivity, and call the Dt in Soret effect thermal diffusion coefficient(m2/s.k). And this can be found in many paper about Soret effect(thermal diffusion). And according to the Flunet 6.3 user's guide, thermal diffusion coefficient should be (m2/s.k), but when comes to the input interface, it change into (kg/m.s).

November 23, 2011, 22:14		#5
sasenbb New Member 李炳威 Join Date: Nov 2011 Posts: 3 Rep Power: 3	Thank you again Amir, I think I get it. According to <The soret effect: A review of recent experimental results>Eq.(1) ,and Eq. 8.9-1 in the Fluent 6.3 user's guide, DT,i in the Eq. 8.9-1 is not the parameter in the St=DT/D. And DT,i = rhoDTC0(1-C0)*T, C0 is the mass fraction , DT,i(kg/m.s) DT(m2/s.k)