Resumo:
In this work, the finite element method using triangular elements and the Galerkin
formulation is used to study the mixed convection heat transfer in horizontal and vertical ducts.
The flow is considered two-dimensional, developed, laminar and unsteady. The fluid physical
properties are considered constant except the density in the buoyancy terms to witch the
Boussinesq approximation is applied. The fluid used in this work is air (Pr = 0,7). Computational
programs in FORTRAN language were developed to calculate, according to each case studied,
the local and average Nusselt number, the product of the average friction factor by Reynolds
number, the average nondimensional axial velocity and the average nondimensional temperature.
Moreover, the distributions of stream function, nondimensional axial velocity, nondimensional
temperature and vorticity were also calculated. Three mixed convection problems in horizontal
ducts were studied. They are as follows – an horizontal rectangular duct with a heated inner
cylinder, an horizontal rectangular duct with an inclined cross section and a heated inner
cylinder, and a heated horizontal tube with an insulated eccentric inner plate. For the three cases
mentioned previously, heating is realized with an uniform axial heat flux and circumferenttial
uniform temperature. Four cases of mixed convection heat transfer with vertical flow were
studied. They are as follow – a vertical elliptic duct, a bank of aligned tubes between vertical
plates, a bank of aligned tubes in a vertical flow, and a bank of alternate tubes in a vertical flow.
The elliptic duct has uniform axial heat flux and peripheral uniform temperature. In the case of
the tube banks, the tube walls are kept isothermal, the plates are insulated and the entry fluid has
uniform temperature and uniform velocity profiles. The computational programs are validated for
several problems and the differences found are small enough to turn these programs applicable to
the proposed problems.
