Turbine is one of the core parts of turbocharger, usually using open or semi-open impeller, this is due to the simple structure, good strength and rigidity.
However, due to the gap between the blade top and the turbine shell, the gap leakage loss is large. Especially in small impellers, the relative blade top clearance is higher and the loss is greater.
Studies have shown that the loss caused by the blade top clearance in the axial turbine accounts for more than one-third of the total loss, which greatly affects the turbine efficiency and life.
And with the advancement of technology, 3D printing technology has gradually matured, providing the possibility for the processing of complex runners, and small closed complex runner impellers have also received attention from some people.
Firstly, the preliminary performance and strength calculation of this turbine is carried out, and for the 60kW gas turbine turbine stage designed, under the condition of an inlet temperature of 1350°C, the turbine efficiency is preliminarily calculated to be 81.2%, and the cycle efficiency is 38.03%, which is higher than that of conventional micro gas turbines.
This is a solution for miniaturization of gas turbines, which solves the problems of small turbines that are too compact to make it difficult to arrange blade cooling devices, and higher blade top clearances that make the leakage account for a larger proportion.
First, the rotor system airflow excitation dynamic analysis
1. Analysis of fluid excitation dynamics of fluid intersection of streamlined tunnel turbine rim
The streamlined tunneled turbine model is established based on the provided parameter relationships, as shown in Figure 1. The flow mechanism within the wheel flange clearance (RC) is closer to the sealing clearance, and the excitation force generated is more uniform than that of the blade turbine because it is not affected by the internal flow channel.
The difference between tunnel turbine flange clearance and sealing gap is that the former is irregularly shaped, making it difficult to directly calculate the stiffness and damping coefficient. However, the rotor vortex trajectories are similar, as shown in Figure 2.
The rotor rotates at speed ω, the axis is Or, the vortex velocity is Ω, the vortex trajectory is approximately a circle, the center Os is the geometric center of the turbine shell, and the radius is the vortex eccentricity e.
The tunnel turbine flange gap excitation force can be calculated by CFD method, if the motion under a fixed coordinate system (as shown in Figure 2(a)) is used, the dynamic grid needs to be used for transient calculation, and the rotational coordinate system (as shown in Figure 2(b)) can be applied to solve the problem by steady-state calculation, which greatly saves computing resources and shortens the calculation time, and the rotation speed of the coordinate system is Ω vortex velocity.
Second, the rotor system model
Taking a certain type of turbocharger as the research object, the three-dimensional model of the rotor was established by using Creo software, and the rotor dynamics calculation was carried out by Samcef software.
1. Basic parameters of rotor
The overall structure of the rotor system is shown in Figure 3. The support bearings are ball bearings, with two sealing rings installed at the compressor end and one sealing ring installed at the turbine end. At the same time, a tunnel turbine rotor model was established, as shown in Figure 4.
Due to the smaller flow area of tunnel turbines, they are amplified proportionally compared to blade turbines. The common working speed of this type of supercharger is about 140000r/min-1, and it works between the second-order and third-order critical speeds.
In terms of material selection, the SiC ceramic used in the model and the tunnel turbine of K418 superalloy with the same K418 superalloy as the original turbine were considered to study the influence of turbines of different structures and materials on the dynamic characteristics of the rotor system. The material properties of each part of the rotor system are shown in Table 1.
In the imbalance response analysis, the center of gravity of the compressor and the center of gravity of the turbine add an unbalanced mass respectively, in which the maximum allowable unbalance at the compressor end is 0.4g∙mm, and the maximum allowable unbalance at the turbine end is 0.55g∙mm.
2. Rotor stiffness and damping parameters
Fluid excitation can be reflected by stiffness and damping, where cross stiffness and main damping coefficient have a great influence on rotor stability. The stiffness and damping coefficient of the sealing fluid and tip gap excitation in the blade turbine rotor are shown in Tables 2 and 3.
For the tunnel turbine, the gap mesh models with clearances of 0.2, 0.4, 0.6, 0.8 and 1mm were established, and the excitation force of the wheel flange gap was calculated by Fluent software, and the excitation parameters were obtained by linear fitting.
The calculated data points and their fitted curves are shown in Figure 5. It can be seen that with the increase of vortex speed and the decrease of the wheel flange clearance, the absolute value of the excitation force increases.
Combined with the fitted curve and formula, the equivalent stiffness and damping at each gap can be obtained, as shown in Table 4. Among them, the main stiffness and main damping increase with the decrease of the wheel flange clearance, and the cross stiffness is lower than that of the blade turbine.
bibliography
[1] HUANG Ruo, ZHANG Weili, XING Weidong, et al. Effect of airflow excitation on rotordynamic characteristics of ball bearing turbocharger[J].Journal of Vibration and Shock,2014,33(19):140-146.)
[2] LIU Yang, TAO Jizhong, ZHENG Yueqing, et al. Dynamic analysis of viscoelastic supported flexible rotor[J].Mechanical Design and Manufacturing,2014(06):191-194.)
