conjugate heat transfer in porous media-variable HTC

hmasenger · June 20, 2014, 02:44

hi all
i am modeling a compact heat exchanger that cools down air temperature by water passing trough rows of coil. a conjugate heat transfer containing two domains (a porous media for air passage involving parameters such as pressure loose in finns installed on coils and HTC ,and one for water passing in coils) has been used .the problem is that i don't know how to set a variable heat transfer coefficient for porous media so it can model the heat flux to water as its temperature is not constant along the passage.i can calculate HTC and pressure loose at finnes by a previous microscopic simulation, but as you know ,as water tempreture in coils varies , the finnes temprature and its HTC will change.so can you guid me how to relate the simplified HTC in porous media (that have gained form varius water tempratures in previous microscopic simulations)to coil's wall temprature or any other coil's parameter?

ghorrocks · June 21, 2014, 06:57

The convective boundary condition does not give constant heat flux. It assumes a heat transfer coefficient which relates the heat transfer to the local temperature difference (wall to fluid). This is usually reasonably general so a constant HTC is good enough for many cases.

hmasenger · June 21, 2014, 07:05

The problem is that HTC should be calculated by a log mean temprature of coils. So how is it posible while i dont know the water temp at coil outlet.? I dont know if you get my point ?

ghorrocks · June 21, 2014, 08:57

Are you confusing heat exchanger design coefficients with the fluid mechanics heat transfer coefficient? The heat transfer coefficient in CFX is purely a local thing - the heat transfer at that part of the interface is just a function of the HTC and local temperature difference. I realise heat exchanger design often uses global parameters with inlet and outlet temperatures, but that is not what CFX uses for its HTC.

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June 21, 2014, 06:57		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	The convective boundary condition does not give constant heat flux. It assumes a heat transfer coefficient which relates the heat transfer to the local temperature difference (wall to fluid). This is usually reasonably general so a constant HTC is good enough for many cases.

June 21, 2014, 07:05		#3
hmasenger Senior Member hamed Join Date: Apr 2009 Posts: 148 Rep Power: 17	The problem is that HTC should be calculated by a log mean temprature of coils. So how is it posible while i dont know the water temp at coil outlet.? I dont know if you get my point ?

June 21, 2014, 08:57		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Are you confusing heat exchanger design coefficients with the fluid mechanics heat transfer coefficient? The heat transfer coefficient in CFX is purely a local thing - the heat transfer at that part of the interface is just a function of the HTC and local temperature difference. I realise heat exchanger design often uses global parameters with inlet and outlet temperatures, but that is not what CFX uses for its HTC.