Bestimmung des Wandwärmeübergangskoeffizienten von durchströmten Kugelschüttungen

The prediction of the temperature distribution in a packed bed requieres the knowledge of the effective thermal conductivity in the core and the wall heat transfer coefficient. Empirical correlations published previously yield strongly differing results varying by a factor of ten at low Reynolds numbers. Very high Reynolds numbers had not yet been investigated. Wall heat transfer coefficients in pebble beds were achieved by applying the analogy between heat and mass transfer on sublimation of napthahlene into air. The use of mass transfer instead of heat transfer techniques provides some advantages concerning secondary effects. Two series of testsapplying napthalene mass transfer were performed in the ranges of Reynolds Rnumbere5 $\cdot$ 10$^{1}$ $\le$ 6 $\cdot$ 10$^{3}$ and 2 $\cdot$ 10$^{3}$ $\le$ Re $\le$ 2 $\cdot$ 10$^{4}$. Moreover, the mass transfer in the inlet section was investigated. The results of both test series match well. A good agreement is observed with earlier published data, which were not affected by entrance effects. The data are correlated for the range of Reynolds numbers 5 $\cdot$ 10$^{1}$ $\le$ Re $\le$ 2 $\cdot$ 10$^{4}$. The transition effect observed at Re $\approx$ 4 $\cdot$ 10$^{3}$ is explained by a change of the flow structure. A correlation is given for the mass and heat transfer in the entrance section