Extremely Small Constant Increase of Heat Transfer Coefficient With Concentration

Shomaz ul Haq · June 25, 2017, 06:17

Dear all,

Hello. Hope all are well. I have been simulating laminar nanofluid single phase flow with constant wall heat flux in a pipe for some time now. I have achieved a high quality mesh, did a grid independence study, and have got enhancement in heat transfer based on heat transfer coefficient (h). The enhancement is more at the inlet and decreases along the pipe till the end. The weird thing is I am getting an almost exactly constant increase of just 3-4 W/(m^2.K^-1) when I increase concentration from 1% to 2% to 3% to 4% to 5%. For each % increase in concentration, the increase in h is almost constant and linear. I know it should increase more at higher concentration according to both experiments and other simulations. I have defined density, specific heat, thermal conductivity, and viscosity as a function of concentration by defining a new variable (by copying material water and changing the properties - I didn't change molar mass and thermal diffusivity though). I have defined both local and average heat transfer coefficient in CFD-Post. I would be grateful if anyone could help me out in figuring what am I doing forgetting and/or doing wrong. Somebody might have faced a similar problem. Hope to hear on this matter. Thanks.

Shomaz ul Haq · June 29, 2017, 03:53

Hello everyone. Would be grateful for any help in this regard. Thanks.

evcelica · June 29, 2017, 13:36

Is the thermal conductivity of the second fluid higher? I would expect this behavior if so.

Shomaz ul Haq · June 30, 2017, 10:49

Thanks for the reply Erik. The fluid is a single nanofluid whose effective thermophysical properties are modeled as a function of base fluid properties and concentration. And yes the thermal conductivity of Al2O3 is higher than water. Would be grateful if you could help me find out why there is this strange trend whereas literature says otherwise. Hope to hear. Thanks.

Shomaz ul Haq · July 4, 2017, 15:16

Hey everyone. Hope everyone is well. I would be grateful if somebody could help me figure out why my HTC isn't matching literature values. Would be grateful. Erik or anyone? Thanks.

Shomaz ul Haq · July 7, 2017, 10:23

Hello everyone,

Hope all are well. Please apologize my asking it again and again. I would be grateful if somebody could help me out here. Have tried everything. Haven't got a clue why my results for htc much below the value obtained by other researchers. Anyone? Erik, Glenn, Maxim? Thanks.

ghorrocks · July 9, 2017, 09:06

This is not a question which should be asked on the forum. This is a research question, and will require review of the literature, analysis of your simulation, validation and verification and some careful thought to work out the problem. You cannot do work of this detail over the forum, so we will not be able to help you much.

Have a look at the FAQ: https://www.cfd-online.com/Wiki/Ansy...ible_answer.3F

Especially the links in the FAQ.

We can help you with more specific questions.

June 25, 2017, 06:17	Extremely Small Constant Increase of Heat Transfer Coefficient With Concentration	#1
Shomaz ul Haq Senior Member Shomaz ul Haq Join Date: Aug 2015 Location: Islamabad, Pakistan Posts: 205 Rep Power: 12	Dear all, Hello. Hope all are well. I have been simulating laminar nanofluid single phase flow with constant wall heat flux in a pipe for some time now. I have achieved a high quality mesh, did a grid independence study, and have got enhancement in heat transfer based on heat transfer coefficient (h). The enhancement is more at the inlet and decreases along the pipe till the end. The weird thing is I am getting an almost exactly constant increase of just 3-4 W/(m^2.K^-1) when I increase concentration from 1% to 2% to 3% to 4% to 5%. For each % increase in concentration, the increase in h is almost constant and linear. I know it should increase more at higher concentration according to both experiments and other simulations. I have defined density, specific heat, thermal conductivity, and viscosity as a function of concentration by defining a new variable (by copying material water and changing the properties - I didn't change molar mass and thermal diffusivity though). I have defined both local and average heat transfer coefficient in CFD-Post. I would be grateful if anyone could help me out in figuring what am I doing forgetting and/or doing wrong. Somebody might have faced a similar problem. Hope to hear on this matter. Thanks. ramakant likes this.

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June 29, 2017, 03:53		#2
Shomaz ul Haq Senior Member Shomaz ul Haq Join Date: Aug 2015 Location: Islamabad, Pakistan Posts: 205 Rep Power: 12	Hello everyone. Would be grateful for any help in this regard. Thanks.

June 29, 2017, 13:36		#3
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,186 Rep Power: 23	Is the thermal conductivity of the second fluid higher? I would expect this behavior if so.

June 30, 2017, 10:49		#4
Shomaz ul Haq Senior Member Shomaz ul Haq Join Date: Aug 2015 Location: Islamabad, Pakistan Posts: 205 Rep Power: 12	Thanks for the reply Erik. The fluid is a single nanofluid whose effective thermophysical properties are modeled as a function of base fluid properties and concentration. And yes the thermal conductivity of Al2O3 is higher than water. Would be grateful if you could help me find out why there is this strange trend whereas literature says otherwise. Hope to hear. Thanks.

July 4, 2017, 15:16		#5
Shomaz ul Haq Senior Member Shomaz ul Haq Join Date: Aug 2015 Location: Islamabad, Pakistan Posts: 205 Rep Power: 12	Hey everyone. Hope everyone is well. I would be grateful if somebody could help me figure out why my HTC isn't matching literature values. Would be grateful. Erik or anyone? Thanks.

July 7, 2017, 10:23		#6
Shomaz ul Haq Senior Member Shomaz ul Haq Join Date: Aug 2015 Location: Islamabad, Pakistan Posts: 205 Rep Power: 12	Hello everyone, Hope all are well. Please apologize my asking it again and again. I would be grateful if somebody could help me out here. Have tried everything. Haven't got a clue why my results for htc much below the value obtained by other researchers. Anyone? Erik, Glenn, Maxim? Thanks.

July 9, 2017, 09:06		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,852 Rep Power: 144	This is not a question which should be asked on the forum. This is a research question, and will require review of the literature, analysis of your simulation, validation and verification and some careful thought to work out the problem. You cannot do work of this detail over the forum, so we will not be able to help you much. Have a look at the FAQ: https://www.cfd-online.com/Wiki/Ansy...ible_answer.3F Especially the links in the FAQ. We can help you with more specific questions.