and I is the intensity of solar radiation.
A study on the influence of cross section shape of heat pipe was
carried out by Hussein et al. [49]. They analyzed three groups of
heat pipe having circular, elliptical and semi-circular cross section.
Systems were tested using water as working fluid at filling ratio of
10%, 20% and 35%. Two groups of experiments were carried out:
the first one, at fixed mass flow rate of cooling water and low inlet
temperature, in order to investigate on instantaneous performance
of the wickless heat pipe solar collectors during a day; the second
group of experiments regarded efficiency, at two mass flow rates
and different inlet cooling water temperatures, at quasi-steady
state condition around solar noon.