Session: 38-01 Radial & Mixed Flow Turbines

Paper Number: 102360

102360 - Windage Loss and Flow Characteristics in Back Gap of Radial Inflow Impellers With Steam Medium 

    The wheel back gap of radial inflow turbines generates disk-type windage loss, and it is necessary to accurately predict and minimize the loss. In this paper, a steady-state numerical analysis for a two-stage radial inflow turbine with steam as working medium is carried out, and a modified model for predicting the windage loss is proposed and validated.

    Firstly, the numerical results show that at the design condition, the windage loss affects obviously the turbine power output, the power of the first stage reduced by 7%, the second stage reduced by 5%. However, the back gap will not only produce windage loss, but also have an impact on the blade's working ability. Compared with the model without back gap, the pressure at the surface of rotor blade leading edge is reduced, and a periodic high-pressure area appears at the leading edge. Furthermore, at 50% rotor blade height of meridional plane, the gap leads to the shrink of the low-pressure area at leading edge of suction surface, which decreases the working ability of the rotor. Moreover, the tangential velocity component near the stator wall no longer tends to theoretical value (zero). The throughflow caused by pressure drop between inlet and outlet affects the axial distribution of the tangential component of velocity in back gap, consequently making the distribution shows a different state from an enclosed disk.

    Secondly, within the back gap, the flow impinges the stator wall and moves outward radially in the form of a wall jet. As the pressure and rotational speed increase, the windage loss in back gap gradually increases. The flow gradually changes from laminar to turbulent, and a core rotation zone generated near the rotor wall of the disk inlet. Such flow has been observed to gradually expand with increasing pressure and rotational speed. Similarly, there is a vortex at the inlet of stator as the velocity increases. It is also found that the vorticity in the whole gap increases with the rotational speed increase. Due to the strengthening of centrifugal force and the decrease of viscous force, it is easier to cause unsteady flow and increase the windage loss. Afterwards, two empirical models from Daily and Coren for calculating windage loss in disk-type gap are validated, respectively. It is observed that Daily and Coren models have maximum deviations of 59.5% and 8.3%, respectively, which suggests Coren model is more acceptable to predicate back gap windage loss of radial inflow turbine.

    Finally, models from Daily and Coren indicate that relative gap (S/R₀) and throughflow Reynolds number (Re_w) are important influencing parameters for windage loss in disk-type gap with throughflow. Therefore, based on the simulation results, a model is proposed to improve the accuracy of windage loss model. And the model containing these two parameters can accurately predict the windage loss with a maximum deviation of 5%. Despite its preliminary character, the model has an important reference value for the design of radial inflow turbomachinery.

Presenting Author: Zhuobin Zhao Xi'an Jiaotong University

Presenting Author Biography: Zhuobin Zhao, Xi’an Jiaotong University master degree, now is a Ph.D. candidate at Shaanxi Engineering Laboratory of Turbomachinery and Power Equipment, Institute of Turbomachinery, Xi’an Jiaotong University, China, under the supervision of Associate Prof. Qinghua Deng. His research topic is focused on the aerothermodynamic of turbomachinery, and the optimization design of supercritical CO2 turbines.

Authors:

Zhuobin Zhao Xi'an Jiaotong University
Qinghua Deng Xi'an Jiaotong University
Lehao Hu Xi'an Jiaotong University
Jun Li Xi'an Jiaotong University
Zhenping Feng Xi'an Jiaotong University