The penetration lengths of a jet issuing from upward and downward injection nozzles were measured in a dense fluidized bed of Geldart A to Geldart B particles, operated at superficial velocity well beyond the minimum bubbling velocity. Nozzle orientation, injection velocity and injected gas density were found to be the parameters having the most influence on the jet penetration lengths. Three distinct jet penetration lengths were determined for the upward nozzle: L-min, L-max and L-b, in accordance with Knowlton and Hirsan's (1980) definition [1], while for the downward nozzle, only L-min and L-max were observed. The jet penetration lengths were correlated with respect to dimensionless groups in a systematic approach in an effort to identify the most important terms. For each nozzle orientation, the analysis yielded unique correlation format which could be applied to each characteristic jet length by changing the correlation parameters. Fundamental distinctions between the upward and downward nozzles were uncovered. The mechanism responsible for the jet momentum dissipation was found to be gravitational forces acting on the jet volume for the upward nozzle and drag forces exerted on the entrained particles for the downward nozzle. Five new correlations were derived for the prediction of the characteristic jet lengths for upward and downward nozzles. The correlations retained for the upward nozzle were also found to be in good agreement with data from a high pressure fluidized bed. (C) 2012 Elsevier B.V. All rights reserved.