Abstract
The performance of a radial flow turbine, operating on compressed air, is measured over a range of pressure ratio and speed. Separate tests on the nozzle assembly alone enable the rotor performance to be separated from the overall characteristics. In particular, the conditions at each pressure ratio for which the nozzle exit whirl velocity is equal to the rotor tip speed may be deduced. Also, it is possible to evaluate the enthalpy ‘loss’ in the rotor, that is, the difference between the specific enthalpy drop actually occurring in the rotor and its isentropic value.
An attempt is then made to relate this loss with the relative velocities within the rotor passages as predicted by inviscid, two-dimensional analytical techniques. The method developed by Stanitz and co-workers has been programmed to analyse the rotor internal fluid flow properties in the blade-to-blade plane for the conditions of zero incidence. The region investigated does not include the whole rotor, but extends from a boundary upstream of the blade tip to a downstream boundary at a radius equal to 72 per cent of the tip radius. The analysis is performed for overall turbine static to static pressure ratios of 1·2 to 1·6.
The result of the theoretical analysis is to show that the variable ratio of rotor outlet to inlet specific volume with changing pressure ratio is accommodated almost entirely without any change in the flow distribution; that is, over the region of the impeller investigated, and at overall turbine pressure ratios up to 1·6, the compressible flow through the rotor may be considered as being through stream tubes of fixed geometry. There is thus a sound basis for the correlation of rotor loss coefficient in terms of parameters which describe compressible flow with friction along variable area pipes. Consideration is given to the manner in which the zero incidence loss coefficient may be defined in order to remain constant at varying pressure ratios.
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