Turbulent fluid flow through low loss bends.

In the Introduction to this thesis the steady flow of Newtonian fluids in duct systems is discussed, with particular consideration to rectangular section bends. The considerable economic savings, both capital and annual that can be obtained by a small reduction in the head loss of an individual system element is indicated. Reliable experimental data on the value of head loss coefficients for incompressible fluid flow through rectangular section bends is very limited and correlation of different experiments is almost impossible because of the different definitions of loss coefficient and the various experimental procedures used. At present theoretical methods of analysing the three-dimensional flow problem occuring in a bend are non-existent and only a limited contribution is made by two-dimensional approaches. Reasons for this situation are discussed together with possible areas for future development. A computer based data logging instrumentation system is used to record the necessary data required to calculate the value of the bend loss coefficient for various combinations of inner and outer radius ratio. The aspect ratio and Reynolds number of 2 and 2 x 105 respectively are representative of a number of practical situations. An indication of the accuracy of the values of bend loss coefficient quoted is given by the corresponding uncertainty interval which is based on that selected, for given confidence limits, in the collected data. For the combinations of inner and outer radii the variable area bends are modelled by a combination of a diffuser, constant area bend and a contraction in the above or reversed order. The calculated value of the model loss coefficient agrees surprisingly well with the experimental values. Based on the extension of the model calculations the optimum geometrical shapes for circular cross-section 90° bends are suggested.