Experiments and CFD Analyses on Condensation Heat Transfer in a Square Cross Section Channel

Full text of publication follows: Condensation in the presence of noncondensable gases has a well known relevance in industry; in particular, it is important for the transient analysis of containment loads in light water reactor plants of both commercial and innovative design. The phenomenon has been thoroughly studied in the past from both the theoretical and the experimental points of view. In particular, the University of Pisa has participated in past activities regarding the condensation phenomena like the support to the safety analysis of the AP600 reactor, International Standard Problems, severe accident research in the SARNET EU Project, etc.. Recently, an experimental facility has been set up at the University of Pisa in the frame of these studies, to provide original condensation data to be compared with CFD prediction. In this aim the test section of the facility has been conceived with a simple geometry, consisting in a 2 m long square channel with a side of 34 cm. The channel has one of the sides consisting in an aluminium 4.5 cm thick plate, cooled on the back surface by water. The channel is enclosed inside a semi-cylindrical aluminium shell welded to the plate, thus forming a vessel which can withstand moderate pressures. The test section is included within a closed loop, in which a blower provides the necessary head for circulating the mixture of air and steam and a steam generator reintegrates the amount of water vapour needed to compensate condensation. Present experimental tests have been performed at atmospheric pressure, established in the plant by the introduction of a pipe connecting it to the outer environment. In such conditions, the electrical power fed to the steam generator, the secondary coolant temperature and the air-steam mixture velocity define the heat and mass transfer conditions, hence establishing the vapour concentration at the inlet of the test channel. Therefore, variation of these independent parameters allows obtaining various operating conditions. Thermocouples placed within the thickness of the thick aluminium plate allow the measurement of the heat flux distribution along the length and the width of the plate itself. In addition the collection and measurement of the overall condensate flow provides integral data on heat transfer to be compared with those obtained by the energy balance on the secondary fluid. The obtained results have been analysed making use of the FLUENT computational fluid-dynamic code. The model developed in this purpose involves the simulation of heat conduction within the plate as well as of heat and mass transfer in the channel in 2D geometry. The two-dimensional approach has been chosen after previous 3D calculations had shown no relevant difference with respect to 2D ones. On the other hand, the inclusion of the plate in the calculation domain allows for a clear definition of the boundary conditions in terms of secondary coolant convective heat transfer coefficient and temperature. In the paper both the experimental data and the obtained calculation results are reported. A good match between them is been observed, representing a very encouraging result which support the adequacy of the adopted code models. The comparison of the experimental data with engineering correlations provides further interesting information about the phenomenon of condensation in the addressed conditions. (authors)