WE-NET Home Page/Summary of Annual Reports 1997

5.4 Development of devices for common use

5.4.1 R & D Goals

Goals of studies in the Phase I are that to make a conceptual design of devices and equipment necessary for constructing large-scale liquid hydrogen transportation-storage system, and to select technical items to be developed.

A large liquid hydrogen pump is required to be capable of providing a stable supply of liquid hydrogen for a long time. Meanwhile, liquid hydrogen as operating fluid is cryogenic with a temperature of 20K, low viscous with viscosity of 1.3e-5 kg/m*s, and has low density of 71 kg/m³. The bearing for the pump, therefore, faces very severe lubricating conditions, and developing a bearing which is capable of operating for a long time under this conditions is essential for preparing the liquid hydrogen pump. One possibility for such a bearing is a magnetically floating bearing.

In fiscal 1997, an experimental study on a magnetic bearing in the liquid hydrogen was conducted in order to confirm the design suitability and technological problems anticipated.

5.4.2 Results in fiscal year 1997

5.4.2.1 Design and Preparation of a Bearing

The designed and prepared magnetic bearing is shown in Fig 5-4-1. As shown in the figure, the bearing consists of two radial bearings, one thrust bearing, pump impellers and a turbine blades on each shaft end. Because of restrictions in terms of the control characteristics, the overall configuration is designed to ensure that the primary specific frequency of bending mode of the rotor is not below the target rotation frequency. The materials employed are titanium for the rotor and the turbine, permalloy for the electromagnet and aluminum for the casing.

Concerning the float control, each parameter of penta-axial control is set to keep the stable condition. The position sensors, which are as same as those used in the liquid hydrogen pump for a rocket engine, are placed facing each other and to compensate in terms of their temperature characteristics.

5.4.2.2 Bearing Tests

The LN₂ and LH₂ tests were conducted using the prototype bearing prepared in fiscal 1997. Fig 5-4-2 shows the test device.

Because of delivery problems of the pump impellers, a dummy pump was installed to conduct the rotation test. This limited the maximum rotation frequency in the LH₂ test up to 19,000 rpm as described later.

(1) Test Details

[Ambient Temperature Test]
To obtain characteristic data on the magnetic bearing and to conduct the floating test.

[LN₂ Test]
To conduct the floating test after obtaining characteristic data on the displacement sensors and electromagnet at the LN₂ temperature, followed by an attempt to confirm the stable operation of the magnetic bearing under the load condition with the bearing being rotated by gaseous N₂.

[LH₂ Test]
To conduct the floating test after obtaining characteristic data at the LH₂ temperature, followed by an attempt to confirm the stable operation of the magnetic bearing with the turbine being operated by gaseous H₂.

(2) Test Results

The characteristics of the sensors and electromagnet at an ambient temperature, LN₂ temperature and LH₂ temperature were obtained. In addition, stable floating at each temperature was confirmed by the relevant floating test.

In the case of the LN₂ test at 77K, the maximum turbine rotation reached 14,000 rpm. When the rotation passed through the first and secondary critical speeds (6,000 - 10,000 rpm), the shaft vibration temporarily increased to 60mm p-p, and went below 20mm p-p thereafter. At around 14,000 rpm, the pump began to stall, increasing the rotation to 18,000 rpm with the shaft vibration reaching 60mm p-p. At this point, the test was suspended.

In the case of the LH₂ test at 20K, the rotation test was conducted with gaseous H₂ and the maximum rotation reached 19,000 rpm. As in the case of the LN₂ test, the shaft vibration increased to 70mm p-p when the rotation frequency passed the critical speed. The whirling of the shaft in this instance is shown in Fig 5-4-3. After passing the critical speed, the shaft vibration dropped. The shaft whirling at the rotation of 19,000 rpm was less than 20mm p-p as shown in Fig 5-4-3, confirming that the magnetic bearing can support the shaft.

(3) Conclusions

The test results can be summarized as follows:

An experimental magnetic bearing for cryogenic application was prepared and the appropriate working of the sensors and electromagnet under cryogenic conditions involving those of LN₂ and LH₂ was confirmed.
The rotation test under cryogenic conditions achieved rotation speed up to 19,000 rpm.
The critical speed of the shaft at around 6,000 - 10,000 rpm was safely passed. Further improvements will be made through fine tuning of the control system.
The tests in fiscal 1997 were restricted to the maximum rotation speed of 20,000 rpm because of the reasons given below. In the next year and thereafter, the achievement of rated operation at 36,000 rpm is aimed at using a normal pump or adding an inducer along with measures to prevent stalling.

Instead of a normal pump, a dummy impeller were installed to create the load. As the balance adjustment of the impeller was not conducted, they caused an imbalance.
The dummy impeller has a shape liable to make stalling. If the pump stalls at a rotation speed of more than 20,000 rpm, excessive rotation may result, exceeding the rated rotation speed of 36,000 rpm. When such a situation occurs, the pump enters the unstable control range and becomes uncontrollable.

5.4.3 Research plan for fiscal year 1998

Elemental development of liquid hydrogen pumps will be continued in fiscal year 1998. The magnetic bearing device will be modified with an inducer and evaluated its performance up to 36,000rpm with liquid hydrogen.