Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.
In this paper, a numerical study is carried out to characterize
the transient local heat transfer coefficient at the fluid-solid
wall interface of a solar collector. For that purpose, the
considered collector wall geometry is a flat plate with nonnegligible
thickness which is subjected to a variable solar heat
flux. The transient conjugated conduction-convection heat
transfer has been taken into account. The heat transfer
coefficient is calculated as a function of the plate thickness as
well as the position along the plate. A good agreement has been
found between the calculated temperatures and other
experimental results. The heat transfer coefficient evolutions, as
a function of time, have been obtained for various positions
along the plate. The results showed that at first, high values of
the heat coefficient are reached, and then it decreases and tends
to constant values. It has been also noticed that at a fixed value
of time, the heat transfer decreases when the position is
increased from the beginning of the plate towards its end. The
parametric study allowed obtaining a correlation of the
transient convective heat transfer coefficient as a function of
the steady state coefficient (which depends on the flow velocity
and the coordinate of the considered point on the plate),
multiplied by a function of time and the plate properties. The
results have been used to optimize the heat transfer coefficient
measurement technique using the pulse method. The pulse
method consists in imposing a heat flux on a wall, and then to
calculate, by an inverse method, the heat transfer coefficient
from the time evolution of surface temperature (thermo-gram).
Measurement of the heat transfer coefficient is based on the
introduction into the inverse model of a function that represents
the theoretical evolution of this coefficient due to the energy
excitation. This function is deduced from the numerical study
conducted in this work.
