Separation is an important characteristic of the type of flow encountered in many modern heat transfer devices. Design requirements of compactness have resulted in the rapid growth of the use of complex geometrical heat transfer surfaces, which have developed from the single tube and tube bank placed across the line of flow. A single tube or cylinder placed in a cross-flow is completely submerged in the fluid and it therefore forms an obstacle around which the fluid must flow. A boundary layer exists on the cylindrical surface with free stream velocity at its extreme and zero velocity at the wall. However, the free stream velocity increases around the front of the cylinder and at low approach velocities flow within the boundary layer also accelerates. Behind the cylinder free stream and boundary layer flow decelerates again in a more or less reverse pattern. At higher approach velocities the increased velocity around the front of the cylinder which is accompanied by a drop in static pressure is not followed by a similar increase in velocity in the boundary layer, due to the increased viscous stress at the higher velocity gradients. Thus, in the boundary layer the fluid has lost velocity before it starts to decelerate behind the cylinder and it is then opposed by a ‘surplus’ of static pressure which forces the boundary layer away from the surface. Separation, or break-away, results in the formation of turbulent eddies which are carried downstream behind the cylinder. Separation occurs nearer the front of the cylinder as the approach velocity increases, and occurs much more readily in flow over blunt ended obstacles.
Unable to display preview. Download preview PDF.
- 2.Snyder, N. W. Chem. Eng. Progr., Symposium Series, Vol. 49, No. 5, 11 (1953).Google Scholar
- 3.Schenck, H. Jnr. Heat Transfer Engineering, Longmans, Green and Co. Ltd. (1960).Google Scholar
- 5.Douglas, M. J. M. and Churchill, S. W. Chem. Eng. Propr., Symposium Series, Vol. 52, No. 18, 23 (1956).Google Scholar
- 6.Hsu, S. T. Engineering Heat Transfer, D. Van Nostrand Company, Inc., Princeton (1963).Google Scholar
- 7.Colburn, A. P. Trans. AIChE, Vol. 29, 174 (1933).Google Scholar
- 8.Grimison, E. D. Trans. ASNE, Vol. 59, 583 (1937).Google Scholar
- 10.Kays, W. M. and London, A. L. Compact Heat Exchangers, McGraw-Hill Book Company, Inc., New York (1964).Google Scholar