STUDY OF MIXING AND AUGMENTATION OF NATURAL CONVECTION HEAT TRANSFER BY A FORCED JET IN A LARGE ENCLOSURE

摘要

This paper presents experimental study of gas mixing process and heat transfer augmentation by a forced jet in a large enclosure. In this experiment, a cylindrical enclosure was constructed with a horizontal copper plate installed at the bottom to provide an isothermal heating/cooling surface. Cold/hot air was injected at several positions with varying pipe diameter and injecting orientation, and was removed from the top of the enclosure. Under natural convection, the mean Nusselt number inside the large enclosure with heating surface can be correlated by the enclosure Rayleigh number Ra_D~(1/3), and under strong forced convection, by the jet Reynolds number Re_j~(2/3). A combining rule for mixed convection is used to balance the contributions from the separate correlations, given by Nu_D~n = Nu_(fc)~n +- Nu_(nc)~n. The positive sign represents the destabilizing effect of buoyancy force (heating surface), and negative sign represents the stabilizing effect of buoyancy force (cooling surface). By using the ratio of forced and free convection Nusselt numbers, this form can be further reduced to predict the augmentation ratio as a function of Archimedes number Ar=Re_j~2/Gr_D and a correcting factor accounting for jet orientation and enclosure geometry.