This paper presents the results of numerical simulation of flow pattern in cyclones of different height, which have recently been tested for separation efficiency without any information about flow field. Results show that the difference between tangential velocity in the upper cylinder and in the lower cone of a certain cyclone is not significant, suggesting that reduced cross-section in the cone does not lead to tangential velocity acceleration. Tangential velocity in cyclones of different height is quite different. Tangential velocity decreases with increasing cyclone height. and this should be responsible for the lower separation efficiency observed in long cyclones. However, if there exists considerable shortcut flow at the entrance of the gas exit tube, the higher tangential velocity will not give rise to the higher separation efficiency, for example, in the case of the shortest cyclone covered in this study, whose gas exit tube protrudes into the conical part. (c) 2004 Elsevier B.V. All rights reserved.