Turbocharger Supported on Floating Ring Bearings

A turbocharger is usually supported by the hydrodynamic journal bearings — very often the floating ring bearings. These bearings are the extension of the plain journal bearings in which a ring is inserted between the journal and bushing surfaces. Therefore, such a bearing can be considered as two plain journal bearings working in series. However, the both films are not completely disconnected rather they are connected through the oil channels in the ring.

A time dependent analysis is performed to analyze the dynamics of the rotor. Results include stress in the rotor, pressure in the bearing, ring speed and torque, and flow through channels in the ring.

Model Definition

The model consists of a turbocharger rotor supported by two floating ring bearings, one near the compressor and another near the turbine, making both the compressor and turbine overhung on the shaft. The schematic of the rotor is shown in Figure 1.

Figure 1: Rotor sketch.

The rotor of the turbocharger is modeled using the Beam Rotor interface and the bearings are modeled using the Floating Ring Bearing feature. The connection between the inner and outer film is established using the Inner-Outer Film Connection feature on the Floating Ring Bearing. The coupling between the rotor and the bearing is handled using the multiphysics coupling feature. The Disk feature is used to model the inertia of the turbine and the compressor. The properties of the rotor are given in Table 1.

Table 1: Rotor properties.
Parameter	value
Young’s modulus of the rotor, E	205 GPa
Poisson’s ratio of the rotor, ν	0.3
Density of the rotor, ρ	7800 kg/m3
Length of the rotor, L	0.15 m
Location of the turbine	0.1 L
Location of the bearing near turbine	0.3 L
Location of the bearing near compressor	0.7 L
Location of the compressor	0.9 L
Mass of the turbine, mt	1.4 kg
Transverse moment of inertia of the turbine, Idt	6.3 x 10-4 kg.m2
Polar moment of inertia of the turbine, Ipt	1.26 x 10-4 kg.m2
Mass of the compressor, mc	1.0 kg
Transverse moment of inertia of the compressor, Idc	4.5 x 10-4 kg.m2
Polar moment of inertia of the compressor, Ipc	9 x10-4 kg.m2

Bearing properties are given in Table 2.

Table 2: Bearing properties.
Parameter	value
Mass of the ring, mr	0.02 kg
Outer radius of the ring, Ro	9 mm
Inner radius of the ring, Ri	6 mm
Outer bearing clearance, Co	0.08 mm
Inner bearing clearance, Ci	0.02 mm
Viscosity of the lubricant, μ	0.06 Pa.s
Length of the bearing, Lb	0.01 m

The rotor is rotating at 8000 rpm and is initially concentric with the bearing. A gravity load acts on the rotor. A time dependent analysis is performed to analyze the response of the turbocharger system.

Results and Discussion

The pressure in the bearings and the stress in the rotor is shown in Figure 2. The maximum stress occurs in the central region of the rotor. The pressure is maximum in the bottom of the bearing because the rotor moves downward from the bearing center due to gravitational force. The clearance between the journal and the ring as well as the ring and the bushing decreases due to this motion.

Figure 2: Pressure and stress in the turbocharger.

The variation of the axial rotational speed of the ring is shown in Figure 3. Initially the ring and bushing are stationary but the journal is moving at the prescribed angular velocity. Due to this there is a relative slip motion in the inner film whereas in the outer film there is no slip motion. Because of the slip in the inner film a viscous torque acts on the ring and there is zero torque from the outer film at this stage. The torque on the ring is shown in Figure 4. Due to the net torque on the ring, it starts rotating axially. The ring rotation decreases the relative slip in the inner film but increases it in the outer film. As a result, the torque from the inner film reduces and at the same time torque from the outer film increases. Also, the torque from the outer film opposes the torque from the inner film. Thus the net angular acceleration of the ring decreases as the ring picks up speed. Eventually, the ring will attain a speed at which the torque from both the inner and outer films are equal and opposite and at this stage the net angular acceleration of the ring becomes zero. Therefore, ring continues to rotate at this speed after this.

Figure 3: Ring speed.

The orbit of rotor in the ring, shown in Figure 5, shows that after going through the initial transient phase, the rotor performs a small amplitude whirl in the ring about an equilibrium point.

Figure 4: Torque on the ring.

Figure 5: Rotor orbit in the ring.

The ring orbit characteristics are also similar to the rotor shown in Figure 6. However, in steady state the whirling of the ring has a very small amplitude as compared to the rotor.

Figure 6: Ring orbit.

Figure 7: Mass flow rate through channels.

Mass flow rate in the first channel of both the bearings is shown in Figure 7. During the transient response we see a non periodic behavior in the flow rate, but as it reaches steady state the flow rate variation is periodic. Also, flow rate changes from positive to negative values during a cycle. This is an indication of a flow happening both ways from inner film to outer film and vice versa.

Rotordynamics_Module/Automotive_and_Aerospace/turbocharger_transient_analysis

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