Design and development of liquid hydrogen mixer unit

A dc brushless motor driven mixer unit (a vaneaxial fan) was designed and evaluated for use in cryogenic fluids (liquids and gases). It was found to operate well in all fluids in which it was tested. The test fluids were liquid and gaseous helium, hydrogen in the liquid phase gas phase and mixtures of the two phases, and liquid nitrogen. It operated for over 100 hr at cryogenic temperatures without damage to the bearings and at the conclusion of testing the condition of the bearings was such that an operational life of 5000 hr appeared possible. The unit demonstrated that (with the brushless dc motor principle) useful pumping of cryogenic fluids could be accomplished with very small power inputs to the mixer motor. During test, the motor input power varied from approximately 0.5 W to 2.5 W, depending on fluid density. The high power input produced a mixer efficiency of over 49 percent. In an earlier program (see NASA Report CR-72365), a mixer using an ac induction motor demonstrated an efficiency of 17.7 percent under the same conditions, illustrating the advantage of the dc brushless motor driven unit. The mixer also demonstrated its ability to automatically vary speed as a function of the density of the fluid being pumped. This causes the unit to deliver higher volumetric flow rates as fluid density decreases.