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CFX CHT increased mass flow rate for water matters little for outlet air temperature

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Old   December 4, 2013, 00:58
Default CFX CHT increased mass flow rate for water matters little for outlet air temperature
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Nicholas Lee
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i am doing the conjugate heat transfer simulation using CFX, by using Conservative Interface Flux and Solid timescale fator 100, it can converge normally,

but when i just changed the mass flow rate for inlet water(from 0.15kg/s to 0.3kg/s) with others kept unchanged, the temperature for outlet air hardly changed.

any replies will be appreciated greatly.

attached is the basic structure.
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Old   December 4, 2013, 01:10
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ghorrocks have mentioned that the Imbalance should be concerned,

and i have uplowned the imbalance for my model, could any one help me to get through it of ?
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Old   December 4, 2013, 06:26
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Glenn Horrocks
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Have you looked at the flow in the post processor to see what the differences are?
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Old   December 4, 2013, 06:56
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Quote:
Originally Posted by ghorrocks View Post
Have you looked at the flow in the post processor to see what the differences are?
thank you ghorrocks and sorry for troubling you so much today that quoting your words for many times today.

actually i have not checking the flow in the CFX-post, for i was just concerning about the outlet temperature for air and cooling water and neglecting others.

i will have a check about the flow tomorrow
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Old   December 4, 2013, 07:45
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Quote:
Originally Posted by ghorrocks View Post
Have you looked at the flow in the post processor to see what the differences are?
dear ghorrocks, supposing that the flow situation has been changed, but generally judging the question, there should be some changes for temperature of outlet air and the cooling water, in other words the with the doubled flow speed of cooling water, the temperature of outlet air should be decreased greatly?

am i right?
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Old   December 4, 2013, 16:34
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I do not know the details of what you are modelling so I have no idea what will happen. But I do know that the first thing you should do with any weird result is to look at it in detail with the post processor and try to work out if what you have modelled is real or not, and if real try to understand what is going on.
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Old   December 4, 2013, 18:44
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If the solution has converged correctly, and the exterior of the heat exchanger is adiabatic, the following should be true (conservation of energy in steady state):

Change of massFlowInt(Static Enthalpy) between inlet and outlet of stream 1 = Change of massFlowInt(Static Enthalpy) between inlet and outlet of stream 2

That is, whatever energy leaves stream 1 must end on stream 2. Is this the case ?

Once that is verified, you must know what is the maximum possible heat transfer rate for this heat exchanger (this is a function of inlet conditions and a thermodynamic restriction based on pure physics). If the previous heat transfer rate is near the maximum, you will only see major changes if you change the mass flow of the critical stream (the one that defines the maximum heat transfer rate). You can check details on a standard heat transfer book, for example Incropera & DeWitt.
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Old   December 4, 2013, 20:04
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got it.

it is really kind of you, i appreciate it greatly that your replies were so timely and significant.

Best regards.
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