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approach to heat transfer coefficients

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Old   September 30, 2004, 05:25
Default approach to heat transfer coefficients
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The problem with usage heat transfer coefficient obtained from CFD

q = h (Tnw Tw) (1)

is that h is useless outside CFD. The problem is that Tnw is a function of Tw. This way to calculate flux one has to know not only HTC but Tnw as well ( in other words for each wall temperature CFD simulation have be run). It is inconvenient for practical tasks. It looks that there is more convenient way to use CFX results.

Let there are exist such local h T that next equation will be true for a range of wall temperature values Tw:

q = h (T Tw i.e. h T are independent of Tw. (2)

The system of two equation may be written down :

q1 = h (T Tw1) (3)

q2 = h (T Tw2) (4)

The solution is:

h = (q1 q2)/( Tw2 Tw1); (5)

T = Tw1 + q1/ h (6)

It may be presented as a strait line in (T, Q,) plain, that passed through points (q1, Tw1) (q2, Tw2) and crossed X-axes in T=Te point.

This idea was checked on case of gas turbine nozzle vane simulation. Tree cases was ran , T and h values were calculated with two cases results and verified with third case. Calculated T and h values remains stable in 300C range of wall temperature.

Next check was done by direct definition of T with CFD simulation. The wall BC was defined as q=0 (adiabatic wall). For this case T= Tw= Tnw.

The found Tw and Tnw distributios along vane cross-section curves well coincide with calculated T values.

Does anyone see hidden problems in solution mentioned above ?

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