The present study deals with a meanline and a throughflow analysis for loss prediction of mixed and axial turbomachines, respectively.
The flow field in axial turbines is extremely complicated in that it contains unsteady, three-dimensional and rotational effects. The unsteadiness due to stator-rotor interaction plays a significant role in determining the turbine stage flow. Though unsteady numerical analysis with a three- dimensional Navier-Stokes code for a turbine stage may be executed, it requires too much calculation cost. Therefore, a simple and fast flow analysis with a good accuracy is still essential for the turbine design at the initial design phase. In this sense, it is necessary to formulate models that enable reasonable estimates of various unsteady, three-dimensional viscous effects on the turbine performance.
Although Park and Chung's deviation model is good for prediction of stator downstream flows, it yields great discrepancies for rotor downstream flows because it is based on the classical secondary flow theory without consideration of the unsteadiness and because it takes no account of the distribution of the deviation angle and the loss coefficient due to the tip leakage flow. In the present study, a simple empirical deviation model of the rotor downstream flow has been developed so that it may be used in the streamline curvature method for flow analysis of single- stage, subsonic axial turbines with wide ranges of turning angle, aspect ratio and blading type.
New models to distribute deviation angles and pressure loss coefficients due to the tip leakage flow have been proposed to be used in association with the streamline curvature method as a throughflow analysis. Deviation angle modeling is based on Lakshminarayana's theoretical concept that the leakage flow originating from the tip clearance along the chord forms vortex layers which roll up into spiral to form a core of rotating fluid below the suction surface and inboard of the blade tip.
Spanwise mixing is included in the present throughflow calculation. Both convective secondary flow and turbulent diffusion are considered in the spanwise mixing process.
For rotor exit flow analysis, a tip leakage loss coefficient distribution model and a spanwise mixing model are incorporated into the streamline curvature method. The calculated performance results are compared with those of five test turbines of which geometries and experimental data have been published in the open literature. By applying these models to these five test turbines, the spanwise variations of flow angle, axial velocity and loss coefficients at rotor exit are predicted with good accuracy, being comparable to a steady three-dimensional Navier-Stokes analysis.
Also, in order to improve the performance prediction of mixed-flow pumps, a new separation loss model and a modified recirculation loss model are proposed in the present study. The meanline analysis incorporating with the proposed loss models was used to predict the performance of four mixed-flow pumps with different specific speeds. The present prediction method is also compared with that based on two-dimensional cascade theory. Predicted performance curves by the proposed loss models agree well with experimental data in the normal operating range.