Abstract
A vibration measurement of a mechanical element in liquid hydrogen was conducted using a fiber Bragg grating (FBG) sensor. The monitored mechanical element was an 8-mm diameter stainless-steel rod connected to a bearing cartridge of a ball-bearing, which is an element of a liquid rocket engine. The rotation of the ball-bearing vibrates both the cartridge and the rod to be monitored. The vibration was measured during a rotational test where the test temperature and the maximum rotational speed were 26 K and 30,000 rpm, respectively. The FBG-sensor signals were recorded at the sampling frequency of 50 kHz. The vibration of the rod was measured successfully up to 25 kHz despite the root-mean-square level of the dynamic strain smaller than 10x10-6. The frequency analysis revealed that the FBG-sensor signal corresponded well with the rotational speed of the ball bearing. The peak frequency agreed with the first-order frequency of the rotational speed of the ball-bearing. The experiment demonstrates that FBG sensors are available to mechanical measurements of liquid rocket engines during operational tests, and also seem to be useful for mechanical measurements of fuel cell cars and hydrogen engine cars.
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