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

2. 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.

3. 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

4. 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.

5. Complete design of the proposed system

6. Two Stage CryoCooler Neon RTBC and Helium RTBC

7. 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.

8. Thermodynamics schematic

9. 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

10. 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.

11. Schematic of the bottom cycle showing the four stage Helium compressor

12. Compressor Design and CFD Analysis

13. 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.

14. Existing Single Stage Design

15. Single Stage Centrifugal Compressor Development
Motor
Coupler
Compressor

16. Impeller
Diffuser
Inlet Guide Vane

17. Experimental Set-up

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

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

20. 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.

21. Conceptual Design of the Gas Foil Bearings

22. Schematic of the conceptual design

23. 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’.

25. Motor Design

26. 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.

27. 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.

28. 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

29. Shaft Structure

30. 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 .

31. 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.

32. Simulated Back EMF- Litz Rectangular Wire Configuration

33. Simulated Current- Litz Rectangular Wire Configuration

34. 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.

35. Development of Tribological Coatings

36. 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 .

37. 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.

38. 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
200.21
HV –Vicker’s Hardness
Films have shown good stoichiometric ratio of Ti & N

39. 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.

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

41. 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

42. 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.

43. 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.