[CellZone]

Hello,

I do not really get the idea behind the heat transfer coefficient:

q=htc*(T_Wall - T_Fluid)

If I have a CFD simulation and I have a for example a heated block:

a) with T_Wall = 40°C

b) with T_Wall = 45°C

The flow field with T_Fluid = 20°C in the simulation remains the same (yes there are some effects due to the fact, that the warmer block influences the air arround it) , also the mass flow (enthalpy) entering the system remains constant.

Is the htc in both cases the same?


So if I solve for htc=q / (T_Wall - T_Fluid)
-- > then q remains the same, according the energy balance in cfd
-- > T_Fluid is nearly the same
-- > T_Wall is different

-- > so the htc is slightly changeing

But I thought the htc does mainly depend on the flow field (laminar/turbulent) and not the surrounding temperature.

Can someone help me?

Thank you!

[Klay]

These are coefficiants that engineers have balanced to try to account for the reality of heat transfer: D.

The coefficiant, denoted h, depends on the state of the surface, the velocity of the fluid and other factors, but it is often considered as invariant to facilitate calculations and allows a good approximation in some cases.


The determination of this convective flow makes it possible, for example, to quantify the amount of heat exchanged between a hot wall and the ambient air. I inject a fluid at a rate Qm and a temperature T input in an isolated system (no heat loss) consisting of a radiator for example. And I make it resort to another side by aspiring (constant density). At what temperature T output, will my fluid come out?


I know that the total energy exchange according to the thermodynamics is, at constant pressure: Q = Qm * Cp * [T output - T input] and I know that my fluid has gained heat through the radiator by convection Q = h * [T radiator-T input ].


It allows you to calculate that kind of thing. What you must remember roughly transfers heat is that the transfer is always from the hottest to the coldest.
And the amount of energy required to raise a fluid at a constant pressure by one degree is not the same for all fluids.

We find the same expressions for transfers of matter, it goes always from the most concentrated solution to the least concentrated (Fick's law).

Tell us a little more about the law of Fourrier, Newton and the phenomena of transport of heat, there are many books that will explain to you that me ^

[LuckyTran]

Originally Posted by CellZone

So if I solve for htc=q / (T_Wall - T_Fluid)
-- > then q remains the same, according the energy balance in cfd
-- > T_Fluid is nearly the same
-- > T_Wall is different

q will not be the same because it depends on T_wall. When you change the T_wall, you will get a different q. I'm not sure why others did not point this out sooner, sorry.