Paper 16: An Analysis of the Losses in a Radial Gas Turbine

A discussion of the flow pattern in the rotor passages of a radial gas turbine showed that loss data for predicting turbine performance could only be adequately obtained from rotating cascades or turbine tests. The details are given of an analysis of the radial gas turbine tests described by Hiett and Johnston. The analysis showed that the deviation in the nozzle gas leaving angle was dependent not only on the nozzle geometry and gas leaving angle but also on the rotor speed and geometry. Two rotor loss coefficients were obtained, one, ζ _RA , was similar in form to the conventional axial turbine type loss coefficient, the other, ζ _RB , could be used with Wallace's theory for predicting the off-design characteristics of the turbine. The two loss coefficients are shown to be related. The results showed that ζ _RB could be expressed as a function of the gas incident angle and the relative velocity at the rotor tip for each turbine. The conventional loss coefficient ζ _RA had a minimum value at negative incidence to the blade but was nearly constant over a wide incidence range. At high positive and negative incidence the coefficient was strongly dependent on the overall pressure ratio across the turbine.

At zero incidence the two loss coefficients were equal. The tests showed that for this condition the rotor loss coefficients decreased with increase in flow coefficient and with increase in mean acceleration across the rotor. For low mass flow rates there was a rapid increase in the loss coefficient. Finally, it was observed that there was a correlation between the deviation in the nozzle gas leaving angle and the optimum incident gas angle to the rotor.