MINIATURE ENGINEERING SYSTEMS GROUP Two-Stage CryoCooler Development for Liquid Hydrogen Systems This work is supported by NASA Hydrogen Research at Florida Universities.

List of Authors L.An, Q.Chen, J.Cho, L.Chow, N.Dhere, C.Ham, J.Kapat, K.B.Sundaram, T.Wu, K.Finney, X.Y.Li, K.Murty, A.Pai, H.Seigneur, L.Zhao, L.Zheng, L.Zhou.   Dept. of Mechanical, Materials and Aerospace Engineering, School of Electrical Engineering and Computer Science, and Florida Solar Energy Center.

Outline of the presentation Introduction Complete design of the proposed system Compressor design and CFD analysis Conceptual design of Gas Foil Bearings Motor design Development of tribological coatings Conclusion

Introduction Spaceport of future will use large quantities of liquid hydrogen. Efficient storage and transfer of liquid hydrogen is necessary for reducing launch costs. An overall design of a two-stage cyrocooler for application in zero boil-off for long duration storage of liquid hydrogen systems is presented here. Primary focus of the presentation is on the design concept of centrifugal helium compressor for bottom stage of the working cycle, motor for driving the compressor, bearings and tribological coatings for the system.

Complete design of the proposed system

Two Stage CryoCooler Neon RTBC and Helium RTBC

Advantages Over Existing CryoCoolers for Liquid Hydrogen Systems High COP for the overall system due to high efficiency values of compressors and motors compared to what is available commercially. High system reliability because of a “DC” cryo-cycle, rotary components, gas foil bearings and advanced tribological coatings.

Thermodynamics schematic

System Performance Top cycle is capable of removing heat at liquid Nitrogen temperature with cooling power ~ 1000 W 2-stage RTBC cycle is capable of removing heat at liquid Hydrogen temperature with cooling power ~ 50W COP ~ 0.007

Design Features Top cycle can work separately as a liquid nitrogen cryocooler; or it can work with bottom cycle as a liquid hydrogen cryocooler. State-of-the-art aerodynamics design of the 2-stage intercooled neon centrifugal compressor and the 4-stage intercooled helium centrifugal compressor. Integrated motor and oil-free non-contact bearings for high speed and efficiency. Hard, low friction and durable coatings at cryogenic temperature. Innovative micro-channel high effectiveness heat exchanger.

Schematic of the bottom cycle showing the four stage Helium compressor

Compressor Design and CFD Analysis

Single Stage Compressor Single stage compressor is being developed first to aid the design of more complex two and four stage compressors. Plastic models have been created showing the conceptual idea. They indicate the small size of the parts.

Existing Single Stage Design

Single Stage Centrifugal Compressor Development Motor Coupler Compressor

Impeller Diffuser Inlet Guide Vane

Experimental Set-up

Fully Structured 3D Grid CFD Simulation of IGV click to see movie Fully Structured 3D Grid (Created in GAMBIT, 330K)

Reverse flow occurs at outlet of IGV. (Solved by Fluent 6.0)

CFD-IGV CFD simulation results show that pressure loss through IGV is about 5000 Pa. As expected, IGV creates an acceptable flow angle at the eye of impeller. However, certain amount of reverse flow still exists in spite of careful design. This may be eliminated by the interaction of IGV and rotor, which would be simulated in the next stage. If the flow reversal still persists, IGV design will be modified by adjusting angle of IGV vanes.

Conceptual Design of the Gas Foil Bearings

Schematic of the conceptual design

Conceptual Design Configuration It contains an outer hollow cylinder to which the foils are attached. An inner hollow cylinder would have long cut grooves extending to about 90% of its length through which the foils would pass and hold the shaft in position during start-up and at stop. The outer hollow cylinder can be rotated about the shaft center axis of rotation and the rotation of which would cause the foils to lose contact with the shaft thus making the same bearing as ‘Gas Bearing’ and also as a ‘Gas Foil Bearing’.

Motor Design

Specifications of the Motor Output Shaft Power 2000W Shaft Speed 200krpm Shaft Diameter 10-20mm Max. Length 100mm Max. Outer Diameter 44mm DC Power Supply 28V The motor efficiency needs to be as high as possible. Size and weight are also important issues.

Some Popular Motor Types Induction motor (IM) : low cost, but low efficiency at high speed due to higher iron loss. Switched reluctance motor (SRM): high reliability, but iron loss is very critical at high speed. Permanent magnet synchronous motor (PMSM): very high efficiency due to no exciting copper loss in the rotor. High power density with high energy density permanent magnet Nd-Fe-B. Brushless DC motor (BLDC): high power density as PMSM, but the large harmonics will reduce efficiency significantly at high speed.

Radial Flux PMSM Structure Shaft Stator Outer Diameter = 36mm Stator Inner Diameter = 26.3mm Rotor Diameter = 16mm PM Width = 7mm PM Height = 15mm Motor Active Length = 25.4mm PM Winding Laminated low loss core

Shaft Structure

Winding Method 2-pole, 3-phase. 5 coils/phase/pole. Two layer lap winding. Pitch factor: 23/30. First coil: Top1 -> Btm12. Rectangular Litz wire. 50 strands AWG30. Outer dimensions: 1.78x2.27mm2 .

Wire Selection Solid copper wire AWG13(Do:75mil, R:2mΩ/ft) AWG14(Do:67mil,R:2.6mΩ/ft) Litz wire (multi-strand configuration) Round Litz Wire Rectangular Benefit of using Litz-wire Easy shape. Reduce skin effect, proximity effect. Each strand tends to take all possible positions in the cross-section of the entire conductor.

Simulated Back EMF- Litz Rectangular Wire Configuration

Simulated Current- Litz Rectangular Wire Configuration

Efficiency Copper Loss 16.9W Iron Loss 16.4W Windage Loss 21W Motor Efficiency 97.3% Control Efficiency 95% Total Efficiency 92.5% *The bearing loss is not considered here, since we will use gas foil bearing later.

Development of Tribological Coatings

Objective The goal is to develop tribological coatings having extremely high hardness, extremely low coefficient of friction, and high durability at temperatures <60 K To fulfill these functional demands, an adequate adhesion between coating and substrate as well as an adequate load carrying capacity are both essential. Extremely hard and extremely low friction coatings of titanium nitride (TiN) and molybdenum disulphide (MoS2) as well as diamond-like-carbon (DLC) were chosen for this research based on literature for friction behavior and wear resistance under cryogenic temperatures .

Titanium Nitride (TiN) Coatings - By DC Magnetron Sputtering DC Magnetron Sputtering was used to achieve film thicknesses of approximately 1 micron. Number of Depositions have been carried out by DC Magnetron Sputtering under varying conditions. Achieved film thickness of > 1 micron. Peel test has shown good adhesion of TiN coatings with glass substrates. Dektak Profilometer have shown good uniformity of TiN films. Analysis by Energy Dispersive Spectroscopy (EDS) and microhardness measurements have been carried out. Results for three samples are shown in the following slides.

Titanium Nitride (TiN) Coatings EDS analysis and results of microhardness measurement Sample ID N2 : Ar Ratio Atomic Percent Nitrogen: Argon Average Hardness Elastic Modulus (GPa) GPa HV (Kgf/mm2) 1 0.5: 6 N2 :Ti = 50.3:49.7 9.32 878.47 144.20 2 0.5 : 4 N2:Ti = 53.05:46.95 ----- 3 1: 4 N2:Ti = 52:48 16.62 1567.02 200.21 HV –Vicker’s Hardness Films have shown good stoichiometric ratio of Ti & N

Titanium Nitride (TiN) Coatings Several more depositions of TiN films by DC magnetron sputtering were carried out. The limit in terms of varying the argon to nitrogen ratio was reached as the films indicated greater porosity and signs of peeling off. Characteristic golden color of TiN films was achieved. XRD analysis of the above samples indicated fully reacted microcrystalline TiN nature that may provide excellent hardness. Additional samples on aluminum substrates will be prepared using optimized parameters based on the above observations for XRD, microhardness, wear and coefficient of friction analysis. Mask required for deposition of TiN coatings on three bump on 1cm x 1 cm silicon wafer to minimize the contact area between two rubbing samples and providing more accurate coefficient of friction and wear measurements has been designed and procured.

Titanium Nitride (TiN) Coatings TiN coatings deposited on Aluminum substrates

Molybdenum Disulphide (MoS2) Coatings Depositions of MoS2 by RF magnetron sputtering were carried out. XRD analysis of the samples indicated fully reactive microcrystalline MoS2 nature. Deposition of bilayer coatings of TiN and MoS2 on a glass substrate have been carried out. Testing of the above film will be carried out for satisfying requirements of good wear resistance and low coefficient of friction coatings. Hard Coatings at Cryogenic Temperatures Cryogenic environment leads to increase in the coefficient of friction and DLC coatings have lower coefficient of friction and good wear resistance as compared to hard coatings of nitrides at cryogenic temperatures. A special cryogenic tribometer is required for the study of friction and wear at cryogenic temperatures

Vacuum Gauge Controller Microwave CVD Setup 5’ Vacuum Gauge Controller Fw Pw Ref Pw 12” 3’ Microwave Generator 4 Stub Tuner Plasma Applicator Sliding Short Circuit Dual Directional Coupler RF Substrate Biasing Baffle MW Control Panel Turbo- Molecular Assembly Pump & Exhaust (1.5 “ tube) Mechanical Pump 1“ feedthrough hole for water inlet and substrate biasing Convectron Gauge Ionization Gauge Six-Way Cross-Chamber Fixed Flanges, 3 places Rotatable Flanges, Microwave assisted plasma chemical vapor deposition system (MWCVD) has been ordered for depositions of diamond-like carbon (DLC) coatings.

Conclusion An innovative concept of a Two Stage CryoCooler for maintaining Hydrogen at liquid state is presented. The system is highly efficient and reliable for manufacture and storage of liquid hydrogen.